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81. Electrical Appliances and Equipment

81. Electrical Appliances and Equipment (7)

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81. Electrical Appliances and Equipment

Chapter Editor: N. A. Smith


Table of Contents

Tables and Figures

General Profile
N. A. Smith

Lead-Acid Battery Manufacture
Barry P. Kelley

Batteries
N. A. Smith

Electric Cable Manufacture
David A. O’Malley

Electric Lamp and Tube Manufacture
Albert M. Zielinski

Domestic Electrical Appliance Manufacture
N. A. Smith and W. Klost

Environmental and Public Health Issues
N. A. Smith

Tables

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1. Composition of common batteries
2. Manufacture: domestic electrical appliances

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82.  Metal Processing and Metal Working Industry

82. Metal Processing and Metal Working Industry (14)

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82.  Metal Processing and Metal Working Industry

Chapter Editor: Michael McCann


Table of Contents

Tables and Figures

General Profile

Smelting  and Refining Operations

Smelting and Refining
Pekka Roto

Copper, Lead and Zinc Smelting and Refining

Aluminium Smelting and Refining
Bertram D. Dinman

Gold Smelting and Refining
I.D. Gadaskina and L.A. Ryzik

Metal  Processing and Metal Working

Foundries
Franklin E. Mirer

Forging and Stamping
Robert M. Park

Welding and Thermal Cutting
Philip A. Platcow and G.S. Lyndon

Lathes
Toni Retsch

Grinding and Polishing
K. Welinder

Industrial Lubricants, Metal Working Fluids and Automotive Oils
Richard S. Kraus

Surface Treatment of  Metals
J.G. Jones, J.R. Bevan, J.A. Catton, A. Zober, N. Fish, K.M. Morse, G. Thomas, M.A. El Kadeem and Philip A. Platcow

Metal Reclamation
Melvin E. Cassady and Richard D. Ringenwald, Jr.

Environmental Issues in Metal Finishing and Industrial Coatings
Stewart Forbes

Tables

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1. Inputs & outputs for copper smelting
2. Inputs & outputs for lead smelting
3. Inputs & outputs for zinc smelting
4. Inputs & outputs for aluminium smelting
5. Types of foundry furnaces
6. Process materials inputs and pollution outputs
7. Welding processes: Description & hazards
8. Summary of the hazards
9. Controls for aluminium, by operation
10. Controls for copper, by operation
11. Controls for lead, by operation
12. Controls for zinc, by operation
13. Controls for magnesium, by operation
14. Controls for mercury, by operation
15. Controls for nickel, by operation
16. Controls for precious metals
17. Controls for cadmium, by operation
18. Controls for selenium, by operation
19. Controls for cobalt, by operation
20. Controls for tin, by operation
21. Controls for titanium, by operation

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83. Microelectronics and Semiconductors

83. Microelectronics and Semiconductors (7)

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83. Microelectronics and Semiconductors

Chapter Editor: Michael E. Williams


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Tables and Figures

General Profile
Michael E. Williams

Silicon Semiconductor Manufacturing
David G. Baldwin, James R. Rubin and Afsaneh Gerami

Liquid Crystal Displays
David G. Baldwin, James R. Rubin and Afsaneh Gerami

III-V Semiconductor Manufacturing
David G. Baldwin, Afsaneh Geramiand James R. Rubin

Printed Circuit Board and Computer Assembly
Michael E. Williams

Health Effects and Disease Patterns
Donald V. Lassiter

Environmental and Public Health Issues
Corky Chew

Tables

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1. Photoresist systems
2. Photoresist strippers
3. Wet chemical etchants
4. Plasma etching gases & etched materials
5. Junction formation dopants for diffusion
6. Major categories of silicon epitaxy
7. Major categories of CVD
8. Cleaning of flat panel displays
9. PWB process: Environmental, health & safety
10. PWB waste generation & controls
11. PCB waste generation & controls
12. Waste generation & controls
13. Matrix of priority needs

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84. Glass, Pottery and Related Materials

84. Glass, Pottery and Related Materials (3)

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84. Glass, Pottery and Related Materials

Chapter Editors: Joel Bender and Jonathan P. Hellerstein


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Tables and Figures

Glass, Ceramics and Related Materials
Jonathan P. Hellerstein, Joel Bender, John G. Hadley and Charles M. Hohman

     Case Study: Optical Fibres
     George R. Osborne

     Case Study: Synthetic Gems
     Basil Dolphin

Tables

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1. Typical body constituents
2. Manufacturing processes
3. Selected chemical additives
4. Refractory usage by industry in the USA
5. Potential health & safety hazards
6. Nonfatal occupational injury & illness

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85. Printing, Photography and Reproduction Industry

85. Printing, Photography and Reproduction Industry (6)

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85. Printing, Photography and Reproduction Industry

Chapter Editor: David Richardson


Table of Contents

Tables and Figures

Printing and Publication
Gordon C. Miller

Reproduction and Duplicating Services
Robert W. Kilpper

Health Issues and Disease Patterns
Barry R. Friedlander

Overview of Environmental Issues
Daniel R. English

Commercial Photographic Laboratories
David Richardson

Tables

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1. Exposures in the printing industry
2. Printing trade mortality risks
3. Chemical exposure in processing

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86. Woodworking

86. Woodworking (5)

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86. Woodworking

Chapter Editor: Jon Parish


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Tables and Figures

General Profile
Debra Osinsky

Woodworking Processes
Jon K. Parish

Routing Machines
Beat Wegmüller

Wood Planing Machines
Beat Wegmüller

Health Effects and Disease Patterns
Leon J. Warshaw

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1. Poisonous, allergenic & biologically active wood varieties

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Wednesday, 16 March 2011 21:30

Welding and Thermal Cutting

Written by

This article is a revision of the 3rd edition of the Encyclopaedia of Occupational Health and Safety article “Welding and thermal cutting” by G.S. Lyndon.

Process Overview

Welding is a generic term referring to the union of pieces of metal at joint faces rendered plastic or liquid by heat or pressure, or both. The three common direct sources of heat are:

  1. flame produced by the combustion of fuel gas with air or oxygen
  2. electrical arc, struck between an electrode and a workpiece or between two electrodes
  3. electrical resistance offered to passage of current between two or more workpieces.

 

Other sources of heat for welding are discussed below (see table 1).

Table 1. Process materials inputs and pollution outputs for lead smelting and refining

Process

Material input

Air emissions

Process wastes

Other wastes

Lead sintering

Lead ore, iron, silica, limestone flux, coke, soda, ash, pyrite, zinc, caustic, baghouse dust

Sulphur dioxide, particulate matter contain-ing cadmium and lead

   

Lead smelting

Lead sinter, coke

Sulphur dioxide, particulate matter contain-ing cadmium and lead

Plant washdown wastewater, slag granulation water

Slag containing impurities such as zinc, iron, silica and lime, surface impoundment solids

Lead drossing

Lead bullion, soda ash, sulphur, baghouse dust, coke

   

Slag containing such impurities as copper, surface impoundment solids

Lead refining

Lead drossing bullion

     

 

In gas welding and cutting, oxygen or air and a fuel gas are fed to a blowpipe (torch) in which they are mixed prior to combustion at the nozzle. The blowpipe is usually hand held (see figure 1). The heat melts the metal faces of the parts to be joined, causing them to flow together. A filler metal or alloy is frequently added. The alloy often has a lower melting point than the parts to be joined. In this case, the two pieces are generally not brought to fusion temperature (brazing, soldering). Chemical fluxes may be used to prevent oxidation and facilitate the joining.

Figure 1. Gas welding with a torch & rod of filter metal. The welder is protected by a leather apron, gauntlets and goggles

MET040F1

In arc welding, the arc is struck between an electrode and the workpieces. The electrode can be connected to either an alternating current (AC) or direct current (DC) electric supply. The temperature of this operation is about 4,000°C when the workpieces fuse together. Usually it is necessary to add molten metal to the joint either by melting the electrode itself (consumable electrode processes) or by melting a separate filler rod which is not carrying current (non-consumable electrode processes).

Most conventional arc welding is done manually by means of a covered (coated) consumable electrode in a hand-held electrode holder. Welding is also accomplished by many semi or fully automatic electric welding processes such as resistance welding or continuous electrode feed.

During the welding process, the welding area must be shielded from the atmosphere in order to prevent oxidation and contamination. There are two types of protection: flux coatings and inert gas shielding. In flux-shielded arc welding, the consumable electrode consists of a metal core surrounded by a flux coating material, which is usually a complex mixture of mineral and other components. The flux melts as welding progresses, covering the molten metal with slag and enveloping the welding area with a protective atmosphere of gases (e.g., carbon dioxide) generated by the heated flux. After welding, the slag must be removed, often by chipping.

In gas-shielded arc welding, a blanket of inert gas seals off the atmosphere and prevents oxidation and contamination during the welding process. Argon, helium, nitrogen or carbon dioxide are commonly used as the inert gases. The gas selected depends upon the nature of the materials to be welded. The two most popular types of gas-shielded arc welding are metal- and tungsten inert gas (MIG and TIG).

Resistance welding involves using the electrical resistance to the passage of a high current at low voltage through components to be welded to generate heat for melting the metal. The heat generated at the interface between the components brings them to welding temperatures.

Hazards and Their Prevention

All welding involves hazards of fire, burns, radiant heat (infrared radiation) and inhalation of metal fumes and other contaminants. Other hazards associated with specific welding processes include electrical hazards, noise, ultraviolet radiation, ozone, nitrogen dioxide, carbon monoxide, fluorides, compressed gas cylinders and explosions. See table 2 for additional detail.

Table 2. Description and hazards of welding processes

Welding Process

Description

Hazards

Gas welding and cutting

Welding

The torch melts the metal surface and filler rod, causing a joint to be formed.

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns, infrared radiation, fire, explosions

Brazing

The two metal surfaces are bonded without melting the metal. The melting temperature of the filler metal is above 450 °C. Heating is done by flame heating, resistance heating and induction heating.

Metal fumes (especially cadmium), fluorides, fire, explosion, burns

Soldering

Similar to brazing, except the melting temperature of the filler metal is below 450 °C. Heating is also done using a soldering iron.

Fluxes, lead fumes, burns

Metal cutting and flame gouging

In one variation, the metal is heated by a flame, and a jet of pure oxygen is directed onto the point of cutting and moved along the line to be cut. In flame gouging, a strip of surface metal is removed but the metal is not cut through.

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns, infrared radiation, fire, explosions

Gas pressure welding

The parts are heated by gas jets while under pressure, and become forged together.

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns, infrared radiation, fire, explosions

Flux-shielded arc welding

Shielded metal arc welding (SMAC); “stick” arc welding; manual metal arc welding (MMA); open arc welding

Uses a consumable electrode consisting of a metal core surrounded by a flux coating

Metal fumes, fluorides (especially with low-hydrogen electrodes), infrared and ultraviolet radiation, burns, electrical, fire; also noise, ozone, nitrogen dioxide

Submerged arc welding (SAW)

A blanket of granulated flux is deposited on the workpiece, followed by a consumable bare metal wire electrode. The arc melts the flux to produce a protective molten shield in the welding zone.

Fluorides, fire, burns, infrared radiation, electrical; also metal fumes, noise, ultraviolet radiation, ozone, and nitrogen dioxide

Gas-shielded arc welding

Metal inert gas (MIG); gas metal arc welding (GMAC)

The electrode is normally a bare consumable wire of similar composition to the weld metal and is fed continuously to the arc.

Ultraviolet radiation, metal fumes, ozone, carbon monoxide (with CO2 gas), nitrogen dioxide, fire, burns, infrared radiation, electrical, fluorides, noise

Tungsten inert gas (TIG); gas tungsten arc welding (GTAW); heliarc

The tungsten electrode is non-consumable, and filler metal is introduced as a consumable into the arc manually.

Ultraviolet radiation, metal fumes, ozone, nitrogen dioxide, fire, burns, infrared radiation, electrical, noise, fluorides, carbon monoxide


Plasma arc welding (PAW) and plasma arc  spraying; tungsten arc cutting

Similar to TIG welding, except that the arc and stream of inert gases pass through a small orifice before reaching the workpiece, creating a “plasma” of highly ionized gas which can achieve temperatures of over 33,400°C.This is also used for metallizing.

Metal fumes, ozone, nitrogen dioxide, ultraviolet and infrared radiation, noise; fire, burns, electrical, fluorides, carbon monoxide, possible x rays

Flux core arc welding (FCAW); metal active gas welding (MAG)

Uses a flux-cored consumable electrode; may have carbon dioxide shield (MAG)

Ultraviolet radiation, metal fumes, ozone, carbon monoxide (with CO2 gas), nitrogen dioxide, fire, burns, infrared radiation, electrical, fluorides, noise

Electric resistance welding

Resistance welding (spot, seam, projection or butt welding)

A high current at low voltage flows through the two components from electrodes. The heat generated at the interface between the components brings them to welding temperatures. During the passage of the current, pressure by the electrodes produces a forge weld. No flux or filler metal is used.

Ozone, noise (sometimes), machinery hazards, fire, burns, electrical, metal fumes

Electro-slag welding

Used for vertical butt welding. The workpieces are set vertically, with a gap between them, and copper plates or shoes are placed on one or both sides of the joint to form a bath. An arc is established under a flux layer between one or more continuously fed electrode wires and a metal plate. A pool of molten metal is formed, protected by molten flux or slag, which is kept molten by resistance to the current passing between the electrode and the workpieces. This resistance-generated heat melts the sides of the joint and the electrode wire, filling the joint and making a weld. As welding progresses, the molten metal and slag are retained in position by shifting the copper plates.

Burns, fire, infrared radiation, electrical, metal fumes

Flash welding

The two metal parts to be welded are connected to a low-voltage, high-current source. When the ends of the components are brought into contact, a large current flows, causing “flashing” to occur and bringing the ends of the components to welding temperatures. A forge weld is obtained by pressure.

Electrical, burns, fire, metal fumes


Other welding processes

Electron beam welding

A workpiece in an vacuum chamber is bombarded by a beam of electrons from an electron gun at high voltages. The energy of the electrons is transformed into heat upon striking the workpiece, thus melting the metal and fusing the workpiece.

X rays at high voltages, electrical, burns, metal dusts, confined spaces

Arcair cutting

An arc is struck between the end of a carbon electrode (in a manual electrode holder with its own supply of compressed air) and the workpiece. The molten metal produced is blown away by jets of compressed air.

Metal fumes, carbon monoxide, nitrogen dioxide, ozone, fire, burns, infrared radiation, electrical

Friction welding

A purely mechanical welding technique in which one component remains stationary while the other is rotated against it under pressure. Heat is generated by friction, and at forging temperature the rotation ceases. A forging pressure then effects the weld.

Heat, burns, machinery hazards

Laser welding and drilling

Laser beams can be used in industrial applications requiring exceptionally high precision, such as miniature assemblies and micro techniques in the electronics industry or spinnerets for the artificial fibre industry. The laser beam melts and joins the workpieces.

Electrical, laser radiation, ultraviolet radiation, fire, burns, metal fumes, decomposition products of workpiece coatings

Stud welding

An arc is struck between a metal stud (acting as the electrode) held in a stud welding gun and the metal plate to be joined, and raises the temperature of the ends of the components to melting point. The gun forces the stud against the plate and welds it. Shielding is provided by a ceramic ferrule surrounding the stud.

Metal fumes, infrared and ultraviolet radiation, burns, electrical, fire, noise, ozone, nitrogen dioxide

Thermite welding

A mixture of aluminium powder and a metal oxide powder (iron, copper, etc.) is ignited in a crucible, producing molten metal with the evolution of intense heat. The crucible is tapped and the molten metal flows into the cavity to be welded (which is surrounded by a sand mould). This is often used to repair castings or forgings.

Fire, explosion, infrared radiation, burns

 

Much welding is not done in shops where conditions can generally be controlled, but in the field in the construction or repair of large structures and machinery (e.g., frameworks of buildings, bridges and towers, ships, railroad engines and cars, heavy equipment and so on). The welder may have to carry all his or her equipment to the site, set it up and work in confined spaces or on scaffolds. Physical strain, inordinate fatigue and musculoskeletal injuries may follow being required to reach, kneel or work in other uncomfortable and awkward positions. Heat stress may result from working in warm weather and the occlusive effects of the personal protective equipment, even without the heat generated by the welding process.

Compressed gas cylinders

In high-pressure gas welding installations, oxygen and the fuel gas (acetylene, hydrogen, town gas, propane) are supplied to the torch from cylinders. The gases are stored in these cylinders at high pressure. The special fire and explosion hazards and precautions for the safe use and storage of the fuel gases are also discussed elsewhere in this Encyclopaedia. The following precautions should be observed:

  • Only pressure regulators designed for the gas in use should be fitted to cylinders. For example, an acetylene regulator should not be used with coal gas or hydrogen (although it may be used with propane).
  • Blowpipes must be kept in good order and cleaned at regular intervals. A hardwood stick or soft brass wire should be used for cleaning the tips. They should be connected to regulators with special canvas-reinforced hoses placed in such a way that they are unlikely to be damaged.
  • Oxygen and acetylene cylinders must be stored separately and only on fire-resistant premises devoid of flammable material and must be so located that they may be readily removed in case of fire. Local building and fire protection codes must be consulted.
  • The colour coding in force or recommended for identification of cylinders and accessories should be scrupulously observed. In many countries, the internationally accepted colour codes used for the transport of dangerous materials are applied in this field. The case for enforcement of uniform international standards in this respect is strengthened by safety considerations bound up with the increasing international migration of industrial workers.

 

Acetylene generators

In the low-pressure gas welding process, acetylene is generally produced in generators by reaction of calcium carbide and water. The gas is then piped to the welding or cutting torch into which oxygen is fed.

Stationary generating plants should be installed either in the open air or in a well-ventilated building away from the main workshops. The ventilation of the generator house should be such as to prevent the formation of an explosive or toxic atmosphere. Adequate lighting should be provided; switches, other electrical gear and electrical lamps should either be located outside the building or be explosion-proof. Smoking, flames, torches, welding plant or flammable materials must be excluded from the house or from the vicinity of an open-air generator. Many of these precautions also apply to portable generators. Portable generators should be used, cleaned and recharged only in the open air or in a well-ventilated shop, away from any flammable material.

Calcium carbide is supplied in sealed drums. The material should be stored and kept dry, on a platform raised above the floor level. Stores must be situated under cover, and if they adjoin another building the party wall must be fireproof. The storeroom should be suitably ventilated through the roof. Drums should be opened only immediately before the generator is charged. A special opener should be provided and used; a hammer and chisel should never be used to open drums. It is dangerous to leave calcium carbide drums exposed to any source of water.

Before a generator is dismantled, all calcium carbide must be removed and the plant filled with water. The water should remain in the plant for at least half an hour to ensure that every part is free from gas. The dismantling and servicing should be carried out only by the manufacturer of the equipment or by a specialist. When a generator is being recharged or cleaned, none of the old charge must be used again.

Pieces of calcium carbide wedged in the feed mechanism or adhering to parts of the plant should be carefully removed, using non-sparking tools made of bronze or another suitable non-ferrous alloy.

All concerned should be fully conversant with the manufacturer’s instructions, which should be conspicuously displayed. The following precautions should also be observed:

  • A properly designed back-pressure valve must be fitted between the generator and each blowpipe to prevent backfire or reverse flow of gas. The valve should be regularly inspected after backfire, and the water level checked daily.
  • Only blowpipes of the injector type designed for low-pressure operation should be used. For heating and cutting, town gas or hydrogen at low pressure are sometimes employed. In these cases, a non-return valve should be placed between each blowpipe and the supply main or pipeline.
  • An explosion may be caused by “flash-back”, which results from dipping the nozzle-tip into the molten metal pool, mud or paint, or from any other stoppage. Particles of slag or metal that become attached to the tip should be removed. The tip should also be cooled frequently.
  • Local building and fire codes should be consulted.

 

Fire and explosion prevention

In locating welding operations, consideration should be given to surrounding walls, floors, nearby objects and waste material. The following procedures should be followed:

  • All combustible material must be removed or adequately protected by sheet metal or other suitable materials; tarpaulins should never be used.
  • Wood structures should be discouraged or similarly protected. Wood floors should be avoided.
  • Precautionary measures should be taken in the case of openings or cracks in walls and floors; flammable material in adjoining rooms or on the floor below should be removed to a safe position. Local building and fire codes should be consulted.
  • Suitable fire-extinguishing apparatus should always be at hand. In the case of low-pressure plant using an acetylene generator, buckets of dry sand should also be kept available; fire extinguishers of dry powder or carbon dioxide types are satisfactory. Water must never be used.
  • Fire brigades may be necessary. A responsible person should be assigned to keep the site under observation for at least half an hour after completion of the work, in order to deal with any outbreak of fire.
  • Since explosions can occur when acetylene gas is present in air in any proportion between 2 and 80%, adequate ventilation and monitoring are required to ensure freedom from gas leaks. Only soapy water should be used to search for gas leaks.
  • Oxygen must be carefully controlled. For example, it should never be released into the air in a confined space; many metals, clothing and other materials become actively combustible in the presence of oxygen. In gas cutting, any oxygen which may not be consumed will be released into the atmosphere; gas cutting should never be undertaken in a confined space without proper ventilation arrangements.
  • Alloys rich in magnesium or other combustible metals should be kept away from welding flames or arcs.
  • Welding of containers can be extremely hazardous. If the previous contents are unknown, a vessel should always be treated as if it had contained a flammable substance. Explosions may be prevented either by removing any flammable material or by making it non-explosive and non-flammable.
  • The mixture of aluminium and iron oxide used in thermite welding is stable under normal conditions. However, in view of the ease with which aluminium powder will ignite, and the quasi-explosive nature of the reaction, appropriate precautions should be taken in handling and storage (avoidance of exposure to high heat and possible ignition sources).
  • A written hot-work permit programme is required for welding in some jurisdictions. This programme outlines the precautions and procedures to be followed during welding, cutting, burning and so on. This programme should include the specific operations conducted along with the safety precautions to be implemented. It must be plant specific and may include an internal permit system that must be completed with each individual operation.

 

Protection from heat and burn hazards

Burns of the eyes and exposed parts of the body may occur due to contact with hot metal and spattering of incandescent metal particles or molten metal. In arc welding, a high-frequency spark used to initiate the arc can cause small, deep burns if concentrated at a point on the skin. Intense infrared and visible radiation from a gas welding or cutting flame and incandescent metal in the weld pool can cause discomfort to the operator and persons in the vicinity of the operation. Each operation should be considered in advance, and necessary precautions designed and implemented. Goggles made specifically for gas welding and cutting should be worn to protect the eyes from heat and light radiated from the work. Protective covers over filter glass should be cleaned as required and replaced when scratched or damaged. Where molten metal or hot particles are emitted, the protective clothing being worn should deflect spatter. The type and thickness of fire-resistant clothing worn should be chosen according to the degree of hazard. In cutting and arc welding operations, leather shoe coverings or other suitable spats should be worn to prevent hot particles from falling into boots or shoes. For protecting the hands and forearms against heat, spatter, slag and so on, the leather gauntlet type of glove with canvas or leather cuffs is sufficient. Other types of protective clothing include leather aprons, jackets, sleeves, leggings and head covering. In overhead welding, a protective cape and cap are necessary. All protective clothing should be free from oil or grease, and seams should be inside, so as not to trap globules of molten metal. Clothing should not have pockets or cuffs that could trap sparks, and it should be worn so sleeves overlap gloves, leggings overlap shoes and so on. Protective clothing should be inspected for burst seams or holes through which molten metal or slag may enter. Heavy articles left hot on completion of welding should always be marked “hot” as a warning to other workers. With resistance welding, the heat produced may not be visible, and burns can result from handling of hot assemblies. Particles of hot or molten metal should not fly out of spot, seam or projection welds if conditions are correct, but non-flammable screens should be used and precautions taken. Screens also protect passers-by from eye burns. Loose parts should not be left in the throat of the machine because they are liable to be projected with some velocity.

Electrical safety

Although no-load voltages in manual arc welding are relatively low (about 80 V or less), welding currents are high, and transformer primary circuits present the usual hazards of equipment operated at power supply line voltage. The risk of electric shock should therefore not be ignored, especially in cramped spaces or in insecure positions.

Before welding commences, the grounding installation on arc welding equipment should always be checked. Cables and connections should be sound and of adequate capacity. A proper grounding clamp or bolted terminal should always be used. Where two or more welding machines are grounded to the same structure, or where other portable electric tools are also in use, grounding should be supervised by a competent person. The working position should be dry, secure and free from dangerous obstructions. A well-arranged, well-lighted, properly ventilated and tidy workplace is important. For work in confined spaces or dangerous positions, additional electrical protection (no-load, low-voltage devices) can be installed in the welding circuit, ensuring that only extremely low-voltage current is available at the electrode holder when welding is not taking place. (See discussion of confined spaces below.) Electrode holders in which the electrodes are held by a spring grip or screw thread are recommended. Discomfort due to heating can be reduced by effective heat insulation on that part of the electrode holder which is held in the hand. Jaws and connections of electrode holders should be cleaned and tightened periodically to prevent overheating. Provision should be made to accommodate the electrode holder safely when not in use by means of an insulated hook or a fully insulated holder. The cable connection should be designed so that continued flexing of the cable will not cause wear and failure of the insulation. Dragging of cables and plastic gas supply tubes (gas-shielded processes) across hot plates or welds must be avoided. The electrode lead should not come in contact with the job or any other earthed object (ground). Rubber tubes and rubber-covered cables must not be used anywhere near the high-frequency discharge, because the ozone produced will rot the rubber. Plastic tubes and polyvinyl chloride (PVC) covered cables should be used for all supplies from the transformer to the electrode holder. Vulcanized or tough rubber-sheathed cables are satisfactory on the primary side. Dirt and metallic or other conducting dust can cause a breakdown in the high-frequency discharge unit. To avoid this condition, the unit should be cleaned regularly by blowing-out with compressed air. Hearing protection should be worn when using compressed air for more than a few seconds. For electron-beam welding, the safety of the equipment used must be checked prior to each operation. To protect against electric shock, a system of interlocks must be fitted to the various cabinets. A reliable system of grounding of all units and control cabinets is necessary. For plasma welding equipment used for cutting heavy thicknesses, the voltages may be as high as 400 V and danger should be anticipated. The technique of firing the arc by a high-frequency pulse exposes the operator to the dangers of an unpleasant shock and a painful, penetrating high-frequency burn.

Ultraviolet radiation

The brilliant light emitted by an electric arc contains a high proportion of ultraviolet radiation. Even momentary exposure to bursts of arc flash, including stray flashes from other workers’ arcs, may produce a painful conjunctivitis (photo-ophthalmia) known as “arc eye” or “eye flash”. If any person is exposed to arc flash, immediate medical attention must be sought. Excessive exposure to ultraviolet radiation may also cause overheating and burning of the skin (sunburn effect). Precautions include:

  • A shield or helmet fitted with correct grade of filter should be used (see the article “Eye and face protection” elsewhere in this Encyclopaedia). For the gas-shielded arc welding processes and carbon-arc cutting, flat handshields provide insufficient protection from reflected radiation; helmets should be used. Filtered goggles or eyeglasses with sideshields should be worn under the helmet to avoid exposure when the helmet is lifted up for inspection of the work. Helmets will also provide protection from spatter and hot slag. Helmets and handshields are provided with a filter glass and a protective cover glass on the outside. This should be regularly inspected, cleaned and replaced when scratched or damaged.
  • The face, nape of the neck and other exposed parts of the body should be properly protected, especially when working close to other welders.
  • Assistants should wear suitable goggles at a minimum and other PPE as the risk requires.
  • All arc welding operations should be screened to protect other persons working nearby. Where the work is carried out at fixed benches or in welding shops, permanent screens should be erected where possible; otherwise, temporary screens should be used. All screens should be opaque, of sturdy construction and of a flame-resistant material.
  • The use of black paints for the inside of welding booths has become an accepted practice, but the paint should produce a matte finish. Adequate ambient lighting should be provided to prevent eye strain leading to headaches and accidents.
  • Welding booths and portable screens should be checked regularly to ensure that there is no damage which might result in the arc affecting persons working nearby.

 

Chemical hazards

Airborne contaminants from welding and flame cutting, including fumes and gases, arise from a variety of sources:

  • the metal being welded, the metal in the filler rod or constituents of various types of steel such as nickel or chromium)
  • any metallic coating on the article being welded or on the filler rod (e.g., zinc and cadmium from plating, zinc from galvanizing and copper as a thin coating on continuous mild steel filler rods)
  • any paint, grease, debris and the like on the article being welded (e.g., carbon monoxide, carbon dioxide, smoke and other irritant breakdown products)
  • flux coating on the filler rod (e.g., inorganic fluoride)
  • the action of heat or ultraviolet light on the surrounding air (e.g., nitrogen dioxide, ozone) or on chlorinated hydrocarbons (e.g., phosgene)
  • inert gas used as a shield (e.g., carbon dioxide, helium, argon).

 

Fumes and gases should be removed at the source by LEV. This can be provided by partial enclosure of the process or by the installation of hoods which supply sufficiently high air velocity across the weld position so as to ensure capture of the fumes.

Special attention should be paid to ventilation in the welding of non-ferrous metals and certain alloy steels, as well as to protection from the hazard of ozone, carbon monoxide and nitrogen dioxide which may be formed. Portable as well as fixed ventilation systems are readily available. In general, the exhausted air should not be recirculated. It should be recirculated only if there are not hazardous levels of ozone or other toxic gases and the exhaust air is filtered through a high-efficiency filter.

With electron-beam welding and if materials being welded are of a toxic nature (e.g., beryllium, plutonium and so on), care must be taken to protect the operator from any dust cloud when opening the chamber.

When there is a risk to health from toxic fumes (e.g., lead) and LEV is not practicable—for example, when lead-painted structures are being demolished by flame cutting—the use of respiratory protective equipment is necessary. In such circumstances, an approved, high-efficiency full-facepiece respirator or ahigh-efficiency positive pressure powered air-purified respirator (PAPR) should be worn. A high standard of maintenance of the motor and the battery is necessary, especially with the original high-efficiency positive pressure power respirator. The use of positive pressure compressed air line respirators should be encouraged where a suitable supply of breathing-quality compressed air is available. Whenever respiratory protective equipment is to be worn, the safety of the workplace should be reviewed to determine whether extra precautions are necessary, bearing in mind the restricted vision, entanglement possibilities and so on of persons wearing respiratory protective equipment.

Metal fume fever

Metal fume fever is commonly seen in workers exposed to the fumes of zinc in the galvanizing or tinning process, in brass founding, in the welding of galvanized metal and in metallizing or metal spraying, as well as from exposure to other metals such as copper, manganese and iron. It occurs in new workers and those returning to work after a weekend or holiday hiatus. It is an acute condition that occurs several hours after the initial inhalation of particles of a metal or its oxides. It starts with a bad taste in the mouth followed by dryness and irritation of the respiratory mucosa resulting in cough and occasionally dyspnoea and “tightness” of the chest. These may be accompanied by nausea and headache and, some 10 to 12 hours after the exposure, chills and fever which may be quite severe. These last several hours and are followed by sweating, sleep and often by polyuria and diarrhoea. There is no particular treatment, and recovery is usually complete in about 24 hours with no residua. It can be prevented by keeping exposure to the offending metallic fumes well within the recommended levels through the use of efficient LEV.

Confined spaces

For entry into confined spaces, there may be a risk of the atmosphere being explosive, toxic, oxygen deficient or combinations of the above. Any such confined space must be certified by a responsible person as safe for entry and for work with an arc or flame. A confined-space entry programme, including an entry permit system, may be required and is highly recommended for work that must be carried out in spaces that are typically not constructed for continuous occupancy. Examples include, but are not limited to, manholes, vaults, ship holds and the like. Ventilation of confined spaces is crucial, since gas welding not only produces airborne contaminants but also uses up oxygen. Gas-shielded arc welding processes can decrease the oxygen content of the air. (See figure 2.)

Figure 2. Welding in an enclosed space

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S. F. Gilman

Noise

Noise is a hazard in several welding processes, including plasma welding, some types of resistance welding machines and gas welding. In plasma welding, the plasma jet is ejected at very high speeds, producing intense noise (up to 90 dBA), particularly in the higher frequency bands. The use of compressed air to blow off dust also creates high noise levels. To prevent hearing damage, ear plugs or muffs must be worn and a hearing conservation programme should be instituted, including audiometric (hearing capacity) examinations and employee training.

Ionizing radiation

In welding shops where welds are inspected radiographically with x-ray or gamma-ray equipment, the customary warning notices and instructions must be strictly observed. Workers must be kept at a safe distance from such equipment. Radioactive sources must be handled only with the required special tools and subject to special precautions.

Local and governmental regulations must be followed. See the chapter Radiation, ionizing elsewhere in this Encyclopaedia.

Sufficient shielding must be provided with electron-beam welding to prevent x rays from penetrating the walls and windows of the chamber. Any parts of the machine providing shields against x-ray radiation should be interlocked so that the machine cannot be energized unless they are in position. Machines should be checked at the time of installation for leaks of x-ray radiation, and regularly thereafter.

Other hazards

Resistance welding machines have at least one electrode, which moves with considerable force. If a machine is operated while a finger or hand is lying between the electrodes, severe crushing will result. Where possible, a suitable means of guarding must be devised to safeguard the operator. Cuts and lacerations can be minimized by first deburring components and by wearing protective gloves or gauntlets.

Lockout/tagout procedures should be used when machinery with electrical, mechanical or other energy sources is being maintained or repaired.

When slag is being removed from welds by chipping and so on, the eyes should be protected by goggles or other means.

 

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Wednesday, 16 March 2011 21:40

Lathes

Written by

Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

The important part lathes play in metalworking shops is best illustrated by the fact that 90 to 95% of the swarf (metal shavings) produced in the valves and fittings industry originates from lathes. About one-tenth of the accidents reported in this industry are due to lathes; this corresponds to one-third of all machine accidents. According to a study of the relative accident frequency per machine unit carried out in a plant manufacturing small precision parts and electrical equipment, lathes rank fifth after woodworking machines, metal-cutting saws, power presses and drilling machines. The need for protective measures on lathes is therefore beyond doubt.

Turning is a machine process in which the diameter of material is reduced by a tool with a special cutting edge. The cutting movement is produced by rotating the workpiece, and the feed and traverse movements are produced by the tool. By varying these three basic movements, and also by choosing the appropriate tool cutting-edge geometry and material, it is possible to influence the rate of stock removal, surface quality, shape of the chip formed and tool wear.

Structure of Lathes

A typical lathe consists of:

  • a bed or base with machined slideways for the saddle and tailstock
  • a headstock mounted on the bed, with the spindle and chuck
  • a feed gearbox attached to the front of the bed for transmitting the feed movement as a function of the cutting speed through the leadscrew or feed shaft and apron to the saddle
  • a saddle (or carriage) carrying the cross slide which performs the traverse movement
  • a toolpost mounted on the cross slide (see figure 1).

 

Figure 1. Lathes and similar machines

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This basic model of a lathe can be infinitely varied, from the universal machine to the special automatic lathe designed for one type of work only.

The most important types of lathe are as follows:

  • Centre lathe. This is the most frequently used turning machine. It corresponds to the basic model with horizontal turning axis. The work is held between centres, by a faceplate or in a chuck.
  • Multiple-tool lathe. This enables several tools to be engaged at the same time.
  • Turret lathe, capstan lathe. Machines of this type enable a workpiece to be machined by several tools which are engaged one after the other. The tools are held in the turret, which rotates for bringing them into cutting position. The turrets are generally of the disc or crown type, but there are also drum-type turret lathes.
  • Copy-turning lathes. The desired shape is transmitted by tracer control from a template to the work.
  • Automatic lathe. The various operations, including the change of the work, are automated. There are bar automatics and chucking automatics.
  • Vertical lathe (boring and turning mill). The work turns about a vertical axis; it is clamped to a horizontal revolving table. This type of machine is generally used for machining large castings and forgings.
  • NC and CNC lathes. All the aforementioned machines can be equipped with a numerical control (NC) or computer-assisted numerical control (CNC) system. The result is a semi-automated or fully automated machine which can be used rather universally, thanks to the great versatility and easy programmability of the control system.

 

The future development of the lathe will probably concentrate on control systems. Contact controls will be increasingly replaced by electronic control systems. As regards the latter, there is a trend in evolution from interpolation-programmed to memory-programmed controls. It is foreseeable in the long run that the use of increasingly efficient process computers will tend to optimize the machining process.

Accidents

Lathe accidents are generally caused by:

  • disregard for safety regulations when the machines are installed in workshops (e.g., not enough space between machines, no power disconnect switch for each machine)
  • missing guards or the absence of auxiliary devices (severe injuries have been caused to workers who tried to brake the spindle of their lathes by pressing one of their hands against unguarded belt pulleys and to operators who inadvertently engaged unguarded clutch levers or pedals; injuries due to flying chips because of the absence of hinged or sliding covers have also occurred)
  • inadequately located control elements (e.g., a turner’s hand can be pierced by the tailstock centre if the pedal controlling the chuck is mistaken for the one controlling the hydraulic circuit of the tailstock centre movement)
  • adverse conditions of work (i.e., shortcomings from the point of view of occupational physiology)
  • lack of PPE or wearing unsuitable work clothing (severe and even fatal injuries have been caused to lathe operators who wore loose clothes or had long, free-hanging hair)
  • insufficient instruction of personnel (an apprentice was fatally injured when he filed a short shaft which was fixed between centres and rotated by a cranked carrier on the spindle nose and a straight one on the shaft; the lathe carrier seized his left-hand sleeve, which was wrapped around the workpiece, dragging the apprentice violently into the lathe)
  • poor work organization leading to the use of unsuitable equipment (e.g., a long bar was machined on a conventional production lathe; it was too long for this lathe, and it projected more than 1 m beyond the headstock; moreover, the chuck aperture was too large for the bar and was made up by inserting wooden wedges; when the lathe spindle started rotating, the free bar end bent by 45° and struck the operator’s head; the operator died during the following night)
  • defective machine elements (e.g., a loose carrier pin in a clutch may cause the lathe spindle to start rotating while the operator is adjusting a workpiece in the chuck).

 

Accident Prevention

The prevention of lathe accidents starts at the design stage. Designers should give special attention to control and transmission elements.

Control elements

Each lathe must be equipped with a power disconnect (or isolating) switch so that maintenance and repair work may be carried out safely. This switch must disconnect the current on all poles, reliably cut the pneumatic and hydraulic power and vent the circuits. On large machines, the disconnect switch should be so designed that it can be padlocked in its out position—a safety measure against accidental reconnection.

The layout of the machine controls should be such that the operator can easily distinguish and reach them, and that their manipulation presents no hazard. This means that controls must never be arranged at points which can be reached only by passing the hand over the working zone of the machine or where they may be hit by flying chips.

Switches which monitor guards and interlock them with the machine drive should be chosen and installed in such a way that they positively open the circuit as soon as the guard is shifted from its protecting position.

Emergency stop devices must cause the immediate standstill of the dangerous movement. They must be designed and located in such a way that they can be easily operated by the threatened worker. Emergency stop buttons must be easily reached and should be in red.

The actuating elements of control gear which may trip a dangerous machine movement must be guarded so as to exclude any inadvertent operation. For instance, the clutch engaging levers on the headstock and apron should be provided with safety locking devices or screens. A push-button can be made safe by lodging it in a recess or by shrouding it with a protective collar.

Hand-operated controls should be designed and located in such a way that the hand movement corresponds to the controlled machine movement.

Controls should be identified with easily readable and understandable markings. To avoid misunderstandings and linguistic difficulties, it is advisable to use symbols.

Transmission elements

All moving transmission elements (belts, pulleys, gears) must be covered with guards. An important contribution to the prevention of lathe accidents can be made by the persons responsible for the installation of the machine. Lathes should be so installed that the operators tending them do not hinder or endanger each other. The operators should not turn their backs towards passageways. Protective screens should be installed where neighbouring workplaces or passageways are within the range of flying chips.

Passageways must be clearly marked. Enough space should be left for materials-handling equipment, for stacking workpieces and for tool boxes. Bar-stock guides must not protrude into the passageways.

The floor on which the operator stands must be insulated against cold. Care should be taken that the insulation forms no stumbling obstacle, and the flooring should not become slippery even when covered with a film of oil.

Conduit and pipework should be installed in such a way that they do not become obstacles. Temporary installations should be avoided.

Safety engineering measures on the shop floor should be directed in particular at the following points:

  • work-holding fixtures (faceplates, chucks, collets) should be dynamically balanced before use
  • the maximum permissible speed of a chuck should be indicated on the chuck by the manufacturer and respected by the lathe operator
  • when scroll chucks are used, it should be ensured that the jaws cannot be slung out when the lathe is started
  • chucks of this type should be designed in such a manner that the key cannot be taken off before the jaws have been secured. The chuck keys in general should be so designed that it is impossible to leave them in the chuck.

 

It is important to provide for auxiliary lifting equipment to facilitate mounting and removing of heavy chucks and faceplates. To prevent chucks from running off the spindle when the lathe is suddenly braked, they must be securely fixed. This can be achieved by putting a retaining nut with left-hand thread on the spindle nose, by using a “Camlock” quick-action coupling, by fitting the chuck with a locking key or by securing it with a two-part locking ring.

When powered work-holding fixtures are used, such as hydraulically operated chucks, collets and tailstock centres, measures must be taken which make it impossible for the hands to be introduced into the danger zone of closing fixtures. This can be achieved by limiting the travel of the clamping element to 6 mm, by choosing the location of deadman’s controls so as to exclude the introduction of the hands into the danger zone or by providing a moving guard which has to be closed before the clamping movement can be started.

If starting the lathe while the chuck jaws are open presents a danger, the machine should be equipped with a device which prevents the spindle rotation being started before the jaws are closed. The absence of power must not cause the opening or closure of a powered work-holding fixture.

If the gripping force of a power chuck diminishes, the spindle rotation must be stopped, and it must be impossible to start the spindle. Reversing the gripping direction from inside to outside (or vice versa) while the spindle rotates must not cause the chuck to be dislodged from the spindle. Removal of holding fixtures from the spindle should be possible only when the spindle has ceased rotating.

When machining bar stock, the portion projecting beyond the lathe must be enclosed by bar-stock guides. Bar feed weights must be guarded by hinged covers extending to the floor.

Carriers

To prevent serious accidents—in particular, when filing work in a lathe—unprotected carriers must not be used. A centring safety carrier should be used, or a protective collar should be fitted to a conventional carrier. It is also possible to use self-locking carriers or to provide the carrier disc with a protective cover.

Working zone of the lathe

Universal-lathe chucks should be guarded by hinged covers. If possible, protective covers should be interlocked with spindle drive circuits. Vertical boring and turning mills should be fenced with bars or plates to prevent injury from revolving parts. To enable the operator to watch the machining process safely, platforms with railings must be provided. In certain cases, TV cameras can be installed so that the operator may monitor the tool edge and tool in-feed.

The working zones of automatic lathes, NC and CNC lathes should be completely enclosed. Enclosures of fully automatic machines should only have openings through which the stock to be machined is introduced, the turned part ejected and the swarf removed from the working zone. These openings must not constitute a hazard when work passes through them, and it must be impossible to reach through them into the danger zone.

The working zones of semi-automatic, NC and CNC lathes must be enclosed during the machining process. The enclosures are generally sliding covers with limit switches and interlocking circuit.

Operations requiring access to the working zone, such as change of work or tools, gauging and so on, must not be carried out before the lathe has been safely stopped. Zeroing a variable-speed drive is not considered a safe standstill. Machines with such drives must have locked protective covers that cannot be unlocked before the machine is safely stopped (e.g., by cutting the spindle-motor power supply).

If special tool-setting operations are required, an inching control is to be provided which enables certain machine movements to be tripped while the protective cover is open. In such cases, the operator can be protected by special circuit designs (e.g., by permitting only one movement to be tripped at a time). This can be achieved by using two-hand controls.

Turning swarf

Long turning chips are dangerous because they may get entangled with arms and legs and cause serious injury. Continuous and ravelled chips can be avoided by choosing appropriate cutting speeds, feeds and chip thicknesses or by using lathe tools with chip breakers of the gullet or step type. Swarf hooks with handle and buckle should be used for removing chips.

Ergonomics

Every machine should be so designed that it enables a maximal output to be obtained with a minimum of stress on the operator. This can be achieved by adapting the machine to the worker.

Ergonomic factors must be taken into account when designing the human-machine interface of a lathe. Rational workplace design also includes providing for auxiliary handling equipment, such as loading and unloading attachments.

All controls must be located within the physiological sphere or reach of both hands. The controls must be clearly laid out and should be logical to operate. Pedal-operated controls should be avoided in machines tended by standing operators.

Experience has shown that good work is performed when the workplace is designed for both standing and sitting postures. If the operator has to work standing up, he or she should be given the possibility of changing posture. Flexible seats are in many cases a welcome relief for strained feet and legs.

Measures should be taken to create optimal thermal comfort, taking into account the air temperature, relative humidity, air movement and radiant heat. The workshop should be adequately ventilated. There should be local exhaust devices to eliminate gaseous emanations. When machining bar stock, sound-absorbent-lined guide tubes should be used.

The workplace should preferably be provided with uniform lighting, affording an adequate level of illumination.

Work Clothing and Personal Protection

Overalls should be close fitting and buttoned or zipped to the neck. They should be without breast pockets, and the sleeves must be tightly buttoned at the wrists. Belts should not be worn. No finger rings and bracelets should be worn when working on lathes. Wearing of safety spectacles should be obligatory. When heavy workpieces are machined, safety shoes with steel toe caps must be worn. Protective gloves must be worn whenever swarf is being collected.

Training

The lathe operator’s safety depends to a large extent on working methods. It is therefore important that he or she should receive thorough theoretical and practical training to acquire skills and develop a behaviour affording the best possible safeguards. Correct posture, correct movements, correct choice and handling of tools should become routine to such an extent that the operator works correctly even if his or her concentration is temporarily relaxed.

Important points in a training programme are an upright posture, the proper mounting and removal of the chuck and the accurate and secure fixing of workpieces. Correct holding of files and scrapers and safe working with abrasive cloth must be intensively practised.

Workers must be well informed about the hazards of injury which may be caused when gauging work, checking adjustments and cleaning lathes.

Maintenance

Lathes must be regularly maintained and lubricated. Faults must be corrected immediately. If safety is at stake in the event of a fault, the machine should be put out of operation until corrective action has been taken.

Repair and maintenance work must be carried out only after the machine has been isolated from the power supply

.

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Wednesday, 16 March 2011 21:58

Grinding and Polishing

Written by

Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

Grinding generally involves the use of a bonded abrasive to wear away parts of a workpiece. The aim is to give the work a certain shape, correct its dimensions, increase the smoothness of a surface or improve the sharpness of cutting edges. Examples include removal of sprues and rough edges from a foundry casting, removal of surface scale from metals before forging or welding and deburring of parts in sheet metal and machine shops. Polishing is used to remove surface imperfections such as tool marks. Buffing does not remove metal, but uses a soft abrasive blended in a wax or grease base to produce a high-lustre surface.

Grinding is the most comprehensive and diversified of all machining methods and is employed on many materials—predominantly iron and steel but also other metals, wood, plastics, stone, glass, pottery and so on. The term covers other methods of producing very smooth and glossy surfaces, such as polishing, honing, whetting and lapping.

The tools used are wheels of varying dimensions, grinding segments, grinding points, sharpening stones, files, polishing wheels, belts, discs and so on. In grinding wheels and the like, the abrasive material is held together by bonding agents to form a rigid, generally porous body. In the case of abrasive belts, the bonding agent holds the abrasive secured to a flexible base material. Buffing wheels are made from cotton or other textile disks sewn together.

The natural abrasives—natural corundum or emery (aluminium oxides), diamond, sandstone, flint and garnet—have been largely superseded by artificial abrasives including aluminium oxide (fused alumina), silicon carbide (carborundum) and synthetic diamonds. A number of fine-grained materials such as chalk, pumice, tripoli, tin putty and iron oxide are also used, especially for polishing and buffing.

Aluminium oxide is most widely used in grinding wheels, followed by silicon carbide. Natural and artificial diamonds are used for important special applications. Aluminium oxide, silicon carbide, emery, garnet and flint are used in grinding and polishing belts.

Both organic and inorganic bonding agents are used in grinding wheels. The main type of inorganic bonds are vitrified silicate and magnesite. Notable among organic bonding agents are phenol- or urea- formaldehyde resin, rubber and shellac. The vitrified bonding agents and phenolic resin are completely dominating within their respective groups. Diamond grinding wheels can also be metal bonded. The various bonding agents give the wheels different grinding properties, as well as different properties with regard to safety.

Abrasive and polishing belts and discs are composed of a flexible base of paper or fabric to which the abrasive is bonded by means of a natural or synthetic adhesive.

Different machines are used for different types of operations, such as surface grinding, cylindrical (including centreless) grinding, internal grinding, rough grinding and cutting. The two main types are: those where either the grinder or the work is moved by hand and machines with mechanical feeds and chucks. Common equipment types include: surface-type grinders; pedestal-type grinders, polishers and buffers; disk grinders and polishers; internal grinders; abrasive cut-off machines; belt polishers; portable grinders, polishers and buffers; and multiple polishers and buffers.

Hazards and Their Prevention

Bursting

The major injury risk in the use of grinding wheels is that the wheel may burst during grinding. Normally, grinding wheels operate at high speeds. There is a trend towards ever-increasing speeds. Most industrialized nations have regulations limiting the maximum speeds at which the various types of grinding wheels may be run.

The fundamental protective measure is to make the grinding wheel as strong as possible; the nature of the bonding agent is most important. Wheels with organic bonds, in particular phenolic resin, are tougher than those with inorganic bonds and more resistant to impacts. High peripheral speeds may be permissible for wheels with organic bonds.

Very high-speed wheels, in particular, often incorporate various types of reinforcement. For example, certain cup wheels are fitted with steel hubs to increase their strength. During rotation the major stress develops around the centre hole. To strengthen the wheel, the section around the centre hole, which takes no part in the grinding, can thus be made of an especially strong material which is not suitable for grinding. Large wheels with a centre section reinforced in this way are used particularly by the steel works for grinding slabs, billets and the like at speeds up to 80 m/s.

The most common method of reinforcing grinding wheels, however, is to include glass fibre fabric in their construction. Thin wheels, such as those used for cutting, may incorporate glass fibre fabric at the centre or at each side, while thicker wheels have a number of fabric layers depending on the thickness of the wheel.

With the exception of some grinding wheels of small dimensions, either all wheels or a statistical sampling of them must be given speed tests by the manufacturer. In tests the wheels are run over a certain period at a speed exceeding that permitted in grinding. Test regulations vary from country to country, but usually the wheel has to be tested at a speed 50% above the working speed. In some countries, regulations require special testing of wheels that are to operate at higher speeds than normal at a central testing institute. The institute may also cut specimens from the wheel and investigate their physical properties. Cutting wheels are subjected to certain impact tests, bending tests and so on. The manufacturer is also obliged to ensure that the grinding wheel is well balanced prior to delivery.

The bursting of a grinding wheel may cause fatal or very serious injuries to anyone in the vicinity and heavy damage to plant or premises. In spite of all precautions taken by the manufacturers, occasional wheel bursts or breaks may still occur unless proper care is exercised in their use. Precautionary measures include:

  • Handling and storing. A wheel may become damaged or cracked during transit or handling. Moisture may attack the bonding agent in phenolic resin wheels, ultimately reducing their strength. Vitrified wheels may be sensitive to repeated temperature variations. Irregularly absorbed moisture may throw the wheel out of balance. Consequently, it is most important that wheels are carefully handled at all stages and kept in an orderly manner in a dry and protected place.
  • Checking for cracks. A new wheel should be checked to ensure that it is undamaged and dry, most simply by tapping with a wooden mallet. A faultless vitrified wheel will give a clear ring, an organic bonded wheel a less ringing tone; but either can be differentiated from the cracked sound of a defective wheel. In case of doubt, the wheel should not be used and the supplier should be consulted.
  • Testing. Before the new wheel is put into service, it should be tested at full speed with due precautions being observed. After wet grinding, the wheel should be run idle to eject the water; otherwise the water may collect at the bottom of the wheel and cause imbalance, which may result in bursting when the wheel is next used.
  • Mounting. Accidents and breakages occur when grinding wheels are mounted on unsuitable apparatus—for example, on spindle ends of buffing machines. The spindle should be of adequate diameter but not so large as to expand the centre hole of the wheel; flanges should be not less than one-third the diameter of the wheel and made of mild steel or of similar material.
  • Speed. In no circumstances should the maximum permissible operating speed specified by the makers be exceeded. A notice indicating the spindle speed should be fitted to all grinding machines, and the wheel should be marked with the maximum permissible peripheral speed and the corresponding number of revolutions for a new wheel. Special precautions are necessary with variable speed grinding machines and to ensure the fitting of wheels of appropriate permissible speeds in portable grinders.
  • Work rest. Wherever practicable, rigidly mounted work rests of adequate dimensions should be provided. They should be adjustable and kept as close as possible to the wheel to prevent a trap in which the work might be forced against the wheel and break it or, more probable, catch and injure the operator’s hand.
  • Guarding. Abrasive wheels should be provided with guards strong enough to contain the parts of a bursting wheel (see figure 1). Some countries have detailed regulations regarding the design of the guards and the materials to be used. In general, cast iron and cast aluminium are to be avoided. The grinding opening should be as small as possible, and an adjustable nose piece may be necessary. Exceptionally, where the nature of the work precludes the use of a guard, special protective flanges or safety chucks may be used. The spindles and tapered ends of double-ended polishing machines can cause entanglement accidents unless they are effectively guarded.

 

Figure 1. A well guarded, vitrified abrasive wheel mounted in a surface grinder and operating at a peripheral speed of 33 m/s

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Eye injuries

Dust, abrasives, grains and splinters are a common hazard to the eyes in all dry-grinding operations. Effective eye protection by goggles or spectacles and fixed eye shields at the machine are essential; fixed eye shields are particularly useful when wheels are in intermittent use—for example, for tool grinding.

Fire

Grinding of magnesium alloys carries a high fire risk unless strict precautions are taken against accidental ignition and in the removal and drenching of dust. High standards of cleanliness and maintenance are required in all exhaust ducting to prevent risk of fire and also to keep ventilation working efficiently. Textile dust released from buffing operations is a fire hazard requiring good housekeeping and LEV.

Vibration

Portable and pedestal grinders carry a risk of hand-arm vibration syndrome (HAVS), also known as “white finger” from its most noticeable sign. Recommendations include limiting intensity and duration of exposure, redesigning tools, protective equipment and monitoring exposure and health.

Health hazards

Although modern grinding wheels do not themselves create the serious silicosis hazard associated in the past with sandstone wheels, highly dangerous silica dust may still be given off from the materials being ground—for example, sand castings. Certain resin-bonded wheels may contain fillers which create a dangerous dust. In addition, formaldehyde-based resins can emit formaldehyde during grinding. In any event, the volume of dust produced by grinding makes efficient LEV essential. It is more difficult to provide local exhaust for portable wheels, although some success in this direction has been achieved by use of low-volume, high-velocity capture systems. Prolonged work should be avoided and respiratory protective equipment provided if necessary. Exhaust ventilation is also required for most belt sanding, finishing, polishing and similar operations. With buffing in particular, combustible textile dust is a serious concern.

Protective clothing and good sanitary and washing facilities with showers should be provided, and medical supervision is desirable, especially for metal grinders.

 

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The industrial revolution could not have occurred without the development of refined petroleum-based industrial oils, lubricants, cutting oils and greases. Prior to the discovery in the 1860s that a superior lubricant could be produced by distilling crude oil in a vacuum, industry depended on naturally occurring oils and animal fats such as lard and whale sperm oil for lubricating moving parts. These oils and animal products were especially susceptible to melting, oxidation and breakdown from exposure to heat and moisture produced by the steam engines which powered almost all industrial equipment at that time. The evolution of petroleum-based refined products has continued from the first lubricant, which was used to tan leather, to modern synthetic oils and greases with longer service life, superior lubricating qualities and better resistance to change under varying temperatures and climatic conditions.

Industrial Lubricants

All moving parts on machinery and equipment require lubrication. Although lubrication may be provided by dry materials such as Teflon or graphite, which are used in parts such as small electrical motor bearings, oils and greases are the most commonly used lubricants. As the complexity of the machinery increases, the requirements for lubricants and metal process oils become more stringent. Lubricating oils now range from clear, very thin oils used to lubricate delicate instruments, to thick, tar-like oils used on large gears such as those which turn steel mills. Oils with very specific requirements are used both in the hydraulic systems and to lubricate large computer-operated machine tools such as those used in the aerospace industry to produce parts with extremely close tolerances. Synthetic oils, fluids and greases, and blends of synthetic and petroleum-based oils, are used where extended lubricant life is desired, such as sealed-for-life electric motors, where the increased time between oil changes offsets the difference in cost; where extended temperature and pressure ranges exist, such as in aerospace applications; or where it is difficult and expensive to re-apply the lubricant.

Industrial Oils

Industrial oils such as spindle and lubricating oils, gear lubricants, hydraulic and turbine oils and transmission fluids are designed to meet specific physical and chemical requirements and to operate without discernible change for extended periods under varying conditions. Lubricants for aerospace use must meet entirely new conditions, including cleanliness, durability, resistance to cosmic radiation and the ability to operate in extremely cold and hot temperatures, without gravity and in a vacuum.

Transmissions, turbines and hydraulic systems contain fluids which transfer force or power, reservoirs to hold the fluids, pumps to move the fluids from one place to another and auxiliary equipment such as valves, piping, coolers and filters. Hydraulic systems, transmissions and turbines require fluids with specific viscosities and chemical stability to operate smoothly and provide the controlled transfer of power. The characteristics of good hydraulic and turbine oils include a high viscosity index, thermal stability, long life in circulating systems, deposit resistance, high lubricity, anti-foam capabilities, rust protection and good demulsibility.

Gear lubricants are designed to form strong, tenacious films which provide lubrication between gears under extreme pressure. The characteristics of gear oils include good chemical stability, demulsibility and resistance to viscosity increase and deposit formation. Spindle oils are thin, extremely clean and clear oils with lubricity additives. The most important characteristics for way oils—used to lubricate two flat sliding surfaces where there is high pressure and slow speed—are lubricity and tackiness to resist squeezing out and resistance to extreme pressure.

Cylinder and compressor oils combine the characteristics of both industrial and automotive oils. They should resist accumulation of deposits, act as a heat transfer agent (internal combustion engine cylinders), provide lubrication for cylinders and pistons, provide a seal to resist blow-back pressure, have chemical and thermal stability (especially vacuum pump oil), have a high viscosity index and resist water wash (steam-operated cylinders) and detergency.

Automotive Engine Oils

Manufacturers of internal combustion engines and organizations, such as the Society of Automotive Engineers (SAE) in the United States and Canada, have established specific performance criteria for automotive engine oils. Automotive gasoline and diesel engine oils are subjected to a series of performance tests to determine their chemical and thermal stability, corrosion resistance, viscosity, wear protection, lubricity, detergency and high and low temperature performance. They are then classified according to a code system which allows consumers to determine their suitability for heavy-duty use and for different temperatures and viscosity ranges.

Oils for automotive engines, transmissions and gear cases are designed with high viscosity indexes to resist changes in viscosity with temperature changes. Automotive engine oils are especially formulated to resist breakdown under heat as they lubricate internal combustion engines. Internal combustion engine oils must not be too thick to lubricate the internal moving parts when an engine starts up in cold weather, and they must not thin out as the engine heats up when operating. They should resist carbon build-up on valves, rings and cylinders and the formation of corrosive acids or deposits from moisture. Automotive engine oils contain detergents designed to hold carbon and metallic wear particles in suspension so that they can be filtered out as the oil circulates and not accumulate on internal engine parts and cause damage.

Cutting Fluids

The three types of cutting fluids used in industry are mineral oils, soluble oils and synthetic fluids. Cutting oils are typically a blend of high-quality, high-stability mineral oils of various viscosities together with additives to provide specific characteristics depending on the type of material being machined and the work performed. Soluble water-in-oil cutting fluids are mineral oils (or synthetic oils) which contain emulsifiers and special additives including defoamants, rust inhibitors, detergents, bactericides and germicides. They are diluted with water in varying ratios before being used. Synthetic cutting fluids are solutions of non-petroleum-based fluids, additives and water, rather than emulsions, some of which are fire resistant for machining specific metals. Semi-synthetic fluids contain 10 to 15% mineral oil. Some special fluids have both lubricating oil and cutting fluid characteristics due to the tendency of fluids to leak and intermix in certain machine tools such as multi-spindle, automatic screw machines.

The desired characteristics of cutting fluids depend on the composition of the metal being worked on, the cutting tool being used and the type of cutting, planing or shaping operation performed. Cutting fluids improve and enhance the metal working process by cooling and lubrication (i.e., protecting the edge of the cutting tool). For example, when working on a soft metal which creates a lot of heat, cooling is the most important criterion. Improved cooling is provided by using a light oil (such as kerosene) or water-based cutting fluid. Control of the built-up edge on cutting tools is provided by anti-weld or anti-wear additives such as sulphur, chlorine or phosphorus compounds. Lubricity, which is important when working on steel to overcome the abrasiveness of iron sulphide, is provided by synthetic and animal fats or sulphurized sperm oil additives.

Other Metal Working and Process Oils

Grinding fluids are designed to provide cooling and prevent metal build-up on grinding wheels. Their characteristics include thermal and chemical stability, rust protection (soluble fluids), preventing gummy deposits upon evaporation and a safe flashpoint for the work performed.

Quench oils, which require high stability, are used in metal treating to control the change of the molecular structure of steel as it cools. Quenching in lighter oil is used to case harden small, inexpensive steel parts. A slower quench rate is used to produce machine tool steels which are fairly hard on the outside with lower internal stress. A gapped or multi-phase quenching oil is used to treat high carbon and alloy steels.

Roll oils are specially formulated mineral or soluble oils which lubricate and provide a smooth finish to metal, particularly aluminium, copper and brass, as it goes through hot and cold rolling mills. Release oils are used to coat dies and moulds to facilitate the release of the formed metal parts. Tanning oils are still used in the felt and leather-making industry. Transformer oils are specially formulated dielectric fluids used in transformers and large electric breakers and switches.

Heat transfer oils are used in open or closed systems and may last up to 15 years in service. The primary characteristics are good thermal stability as systems operate at temperatures from 150 to 315°C, oxidation stability and high flashpoint. Heat transfer oils are normally too viscous to be pumped at ambient temperatures and must be heated to provide fluidity.

Petroleum solvents are used to clean parts by spraying, dripping or dipping. The solvents remove oil and emulsify dirt and metal particles. Rust preventive oils may be either solvent or water based. They are applied to stainless steel coils, bearings and other parts by dipping or spraying, and leave polarized or wax films on the metal surfaces for fingerprint and rust protection and water displacement.

Greases

Greases are mixtures of fluids, thickeners and additives used to lubricate parts and equipment which cannot be made oil-tight, which are hard to reach or where leaking or splashed liquid lubricants might contaminate products or create a hazard. They have a wide range of applications and performance requirements, from lubricating jet engine bearings at sub-zero temperatures to hot rolling mill gears, and resisting acid or water washout, as well as the continuous friction created by railroad car wheel roller bearings.

Grease is made by the blending of metallic soaps (salts of long-chained fatty acids) into a lubricating oil medium at temperatures of 205 to 315°C. Synthetic greases may use di-esters, silicone or phosphoric esters and polyalkyl glycols as fluids. The characteristics of the grease depend to a great extent upon the particular fluid, metallic element (e.g., calcium, sodium, aluminium, lithium and so on) in the soap and the additives used to improve performance and stability and to reduce friction. These additives include extreme-pressure additives which coat the metal with a thin layer of non-corrosive metallic sulphur compounds, lead naphthenate or zinc dithiophosphate, rust inhibitors, anti-oxidants, fatty acids for added lubricity, tackiness additives, colour dyes for identification and water inhibitors. Some greases may contain graphite or molybdenum fillers which coat the metallic parts and provide lubrication after the grease has run out or decomposed.

Industrial Lubricants, Grease and Automotive Engine Oil Additives

In addition to using high-quality lubricant base stocks with chemical and thermal stability and high viscosity indexes, additives are needed to enhance the fluid and provide specific characteristics required in industrial lubricants, cutting fluids, greases and automotive engine oils. The most commonly used additives include but are not limited to the following:

  • Anti-oxidants. Oxidation inhibitors, such as 2,6-ditertiary butyl, paracresol and phenyl naphthylamine, reduce the rate of deterioration of oil by breaking up the long-chain molecules which form when exposed to oxygen. Oxidation inhibitors are used to coat metals such as copper, zinc and lead to prevent contact with the oil so they will not act as catalysts, speeding up oxidation and forming acids which attack other metals.
  • Foam inhibitors. Defoamants, such as silicones and polyorganic silioxanes, are used in hydraulic oils, gear oils, transmission fluids and turbine oils to reduce surface film tension and remove air entrapped in the oil by pumps and compressors, in order to maintain constant hydraulic pressure and prevent cavitation.
  • Corrosion inhibitors. Anti-rust additives, such as lead naphthenate and sodium sulphonate, are used to prevent rust from forming on metallic parts and systems where circulating oil has been contaminated with water or by moist air which entered system reservoirs as they cooled down when the equipment or machinery was not in use.
  • Anti-wear additives. Anti-wear additives, such as tricresylphosphate, form polar compounds which are attracted to metal surfaces and provide a physical layer of additional protection in the event that the oil film is not sufficient.
  • Viscosity index improvers. Viscosity index improvers help oils resist the effects of temperature changes. Unfortunately, their effectiveness diminishes with extended use. Synthetic oils are designed with very high viscosity indexes, allowing them to maintain their structure over wider temperature ranges and for much longer periods of time than mineral oils with viscosity index improver additives.
  • Demulsifiers. Water inhibitors and special compounds separate water out of oil and prevent gum formation; they contain waxy oils which provide added lubricity. They are used where equipment is subject to water wash or where a large amount of moisture is present, such as in steam cylinders, air compressors and gear cases contaminated by soluble cutting fluids.
  • Colour dyes. Dyes are used to assist users to identify different oils used for specific purposes, such as transmission fluids and gear oils, in order to prevent misapplication.
  • Extreme pressure additives. Extreme pressure additives, such as non-corrosive sulphurized fatty compounds, zinc dithiophosphate and lead naphthenate, are used in automotive, gear and transmission oils to form coatings which protect metal surfaces when the protective oil film thins or is squeezed out and cannot prevent metal to metal contact.
  • Detergents. Metal sulphonate and metal phenate detergents are used to hold dirt, carbon and metallic wear particles in suspension in hydraulic oils, gear oils, engine oils and transmission fluids. These contaminants are typically removed when the oil passes through a filter to prevent their being recirculated through the system where they could cause damage.
  • Tackiness additives. Adhesive or tackiness additives are used to enable oils to adhere to and resist leakage from bearing assemblies, gear cases, large open gears on mills and construction equipment, and overhead machinery. Their tackiness diminishes with extended service.
  • Emulsifiers. Fatty acids and fatty oils are used as emulsifiers in soluble oils to help form solutions with water.
  • Lubricity additives. Fat, lard, tallow, sperm and vegetable oils are used to provide a higher degree of oiliness in cutting oils and some gear oils.
  • Bactericides. Bactericides and germicides, such as phenol and pine oil, are added to soluble cutting oils to prolong the life of the fluid, maintain stability, reduce odours and prevent dermatitis.

 

Manufacturing Industrial Lubricants and Automotive Oils

Industrial lubricants and oils, grease, cutting fluids and automotive engine oils are manufactured in blending and packaging facilities, also called “lube plants” or “blending plants”. These facilities may be located either in or adjacent to refineries which produce lubricant base stocks, or they may be some distance away and receive the base stocks by marine tankers or barges, railroad tank cars or tank trucks. Blending and packaging plants blend and compound additives into lubricating oil base stocks to manufacture a wide range of finished products, which are then shipped in bulk or in containers.

The blending and compounding processes used to manufacture lubricants, fluids and greases depend on the age and sophistication of the facility, the equipment available, the types and formulation of the additives used and the variety and volume of products produced. Blending may require only physical mixing of base stocks and additive packages in a kettle using mixers, paddles or air agitation, or auxiliary heat from electric or steam coils may be needed to help dissolve and blend in the additives. Other industrial fluids and lubricants are produced automatically by mixing base stocks and pre-blended additive and oil slurries through manifold systems. Grease may be either batch produced or continuously compounded. Lube plants may compound their own additives from chemicals or purchase pre-packaged additives from specialty companies; a single plant may use both methods. When lube plants manufacture their own additives and additive packages, there may be a need for high temperatures and pressures in addition to chemical reactions and physical agitation to compound the chemicals and materials.

After production, fluids and lubricants may be held in the blending kettles or placed in holding tanks to ensure that the additives remain in suspension or solution, to allow time for testing to determine whether the product meets quality specifications and certification requirements, and to allow process temperatures to return to ambient levels before products are packaged and shipped. When testing is completed, finished products are released for bulk shipment or packaging into containers.

Finished products are shipped in bulk in railroad tank cars or in tank trucks directly to consumers, distributors or outside packaging plants. Finished products also are shipped to consumers and distributors in railroad box cars or package delivery trucks in a variety of containers, as follows:

  • Metal, plastic and combination metal/plastic or plastic/fibre intermediate bulk containers, which range in size from 227 l to approximately 2,840 l, are shipped as individual units on built-in or separate pallets, stacked 1 or 2 high.
  • Metal, fibre or plastic drums with a capacity of 208 l, 114 l or 180 kg are typically shipped 4 to a pallet.
  • Metal or plastic drums with a capacity of 60 l or 54 kg, and 19 l or 16 kg metal or plastic pails, are stacked on pallets and banded or stretch wrapped to maintain stability.
  • Metal or plastic containers with a capacity of 8 l or 4 l, 1 l plastic, metal and fibre bottles and cans and 2 kg grease cartridges are packaged in cartons which are stacked on pallets and banded or stretch wrapped for shipment.


Some blending and packaging plants may ship pallets of mixed products and mixed sizes of containers and packages directly to small consumers. For example, a single-pallet shipment to a service station could include 1 drum of transmission fluid, 2 kegs of grease, 8 cases of automotive engine oil and 4 pails of gear lubricant.

Product Quality

Lubricant product quality is important to keep machines and equipment operating properly and to produce quality parts and materials. Blending and packaging plants manufacture finished petroleum products to strict specifications and quality requirements. Users should maintain the level of quality by establishing safe practices for the handling, storage, dispensing and transfer of lubricants from their original containers or tanks to the dispensing equipment and to the point of application on the machine or equipment to be lubricated or the system to be filled. Some industrial facilities have installed centralized dispensing, lubrication and hydraulic systems which minimize contamination and exposure. Industrial oils, lubricants, cutting oils and grease will deteriorate from water or moisture contamination, exposure to excessively high or low temperatures, inadvertent mixing with other products and long-term storage which allows additive drop-out or chemical changes to occur.

Health and Safety

Because they are used and handled by consumers, finished industrial and automotive products must be relatively free of hazards. There is a potential for hazardous exposures when blending and compounding products, when handling additives, when using cutting fluids and when operating oil mist lubrication systems.

The chapter Oil and natural gas refineries in this Encyclopaedia gives information regarding potential hazards associated with auxiliary facilities at blending and packaging plants such as boiler rooms, laboratories, offices, oil-water separators and waste treatment facilities, marine docks, tank storage, warehouse operations, railroad tank car and tank truck loading racks and railroad box car and package truck loading and unloading facilities.

Safety

Manufacturing additives and slurries, batch compounding, batch blending and in-line blending operations require strict controls to maintain desired product quality and, along with the use of PPE, to minimize exposure to potentially hazardous chemicals and materials as well as contact with hot surfaces and steam. Additive drums and containers should be stored safely and kept tightly sealed until ready for use. Additives in drums and bags need to be handled properly to avoid muscular strain. Hazardous chemicals should be properly stored, and incompatible chemicals should not be stored where they can mix with one another. Precautions to be taken when operating filling and packaging machinery include using gloves and avoiding catching fingers in devices which crimp covers on kegs and pails. Machine guards and protective systems should not be removed, disconnected or by-passed to expedite work. Intermediate bulk containers and drums should be inspected before filling to make sure they are clean and suitable.

A confined-space permit system should be established for entry into storage tanks and blending kettles for cleaning, inspection, maintenance or repair. A lockout/tagout procedure should be established and implemented before working on packaging machinery, blending kettles with mixers, conveyors, palletizers and other equipment with moving parts.

Leaking drums and containers should be removed from the storage area and spills cleaned up to prevent slips and falls. Recycling, burning and disposal of waste, spilled and used lubricants, automotive engine oils and cutting fluids should be in accordance with government regulations and company procedures. Workers should use appropriate PPE when cleaning spills and handling used or waste products. Drained motor oil, cutting fluids or industrial lubricants which may be contaminated with gasoline and flammable solvents should be stored in a safe place away from sources of ignition, until proper disposal.

Fire protection

While the potential for fire is less in industrial and automotive lubricant blending and compounding than in refining processes, care must be taken when manufacturing metal working oils and greases due to the use of high blending and compounding temperatures and lower flashpoint products. Special precautions should be taken to prevent fires when products are dispensed or containers filled at temperatures above their flashpoints. When transferring flammable liquids from one container to another, proper bonding and grounding techniques should be applied to prevent static build-up and electrostatic discharge. Electrical motors and portable equipment should be properly classified for the hazards present in the area in which they are installed or used.

The potential for fire exists if a leaking product or vapour release in the lube blending and grease processing or storage areas reaches a source of ignition. The establishment and implementation of a hot-work permit system should be considered to prevent fires in blending and packaging facilities. Storage tanks installed inside buildings should be constructed, vented and protected in accordance with government requirements and company policy. Products stored on racks and in piles should not block fire protection systems, fire doors or exit routes.

Storage of finished products, both in bulk and in containers and packages, should be in accordance with recognized practices and fire prevention regulations. For example, flammable liquids and additives which are in solutions of flammable liquids may be stored in outside buildings or separate, specially designed inside or attached storage rooms. Many additives are stored in warm rooms (38 to 65°C) or in hot rooms (over 65°C) in order to keep the ingredients in suspension, to reduce the viscosity of thicker products or to provide for easier blending or compounding. These storage rooms should comply with electrical classification, drainage, ventilation and explosion venting requirements, especially when flammable liquids or combustible liquids are stored and dispensed at temperatures above their flashpoints.

Health

When blending, sampling and compounding, personal and respiratory protective equipment should be considered to prevent exposures to heat, steam, dusts, mists, vapours, fumes, metallic salts, chemicals and additives. Safe work practices, good hygiene and appropriate personal protection may be needed for exposure to oil mists, fumes and vapours, additives, noise and heat when conducting inspection and maintenance activities while sampling and handling hydrocarbons and additives during the production and packaging and when cleaning up spills and releases:

  • Work shoes with oil- or slip-resistant soles should be worn for general work, and approved protective toe safety shoes with oil- or slip-resistant soles should be worn where hazards of foot injuries from rolling or falling objects or equipment exist.
  • Safety goggles and respiratory protection may be needed for hazardous exposures to chemicals, dust or steam.
  • Impervious gloves, aprons, footwear, face shields and chemical goggles should be worn when handling hazardous chemicals, additives and caustic solutions and when cleaning up spills.
  • Head protection may be needed when working in pits or areas where the potential exists for injury to the head.
  • Ready access to appropriate cleaning and drying facilities to handle splashes and spills should be provided.

 

Oil is a common cause of dermatitis, which can be controlled through the use of PPE and good personal hygiene practices. Direct skin contact with any formulated greases or lubricants should be avoided. Lighter oils such as kerosene, solvents and spindle oils defat the skin and cause rashes. Thicker products, such as gear oils and greases, block the pores of the skin, leading to folliculitis.

Health hazards due to microbial contamination of oil may be summarized as follows:

  • Pre-existing skin conditions may be aggravated.
  • Lubricant aerosols of respirable size may cause respiratory illness.
  • Organisms may change the composition of the product so that it becomes directly injurious.
  • Harmful bacteria from animals, birds or humans may be introduced.

 

Contact dermatitis may occur when employees are exposed to cutting fluids during production, work or maintenance and when they wipe oil-covered hands with rags embedded with minute metal particles. The metal causes small lacerations in the skin which may become infected. Water-based cutting fluids on skin and clothing may contain bacteria and cause infections, and the emulsifiers may dissolve fats from the skin. Oil folliculitis is caused by prolonged exposure to oil-based cutting fluids, such as from wearing oil-soaked clothing. Employees should remove and launder clothing that is soaked with oil before wearing it again. Dermatitis may also be caused by using soaps, detergents or solvents to clean the skin. Dermatitis is best controlled by good hygiene practices and minimizing exposure. Medical advice should be sought when dermatitis persists.

In the extensive review conducted as a basis for its criteria document, the US National Institute for Occupational Safety and Health (NIOSH) found an association between exposure to metal working fluids and the risk of developing cancer at several organ sites, including the stomach, pancreas, larynx and rectum (NIOSH 1996). The specific formulations responsible for the elevated cancer risks remain to be determined.

Occupational exposure to oil mists and aerosols is associated with a variety of non-malignant respiratory effects, including lipoid pneumonia, asthma, acute airways irritation, chronic bronchitis and impaired pulmonary function (NIOSH 1996).

Metal working fluids are readily contaminated by bacteria and fungi. They may affect the skin or, when inhaled as contaminated aerosols, they may have systemic effects.

Refinery processes such as hydrofinishing and acid treatment are used to remove aromatics from industrial lubricants, and the use of naphthenic base stocks has been restricted in order to minimize carcinogenicity. Additives introduced in blending and compounding may also create a potential risk to health. Exposures to chlorinated compounds and leaded compounds, such as those used in some gear lubricants and greases, cause irritation of the skin and may be potentially hazardous. Tri-orthocresyl phosphate has caused outbreaks of nerve palsies when lubricating oil was accidentally used for cooking. Synthetic oils consist mainly of sodium nitrite and triethanolamine and additives. Commercial triethanolamine contains diethanolamine, which can react with sodium nitrite to form a relatively weak carcinogen, N-nitrosodiethanolamine, which may create a hazard. Semi-synthetic lubricants present the hazards of both products, as well as the additives in their formulations.

Product safety information is important to employees of both manufacturers and users of lubricants, oils and greases. Manufacturers should have material safety data sheets (MSDSs) or other product information available for all of the additives and base stocks used in blending and compounding. Many companies have conducted epidemiological and toxicological testing to determine the degree of hazards associated with any acute and chronic health effects of their products. This information should be available to workers and users through warning labels and product safety information.

 

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Wednesday, 16 March 2011 22:23

Surface Treatment of Metals

Written by

Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

There is a wide variety of techniques for finishing the surfaces of metal products so that they resist corrosion, fit better and look better (see table 1). Some products are treated by a sequence of several of these techniques. This article will briefly describe some of those most commonly used.

Table 1. Summary of the hazards associated with the different metal treatment methods

Metal treatment method

Hazards

Precautions

Electrolytic polishing

Burns and irritation from caustic and corrosive chemicals

Use appropriate personal protective equipment. Install effective exhaust ventilation.

Electroplating

Exposure to potentially cancer causing chromium and nickel; exposure to cyanides; burns and irritation from caustic and corrosive chemicals; electric shock; the process can be wet, causing slip and fall hazards; potential explosive dust generation; ergonomic hazards

Use appropriate personal protective equipment. Install effective exhaust ventilation, often slotted, push-pull system. Clean up spills immediately. Install non-skid flooring. Use effective design of work procedures and stations to avoid ergonomic stress.

Enamels and glazing

Physical hazards from grinders, conveyers, mills; burn hazard from high temperature liquids and equipment; exposure to dusts that may cause lung disease

Install proper machine guards, including interlocks. Use appropriate personal protective equipment. Install effective exhaust ventilation to avoid dust exposure. HEPA-filtered equipment may be necessary.

Etching

Exposure to hydrofluoric acid; burns and irritation from caustic and corrosive chemicals; burn hazard from high temperature liquids and equipment

Implement a programme to avoid exposure to hydrofluoric acid. Use appropriate personal protective equipment. Install effective exhaust ventilation.

Galvanizing

Burn hazard from high temperature liquids, metals, and equipment; burns and irritation from caustic and corrosive chemicals; metal fume fever; potential lead exposure

Use appropriate personal protective equipment. Install effective exhaust ventilation. Implement a lead exposure reduction/monitoring programme.

Heat treatment

Burn hazard from high temperature liquids, metals and equipment; burns and irritation from caustic and corrosive chemicals; possible explosive atmospheres of hydrogen; potential exposure to carbon monoxide; potential exposure to cyanides; fire hazard from oil quenching

Use appropriate personal protective equipment. Install effective exhaust ventilation. Display signs warning of high temperature equipment and surfaces. Install systems to monitor the concentration of carbon monoxide. Install adequate fire-suppression systems.

Metallizing

Burn hazard from high temperature metals and equipment; possible explosive atmospheres of dust, acetylene; zinc metal fume fever

Install adequate fire suppression systems. Properly separate chemicals and gases. Use appropriate personal protective equipment. Install effective exhaust ventilation.

Phosphating

Burns and irritation from caustic and corrosive chemicals

Use appropriate personal protective equipment. Install effective exhaust ventilation.

Plastics coating

Exposure to chemical sensitizers

Seek alternatives to sensitizers. Use appropriate personal protective equipment. Install effective exhaust ventilation.

Priming

Exposure to various solvents which are potentially toxic and flammable, exposure to chemical sensitizers, exposure to potentially carcinogenic chromium

Seek alternatives to sensitizers. Use appropriate personal protective equipment. Install effective exhaust ventilation. Properly separate chemicals/gases.

 

Before any of these techniques can be applied, the products must be thoroughly cleaned. A number of methods of cleaning are used, individually or in sequence. They include mechanical grinding, brushing and polishing (which produce metallic or oxidic dust—aluminium dust may be explosive), vapour degreasing, washing with organic grease solvents, “pickling” in concentrated acid or alkaline solutions and electrolytic degreasing. The last involves immersion in baths containing cyanide and concentrated alkali in which electrolytically formed hydrogen or oxygen remove the grease, resulting in “blank” metal surfaces that are free from oxides and grease. The cleaning is followed by adequate rinsing and drying of the product.

Proper design of the equipment and effective LEV will reduce some of the risk. Workers exposed to the hazard of splashes must be provided with protective goggles or eye shields and protective gloves, aprons and clothing. Showers and eyewash fountains should be nearby and in good working order, and splashes and spills should be washed away promptly. With electrolytic equipment, the gloves and shoes must be non-conducting, and other standard electrical precautions, such as the installation of ground fault circuit interrupters and lockout/tagout procedures should be followed.

Treatment Processes

Electrolytic polishing

Electrolytic polishing is used to produce a surface of improved appearance and reflectivity, to remove excess metal to accurately fit the required dimensions and to prepare the surface for inspection for imperfections. The process involves preferential anodic dissolution of high spots on the surface after vapour degreasing and hot alkaline cleaning. Acids are frequently used as the electrolyte solutions; accordingly, adequate rinsing is required afterwards.

Electroplating

Electroplating is a chemical or electrochemical process for applying a metallic layer to the product—for example, nickel to protect against corrosion, hard chromium to improve the surface properties or silver and gold to beautify it. Occasionally, non-metallic materials are used. The product, wired as the cathode, and an anode of the metal to be deposited are immersed in an electrolyte solution (which can be acidic, alkaline or alkaline with cyanide salts and complexes) and connected externally to a source of direct current. The positively charged cations of the metallic anode migrate to the cathode, where they are reduced to the metal and deposited as a thin layer (see figure 1). The process is continued until the new coating reaches the desired thickness, and the product is then washed, dried and polished.

Figure 1. Electroplating: Schematic representation

MET070F1

 

Anode: Cu → Cu+2 + 2e- ; Cathode: Cu+2 +  2e- → Cu

In electroforming, a process closely related to electroplating, objects moulded of, for example, plaster or plastic are made conductive by the application of graphite and then are connected as the cathode so that the metal is deposited on them.

In anodization, a process that has become increasingly important in recent years, products of aluminium (titanium and other metals are also used) are connected as the anode and immersed in dilute sulphuric acid. However, instead of the formation of positive aluminium ions and migrating for deposition on the cathode, they are oxidized by the oxygen atoms arising at the anode and become bound to it as an oxide layer. This oxide layer is partially dissolved by the sulphuric acid solution, making the surface layer porous. Subsequently, coloured or light-sensitive materials can be deposited in these pores, as in the fabrication of nameplates, for example.

Enamels and glazes

Vitreous enamel or porcelain enamel is used to give a high heat-, stain- and corrosion-resistant covering to metals, usually iron or steel, in a wide range of fabricated products including bath tubs, gas and electric cookers, kitchen ware, storage tanks and containers, and electrical equipment. In addition, enamels are used in the decoration of ceramics, glass, jewellery and decorative ornaments. The specialized use of enamel powders in the production of such ornamental ware as Cloisonné and Limoges has been known for centuries. Glazes are applied to pottery ware of all kinds.

The materials used in the manufacture of vitreous enamels and glazes include:

  • refractories, such as quartz, feldspar and clay
  • fluxes, such as borax (sodium borate decahydrate), soda ash (anhydrous sodium carbonate), sodium nitrate, fluorspar, cryolite, barium carbonate, magnesium carbonate, lead monoxide, lead tetroxide and zinc oxide
  • colours, such as oxides of antimony, cadmium, cobalt, iron, nickel, manganese, selenium, vanadium, uranium and titanium
  • opacifiers, such as oxides of antimony, titanium, tin and zirconium, and sodium antimoninate
  • electrolytes, such as borax, soda ash, magnesium carbonate and sulphate, sodium nitrite and sodium aluminate
  • flocculating agents, such as clay, gums, ammonium alginate, bentonite and colloidal silica.

 

The first step in all types of vitreous enamelling or glazing is the making of the frit, the enamel powder. This involves preparation of the raw materials, smelting and frit handing.

After careful cleaning of the metal products (e.g., shot blasting, pickling, degreasing), the enamel may be applied by a number of procedures:

  • In the wet process, the object is dipped into the aqueous enamel slip, withdrawn and allowed to drain or, in “slushing”, the enamel slip is thicker and must be shaken from the object.
  • In the dry process, the ground-coated object is heated to the enamelling temperature and then dry enamel powder is dusted through sieves onto it. The enamel sinters into place and, when the object is returned to the furnace, it melts down to a smooth surface.
  • Spray application is being used increasingly, usually in a mechanized operation. It requires a cabinet under exhaust ventilation.
  • Decorative enamels are usually applied by hand, using brushes or similar tools.
  • Glazes for porcelain and pottery articles are usually applied by dipping or spraying. Although some dipping operations are being mechanized, pieces are usually dipped by hand in the domestic porcelain industry. The object is held in the hand, dipped into a large tub of glaze, the glaze is removed by a flick of the wrist and the object is placed in a dryer. An enclosed hood or cabinet with efficient exhaust ventilation should be provided when the glaze is sprayed.

 

The prepared objects are then “fired” in a furnace or kiln, which usually is gas fuelled.

Etching

Chemical etching produces a satin or matte finish. Most frequently, it is used as a pre-treatment prior to anodizing, lacquering, conversion coating, buffing or chemical brightening. It is most frequently applied to aluminium and stainless steel, but is also used for many other metals.

Aluminium is usually etched in alkaline solutions containing various mixtures of sodium hydroxide, potassium hydroxide, trisodium phosphate and sodium carbonate, together with other ingredients to prevent sludge formation. One of the most common processes uses sodium hydroxide at a concentration of 10 to 40 g/l maintained at a temperature of 50 to 85°C with an immersion time as long as 10 minutes.

The alkaline etching is usually preceded and followed by treatment in various mixtures of hydrochloric, hydrofluoric, nitric, phosphoric, chromic or sulphuric acid. A typical acid treatment involves immersions of 15 to 60 seconds in a mixture of 3 parts by volume of nitric acid and 1 part by volume of hydrofluoric acid that is maintained at a temperature of 20°C.

Galvanizing

Galvanizing applies a zinc coating to a variety of steel products to protect against corrosion. The product must be clean and oxide-free for the coating to adhere properly. This usually involves a number of cleaning, rinsing, drying or annealing processes before the product enters the galvanizing bath. In “hot dip” galvanizing, the product is passed through a bath of molten zinc; “cold” galvanizing is essentially electroplating, as described above.

Manufactured products are usually galvanized in a batch process, while the continuous strip method is used for steel strip, sheet or wire. Flux may be employed to maintain satisfactory cleaning of both the product and the zinc bath and to facilitate drying. A prefluxing step may be followed by an ammonium chloride flux cover on the surface of the zinc bath, or the latter may be used alone. In galvanizing pipe, the pipe is immersed in a hot solution of zinc ammonium chloride after cleaning and before the pipe enters the molten zinc bath. The fluxes decompose to form irritating hydrogen chloride and ammonia gas, requiring LEV.

The various types of continuous hot-dip galvanizing differ essentially in how the product is cleaned and whether the cleaning is done on-line:

  • cleaning by flame oxidation of the surface oils with subsequent reduction in the furnace and annealing done in-line
  • electrolytic cleaning done prior to in-line annealing
  • cleaning by acid pickling and alkali cleaning, using a flux prior to the preheat furnace and annealing in a furnace before galvanizing
  • cleaning by acid pickling and alkali cleaning, eliminating the flux and preheating in a reducing gas (e.g., hydrogen) prior to galvanizing.

 

The continuous galvanizing line for light-gauge strip steel omits pickling and the use of flux; it uses alkaline cleaning and maintains the clean surface of the strip by heating it in a chamber or furnace with a reducing atmosphere of hydrogen until it passes below the surface of the molten zinc bath.

Continuous galvanizing of wire requires annealing steps, usually with a molten lead pan in front of the cleaning and galvanizing tanks; air or water cooling; pickling in hot, dilute hydrochloric acid; rinsing; application of a flux; drying; and then galvanizing in the molten zinc bath.

A dross, an alloy of iron and zinc, settles to the bottom of the molten zinc bath and must be removed periodically. Various types of materials are floated on the surface of the zinc bath to prevent oxidation of the molten zinc. Frequent skimming is needed at the points of entry and exit of the wire or strip being galvanized.

Heat treatment

Heat treatment, the heating and cooling of a metal which remains in the solid state, is usually an integral part of the processing of metal products. It almost always involves a change in the crystalline structure of the metal which results in a modification of its properties (e.g., annealing to make the metal more malleable, heating and slow cooling to reduce hardness, heating and quenching to increase hardness, low-temperature heating to minimize internal stresses).

Annealing

Annealing is a “softening” heat treatment widely used to allow further cold working of the metal, improve machinability, stress-relieve the product before it is used and so on. It involves heating the metal to a specific temperature, holding it at that temperature for a specific length of time and allowing it to cool at a particular rate. A number of annealing techniques are used:

  • Blue annealing, in which a layer of blue oxide is produced on the surface of iron-based alloys
  • Bright annealing, which is carried out in a controlled atmosphere to minimize surface oxidation
  • Close annealing or box annealing, a method in which both ferrous and non-ferrous metals are heated in a sealed metal container with or without a packing material and then slowly cooled
  • Full annealing, usually carried out in a protective atmosphere, aimed at obtaining the maximum softness economically feasible
  • Malleablizing, a special kind of anneal given to iron castings to make them malleable by transforming the combined carbon in the iron to fine carbon (i.e., graphite)
  • Partial annealing, a low-temperature process to remove internal stresses induced in the metal by cold working
  • Sub-critical or spheroidizing annealing, which produces improved machinability by allowing the iron carbide in the crystalline structure to acquire a spheroid shape.

 

Age-hardening

Age-hardening is a heat treatment often used on aluminium-copper alloys in which the natural hardening that takes place in the alloy is accelerated by heating to about 180°C for about 1 hour.

Homogenizing

Homogenizing, usually applied to ingots or powdered metal compacts, is designed to remove or greatly reduce segregation. It is achieved by heating to a temperature about 20°C below the metal’s melting point for about 2 hours or more and then quenching.

Normalizing

A process similar to full annealing, ensures the uniformity of the mechanical properties to be obtained and also produces greater toughness and resistance to mechanical loading.

Patenting

Patenting is a special type of annealing process that is usually applied to materials of small cross-section which are intended to be drawn (e.g., 0.6% carbon steel wire). The metal is heated in an ordinary furnace to above the transformation range and then passes from the furnace directly into, for example, a lead bath held at a temperature of about 170°C.

Quench-hardening and tempering

An increase in hardness can be produced in an iron-based alloy by heating to above the transformation range and rapidly cooling to room temperature by quenching in oil, water or air. The article is often too highly stressed to be put into service and, in order to increase its toughness, it is tempered by reheating to a temperature below the transformation range and allowing it to cool at the desired rate.

Martempering and austempering are similar processes except that the article is quenched, for example, in a salt or lead bath held at a temperature of 400°C.

Surface- and case-hardening

This is another heat-treatment process applied most frequently to iron-based alloys, which allows the surface of the object to remain hard while its core remains relatively ductile. It has a number of variations:

  • Flame hardening involves hardening the surfaces of the object (e.g., gear teeth, bearings, slideways) by heating with a high-temperature gas torch and then quenching in oil, water or another suitable medium.
  • Electrical induction hardening is similar to flame hardening except that the heating is produced by eddy currents induced in the surface layers.
  • Carburizing increases the carbon content of the surface of an iron-based alloy by heating the object in a solid, liquid or gaseous carbonaceous medium (e.g., solid charcoal and barium carbonate, liquid sodium cyanide and sodium carbonate, gaseous carbon monoxide, methane and so on) at a temperature of about 900°C.
  • Nitriding increases the nitrogen content of the surface of a special low-alloy cast iron or steel object by heating it in a nitrogenous medium, usually ammonia gas, at about 500 to 600°C.
  • Cyaniding is a method of case-hardening in which the surface of a low-carbon steel object is enriched in both carbon and nitrogen simultaneously. It usually involves heating the object for 1 hour in a bath of molten 30% sodium cyanide at 870°C, and then quenching in oil or water.
  • Carbo-nitriding is a gaseous process for the simultaneous absorption of carbon and nitrogen into the surface layer of steel by heating it to 800 to 875°C in an atmosphere of a carburizing gas (see above) and a nitriding gas (e.g., 2 to 5% anhydrous ammonia).

 

Metallizing

Metallizing, or metal spraying, is a technique for applying a protective metallic coating to a mechanically roughened surface by spraying it with molten droplets of metal. It is also used to build up worn or corroded surfaces and for salvaging badly-machined component parts. The process is widely known as Schooping, after the Dr. Schoop who invented it.

It uses the Schooping gun, a hand-held, pistol-shaped spray gun through which the metal in wire form is fed into a fuel gas/oxygen blowpipe flame which melts it and, using compressed air, sprays it onto the object. The heat source is a mixture of oxygen and either acetylene, propane or compressed natural gas. The coiled wire is usually straightened before being fed into the gun. Any metal that can be made into a wire may be used; the gun can also accept the metal in powder form.

Vacuum metallizing is a process in which the object is placed in a vacuum jar into which the coating metal is sprayed.

Phosphating

Phosphating is used mainly on mild and galvanized steel and aluminium to augment the adhesion and corrosion resistance of paint, wax and oil finishes. It is also used to form a layer which acts as a parting film in the deep drawing of sheet metal and improves its wear resistance. It essentially consists of allowing the metal surface to react with a solution of one or more phosphates of iron, zinc, manganese, sodium or ammonium. Sodium and ammonium phosphate solutions are used for combined cleaning and phosphating. The need to phosphate multi-metal objects and the desire to increase line speeds in automated operations have led to reducing reaction times by the addition of accelerators such as fluorides, chlorates, molybdates and nickel compounds to the phosphating solutions.To reduce crystal size and, consequently, increase the flexibility of zinc phosphate coatings, crystal refining agents such as tertiary zinc phosphate or titanium phosphate are added to the pre-treatment rinse.

The phosphating sequence typically includes the following steps:

  • hot caustic cleaning
  • brushing and rinsing
  • further hot caustic cleaning
  • conditioning water rinse
  • spraying or dipping in hot solutions of acid phosphates
  • cold water rinse
  • warm chromic acid rinse
  • another cold water rinse
  • drying.

 

Priming

Organic paint primers are applied to metal surfaces to promote the adhesion of subsequently applied paints and to retard corrosion at the paint-metal interface. The primers usually contain resins, pigments and solvents and may be applied to the prepared metal surfaces by brush, spray, immersion, roller coating or electrophoresis.

The solvents may be any combination of aliphatic and aromatic hydrocarbons, ketones, esters, alcohols and ethers. The most commonly used resins are polyvinyl butynol, phenolic resins, drying oil alkyds, epoxidized oils, epoxyesters, ethyl silicates and chlorinated rubbers. In complex primers, cross-linking agents such as tetraethylene pentamine, pentaethylene hexamine, isocyanates and urea formaldehyde are used. Inorganic pigments used in primer formulations include lead, barium, chromium, zinc and calcium compounds.

Plastic coating

Plastic coatings are applied to metals in liquid form, as powders which are subsequently cured or sintered by heating, or in the form of fabricated sheets which are laminated to the metal surface with an adhesive. The most commonly used plastics include polyethylene, polyamides (nylons) and PVC. The latter may include plasticizers based on monomeric and polymeric esters and stabilizers such as lead carbonate, fatty acid salts of barium and cadmium, dibutyltin dilaurate, alkyltin mercaptides and zinc phosphate. Although generally of low toxicity and non-irritating, some of the plasticizers are skin sensitizers.

Hazards and Their Prevention

As might be deduced from the complexity of the processes outlined above, there is a large variety of safety and health hazards associated with the surface treatment of metals. Many are regularly encountered in manufacturing operations; others are presented by the uniqueness of the techniques and materials employed. Some are potentially life threatening. By and large, however, they can be prevented or controlled.

Workplace design

The workplace should be designed to allow the delivery of raw materials and supplies and the removal of the finished products without interfering with the ongoing processing. Since many of the chemicals are flammable or prone to react when mixed, proper separation in storage and in transit is essential. Many of the metal finishing operations involve liquids, and when leaks, spills or splashes of acids or alkalis occur they must be washed away promptly. Accordingly, adequately drained, slip-resistant floors must be provided. Housekeeping must be diligent to keep the work areas and other spaces clean and free from accumulations of materials. Systems for disposal of solid and liquid wastes and effluents from furnaces and exhaust ventilation must be designed with environmental concerns in mind.

Work stations and work assignments should use ergonomic principles to minimize strains, sprains, excessive fatigue and RSIs. Machine guards must have automatic lockout so the machine is de-energized if the guard is removed. Splash guards are essential. Because of the danger of splashes of hot acid and alkali solutions, eyewash fountains and whole-body showers must be installed within easy reach. Signs should be posted to warn other production and maintenance personnel of such dangers as chemical baths and hot surfaces.

Chemical assessment

All chemicals should be evaluated for potential toxicity and physical hazards, and less hazardous materials should be substituted where possible. However, since the less toxic material may be more flammable, the hazard of fire and explosion must also be considered. In addition, the chemical compatibility of materials must be considered. For example, mixing of nitrate and cyanide salts by accident could cause an explosion due to the strong oxidizing properties of nitrates.

Ventilation

Most of the metal coating processes require LEV that is strategically placed to draw the vapours or other contaminants away from the worker. Some systems push fresh air across the tank to “push” airborne contaminants to the exhaust side of the system. Fresh air intakes must be located away from exhaust vents so that potentially toxic gases are not recirculated.

Personal protective equipment

Processes should be engineered to prevent potentially toxic exposures, but since they cannot always be totally avoided, employees will have to be provided with appropriate PPE (e.g., goggles with or without face shields as appropriate, gloves, aprons or coveralls and shoes). Because many of the exposures involve hot corrosive or caustic solutions, the protective items should be insulated and chemical-resistant. If there is possible exposure to electricity, PPE should be non-conductive. PPE must be available in adequate quantity to allow contaminated, wet items to be cleaned and dried before re-using them. Insulated gloves and other protective clothing should be available where there is the risk of thermal burns from hot metal, furnaces and so on.

An important adjunct is the availability of wash-up facilities and clean lockers and dressing rooms, so that workers’ clothing remains uncontaminated and workers do not carry toxic materials back into their homes.

Employee training and supervision

Employee education and training are essential both when new to the job or when there have been changes in the equipment or the process. MSDSs must be provided for each of the chemical products which explain the chemical and physical hazards, in languages and at educational levels that ensure they will be understood by the workers. Competence testing and periodic retraining will assure that workers have retained the needed information. Close supervision is advisable to make sure that the proper procedures are being followed.

Selected hazards

Certain hazards are unique to the metal coating industry and deserve special consideration.

Alkaline and acid solutions

The heated alkaline and acid solutions used in cleaning and treatment of metals are particularly corrosive and caustic. They are irritating to the skin and mucous membranes and are especially dangerous when splashed into the eye. Eyewash fountains and emergency showers are essential. Proper protective clothing and goggles will guard against the inevitable splashes; when a splash reaches the skin, the area should be immediately and copiously rinsed with cool, clean water for at least 15 minutes; medical attention may be necessary, particularly when the eye is involved.

Care should be exercised when utilizing chlorinated hydrocarbons as phosgene may result from a reaction of the chlorinated hydrocarbon, acids and metals. Nitric and hydrofluoric acid are particularly dangerous when their gases are inhaled, because it may take 4 hours or more before the effects on the lungs become apparent. Bronchitis, pneumonitis and even potentially fatal pulmonary oedema may appear belatedly in a worker who apparently had no initial effect from the exposure. Prompt prophylactic medical treatment and, often, hospitalization are advisable for workers who have been exposed. Skin contact with hydrofluoric acid can cause severe burns without pain for several hours. Prompt medical attention is essential.

Dust

Metallic and oxidic dusts are a particular problem in grinding and polishing operations, and are most effectively removed by LEV as they are created. Ductwork should be designed to be smooth and air velocity should be sufficient to keep the particulates from settling out of the air stream. Aluminium and magnesium dust may be explosive and should be collected in a wet trap. Lead has become less of a problem with the decline of its use in ceramics and porcelain glazes, but it remains the ubiquitous occupational hazard and must always be guarded against. Beryllium and its compounds have received interest recently due to the possibility of carcinogenicity and chronic beryllium disease.

Certain operations present a risk of silicosis and pneumoconiosis: the calcining, crushing and drying of flint, quartz or stone; the sieving, mixing and weighing out of these substances in the dry state; and the charging of furnaces with such materials. They also represent a danger when they are used in a wet process and are splashed about the workplace and on workers’ clothing, to become dusts again when they dry out. LEV and rigorous cleanliness and personal hygiene are important preventive measures.

Organic solvents

Solvents and other organic chemicals used in degreasing and in certain processes are dangerous when inhaled. In the acute phase, their narcotic effects may lead to respiratory paralysis and death. In chronic exposure, toxicity of the central nervous system and liver and kidney damage are most frequent. Protection is provided by LEV with a safety zone of at least 80 to 100 cm between the source and the breathing area of the worker. Bench ventilation must also be installed to remove residual vapours from the finished workpieces. Defatting of the skin by organic solvents may be a precursor of dermatitis. Many solvents are also flammable.

Cyanide

Baths containing cyanides are frequently used in electrolytic degreasing, electroplating and cyaniding. Reaction with acid will form the volatile, potentially lethal hydrogen cyanide (prussic acid). The lethal concentration in air is 300 to 500 ppm. Fatal exposures may also result from skin absorption or ingestion of cyanides. Optimum cleanliness is essential for workers using cyanide. Food should not be eaten before washing, and should never be in the work area. Hands and clothing must be carefully cleaned following a potential cyanide exposure.

First aid measures for cyanide poisoning include transport into the open air, removal of contaminated clothing, copious washing of the exposed areas with water, oxygen therapy and inhalation of amyl nitrite. LEV and skin protection are essential.

Chromium and nickel

Chromic and nickel compounds used in galvanic baths in electroplating may be hazardous. Chromium compounds can cause burns, ulceration and eczema of the skin and mucosa and a characteristic perforation of the nasal septum. Bronchial asthma may occur. Nickel salts can cause obstinate allergic or toxic-irritative skin injury. There is evidence that both chromium and nickel compounds may be carcinogenic. LEV and skin protection are essential.

Furnaces and ovens

Special precautions are needed when working with the furnaces employed, for example, in the heat treatment of metals where components are handled at high temperatures and the materials used in the process may either be toxic or explosive or both. The gaseous media (atmospheres) in the furnace may react with the metal charge (oxidizing or reducing atmospheres) or they may be neutral and protective. Most of the latter contain up to 50% hydrogen and 20% carbon monoxide, which, in addition to being combustible, form highly explosive mixtures with air at elevated temperatures. The ignition temperature varies from 450 to 750 °C, but a local spark may cause ignition even at lower temperatures. The danger of explosion is greater when the furnace is being started up or shut down. Since a cooling furnace tends to suck in air (a particular danger when the fuel or power supply is interrupted), a supply of inert gas (e.g., nitrogen or carbon dioxide) should be available for purging when the furnace is shut down as well as when a protective atmosphere is introduced into a hot furnace.

Carbon monoxide is perhaps the greatest hazard from furnaces and ovens. Since it is colourless and odourless, it frequently reaches toxic levels before the worker becomes aware of it. Headache is one of the earliest symptoms of toxicity, and, therefore, a worker developing a headache on the job should immediately be removed into fresh air. Danger zones include recessed pockets in which the carbon monoxide may collect; it should be remembered that brickwork is porous and may retain the gas during normal purging and emit it when the purging is completed.

Lead furnaces may be dangerous since lead tends to vaporize quite rapidly at temperatures above 870°C. Accordingly, an effective fume extraction system is required. A pot breakage or failure may also be hazardous; a sufficiently large well or pit should be provided to capture the molten metal if this occurs.

Fire and explosion

Many of the compounds used in metal coating are flammable and, under certain circumstances, explosive. For the most part, the furnaces and drying ovens are gas fired, and special precautions such as flame-failure devices at burners, low-pressure cut-off valves in the supply lines and explosion relief panels in the structure of the stoves should be installed. In electrolytic operations, hydrogen formed in the process may collect at the surface of the bath and, if not exhausted, may reach explosive concentrations. Furnaces should be properly ventilated and burners protected from being clogged by dripping material.

Oil quenching is also a fire hazard, especially if the metal charge is not completely immersed. Quenching oils should have a high flashpoint, and their temperature should not exceed 27°C.

Compressed oxygen and fuel gas cylinders used in metallizing are fire and explosion hazards if not stored and operated properly. See the article “Welding and thermal cutting” in this chapter for detailed precautions.

As required by local ordinances, firefighting equipment, including alarms, should be provided and maintained in working order, and the workers drilled in using it properly.

Heat

The use of furnaces, open flames, ovens, heated solutions and molten metals inevitably presents the risk of excessive heat exposure, which is compounded in hot, humid climates and, particularly, by occlusive protective garments and gear. Complete air conditioning of a plant may not be economically feasible, but supplying cooled air in local ventilation systems is helpful. Rest breaks in cool surroundings and adequate fluid intake (fluids taken at the work station should be free of toxic contaminants) will help to avert heat toxicity. Workers and supervisors should be trained in the recognition of heat stress symptoms.

Conclusion

Surface treatment of metals involves a multiplicity of processes entailing a broad range of potentially toxic exposures, most of which can be prevented or controlled by the diligent application of well-recognized preventive measures.

 

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Saturday, 19 March 2011 19:54

Metal Reclamation

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Metal reclamation is the process by which metals are produced from scrap. These reclaimed metals are not distinguishable from the metals produced from primary processing of an ore of the metal. However, the process is slightly different and the exposure could be different. The engineering controls are basically the same. Metal reclamation is very important to the world economy because of the depletion of raw materials and the pollution of the environment created by scrap materials.

Aluminium, copper, lead and zinc comprise 95% of the production in the secondary non-ferrous metal industry. Magnesium, mercury, nickel, precious metals, cadmium, selenium, cobalt, tin and titanium are also reclaimed. (Iron and steel are discussed in the chapter Iron and steel industry. See also the article “Copper, lead and zinc smelting and refining” in this chapter.)

Control Strategies

Emission/exposure control principles

Metal reclamation involves exposures to dust, fumes, solvents, noise, heat, acid mists and other potential hazardous materials and risks. Some process and/or material handling modifications may be feasible to eliminate or reduce the generation of emissions: minimizing handling, lowering pot temperatures, decreasing dross formation and surface generation of dust, and modifying plant layout to reduce material handling or re-entrainment of settled dust.

Exposure can be reduced in some cases if machines are selected to perform high-exposure tasks so that employees may be removed from the area. This can also reduce ergonomic hazards due to materials handling.

To prevent cross contamination of clean areas in the plant, it is desirable to isolate processes generating significant emissions. A physical barrier will contain emissions and reduce their spread. Thus, fewer people are exposed, and the number of emission sources contributing to exposure in any one area will be reduced. This simplifies exposure evaluations and makes the identification and control of major sources easier. Reclaim operations are often isolated from other plant operations.

Occasionally, it is possible to enclose or isolate a specific emission source. Because enclosures are seldom air tight, a negative draught exhaust system is often applied to the enclosure. One of the most common ways to control emissions is to provide local exhaust ventilation at the point of emission generation. Capturing emissions at their source reduces the potential for emissions to disperse into the air. It also prevents secondary employee exposure created by the re-entrainment of settled contaminants.

The capture velocity of an exhaust hood must be great enough to prevent fumes or dust from escaping the air flow into the hood. The air flow should have enough velocity to carry fume and dust particles into the hood and to overcome the disrupting effects of cross drafts and other random air movements. The velocity required to accomplish this will vary from application to application. The use of recirculation heaters or personal cooling fans which can overcome local exhaust ventilation should be restricted.

All exhaust or dilution ventilation systems also require replacement air (known also as “make-up” air systems). If the replacement air system is well designed and integrated into natural and comfort ventilation systems, more effective control of exposures can be expected. For example, replacement air outlets should be placed so clean air flows from the outlet across the employees, towards the emission source and to the exhaust. This technique is often used with supplied-air islands and places the employee between clean incoming air and the emission source.

Clean areas are intended to be controlled through direct emission controls and housekeeping. These areas exhibit low ambient contaminant levels. Employees in contaminated areas can be protected by supplied-air service cabs, islands, stand-by pulpits and control rooms, supplemented by personal respiratory protection.

The average daily exposure of workers can be reduced by providing clean areas such as breakrooms and lunchrooms that are supplied with fresh filtered air. By spending time in a relatively contaminant-free area, the employees’ time-weighted average exposure to contaminants can be reduced. Another popular application of this principle is the supplied-air island, where fresh filtered air is supplied to the breathing zone of the employee at the workstation.

Sufficient space for hoods, duct work, control rooms, maintenance activities, cleaning and equipment storage should be provided.

Wheeled-vehicles are significant sources of secondary emissions. Where wheeled-vehicle transport is used, emissions can be reduced by paving all surfaces, keeping surfaces free of accumulated dusty materials, reducing vehicle travel distances and speed, and by re-directing vehicle exhaust and cooling fan discharge. Appropriate paving material such as concrete should be selected after considering factors such as load, use and care of surface. Coatings may be applied to some surfaces to facilitate wash down of roadways.

All exhaust, dilution and make-up air ventilation systems must be properly maintained in order to effectively control air contaminants. In addition to maintaining general ventilation systems, process equipment must be maintained to eliminate spillage of material and fugitive emissions.

Work practice programme implementation

Although standards emphasize engineering controls as a means of achieving compliance, work practice controls are essential to a successful control programme. Engineering controls can be defeated by poor work habits, inadequate maintenance and poor housekeeping or personal hygiene. Employees who operate the same equipment on different shifts can have significantly different airborne exposures because of differences in these factors between shifts.

Work practice programmes, although often neglected, represent good managerial practice as well as good common sense; they are cost effective but require a responsible and cooperative attitude on the part of employees and line supervisors. The attitude of senior management toward safety and health is reflected in the attitude of line supervisors. Likewise, if supervisors do not enforce these programmes, employees attitudes may suffer. Fostering good health and safety attitudes can be accomplished through:

  • a cooperative atmosphere in which employees participate in the programmes
  • formal training and educational programmes
  • emphasizing the plant safety and health programme. Motivating employees and obtaining their trust is necessary in order to have an effective programme.

 

Work practice programmes cannot be simply “installed”. Just as with a ventilation system, they must be maintained and continually checked to insure that they are functioning properly. These programmes are the responsibility of management and employees. Programmes should be established to teach, encourage and supervise “good” (i.e., low exposure) practices.

Personal protective equipment

Safety glasses with side shields, coveralls, safety shoes and work gloves should be routinely worn for all jobs. Those engaged in casting and melting, or in casting alloys, should wear aprons and hand protection made of leather or other suitable materials to protect against the splatter of molten metal.

In operations where engineering controls are not adequate to control dust or fume emissions, appropriate respiratory protection should be worn. If noise levels are excessive, and cannot be engineered out or noise sources cannot be isolated, hearing protection should be worn. There should also be a hearing conservation programme, including audiometric testing and training.

Processes

Aluminium

The secondary aluminium industry utilizes aluminium-bearing scrap to produce metallic aluminium and aluminium alloys. The processes used in this industry include scrap pre-treatment, remelting, alloying and casting. The raw material used by the secondary aluminium industry includes new and old scrap, sweated pig and some primary aluminium. New scrap consists of clippings, forging and other solids purchased from the aircraft industry, fabricators and other manufacturing plants. Borings and turnings are by-product of the machining of castings, rods and forging by the aircraft and automobile industry. Drosses, skimmings and slags are obtained from primary reduction plants, secondary smelting plants and foundries. Old scrap includes automobile parts, household items and airplane parts. The steps involved are as follows:

  • Inspection and sorting. Purchased aluminium scrap undergoes inspection. Clean scrap requiring no pre-treatment is transported to storage or is charged directly into the smelting furnace. The aluminium that needs pre-treatment is manually sorted. Free iron, stainless steel, zinc, brass and oversized materials are removed.
  • Crushing and screening. Old scrap, especially casting and sheet contaminated with iron, are inputs to this process. Sorted scrap is conveyed to a crusher or hammer mill where the material is shredded and crushed, and the iron is torn away from the aluminium. The crushed material is passed over vibrating screens to remove dirt and fines.
  • Baling. Specially designed baling equipment is used to compact bulky aluminium scrap such as scrap sheet, castings and clippings.
  • Shredding/classifying. Pure aluminium cable with steel reinforcement or insulation is cut with alligator-type shears, then granulated or further reduced in hammer mills to separate the iron core and plastic coating from the aluminium.
  • Burning/drying. Borings and turning are pre-treated in order to remove cutting oils, greases, moisture and free iron. The scrap is crushed in a hammer mill or ring crusher, the moisture and organics are volatilized in a gas- or oil-fired rotary dryer, the dried chips are screened to remove aluminium fines, the remaining material is magnetically treated for iron removal, and the clean, dried borings are sorted in tote boxes.
  • Hot-dross processing. Aluminium can be removed from the hot dross discharged from the refining furnace by batch fluxing with a salt-cryolite mixture. This process is carried out in a mechanically rotated, refractory-lined barrel. The metal is tapped periodically through a hole in its base.
  • Dry milling. In the dry-milling process, cold aluminium-laden dross and other residues are processed by milling, screening and concentrating to obtain a product containing a minimum aluminium content of 60 to 70%. Ball mills, rod mills or hammer mills can be used to reduce the oxides and non-metallics to fine powders. Separation of dirt and other non-recoverables from the metal is achieved by screening, air classification and/or magnetic separation.
  • Roasting. Aluminium foil backed with paper, gutta-percha or insulation is an input in this process. In the roasting process, carboneous materials associated with aluminium foils are charged and then separated from the metal product.
  • Aluminium sweating. Sweating is a pyrometallurgical process which is used to recover aluminium from high-iron-content scrap. High-iron aluminium scrap, castings and dross are inputs in this process. Open-flame reverberatory furnaces with sloping hearths are generally employed. Separation is accomplished as aluminium and other low-melting constituents melt and trickle down the hearth, through a grate and into air-cooled moulds, collecting pots or holding wells. The product is termed “sweated pig”. The higher-melting materials including iron, brass and oxidation products formed during the sweating process are periodically tapped from the furnace.
  • Reverberatory (chlorine) smelting-refining. Reverberatory furnaces are used to convert clean sorted scrap, sweated pigs or, in some cases, untreated scrap into specification alloys. The scrap is charged to the furnace by mechanical means. Materials are added for processing by batch or continuous feed. After the scrap is charged a flux is added to prevent contact with and subsequent oxidation of the melt by air (cover flux). Solvent fluxes are added which react with non-metallics, such as residues from burned coatings and dirt, to form insolubles which float to the surface as slag. Alloying agents are then added, depending on the specifications. Demagging is the process which reduces the magnesium content of the molten charge. When demagging with chlorine gas, chlorine is injected through carbon tubes or lances and reacts with magnesium and aluminium as it bubbles. In the skimming step impure semi-solid fluxes are skimmed off the surface of the melt.
  • Reverberatory (fluorine) smelting-refining. This process is similar to the reverberatory (chlorine) smelting-refining process except that aluminium fluoride rather than chlorine is employed.

 

Table 1 lists exposure and controls for aluminium reclamation operations.

Table 1. Engineering/administrative controls for aluminium, by operation

Process equipment

Exposure

Engineering/administrative controls

Sorting

Torch desoldering—metal fumes such as lead and cadmium

Local exhaust ventilation during desoldering; PPE—respiratory protection when desoldering

Crushing/screening

Non-specific dusts and aerosol, oil mists, metal particulates, and noise

Local exhaust ventilation and general area ventilation, isolation of noise source; PPE—hearing protection

Baling

No known exposure

No controls

Burning/drying

Non-specific particulate matter which may include metals, soot, and condensed heavy organics. Gases and vapours containing fluorides, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes

Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE—hearing protection

Hot-dross processing

Some fumes

Local exhaust ventilation, general area ventilation

Dry milling

Dust

Local exhaust ventilation, general area ventilation

Roasting

Dust

Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE—hearing protection

Sweating

Metal fumes and particulates, non-specific gases and vapours, heat and noise

Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

Reverberatory (chlorine) smelting-refining

Products of combustion, chlorine, hydrogen chlorides, metal chlorides, aluminium chlorides, heat and noise

Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

Reverberatory (fluorine) smelting-refining

Products of combustion, fluorine, hydrogen flluorides, metal fluorides, aluminium fluorides, heat and noise

Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

 

Copper reclamation

The secondary copper industry utilizes copper-bearing scrap to produce metallic copper and copper based alloys. The raw materials used can be classified as new scrap produced in the fabrication of finished products or old scrap from obsolete worn out or salvaged articles. Old scrap sources include wire, plumbing fixtures, electrical equipment, automobiles and domestic appliances. Other materials with copper value include slags, drosses, foundry ashes and sweepings from smelters. The following steps are involved:

  • Stripping and sorting. Scrap is sorted on the bases of its copper content and cleanliness. Clean scrap may be manually separated for charging directly to a melting and alloying furnace. Ferrous components can be separated magnetically. Insulation and lead cable coverings are stripped by hand or by specially designed equipment.
  • Briquetting and crushing. Clean wire, thin plate, wire screen, borings, turnings and chips are compacted for easier handling. The equipment used includes hydraulic baling presses, hammer mills and ball mills.
  • Shredding. The separation of copper wire from insulation is accomplished by reducing the size of the mixture. The shredded material is then sorted by air or hydraulic classification with magnetic separation of any ferrous materials.
  • Grinding and gravity separation. This process accomplishes the same function as shredding but uses an aqueous separation medium and different input materials such as slags, drosses, skimmings, foundry ashes, sweepings and baghouse dust.
  • Drying. Borings, turnings and chips containing volatile organic impurities such as cutting fluids, oils and greases are removed.
  • Insulation burning. This process separates insulation and other coatings from copper wire by burning these materials in furnaces. The wire scrap is charged in batches to a primary ignition chamber or afterburner. Volatile combustion products are then passed through a secondary combustion chamber or baghouse for collection. Non-specific particulate matter is generated which may include smoke, clay and metal oxides. Gases and vapours may contain oxides of nitrogen, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes.
  • Sweating. The removal of low vapour-melting components from scrap is accomplished by heating the scrap to a controlled temperature which is just above the melting point of the metals to be sweated out. The primary metal, copper, is generally not the melted component.
  • Ammonium carbonate leaching. Copper can be recovered from relatively clean scrap by leaching and dissolution in a basic ammonium carbonate solution. Cupric ions in an ammonia solution will react with metallic copper to produce cuprous ions, which can be reoxidized to the cupric state by air oxidation. After the crude solution is separated from the leach residue, the copper oxide is recovered by steam distillation.
  • Steam distillation. Boiling the leached material from the carbonate leaching process precipitates the copper oxide. The copper oxide is then dried.
  • Hydrothermal hydrogen reduction. Ammonium carbonate solution containing copper ions is heated under pressure in hydrogen, precipitating the copper as a powder. The copper is filtered, washed, dried and sintered under a hydrogen atmosphere. The powder is ground and screened.
  • Sulphuric acid leaching. Scrap copper is dissolved in hot sulphuric acid to form a copper sulphate solution for feed to the electrowinning process. After digestion, the undissolved residue is filtered off.
  • Converter smelting. Molten black copper is charged to converter, which is a pear-shaped or cylindrical steel shell lined refractory brick. Air is blown into the molten charges through nozzles called tuyères. The air oxidizes copper sulphide and other metals. A flux containing silica is added to react with the iron oxides to form an iron silicate slag. This slag is skimmed from the furnace, usually by tipping the furnace and then there is a secondary blow and skim. The copper from this process is called blister copper. The blister copper is generally further refined in a fire refining furnace.
  • Fire refining. The blister copper from the converter is fire refined in a cylindrical tilting furnace, a vessel like a reverberatory furnace. The blister copper is charged to the refining vessel in an oxidizing atmosphere. The impurities are skimmed from the surface and a reducing atmosphere is created by the addition of green logs or natural gas. The resulting molten metal is then cast. If the copper is to be electrolytically refined, the refined copper will be cast as an anode.
  • Electrolytic refining. The anodes from the fire refining process are placed in a tank containing sulphuric acid and a direct current. The copper from the anode is ionized and the copper ions are deposited on a pure copper starter sheet. As the anodes dissolve in the electrolyte the impurities settle to the bottom of the cell as a slime. This slime can be additionally processed to recover other metal values. The cathode copper produced is melted and cast into a variety of shapes.

 

Table 2 lists exposures and controls for copper reclamation operations.

Table 2. Engineering/administrative controls for copper, by operation

Process equipment

Exposures

Engineering/administrative controls

Stripping and sorting

Air contaminants from material handling and desoldering or scrap cutting

Local exhaust ventilation, general area ventilation

Briquetting and crushing

Non-specific dusts and aerosol, oil mists, metal particulates and noise

Local exhaust ventilation and general area ventilation, isolation of noise source; PPE—hearing protection and respiratory protection

Shredding

Non-specific dusts, wire insulation material, metal particulates and noise

Local exhaust ventilation and general area ventilation, isolation of noise source; PPE—hearing protection and respiratory protection

Grinding and gravity separation

Non-specific dusts, metal particulates from fluxes, slags and drosses, and noise

Local exhaust ventilation and general area ventilation, isolation of noise source; PPE—hearing protection and respiratory protection

Drying

Non-specific particulate matter, which may include metals, soot and condensed heavy organics
Gases and vapours containing fluorides, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

Insulation burning

Non-specific particulate matter which may include smoke, clay
and metal oxides
Gases and vapours containing oxides of nitrogen, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—respiratory protection

Sweating

Metal fumes and particulates, non-specific gases, vapours and particulates

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

Ammonium carbonate leaching

Ammonia

Local exhaust ventilation, general area ventilation; PPE—respiratory protection

Steam distillation

Ammonia

Local exhaust ventilation, general area ventilation; PPE—glasses with side shields

Hydrothermal hydrogen reduction

Ammonia

Local exhaust ventilation, general area ventilation; PPE—respiratory protection

Sulphuric acid leaching

Sulphuric acid mists

Local exhaust ventilation, general area ventilation

Converter smelting

Volatile metals, noise

Local exhaust ventilation, general area ventilation; PPE—respiratory protection and hearing protection

Electric crucible smelting

Particulate, sulphur and nitrogen oxides, soot, carbon monoxide, noise

Local exhaust ventilation, general area ventilation; PPE—hearing protection

Fire refining

Sulphur oxides, hydrocarbons, particulates

Local exhaust ventilation, general area ventilation; PPE—hearing protection

Electrolytic refining

Sulphuric acid and metals from sludge

Local exhaust ventilation, general area ventilation

 

Lead reclamation

Raw materials purchased by secondary lead smelters may require processing prior to being charged into a smelting furnace. This section discusses the most common raw materials which are purchased by secondary lead smelters and feasible engineering controls and work practices to limit employee exposure to lead from raw materials processing operations. It should be noted that lead dust can generally be found throughout lead reclamation facilities and that any vehicular air is likely to stir up lead dust which can then be inhaled or adhere to shoes, clothing, skin and hair.

Automotive batteries

The most common raw material at a secondary lead smelter is junk automotive batteries. Approximately 50% of the weight of a junk automotive battery will be reclaimed as metallic lead in the smelting and refining process. Approximately 90% of the automotive batteries manufactured today utilize a polypropylene box or case. The polypropylene cases are reclaimed by almost all secondary lead smelters due to the high economic value of this material. Most of these processes can generate metal fumes, in particular lead and antimony.

In automotive battery breaking there is a potential for forming arsine or stibine due to the presence of arsenic or antimony used as hardening agents in grid metal and the potential for having nascent hydrogen present.

The four most common processes for breaking automotive batteries are:

  1. high speed saw
  2. slow speed saw
  3. shear
  4. whole battery crushing (Saturn crusher or shredder or hammer mill).

 

The first three of these processes involve cutting the top off of the battery, then dumping the groups, or lead-bearing material. The fourth process involves crushing the entire battery in a hammer mill and separating the components by gravity separation.

Automotive battery separation takes place after automotive batteries have been broken in order that the lead-bearing material can be separated from the case material. Removing the case may generate acid mists. The most widely used techniques for accomplishing this task are:

  • The manual technique. This is used by the vast majority of secondary lead smelters and remains the most widely used technique in small to mid-sized smelters. After the battery passes through the saw or shear, an employee manually dumps the groups or lead-bearing material into a pile and places the case and top of the battery into another pile or conveyance system.
  • A tumbler device. Batteries are placed into a tumbler device after the tops have been sawed/sheared off to separate the groups from the cases. Ribs inside the tumbler dump the groups as it slowly rotates. Groups fall through the slots in the tumbler while the cases are conveyed to the far end and are collected as they exit. Plastic and rubber battery cases and tops are further processed after being separated from the lead bearing material.
  • A sink/float process. The sink/float process typically is combined with the hammer mill or crushing process for battery breaking. Battery pieces, both lead bearing and cases, are placed in a series of tanks filled with water. Lead bearing material sinks to the bottom of the tanks and is removed by screw conveyor or drag chain while the case material floats and is skimmed off the tank surface.

 

Industrial batteries which were used to power mobile electric equipment or for other industrial uses are purchased periodically for raw material by most secondary smelters. Many of these batteries have steel cases which require removal by cutting the case open with a cutting torch or a hand-held gas powered saw.

Other purchased lead-bearing scrap

Secondary lead smelters purchase a variety of other scrap materials as raw materials for the smelting process. These materials include battery manufacturing plant scrap, drosses from lead refining, scrap metallic lead such as linotype and cable covering, and tetraethyl lead residues. These types of materials may be charged directly into smelting furnaces or mixed with other charge materials.

Raw material handling and transport

An essential part of the secondary lead smelting process is the handling, transportation and storage of raw material. Materials are transported by fork-lifts, front-end loaders or mechanical conveyors (screw, bucket elevator or belt). The primary method of material transporting in the secondary lead industry is mobile equipment.

Some common mechanical conveyance methods which are used by secondary lead smelters include: belt conveying systems that can be used to transport furnace feed material from storage areas to the furnace charring area; screw conveyors for transporting flue dust from the baghouse to an agglomeration furnace or a storage area or bucket elevators and drag chains/lines.

Smelting

The smelting operation at a secondary lead smelter involves the reduction of lead-bearing scrap into metallic lead in a blast furnace or reverberatory.

Blast furnaces are charged with lead-bearing material, coke (fuel) limestone and iron (flux). These materials are fed into the furnace at the top of the furnace shaft or through a charge door in the side of the shaft neat the top of the furnace. Some environmental hazards associated with blast furnace operations are metal fumes and particulates (especially lead and antimony), heat, noise and carbon monoxide. A variety of charge material conveying mechanisms are used in the secondary lead industry. The skip hoist is probably the most common. Other devices in use include vibratory hoppers, belt conveyors and bucket elevators.

Blast furnace tapping operations involve removing the molten lead and slag from the furnace into moulds or ladles. Some smelters tap metal directly into a holding kettle which keeps the metal molten for refining. The remaining smelters cast the furnace metal into blocks and allow the blocks to solidify.

Blast air for the combustion process enters the blast furnace through tuyères which occasionally begin to fill with accretions and must be physically punched, usually with a steel rod, to keep them from being obstructed. The conventional method to accomplish this task is to remove the cover of the tuyères and insert the steel rod. After the accretions have been punched, the cover is replaced.

Reverberatory furnaces are charged with lead-bearing raw material by a furnace charging mechanism. Reverberatory furnaces in the secondary lead industry typically have a sprung arch or hanging arch constructed of refractory brick. Many of the contaminants and physical hazards associated with reverberatory furnaces are similar to those of blast furnaces. Such mechanisms can be a hydraulic ram, a screw conveyor or other devices similar to those described for blast furnaces.

Reverberatory furnace tapping operations are very similar to blast-furnace tapping operations.

Refining

Lead refining in secondary lead smelters is conducted in indirect fired kettles or pots. Metal from the smelting furnaces is typically melted in the kettle, then the content of trace elements is adjusted to produce the desired alloy. Common products are soft (pure) lead and various alloys of hard (antimony) lead.

Virtually all secondary lead refining operations employ manual methods for adding alloying materials to the kettles and employ manual drossing methods. Dross is swept to the rim of the kettle and removed by shovel or large spoon into a container.

Table 3 lists exposures and controls for lead reclamation operations.

Table 3. Engineering/administrative controls for lead, by operation

Process equipment

Exposures

Engineering/administrative controls

Vehicles

Lead dust from roads and splashing water containing lead

Water washdown and keeping areas wetted down. Operator training, prudent work practices and good housekeeping are key elements in minimizing lead emissions when operating mobile equipment. Enclose equipment and provide a positive pressure filtered air system.

Conveyors

Lead dust

It is also preferable to equip belt conveyor systems with self-cleaning tail pulleys or belt wipes if they are used to transport furnace feed materials or flue dusts.

Battery decasing

Lead dust, acid mists

Local exhaust ventilation, general area ventilation

Charge preparation

Lead dust

Local exhaust ventilation, general area ventilation

Blast furnace

Metal fumes and particulates (lead, antimony), heat and noise, carbon monoxide

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—respiratory protection and hearing protection

Reverberatory furnace

Metal fumes and particulates (lead, antimony), heat and noise

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—respiratory protection and hearing protection

Refining

Lead particulates and possibly alloying metals and fluxing agents, noise

Local exhaust ventilation, general area ventilation; PPE—hearing protection

Casting

Lead particulates and possibly alloying metals

Local exhaust ventilation, general area ventilation

 

Zinc reclamation

The secondary zinc industry utilizes new clippings, skimmings and ashes, die-cast skimmings, galvanizers’ dross, flue dust and chemical residue as sources of zinc. Most of the new scrap processed is zinc- and copper-based alloys from galvanizing and die-casting pots. Included in the old scrap category are old zinc engravers’ plates, die castings, and rod and die scrap. The processes are as follows:

  • Reverberatory sweating. Sweating furnaces are used to separate zinc from other metals by controlling the furnace temperature. Scrap die-cast products, such as automobile grilles and licence plate frames, and zinc skins or residues are starting materials for the process. The scrap is charged to the furnace, flux is added and the contents melted. The high-melting residue is removed and the molten zinc flows out of the furnace directly to subsequent processes, such as melting, refining or alloying, or to collecting vessels. Metal contaminants include zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium. Other contaminants are fluxing agents, sulphur oxides, chlorides and fluorides.
  • Rotary sweating. In this process zinc scrap, die-cast products, residues and skimmings are charged to a direct-fired furnace and melted. The melt is skimmed, and zinc metal is collected in kettles situated outside the furnace. Unmeltable material, the slag, is then removed prior to recharging. The metal from this process is sent to distillation or alloying process. Contaminants are similar to those of reverberatory sweating.
  • Muffle sweating and kettle (pot) sweating. In these processes zinc scrap, die-vapour-cast products, residues and skimmings are charged to the muffle furnace, the material sweated and the sweated zinc is sent to refining or alloying processes. The residue is removed by a shaker screen which separates the dross from the slag. Contaminants are similar to those of reverberatory sweating.
  • Crushing/screening. Zinc residues are pulverized or crushed to break down physical bonds between metallic zinc and contaminant fluxes. The reduced material is then separated in a screening or pneumatic classification step. Crushing can produce zinc oxide and minor amounts of heavy metals and chlorides.
  • Sodium carbonate leaching. Residues are chemically treated to leach out and convert zinc to zinc oxide. The scrap is first crushed and washed. In this step, the zinc is leached out of the material. The aqueous portion is treated with sodium carbonate, causing zinc to precipitate. The precipitate is dried and calcined to yield crude zinc oxide. The zinc oxide is then reduced to zinc metal. Various zinc salt contaminants can be produced.
  • Kettle (pot), crucible, reverberatory, electric induction melting. The scrap is charged to the furnace and fluxes are added. The bath is agitated to form a dross that can be skimmed from the surface. After the furnace has been skimmed the zinc metal is poured into ladles or moulds. Zinc oxide fumes, ammonia and ammonium chloride, hydrogen chloride and zinc chloride can be produced.
  • Alloying. The function of this process is to produce zinc alloys from pre-treated scrap zinc metal by adding to it in a refining kettle fluxes and alloying agents either in the solidified or molten form. The contents are then mixed, the dross skimmed, and the metal is cast into various shapes. Particulates containing zinc, alloying metals, chlorides, non-specific gases and vapours, as well as heat, are potential exposures.
  • Muffle distillation. The muffle distillation process is used to reclaim zinc from alloys and to manufacture pure zinc ingots. The process is semi-continuous which involves charging molten zinc from a melting pot or sweating furnace to the muffle section and vaporizing the zinc and condensing the vaporized zinc and tapping from the condenser to moulds. The residue is removed periodically from the muffle.
  • Retort distillation/oxidation and muffle distillation/oxidation. The product of the retort distillation/oxidation and muffle distillation/oxidation processes is zinc oxide. The process is similar to retort distillation through the vaporization step, but, in this process, the condenser is bypassed and combustion air is added. The vapour is discharged through an orifice into an air stream. Spontaneous combustion occurs inside a refractory vapour-lined chamber. The product is carried by the combustion gases and excess air into a baghouse where the product is collected. Excess air is present to insure complete oxidation and to cool the product. Each of these distillation processes can lead to zinc oxide fume exposures, as well as other metal particulate and oxides of sulphur exposure.

 

Table 4 lists exposures and controls for zinc reclamation operations.

Table 4. Engineering/administrative controls for zinc, by operation

Process equipment

Exposures

Engineering/administrative controls

Reverberatory sweating

Particulates containing zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium, contaminants from fluxing agents, sulphur oxides, chlorides and fluorides

Local exhaust ventilation, general area ventilation, heat stress–work/rest regimen, fluids

Rotary sweating

Particulates containing zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium, contaminants from fluxing agents, sulphur oxides, chlorides and fluorides

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Muffle sweating and kettle (pot) sweating

Particulates containing zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium, contaminants from fluxing agents, sulphur oxides, chlorides and fluorides

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Crushing/screening

Zinc oxide, minor amounts of heavy metals, chlorides

Local exhaust ventilation, general area ventilation

Sodium carbonate leaching

Zinc oxide, sodium carbonate, zinc carbonate, zinc hydroxide, hydrogen chloride, zinc chloride

Local exhaust ventilation, general area ventilation

Kettle (pot) melting crucible, reverberatory, electric induction melting

Zinc oxide fumes, ammonia, ammonia chloride, hydrogen chloride, zinc chloride

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Alloying

Particulates containing zinc, alloying metals, chlorides; non-specific gases and vapours; heat

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Retort distillation, retort distillation/oxidation and muffle distillation

Zinc oxide fumes, other metal particulates, oxides of sulphur

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Graphite rod resistor distillation

Zinc oxide fumes, other metal particulates, oxides of sulphur

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

 

Magnesium reclamation

Old scrap is obtained from sources such as scrap automobile and aircraft parts and old and obsolete lithographic plates, as well as some sludges from primary magnesium smelters. New scrap consists of clippings, turnings, borings, skimmings, slags, drosses and defective articles from sheet mills and fabrication plants. The greatest danger in handling magnesium is that of fire. Small fragments of the metal can readily be ignited by a spark or flame.

  • Hand sorting. This process is used to separate magnesium and magnesium-alloy fractions from other metals present in the scrap. The scrap is spread out manually, sorted on the basis of weight.
  • Open pot melting. This process is used to separate magnesium from contaminants in the sorted scrap. Scrap is added to a crucible, heated and a flux consisting of a mixture of calcium, sodium and potassium chlorides is added. The molten magnesium is then cast into ingots.

 

Table 5 lists exposures and controls for magnesium reclamation operations.

Table 5. Engineering/administrative controls for magnesium, by operation

Process equipment

Exposures

Engineering/administrative
controls

Scrap sorting

Dust

Water washdown

Open pot melting

Fumes and dust, a high potential for fires

Local exhaust ventilation and general area ventilation and work practices

Casting

Dust and fumes, heat and a high potential for fires

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

 

Mercury reclamation

The major sources for mercury are dental amalgams, scrap mercury batteries, sludges from electrolytic processes that use mercury as a catalyst, mercury from dismantled chlor-alkali plants and mercury-containing instruments. Mercury vapour can contaminate each of these processes.

  • Crushing. The crushing process is used to release residual mercury from metal, plastic and glass containers. After the containers are crushed, the contaminated liquid mercury is sent to the filtering process.
  • Filtration. Insoluble impurities such as dirt are removed by passing the mercury-vapour bearing scrap through a filter media. The filtered mercury is fed to the oxygenation process and the solids which do not pass through the filters are sent to retort distillation.
  • Vacuum distillation. Vacuum distillation is employed to refine contaminated mercury when the vapour pressures of the impurities are substantially lower than that of mercury. Mercury charge is vaporized in a heating pot and the vapours are condensed using a water-cooled condenser. Purified mercury is collected and sent to the bottling operation. The residue remaining in the heating pot is sent to the retorting process to recover the trace amounts of mercury that were not recovered in the vacuum distillation process.
  • Solution purification. This process removes metallic and organic contaminants by washing the raw liquid mercury with a dilute acid. The steps involved are: leaching the raw liquid mercury with dilute nitric acid to separate metallic impurities; agitating the acid-mercury with compressed air to provide good mixing; decanting to separate the mercury from the acid; washing with water to remove the residual acid; and filtering the mercury in a medium such as activated carbon or silica gel to remove the last traces of moisture. In addition to mercury vapour there can be exposure to solvents, organic chemicals and acid mists.
  • Oxygenation. This process refines the filtered mercury by removing metallic impurities by oxidation with sparging air. The oxidation process involves two steps, sparging and filtering. In the sparging step, contaminated mercury is agitated with air in a closed vessel to oxidize the metallic contaminants. After sparging, the mercury is filtered in a charcoal bed to remove the solid metal oxides.
  • Retorting. The retorting process is used to produce pure mercury by volatilizing the mercury found in solid mercury-bearing scrap. The steps involved in retorting are: heating the scrap with an external heat source in a closed still pot or stack of trays to vaporize the mercury; condensing the mercury vapour in water-cooled condensers; collecting the condensed mercury in a collecting vessel.

 

Table 6 lists exposures and controls for mercury reclamation operations.

Table 6. Engineering/administrative controls for mercury, by operation

Process equipment

Exposures

Engineering/administrative controls

Crushing

Volatile mercury

Local exhaust; PPE—respiratory protection

Filtration

Volatile mercury

Local exhaust ventilation; PPE—respiratory protection

Vacuum distillation

Volatile mercury

Local exhaust ventilation; PPE—respiratory protection

Solution purification

Volatile mercury, solvents, organics and acid mists

Local exhaust ventilation, general area ventilation; PPE—respiratory protection

Oxidation

Volatile mercury

Local exhaust ventilation; PPE—respiratory protection

Retorting

Volatile mercury

Local exhaust ventilation; PPE—respiratory protection

 

Nickel reclamation

The principal raw materials for nickel reclamation are nickel-, copper- and aluminium-vapour based alloys, which can be found as old or new scrap. Old scrap comprises alloys that are salvaged from machinery and airplane parts, while new scrap refers to sheet scrap, turnings and solids which are by-products of the manufacture of alloy products. The following steps are involved in nickel reclamation:

  • Sorting. The scrap is inspected and manually separated from the non-metallic and non-nickel materials. Sorting produces dust exposures.
  • Degreasing. Nickel scrap is degreased by using trichloroethylene. The mixture is filtrated or centrifuged to separate the nickel scrap. The spent solvent solution of trichloroethylene and grease goes through a solvent recovery system. There can be solvent exposure during degreasing.
  • Smelting (electric arc or rotary reverberatory) furnace. Scrap is charged to an electric arc furnace and a reducing agent added, usually lime. The charge is melted and is either cast into ingots or sent directly to a reactor for additional refining. Fumes, dust, noise and heat exposures are possible.
  • Reactor refining. The molten metal is introduced into a reactor where cold-base scrap and pig nickel are added, followed by lime and silica. Alloying materials such as manganese, columbium or titanium are then added to produce the desired alloy composition. Fumes, dust, noise and heat exposures are possible.
  • Ingot casting. This process involves casting the molten metal from the smelting furnace or the refining reactor into ingots. The metal is poured into moulds and allowed to cool. The ingots are removed from the moulds. Heat and metal fume exposures are possible.

 

Exposures and control measures for nickel reclamation operations are listed in table 7.

Table 7. Engineering/administrative controls for nickel, by operation

Process equipment

Exposures

Engineering/administrative controls

Sorting

Dust

Local exhaust and solvent substitution

Degreasing

Solvent

Local exhaust ventilation and solvent substitution and/or recovery, general area ventilation

Smelting

Fumes, dust, noise, heat

Local exhaust ventilation, work/rest regimen, fluids; PPE—respiratory protection and hearing protection

Refining

Fumes, dust, heat, noise

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids; PPE—respiratory protection and hearing protection

Casting

Heat, metal fumes

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

 

Precious metals reclamation

The raw materials for the precious metal industry consist of both old and new scrap. Old scrap includes electronic components from obsolete military and civilian equipment and scrap from the dental industry. New scrap is generated during the fabrication and manufacturing of precious metal products. The products are the elemental metals such as gold, silver, platinum and palladium. Precious metal processing includes the following steps:

  • Hand sorting and shredding. Precious metal-bearing scrap is hand sorted and crushed and shredded in a hammer mill. Hammer mills are noisy.
  • Incineration process. Sorted scrap is incinerated to remove paper, plastic and organic liquid contaminants. Organic chemicals, combustion gases and dust exposures are possible.
  • Blast-furnace smelting. Treated scrap is charged to a blast furnace, along with coke, flux and recycled slag metal oxides. The charge is melted and slagged, producing black copper which contains the precious metals. The hard slag that is formed contains most of the slag impurities. Dust and noise may be present.
  • Converter smelting. This process is designed to further purify the black copper by blowing air through the melt in a converter. Slag-containing metal contaminants are removed and recycled to the blast furnace. The copper bullion containing the precious metals is cast into moulds.
  • Electrolytic refining. Copper bullion serves as the anode of an electrolytic cell. Pure copper thus plates out on the cathode while the precious metals fall to the bottom of the cell and are collected as slimes. The electrolyte used is copper sulphate. Acid mist exposures are possible.
  • Chemical refining. The precious metal slime from the electrolytic refining process is chemically treated to recover the individual metals. Cyanide-based processes are used to recover gold and silver, which can also be recovered by dissolving them in aqua regia solution and/or nitric acid, followed by precipitation with ferrous sulphate or sodium chloride to recover the gold and silver, respectively. The platinum-group metals can be recovered by dissolving them in molten lead, which is then treated with nitric acid and leaves a residue from which the platinum-group metals can be selectively precipitated. The precious metal precipitates are then either melted or ignited in order to collect the gold and silver as grains and the platinum metals as sponge. There can be acid exposures.

 

Exposures and controls are listed, by operation, in table 8 (see also “Gold smelting and refining”).

Table 8. Engineering/administrative controls for precious metals, by operation

Process equipment

Exposures

Engineering/administrative controls

Sorting and shredding

Hammermill is a potential noise hazard

Noise control material; PPE—hearing protection

Incineration

Organics, combustion gases and dust

Local exhaust ventilation and general area ventilation

Blast furnace smelting

Dust, noise

Local exhaust ventilation; PPE—hearing protection and respiratory protection

Electrolytic refining

Acid mists

Local exhaust ventilation, general area ventilation

Chemical refining

Acid

Local exhaust ventilation, general area ventilation; PPE—acid-resistant clothing, chemical goggles and face shield

 

Cadmium reclamation

Old cadmium-bearing scrap includes cadmium-plated parts from junked vehicles and boats, household appliances, hardware and fasteners, cadmium batteries, cadmium contacts from switches and relays and other used cadmium alloys. New scrap is normally cadmium vapour bearing rejects and contaminated by-products from industries which handle the metals. The reclamation processes are:

  • Pre-treatment. The scrap pre-treatment step involves the vapour degreasing of alloy scrap. Solvent vapours generated by heating recycled solvents are circulated through a vessel containing scrap alloys. The solvent and stripped grease are then condensed and separated with the solvent being recycled. There can be exposure to cadmium dust and solvents.
  • Smelting/refining. In the smelting/refining operation, pre-treated alloy scrap or elemental cadmium scrap is processed to remove any impurities and produce cadmium alloy or elemental cadmium. Products of oil and gas combustion exposures and zinc and cadmium dust may be present.
  • Retort distillation. Degreased scrap alloy is charged to a retort and heated to produce cadmium vapours which are subsequently collected in a condenser. The molten metal is then ready for casting. Cadmium dust exposures are possible.
  • Melting/dezincing. Cadmium metal is charged to a melting pot and heated to the molten stage. If zinc is present in the metal, fluxes and chlorinating agents are added to remove the zinc. Among potential exposures are cadmium fumes and dust, zinc fumes and dust, zinc chloride, chlorine, hydrogen chloride and heat.
  • Casting. The casting operation forms the desired product line from the purified cadmium alloy or cadmium metal produced in the previous step. Casting can produce cadmium dust and fumes and heat.

 

Exposures in cadmium reclamation processes and the necessary controls are summarized in table 9.

Table 9. Engineering/administrative controls for cadmium, by operation

Process equipment

Exposures

Engineering/administrative controls

Scrap degreasing

Solvents and cadmium dust

Local exhaust and solvent substitution

Alloy smelting/refining

Products of oil and gas combustion, zinc fumes, cadmium dust and fumes

Local exhaust ventilation and general area ventilation; PPE—respiratory protection

Retort distillation

Cadmium fumes

Local exhaust ventilation; PPE—respiratory protection

Melting/dezincing

Cadmium fumes and dust, zinc fumes and dust, zinc chloride, chlorine, hydrogen chloride, heat stress

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids; PPE—respiratory protection

Casting

Cadmium dust and fumes, heat

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids; PPE—respiratory protection

 

Selenium reclamation

Raw materials for this segment are used xerographic copying cylinders and scrap generated during the manufacture of selenium rectifiers. Selenium dusts may be present throughout. Distillation and retort smelting can produce combustion gases and dust. Retort smelting is noisy. Sulphur dioxide mist and acid mist are present in refining. Metal dusts can be produced from casting operations (see table 10).

Table 10. Engineering/administrative controls for selenium, by operation

Process equipment

Exposures

Engineering/administrative controls

Scrap pretreatment

Dust

Local exhaust

Retort smelting

Combustion gases and dust, noise

Local exhaust ventilation and general area ventilation; PPE—hearing protection; control of burner noise

Refining

SO2, acid mist

Local exhaust ventilation; PPE—chemical goggles

Distillation

Dust and combustion products

Local exhaust ventilation, general area ventilation

Quenching

Metal dust

Local exhaust ventilation, general area ventilation

Casting

Selenium fumes

Local exhaust ventilation, general area ventilation

 

The reclamation processes are as follows:

  • Scrap pre-treatment. This process separates selenium by mechanical processes such as the hammer mill or shot blasting.
  • Retort smelting. This process purifies and concentrates pre-treated scrap in a retort distillation operation by melting the scrap and separating selenium from the impurities by distillation.
  • Refining. This process achieves a purification of scrap selenium based on leaching with a suitable solvent such as aqueous sodium sulphite. Insoluble impurities are removed by filtration and the filtrate is treated to precipitate selenium.
  • Distillation. This process produces a high vapour purity selenium. The selenium is melted, distilled and the selenium vapours are condensed and transferred as molten selenium to a product formation operation.
  • Quenching. This process is used to produce purified selenium shot and powder. The selenium melt is used in producing a shot. The shot is then dried. The steps required to produce powder are the same, except that selenium vapour, rather than molten selenium, is the material which is quenched.
  • Casting. This process is used to produce selenium ingots or other shapes from the molten selenium. These shapes are produced by pouring molten selenium into moulds of the proper size and shape and cooling and solidifying the melt.

 

Cobalt reclamation

The sources of cobalt scrap are super alloy grindings and turnings, and obsolete or worn engine parts and turbine blades. The processes of reclamation are:

  • Hand sorting. Raw scrap is hand sorted to identify and separate the cobalt-base, nickel-base and non-processable components. This is a dusty operation.
  • Degreasing. Sorted dirty scrap is charged to a degreasing unit where vapours of perchloroethylene are circulated. This solvent removes the grease and oil on the scrap. The solvent-oil-grease vapour mixture is then condensed and the solvent is recovered. Solvent exposures are possible.
  • Blasting. Degreased scrap is blasted with grit to remove dirt, oxides and rust. Dusts can be present, depending on the grit used.
  • Pickling and chemical treatment process. Scrap from the blasting operation is treated with acids to remove residual rust and oxide contaminants. Acid mists are a possible exposure.
  • Vacuum melting. Cleaned scrap is charged to a vacuum furnace and melted by electric arc or induction furnace. There can be exposure to heavy metals.
  • casting. Molten alloy is cast into ingots. Heat stress is possible.

 

See table 11 for a summary of exposures and controls for cobalt reclamation.

Table 11. Engineering/administrative controls for cobalt, by operation

Process equipment

Exposures

Engineering/administrative controls

Hand sorting

Dust

Water washdown

Degreasing

Solvents

Solvent recovery, local exhaust and solvent substitution

Blasting

Dust—toxicity dependent upon the grit used

Local exhaust ventilation; PPE for physical hazard and respiratory protection depending on grit used

Pickling and chemical treatment process

Acid mists

Local exhaust ventilation, general area ventilation; PPE—respiratory protection

Vacuum melting

Heavy metals

Local exhaust ventilation, general area ventilation

Casting

Heat

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

 

Tin reclamation

The major sources of raw materials are tin-plated steel trimmings, rejects from tin-can manufacturing companies, rejected plating coils from the steel industry, tin drosses and sludges, solder drosses and sludges, used bronze and bronze rejects and metal type scrap. Tin dust and acid mists can be found in many of the processes.

  • Dealuminization. In this process hot sodium hydroxide is used to leach aluminium from tin-can scrap by contacting the scrap with hot sodium hydroxide, separating the sodium aluminate solution from the scrap residue, pumping the sodium aluminate to a refining operation to recover soluble tin and recovering the dealuminized tin scrap for feed.
  • Batch mixing. This process is a mechanical operation which prepares a feed suitable for charging to the smelting furnace by mixing drosses and sludges with a significant tin content.
  • Chemical detinning. This process extracts the tin in scrap. A hot solution of sodium hydroxide and sodium nitrite or nitrate is added to dealuminized or raw scrap. Draining and pumping the solution to a refining/casting process are performed when the detinning reaction is complete. The detinned scrap is then washed.
  • Dross smelting. This process is used to partially purify drosses and produce crude furnace metal by melting the charge, tapping the crude furnace metal and tapping the mattes and slags.
  • Dust leaching and filtration. This process removes the zinc and chlorine values from flue dust by leaching with sulphuric acid to remove zinc and chlorine, filtering the resulting mixture to separate the acid and dissolved zinc and chlorine from the leached dust, drying the leached dust in a dryer and conveying the tin and lead rich dust back to the batch mixing process.
  • Settling and leaf filtration. This process purifies the sodium stannate solution produced in the chemical detinning process. Impurities such as silver, mercury, copper, cadmium, some iron, cobalt and nickel are precipitated as sulphides.
  • Evapocentrifugation. The sodium stannate is concentrated from the purified solution by evaporation, crystallization of sodium stannate and recovery of sodium stannate is by centrifugation.
  • Electrolytic refining. This process produces cathodic-pure tin from the purified sodium stannate solution by passing the sodium stannate solution through electrolytic cells, removing the cathodes after the tin has been deposited and stripping the tin from the cathodes.
  • Acidification and filtration. This process produces a hydrated tin oxide from the purified sodium stannate solution. This hydrated oxide can either be processed to produce the anhydrous oxide or smelted to produce elemental tin. The hydrated oxide is neutralized with sulphuric acid to form the hydrated tin oxide and filtered to separate the hydrate as filter cake.
  • Fire refining. This process produces purified tin from the cathodic tin by melting the charge, removing the impurities as slag and dross, pouring the molten metal and casting the metallic tin.
  • Smelting. This process is used to produce tin when electrolytic refining is not feasible. This is accomplished by reducing the hydrated tin oxide with a reducing agent, melting the tin metal formed, skimming the dross, pouring the molten tin and casting the molten tin.
  • Calcining. This process converts the hydrated tin oxides to anhydrous stannic oxide by calcining the hydrate and removing and packaging the stannic oxides.
  • Kettle refining. This process is used to purify crude furnace metal by charging a preheated kettle with it, drying the dross to remove the impurities as slag and matte, fluxing with sulphur to remove copper as matte, fluxing with aluminium to remove antimony and casting molten metal into desired shapes.

 

See table 12 for a summary of exposures and controls for tin reclamation.

Table 12. Engineering/administrative controls for tin, by operation

Process equipment

Exposures

Engineering/administrative controls

Dealuminization

Sodium hydroxide

Local exhaust; PPE—chemical goggles and/or face shield

Batch mixing

Dust

Local exhaust ventilation and general area ventilation

Chemical detinning

Caustic

Local exhaust ventilation; PPE—chemical goggles and/or face shield

Dross smelting

Dust and heat

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Dust leaching and filtration

Dust

Local exhaust ventilation, general area ventilation

Settling and leaf filtration

None identified

None identified

Evapocentrifugation

None identified

None identified

Electrolytic refining

Acid mist

Local exhaust ventilation and general area ventilation; PPE—chemical goggles and/or face shield

Acidification and filtration

Acid mists

Local exhaust ventilation and general area ventilation; PPE—chemical goggles and/or face shield

Fire refining

Heat

Work/rest regimen, PPE

Smelting

Combustion gases, fumes and dust, heat

Local exhaust ventilation and general area ventilation, work/rest regimen, PPE

Calcining

Dust, fumes, heat

Local exhaust ventilation and general area ventilation work/rest regimen, PPE

Kettle refining

Dust, fumes, heat

Local exhaust ventilation and general area ventilation, work/rest regimen, PPE

 

Titanium reclamation

The two primary sources of titanium scrap are the home and titanium consumers. Home scrap which is generated by the milling and manufacturing of titanium products includes trim sheets, plank sheet, cuttings, turnings and borings. Consumer scrap consists of recycled titanium products. The reclamation operations include:

  • Degreasing. In this process sized scrap is treated with vapourized organic solvent (e.g., trichloroethylene). Contaminant grease and oil are stripped from the scrap by the solvent vapour. The solvent is recirculated until it can no longer has an ability to degrease. Spent solvent can then be regenerated. The scrap can also be degreased by steam and detergent.
  • Pickling. The acid-pickling process removes oxide scale from the degreasing operation by leaching with a solution of hydrochloric and hydrofluoric acids. The acid treatment scrap is washed with water and dried.
  • Electrorefining. Electrorefining is a titanium scrap pre-treatment process which electro-refines scrap in a fused salt.
  • Smelting. Pre-treated titanium scrap and alloying agents are melted in a electric-arc vacuum furnace to form a titanium alloy. The input materials include pre-treated titanium scrap and alloying materials such as aluminium, vanadium, molybdenum, tin, zirconium, palladium, columbium and chromium.
  • Casting. Molten titanium is poured into moulds. The titanium solidifies into a bar called an ingot.

 

Controls for exposures in titanium reclamation procedures are listed in table 13.

Table 13. Engineering/administrative controls for titanium, by operation

Process equipment

Exposures

Engineering/administrative controls

Solvent degreasing

Solvent

Local exhaust and solvent recovery

Pickling

Acids

Face shields, aprons, long sleeves, safety glasses or goggles

Electrorefining

None known

None known

Smelting

Volatile metals, noise

Local exhaust ventilation and control of noise from burners; PPE—hearing protection

Casting

Heat

PPE

 

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Figure 6. Electroplating: Schematic representation

Metal Finishing

The surface treatment of metals increases their durability and improves their appearance. A single product may undergo more than one surface treatment—for example, an auto body panel may be phosphated, primed and painted. This article deals with the processes used for surface treatment of metals and the methods used to reduce their environmental impact.

Operating a metal finishing business requires cooperation between company management, employees, government and the community to effectively minimize the environmental effect of the operations. Society is concerned with the amount and the long-term effects of pollution entering the air, water and land environment. Effective environmental management is established through detailed knowledge of all elements, chemicals, metals, processes and outputs.

Pollution prevention planning shifts the environmental management philosophy from reacting to problems to anticipating solutions focusing on chemical substitution, process change and internal recycling, using the following planning sequence:

  1. Initiate pollution prevention across all aspects of the business.
  2. Identify waste streams.
  3. Set priorities for action.
  4. Establish root cause of the waste.
  5. Identify and implement changes that reduce or eliminate the waste.
  6. Measure the results.

 

Continuous improvement is achieved by setting new priorities for action and repeating the sequence of actions.

Detailed process documentation will identify the waste streams and allow priorities to be set for waste reduction opportunities. Informed decisions about potential changes will encourage:

  • easy and practical operational improvements
  • process changes involving customers and suppliers
  • changes to less harmful activities where possible
  • reuse and recycling where change is not practical
  • using landfilling of hazardous wastes only as a last resort.

 

Major processes and standard operating processes

Cleaning is required because all metal finishing processes require that parts to be finished be free from organic and inorganic soils, including oils, scale, buffing and polishing compounds. The three basic types of cleaners in use are solvents, vapour degreasers and alkaline detergents.

Solvents and vapour degreasing cleaning methods have been almost totally replaced by alkaline materials where the subsequent processes are wet. Solvents and vapour degreasers are still in use where parts must be clean and dry with no further wet processing. Solvents such as terpenes are in some instances replacing volatile solvents. Less toxic materials such as 1,1,1-trichloroethane have been substituted for more hazardous materials in vapour degreasing (although this solvent is being phased out as an ozone depleter).

Alkaline cleaning cycles usually include a soak immersion followed by an anodic electroclean, followed by a weak acid immersion. Non-etching, non-silicated cleaners are typically used to clean aluminium. The acids are typically sulphuric, hydrochloric and nitric.

Anodizing, an electrochemical process to thicken the oxide film on the metal surface (frequently applied to aluminium), treats the parts with dilute chromic or sulphuric acid solutions.

Conversion coating is used to provide a base for subsequent painting or to passivate for protection against oxidation. With chromating, parts are immersed in a hexavalent chrome solution with active organic and inorganic agents. For phosphating, parts are immersed in dilute phosphoric acid with other agents. Passivating is accomplished through immersion in nitric acid or nitric acid with sodium dichromate.

Electroless plating involves a deposition of metal without electricity. Copper or nickel electroless deposition is used in the manufacture of printed circuit boards.

Electroplating involves the deposition of a thin coat of metal (zinc, nickel, copper, chromium, cadmium, tin, brass, bronze, lead, tin-lead, gold, silver and other metals such as platinum) on a substrate (ferrous or non-ferrous). Process baths include metals in solution in acid, alkaline neutral and alkaline cyanide formulations (see figure 1).

Figure 1. Inputs and outputs for a typical electroplating line

MET110F1

Chemical milling and etching are controlled dissolution immersion processes using chemical reagents and etchants. Aluminium is typically etched in caustic prior to anodizing or chemically brightened in a solution which could contain nitric, phosphoric and sulphuric acids.

Hot-dip coatings involve the application of metal to a workpiece by immersion in molten metal (zinc or tin galvanizing of steel).

Good management practices

Important safety, health and environmental improvements can be achieved through process improvements, such as:

  • using counter-current rinsing and conductivity controls
  • increasing drainage time
  • using more or better wetting agents
  • keeping process temperatures as high as possible to lower viscosity, thus increasing drag-out recovery (i.e., recovery of solution left on metal)
  • using air agitation in rinsing to increase rinsing efficiency
  • using plastic balls in plating tanks to reduce misting
  • using improved filtration on plating tanks to reduce the frequency of purification treatment
  • placing a curb around all process areas to contain spills
  • using separate treatments for recoverable metals such as nickel
  • installing recovery systems such as ion exchange, atmospheric evaporation, vacuum evaporation, electrolytic recovery, reverse osmosis and electrodialysis
  • complementing drag-out recovery systems with reductions in drag-in of contaminants and improved cleaning systems
  • using modern inventory controls to reduce waste and workplace hazards
  • applying standard procedures (i.e., written procedures, regular operating reviews and sound operating logs) to provide the basis for a sound environmental management structure.

 

Environmental planning for specific wastes

Specific waste streams, usually spent plating solutions, can be reduced by:

  • Filtration. Cartridge or diatomaceous earth filters can be used to remove the accumulation of solids, which reduce the efficiency of the process.
  • Carbon treatment can be used to remove organic contaminants (most commonly applied in nickel plating, copper electroplating and zinc and cadmium plating).
  • Purified water. The natural contaminants in water make-up and rinses (e.g., calcium, iron, magnesium, manganese, chlorine and carbonates) can be removed by using deionization, distillation or reverse osmosis. Improving rinse water efficiency reduces the volume of bath sludges requiring treatment.
  • Cyanide bath carbonate freezing. Lowering the bath temperature to –3 °C crystallizes the carbonates formed in cyanide bath by the breakdown of cyanide, excessive anode current densities and the adsorption of carbon dioxide from the air and facilitates their removal.
  • Precipitation. Removal of metal contaminants entering the bath as impurities in anodes can be achieved through precipitation with barium cyanide, barium hydroxide, calcium hydroxide, calcium sulphate or calcium cyanide.
  • Hexavalent chrome alternatives. Hexavalent chromium can be replaced with trivalent chromium plating solutions for decorative plating. Chrome conversion coatings for paint pretreatments can sometimes be replaced by non-chrome conversion coatings or no-rinse chrome chemistries.
  • Non-chelated process chemistries. Instead of chelators being added to process baths to control the concentration of free ions in the solution, non-chelated process chemistries can be used so that it may not be necessary to keep metals in solution. These metals can be allowed to precipitate and can be removed by continuous filtration.
  • Non-cyanide process chemicals. Waste streams containing free cyanide are typically treated using hypochlorite or chlorine to accomplish oxidation, and complex cyanides are commonly precipitated using ferrous sulphate. Using non-cyanide process chemistries both eliminates a treatment step and reduces the sludge volume.
  • Solvent degreasing. Hot alkaline cleaning baths can be used in place of solvent degreasing of workpieces before processing. The effectiveness of alkaline cleaners can be enhanced by applying electrocurrent or ultrasonics. The benefits of avoiding solvent vapours and sludges often outweigh any additional operating costs.
  • Alkaline cleaners. Having to discard alkaline cleaners when the accumulation of oil, grease and soils from use reaches a level which impairs the cleaning efficiency of the bath can be avoided by using skimming devices to remove free-floating oils, settling devices or cartridge filters to remove particulates and oil-water coalescers and by using microfiltration or ultrafiltration to remove emulsified oils.
  • Drag-out reduction. Reducing the volume of drag-out from process baths serves to reduce the amount of valuable process chemicals that contaminates the rinse water, which in turn reduces the amount of sludge that is generated by a conventional metal precipitation treatment process.

 

Several methods of reducing drag-out include:

  • Process bath operating concentration. The chemical concentration should be kept as low as possible to minimize the viscosity (for quicker draining) and the quantity of chemicals (in the film).
  • Process bath operating temperature. The viscosity of the process solution can be reduced by increasing the bath temperature.
  • Wetting agents. The surface tension of the solution can be reduced by adding wetting agents to the process bath.
  • Workpiece positioning. The workpiece should be positioned on the rack so that the adhering film drains freely and does not get trapped in grooves or cavities.
  • Withdrawal or drainage time. The faster a workpiece is removed from the process bath, the thicker the film on the workpiece surface.
  • Air knives. Blowing air at the workpiece as the workpiece rack is raised above the process tank can improve drainage and drying.
  • Spray rinses. These can be used above heated baths so that the rinse flow rate equals the evaporation rate of the tank.
  • Plating baths. Carbonates and organic contaminants should be removed to prevent accumulation of contamination that increases the viscosity of the plating bath.
  • Drainage boards. The spaces between process tanks should be covered with drainage boards to capture process solutions and to return them to the process bath.
  • Drag-out tanks. The workpieces should be placed in drag-out tanks (“static rinse” tanks) before the standard rinsing operation.

 

Drag-out recovery of chemicals uses a variety of technologies. These include:

  • Evaporation. Atmospheric evaporators are most common, and vacuum evaporators offer energy savings.
  • Ion exchange is used for chemical recovery of rinse water.
  • Electrowinning. This is an electrolytic process whereby the dissolved metals in the solution are reduced and deposited on the cathode. The deposited metal is then recovered.
  • Electrodialysis. This utilizes ion-permeable membranes and applied current in order to separate ionic species from the solution.
  • Reverse osmosis. This utilizes a semi-permeable membrane to produce purified water and a concentrated ionic solution. High pressure is used to force the water through the membrane, while most dissolved salts are retained by the membrane.

 

Rinse water

Most of the hazardous waste produced in a metal finishing facility comes from waste water generated by the rinsing operations that follow cleaning and plating. By increasing rinse efficiency, a facility can significantly reduce waste water flow.

Two basic strategies improve rinsing efficiency. First, turbulence can be generated between the workpiece and the rinse water through spray rinses and rinse water agitation. Movement of the rack or forced water or air are used. Second, the contact time between the workpiece and the rinse water can be increased. Multiple rinse tanks set countercurrent in series will reduce the amount of rinse water used.

Industrial Coatings

The term coatings includes paints, varnishes, lacquers, enamels and shellacs, putties, wood fillers and sealers, paint and varnish removers, paint brush cleaners and allied paint products. Liquid coatings contain pigments and additives dispersed in a liquid binder and solvent mixture. Pigments are inorganic or organic compounds that provide coating colour and opacity and influence coating flow and durability. Pigments often contain heavy metals such as cadmium, lead, zinc, chromium and cobalt. The binder increases coating adhesiveness, cohesiveness and consistency and is the primary component that remains on the surface when coating is completed. Binders include a variety of oils, resins, rubbers and polymers. Additives such as fillers and extenders may be added to coatings to reduce manufacturing costs and increase coating durability.

The types of organic solvents used in coatings include aliphatic hydrocarbons, aromatic hydrocarbons, esters, ketones, glycol ethers and alcohols. Solvents disperse or dissolve the binders and decrease the coating viscosity and thickness. Solvents used in coatings formulations are hazardous because many are human carcinogens and are flammable or explosive. Most solvents contained in a coating evaporate when the coating cures, which generates volatile organic compound (VOC) emissions. VOC emissions are becoming increasingly regulated because of the negative effects on human health and the environment. Environmental concerns associated with conventional ingredients, coating application technologies and coating wastes are a driving force for developing pollution prevention alternatives.

Most coatings are used on architectural, industrial or special products. Architectural coatings are used in buildings and building products and for decorative and protective services such as varnishes to protect wood. Industrial facilities incorporate coating operations in various production processes. The automotive, metal can, farm machinery, coil coating, wood and metal furniture and fixtures, and household appliance industries are the major industrial coatings consumers.

Design of a coating formulation depends on the purpose of the coating application. Coatings provide aesthetics, and corrosion and surface protection. Cost, function, product safety, environmental safety, transfer efficiency and drying and curing speed determine formulations.

Coating processes

There are five operations comprising most coating processes: raw materials handling and preparation, surface preparation, coating, equipment cleaning and waste management.

Raw material handling and preparation

Raw material handling and preparation involves inventory storage, mixing operations, thinning and adjusting of coatings and raw material transfer through the facility. Monitoring and handling procedures and practices are needed to minimize the generation of wastes from spoilage, off specification and improper preparation that can result from excessive thinning and consequent wastage. Transfer, whether manual or through a piped system, must be scheduled to avoid spoilage.

Surface preparation

The type of surface preparation technique used depends on the surface being coated—previous preparation, amount of soil, grease, the coating to be applied and the surface finish required. Common preparation operations include degreasing, precoating or phosphating and coating removal. For metal finishing purposes, degreasing involves solvent wiping, cold cleaning or vapour degreasing with halogenated solvents, aqueous alkaline cleaning, semi-aqueous cleaning or aliphatic hydrocarbon cleaning to remove organic soil, dirt, oil and grease. Acid pickling, abrasive cleaning or flame cleaning are used to remove mill scale and rust.

The most common preparation operation for metal surfaces, other than cleaning, is phosphate coating, used to promote adhesion of organic coatings onto metal surfaces and retard corrosion. Phosphate coatings are applied by immersing or spraying metal surfaces with zinc, iron or manganese phosphate solution. Phosphating is a surface finishing process similar to electroplating, consisting of a series of process chemical and rinse baths in which parts are immersed to achieve the desired surface preparation. See the article “Surface treatment of metals” in this chapter.

Coating removal, chemical or mechanical, is conducted on surfaces that require recoating, repair or inspection. The most common chemical coating removal method is solvent stripping. These solutions usually contain phenol, methylene chloride and an organic acid to dissolve the coating from the coated surface. A final water wash to remove the chemicals can generate large quantities of wastewater. Abrasive blasting is the common mechanical process, a dry operation that uses compressed air to propel a blasting medium against the surface to remove the coating.

Surface preparation operations affect the quantity of waste from the specific preparation process. If the surface preparation is inadequate, resulting in poor coating, then removal of the coating and recoating adds to waste generation.

Coating

The coating operation involves transferring the coating to the surface and curing the coating on the surface. Most coating technologies fall into 1 of 5 basic categories: dip coating, roll coating, flow coating, spray coating, and the most common technique, air-atomized spray coating using solvent-based coatings.

Air-atomized spray coatings are usually conducted in a controlled environment because of solvent emissions and overspray. Overspray control devices are fabric filters or water walls, generating either used filters or wastewater from air scrubbing systems.

Curing is performed to convert the coating binder into a hard, tough, adherent surface. Curing mechanisms include: drying, baking or exposure to an electron beam or infrared or ultraviolet light. Curing generates significant VOCs from solvent-based coatings and poses a potential for explosion if the solvent concentrations rise above the lower explosive limit. Consequently, curing operations are equipped with air pollution control devices to prevent VOC emissions and for safety control to prevent explosions.

Environmental and health concerns, increased regulations affecting conventional coating formulations, high solvent costs and expensive hazardous waste disposal have created a demand for alternative coating formulations that contain less hazardous constituents and generate less waste when applied. Alternative coating formulations include:

  • High-solid coatings, containing twice the amount of pigment and resin in the same volume of solvent as conventional coatings. Application lowers VOC emissions between 62 and 85% compared to conventional low-solid solvent-based coatings because the solvent content is reduced.
  • Water-based coatings using water and an organic solvent mixture as the carrier with water used as the base. Compared to solvent-based coatings, water-based coatings generate between 80 and 95% less VOC emissions and spent solvents than conventional low-solid solvent-based coatings.
  • Powder coatings containing no organic solvent, consisting of finely pulverized pigment and resin particles. They are either thermoplastic (high molecular weight resin for thick coatings) or thermosetting (low molecular weight compounds that form a thin layer before chemically cross-linking) powders.

 

Equipment cleaning

Equipment cleaning is a necessary, routine maintenance operation in coating processes. This creates significant amounts of hazardous waste, particularly if halogenated solvents are used for cleaning. Equipment cleaning for solvent-based coatings has traditionally been conducted manually with organic solvents to remove coatings from process equipment. Piping requires flushing with solvent in batches until clean. Coating equipment must be cleaned between product changes and after process shutdowns. The procedures and practices used will determine the level of waste generated from these activities.

Waste management

Several waste streams are generated by coating processes. Solid waste includes empty coating containers, coating sludge from overspray and equipment cleaning, spent filters and abrasive materials, dry coating and cleaning rags.

Liquid wastes include waste water from surface preparation, overspray control or equipment cleaning, off-specification or excess coating or surface preparation materials, overspray, spills and spent cleaning solutions. Onsite closed-loop recycling is becoming more popular for spent solvents as disposal costs rise. Water-based liquids are usually treated onsite prior to discharge to publicly owned treatment systems.

VOC emissions are generated by all conventional coating processes that use solvent-based coatings, requiring control devices such as carbon adsorption units, condensers or thermal catalytic oxidizers.

 

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Saturday, 19 March 2011 20:40

General Profile

Written by

The diversity of processes and products within the microelectronics and semiconductor industry is immense. The focus of the occupational health and safety discussion in this chapter centres on semiconductor integrated circuit (IC) production (both in silicon-based products and valence III-V compounds), printed wiring board (PWB) production, printed circuit board (PCB) assembly and computer assembly.

The industry is composed of numerous major segments. The Electronics Industry Association uses the following delineation in reporting data on pertinent trends, sales and employment within the industry:

  • electronic components
  • consumer electronics
  • telecommunications
  • defence communications
  • computers and peripheral equipment
  • industrial electronics
  • medical electronics.

 

Electronic components include electron tubes (e.g., receiving, special-purpose and television tubes), solid-state products (e.g., transistors, diodes, ICs, light-emitting diodes (LEDs) and liquid-crystal displays (LCDs)) and passive and other components (e.g., capacitors, resistors, coils, transformers and switches).

Consumer electronics include television sets and other home and portable audio and video products, as well as information equipment such as personal computers, facsimile transmission machines and telephone answering devices. Electronic gaming hardware and software, home security systems, blank audio and video cassettes and floppy disks, electronic accessories and total primary batteries also fall under the consumer electronics heading.

In addition to general purpose and specialized computers, computers and peripheral equipment includes auxiliary storage equipment, input/output equipment (e.g., keyboards, mice, optical scanning devices and printers), terminals and so on. While telecommunications, defence communications and industrial and medical electronics utilize some of the same technology these segments also involve specialized equipment.

The emergence of the microelectronics industry has had a profound impact on the evolution and structure of the world’s economy. The pace of change within industrialized nations of the world has been greatly influenced by advances within this industry, specifically in the evolution of the integrated circuit. This pace of change is graphically represented in the timeline of the number of transistors per integrated circuit chip (see figure 1).

Figure 1. Transistors per integrated circuit chip

MICO10F1

The economic importance of worldwide semiconductor sales is significant. Figure 2 is a projection by the Semiconductor Industry Association for worldwide and regional semiconductor sales for 1993 to 1998.

Figure 2. Worldwide semiconductor sales forecast

MICO10F2

The semiconductor IC and computer/electronics assembly industries are unique compared to most other industrial categories in the relative composition of their production workforces. The semiconductor fabrication area has a high percentage of female operators that run the process equipment. The operator-related tasks typically do not require heavy lifting or excess physical strength. Also, many of the job tasks involve fine motor skills and attention to detail. Male workers predominate in the maintenance-related tasks, engineering functions and management. A similar composition is found in the computer/electronics assembly portion of this industry segment. Another unusual feature of this industry is the concentration of manufacturing in the Asia/Pacific area of the world. This is especially true in the final assembly or back-end processes in the semiconductor industry. This processing involves the positioning and placement of the fabricated integrated circuit chip (technically known as a die) on a chip carrier and lead frame. This processing requires precise positioning of the chip, typically through a microscope, and very fine motor skills. Again, female workers predominate this part of the process, with the majority of worldwide production being concentrated in the Pacific Rim, with high concentrations in Taiwan, Malaysia, Thailand, Indonesia and the Philippines, and growing numbers in China and Vietnam.

The semiconductor IC fabrication areas have various unusual properties and characteristics unique to this industry. Namely, the IC processing involves extremely tight particulate control regimens and requirements. A typical modern IC fabrication area may be rated as a Class 1 or less cleanroom. As a method of comparison, an outdoor environment would be greater than Class 500,000; a typical room in a house approximately Class 100,000; and a semiconductor back-end assembly area approximately Class 10,000. To attain this level of particulate control involves actually putting the fabrication worker in totally enclosed bunny suits that have air supply and filtration systems to control the levels of particulates generated by the workers in the fabrication area. The human occupants of the fabrication areas are considered very potent generators of fine particulates from their exhaled air, shedding of skin and hair, and from their clothing and shoes. This requirement for wearing confining clothing and isolating work routines has contributed to employees feeling like they are working in a “non-hospitable” work environment. See figure 3. Also, in the photolithographic area, the processing involves exposing the wafer to a photoactive solution, and then patterning an image on the wafer surface using ultraviolet light. To alleviate unwanted ultraviolet (UV) light from this processing area, special yellow lights are used (they lack the UV wavelength component normally found in indoor lighting). These yellow lights help to make the workers feel they are in a different work environment and can possibly have a disorienting affect on some individuals.

Figure 3. A state-of-the-art cleanroom

MIC010F3

 

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Saturday, 19 March 2011 20:44

Silicon Semiconductor Manufacturing

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Process Overview

The description of silicon semiconductor device processing, either discrete devices (a semiconductor containing only one active device, such as a transistor) or ICs (interconnected arrays of active and passive elements within a single semiconductor substrate capable of performing at least one electronic circuit function), involves numerous highly technical and specific operations. The intent of this description is to provide a basic framework and explanation of the primary component steps utilized in fabricating a silicon semiconductor device and the associated environmental, health and safety (EHS) issues.

The fabrication of an IC involves a sequence of processes that may be repeated many times before a circuit is complete. The most popular ICs use 6 or more masks to complete patterning processes, with 10 to 24 masks being typical. The manufacture of a microcircuit begins with an ultra-high purity silicon wafer 4 to 12 inches in diameter. Perfectly pure silicon is almost an insulator, but certain impurities, called dopants, added in amounts of from 10 to 100 parts per million, make silicon conduct electricity.

An integrated circuit can consist of millions of transistors (also diodes, resistors and capacitors) made of doped silicon, all connected by the appropriate pattern of conductors to create the computer logic, memory or other type of circuit. Hundreds of microcircuits can be made on one wafer.

Six major fabrication processing steps are universal to all silicon semiconductor devices: oxidation, lithography, etching, doping, chemical vapour deposition and metallization. These are followed by assembly, testing, marking, packing and shipping.

Oxidation

Generally, the first step in semiconductor device processing involves the oxidation of the exterior surface of the wafer to grow a thin layer (about one micron) of silicon dioxide (SiO2). This primarily protects the surface from impurities and serves as a mask for the subsequent diffusion process. This ability to grow a chemically stable protective wafer of silicon dioxide on silicon makes silicon wafers the most widely used semiconductor substrate.

Oxidation, commonly called thermal oxidation, is a batch process which takes place in a high-temperature diffusion furnace. The protective silicon dioxide layer is grown in atmospheres containing either oxygen (O2) (dry oxidation) or oxygen combined with water vapour (H2O) (wet oxidation). The temperatures in the furnace range from 800 to 1,300oC. Chlorine compounds in the form of hydrogen chloride (HCl) may also be added to help control unwanted impurities.

The tendency in newer fabrication facilities is towards vertical oxidation furnaces. Vertical furnaces better address the need for greater contamination control, larger wafer size and more uniform processing. They allow a smaller equipment footprint that conserves precious cleanroom floor space.

Dry oxidation

Silicon wafers to be oxidized are first cleaned, using a detergent and water solution, and solvent rinsed with xylene, isopropyl alcohol or other solvents. The cleaned wafers are dried, loaded into a quartz wafer holder called a boat and loaded into the operator end (load end) of the quartz diffusion furnace tube or cell. The inlet end of the tube (source end) supplies high-purity oxygen or oxygen/nitrogen mixture. The “dry” oxygen flow is controlled into the quartz tube and assures that an excess of oxygen is available for the growth of silicon dioxide on the silicon wafer surface. The basic chemical reaction is:

Si + O2 → SiO2

Wet oxidation

Four methods of introducing water vapour are commonly used when water is the oxidizing agent—pyrophoric, high-pressure, bubbler and flash. The basic chemical reactions are:

Pyrophoric and high pressure: Si + 2O2 + 2 H2 → SiO2 + 2H2O

Flash and bubbler: Si + 2H2O → SiO2 + 2H2

Pyrophoric oxidation involves the introduction and combustion of a hydrogen/oxygen gas mixture. Such systems are generally called burnt hydrogen or torch systems. Water vapour is produced when proper amounts of hydrogen and oxygen are introduced at the inlet end of the tube and allowed to react. The mixture must be controlled precisely to guarantee proper combustion and prevent the accumulation of explosive hydrogen gas.

High-pressure oxidation (HiPox) is technically called a water pyrosynthesis system and generates water vapour through the reaction of ultra-pure hydrogen and oxygen. The steam is then pumped into a high-pressure chamber and pressurized to 10 atmospheres, which accelerates the wet oxidation process. De-ionized water may also be used as a steam source.

In bubbler oxidation de-ionized water is placed in a container called a bubbler and maintained at a constant temperature below its boiling point of 100°C through the use of a heating mantle. Nitrogen or oxygen gas enters the inlet side of the bubbler, becomes saturated with water vapour as it rises through the water, and exits through the outlet into the diffusion furnace. Bubbler systems appear to be the most widely used method of oxidation.

In flash oxidation de-ionized water is dripped continuously into the heated bottom surface of a quartz container and the water evaporates rapidly once it hits the hot surface. Nitrogen or oxygen carrier gas flows over the evaporating water and carries the water vapour into the diffusion furnace.

Lithography

Lithography, also known as photolithography or simply masking, is a method of accurately forming patterns on the oxidized wafer. The microelectronic circuit is built up layer by layer, each layer receiving a pattern from a mask prescribed in circuit design.

The printing trades developed the true antecedents of today’s semiconductor device microfabrication processes. These developments relate to the manufacture of printing plates, usually of metal, on which removal of material through chemical etching produces a surface relief pattern. This same basic technique is used in producing master masks used in the fabrication of each layer of processing of a device.

Circuit designers digitize the basic circuitry of each layer. This computerized schematic allows quick generation of the mask circuitry and facilitates any changes that may be needed. This technique is known as computer-aided design (CAD). Utilizing powerful computer algorithms, these on-line design systems permit the designer to lay out and modify the circuitry directly on video display screens with interactive graphic capabilities.

The final drawing, or mask, for each layer of circuitry is created by a computer-driven photoplotter, or pattern generator. These photoplotted drawings are then reduced to the actual size of the circuit, a master mask produced on glass with chrome relief, and reproduced on a work plate which serves for either contact or projection printing on the wafer.

These masks delineate the pattern of the conducting and insulating areas which are transferred to the wafer through photolithography. Most companies do not produce their own masks, but utilize those furnished by a mask producer.

Cleaning

The need for a particulate- and contamination-free exterior wafer surface requires frequent cleaning. The major categories are:

  • de-ionized water and detergent scrubbing
  • solvent: isopropyl alcohol (IPA), acetone, ethanol, terpenes
  • acid: hydrofluoric (HF), sulphuric (H2SO4) and hydrogen peroxide (H2O2), hydrochloric (HCl), nitric (HNO3) and mixtures
  • caustic: ammonium hydroxide (NH4OH).

 

Resist application

Wafers are coated with a resist material of solvent-based polymer and rapidly rotated on a spinner, which spreads a thin uniform layer. The solvents then evaporate, leaving a polymeric film. All resist materials depend on (primarily ultraviolet) radiation-induced changes in the solubility of a synthetic organic polymer in a selected developer rinse. Resist materials are classified as either negative or positive resists, depending on whether the solubility in the developer decreases (negative) or increases (positive) upon exposure to radiation. Table 1 identifies the component makeup of various photoresist systems.

Table 1. Photoresist systems

Ultraviolet

Near (350–450 nm)

Negative

PB
S
D

Azide base aliphatic rubber (isoprene)
n-Butyl acetate, xylene, n-methyl-2-pyrrolidone, ethyl benzene
Xylene, aliphatic hydrocarbons, n-butyl acetate,
Stoddard solvent (petroleum distillates)

 

Positive

PB
S


D

Ortho-diazoketone
Propylene glycol monomethyl ether acetate, ethyl lactate, methyl
methoxy propionate, ethyl ethoxy propionate, n-butyl acetate, xylene,
chlorotoluene
Sodium hydroxide, silicates, potassium hydroxide

Deep (200–250 nm)

Primarily
positive resists

   

Electron-beam (about 100 nm)

 

Negative

PB
S
D

Copolymer-ethyl acrylate and glycidyl methacrylate (COP)
n/a
n/a

 

Positive

PB

S
D

Polymethylmethacrylate, polyfluoralkylmethacrylate, polyalkylaldehyde, poly-cyano ethylacrylate
Propylene glycol monomethyl ether acetate
Alkaline or IPA, ethyl acetate, or methyl isobutyl ketone (MIBK)

X ray (0.5–5 nm)

 

Negative

PB
S
D

Copolymer-ethyl acrylate and glycidyl methacrylate (COP)
n/a
n/a

 

Positive

PB

S
D

Polymethylmethacrylate, ortho-diazoketone, poly
(hexa-fluorobutylmethacrylate), poly (butene-1-sulphone)
Propylene glycol monomethyl ether acetate
n/a

PB = polymer base; S = solvent; D = developer.

Since most photoresists are ultraviolet (UV) light sensitive, the processing area is lit with special yellow lights lacking sensitive UV wavelengths (see figure 1).

Figure 1. Photolithographic “Yellow room” equipment

MIC020F3

Negative and positive UV resists are primarily in use in the industry. E-beam and x-ray resists, however, are gaining in market share because of their higher resolutions. Health concerns in lithography are primarily caused by potential reproductive hazards associated with selected positive resists (e.g., ethylene glycol monoethyl ether acetate as a carrier) that are currently being phased out by the industry. Occasional odours from the negative resists (e.g., xylene) also result in employee concerns. Because of these concerns, a great deal of time is spent by semiconductor industry industrial hygienists sampling photoresist operations. While this is useful in characterizing these operations, routine exposures during spinner and developer operations are typically less than 5% of the airborne standards for occupational exposure for the solvents used in the process (Scarpace et al. 1989).

A 1 hour exposure to ethylene glycol monoethyl ether acetate of 6.3 ppm was found during the operation of a spinner system. This exposure was primarily caused by poor work practices during the maintenance operation (Baldwin, Rubin and Horowitz 1993).

Drying and pre-baking

After the resist has been applied, the wafers are moved on a track or manually moved from the spinner to a temperature-controlled oven with a nitrogen atmosphere. A moderate temperature (70 to 90°C) causes the photoresist to cure (soft bake) and the remaining solvents to evaporate.

To ensure adhesion of the resist layer to the wafer, a primer, hexamethyldisilizane (HMDS), is applied to the wafer. The primer ties up molecular water on the surface of the wafer. HMDS is applied either directly in an immersion or spin-on process or through a vapour prime that offers process and cost advantages over the other methods.

Mask aligning and exposure

The mask and wafer are brought close together using a precise piece of optical/mechanical equipment, and the image on the mask is aligned to any pattern already existing in the wafer beneath the layer of photoresist. For the first mask, no alignment is necessary. In older technologies, alignment for successive layers is made possible by the use of a biscope (dual lens microscope) and precision controls for positioning the wafer with respect to the mask. In newer technologies alignment is done automatically using reference points on the wafers.

Once the alignment is done, a high-intensity ultraviolet mercury vapour or arc lamp source shines through the mask, exposing the resist in places not protected by opaque regions of the mask.

The various methods of wafer alignment and exposure include UV flood exposure (contact or proximity), UV exposure through projection lens for reduction (projection), UV step and repeat reduction exposure (projection), x-ray flood (proximity) and electron beam scan exposure (direct writing). The primary method in use involves UV exposure from mercury vapour and arc lamps through proximity or projection aligners. The UV resists are either designed to react to a broad spectrum of UV wavelengths, or they are formulated to react preferentially to one or more of the main spectrum lines emitted from the lamp (e.g., g-line at 435 nm, h-line at 405 nm and i-line at 365 nm).

The predominant wavelengths of UV light currently used in photomasking are 365 nm or above, but UV lamp spectra also contain significant energy in the wavelength region of health concern, the actinic region below 315 nm. Normally, the intensity of the UV radiation escaping from the equipment is less than both what is present from sunlight in the actinic region and the standards set for occupational exposure to UV.

Occasionally during maintenance, the alignment of the UV lamp requires that it be energized outside the equipment cabinet or without normal protective filters. Exposure levels during this operation can exceed occupational exposure limits, but standard cleanroom attire (e.g., smocks, vinyl gloves, face masks and polycarbonate safety glasses with UV inhibitor) is usually adequate to attenuate the UV light to below exposure limits (Baldwin and Stewart 1989).

While the predominant wavelengths for ultraviolet lamps used in photolithography are 365 nm or above, the quest for smaller features in advanced ICs is leading to the use of exposure sources with smaller wavelengths, such as deep UV and x rays. One new technology for this purpose is the use of krypton-fluoride excimer lasers used in steppers. These steppers use a wavelength of 248 nm with high laser power outputs. However, enclosures for these systems contain the beam during normal operation.

As with other equipment containing high-power laser systems used in semiconductor manufacturing, the main concern is when interlocks for the system must be defeated during beam alignment. High-powered lasers are also one of the most significant electrical hazards in the semiconductor industry. Even after power is off, a significant shock potential exists within the tool. Controls and safety design considerations for these systems are covered by Escher, Weathers and Labonville (1993).

One advanced-technology exposure source used in lithography is x rays. Emission levels from x-ray lithography sources may result in dose rates approaching 50 millisieverts (5 rems) per year in the centre of the equipment. Restricting access to areas inside the shielded wall is recommended to minimize exposure (Rooney and Leavey 1989).

Developing

During the development step the unpolymerized areas of the resist are dissolved and removed. Solvent-based developer is applied to the resist-covered wafer surface by either immersion, spraying or atomization. Developer solutions are identified in table 1. A solvent rinse (n-butyl acetate, isopropyl alcohol, acetone, etc.) is usually applied following the developer to remove any residual material. The resist remaining after developing protect the individual layers during subsequent processing.

Baking

After aligning, exposing and developing the resist, the wafers then move to another temperature-controlled oven with a nitrogen atmosphere. The higher-temperature oven (120 to 135°C) causes the photoresist to cure and fully polymerize on the wafer surface (hard bake).

Photoresist stripping

The developed wafer is then selectively etched using wet or dry chemicals (see “Etching” below). The remaining photoresist must be stripped from the wafer prior to further processing. This is done either by using wet chemical solutions in temperature-controlled baths or through the use of a plasma asher or dry chemical. Table 2 identifies both wet and dry chemical constituents. A discussion of dry chemical plasma etching—using the same equipment and principles of operation as plasma ashing—follows.


Table 2. Photoresist strippers

Wet chemical

 Acid

Sulphuric (H2SO4) and chromic (CrO3)

Sulphuric (H2SO4) and ammonium persulphate ((NH4)2S2O8)

Sulphuric (H2SO4) and hydrogen peroxide (H2O2)

Organics

Phenols, sulphuric acids, trichlorobenzene, perchloroethylene

Glycol ethers, ethanolamine, triethanolamine

Sodium hydroxide and silicates (positive resist)

Dry chemical

Plasma ashing (stripping)

RF (radio frequency) power source—13.56 MHz or 2,450 MHz frequency

Oxygen (O2) source gas

Vacuum pump systems

—Oil lubricated with liquid nitrogen trap (old technology)
—Lubricated with inert perfluoropolyether fluids (newer technology)
—Dry pump (newest technology)


Etching

Etching removes layers of silicon dioxide (SiO2), metals and polysilicon, as well as resists, according to the desired patterns delineated by the resist. The two major categories of etching are wet and dry chemical. Wet etching is predominantly used and involves solutions containing the etchants (usually an acid mixture) at the desired strengths, which react with the materials to be removed. Dry etching involves the use of reactive gases under vacuum in a highly energized chamber, which also removes the desired layers not protected by resist.

Wet chemical

The wet chemical etching solutions are housed in temperature-controlled etch baths made of polypropylene (poly-pro), flame-resistant polypropylene (FRPP) or polyvinyl chloride (PVC). The baths generally are equipped with either ring-type plenum exhaust ventilation or slotted exhaust at the rear of the wet chemical etch station. Vertical laminar flow hoods supply uniformly filtered particulate-free air to the top surface of the etch baths. Common wet etchant chemical solutions are presented in table 3, in relation to the surface layer being etched.

Table 3. Wet chemical etchants

Material to etch

Etchants

Silicon

Polycrystalline silicon (Si)

Hydrofluoric, nitric, acetic acids and iodine
Potassium hydroxide
Ethylene diamine/catechol
Ammonium fluoride, glacial acetic and nitric acids

Silicon dioxide (SiO2)

Buffered oxide etch (BOE) - Hydrofluoric and
ammonium fluoride
BOE, ethylene glycol, monomethyl ether
Hydrofluoric and nitric (P-etch)

Silicon nitride (Si3N4)

Phosphoric and hydrofluoric acids

CVD Oxide or Pad Etch

Ammonium fluoride, acetic and hydrofluoric acids

Metals

Aluminium (Al)

Phosphoric, nitric, acetic and hydrochloric acids
Sodium hydroxide, potassium hydroxide

Chromium-Nickel (Cr/Ni)

Ceric ammonium nitrate and nitric acid
Hydrochloric and nitric acids (aqua regia)

Gold (Au)

Hydrochloric and nitric acids (aqua regia)
Potassium iodide (KI)
Potassium cyanide (KCN) and hydrogen peroxide (H2O2)
Ferric chloride (FeCl3) and hydrochloric acid

Silver (Ag)

Ferric nitrate (FeNO3) and ethylene glycol
Nitric acid

Compound

Formula

Standard concentration (%)

Acetic acid

CH3COOH

36

Ammonium fluoride

NH4F

40

Glacial acetic acid

CH3COOH

99.5

Hydrochloric acid

HCl

36

Hydrofluoric acid

HF

49

Nitric acid

HNO3  

67

Phosphoric acid

H3PO4  

85

Potassium hydroxide

KOH

50 or 10

Sodium hydroxide

NaOH

50 or 10

Sulphuric acid

H2SO4  

96

 

Vertically mounted flow supply hoods, when used in conjunction with splash shields and exhaust ventilation, can create areas of air turbulence within the wet chemical etch station. As a result, a decrease is possible in the effectiveness of the local exhaust ventilation in capturing and routing fugitive air contaminants from the etch baths in use.

The main concern with wet etching is the possibility of skin contact with the concentrated acids. While all the acids used in etching can cause acid burns, exposure to hydrofluoric acid (HF) is of particular concern. The lag time between skin contact and pain (up to 24 hours for solutions less than 20% HF and 1 to 8 hours for 20 to 50% solutions) can result in delayed treatment and more severe burns than expected (Hathaway et al. 1991).

Historically acid burns have been a particular problem within the industry. However, the incidence of skin contact with acids have been reduced in recent years. Some of this reduction was caused by product-related improvements in the etch process, such as the shift to dry etching, the use of more robotics and the installation of chemical dispense systems. The reduction in the rate of acid burns may also be attributed to better handling techniques, greater use of personal protective equipment, better designed wet decks and better training—all of which require continued attention if the rate is to decline further (Baldwin and Williams 1996).

Dry chemical

Dry chemical etching is an area of growing interest and usage due to its ability to better control the etching process and reduce contamination levels. Dry chemical processing effectively etches desired layers through the use of chemically reactive gases or through physical bombardment.

Chemically reactive plasma etching systems have been developed which can effectively etch silicon, silicon dioxide, silicon nitride, aluminium, tantalum, tantalum compounds, chromium, tungsten, gold and glass. Two kinds of plasma etching reactor systems are in use—the barrel, or cylindrical, and the parallel plate, or planar. Both operate on the same principles and primarily vary in configuration only.

A plasma is similar to a gas except that some of the atoms or molecules of the plasma are ionized and may contain a substantial number of free radicals. The typical reactor consists of a vacuum reactor chamber containing the wafer, usually made of aluminium, glass or quartz; a radio-frequency (RF) energy source—usually at 450 kHz, 13.56 MHz or 40.5 MHz and a control module to control processing time, composition of reactant gas, flow rate of gas and RF power level. In addition, an oil-lubricated (older technology) or dry (newer technology) roughing pump vacuum source is in line with the reactor chamber. Wafers are loaded into the reactor, either individually or in cassettes, a pump evacuates the chamber and the reagent gas (usually carbon tetrafluoride) is introduced. Ionization of the gas forms the etching plasma, which reacts with the wafers to form volatile products which are pumped away. The introduction of fresh reactant gas into the chamber maintains etching activity. Table 4  identifies the materials and plasma gases in use for etching various layers.

Table 4. Plasma etching gases and etched materials

Material

Gas

Silicon

Polysilicon (polySi) and Silicon

CF + O2, CCl4 or CF3Cl, CF4 and HCl

Silicon dioxide (SiO2)

C2F6, C3F8, CF4, SiF4, C5F12, CHF3, CCl2F2, SF6, HF

Silicon nitride (Si3N4)

CF4 + Ar, CF4 + O2, CF4 + H2

Metals

Aluminium (Al)

CCl4 or BCl3 + He or Ar

Chromium (Cr)

CCl4

Chromium oxide (CrO3)

Cl2 + Ar or CCl4 + Ar

Gallium arsenide (GaAs)

CCl2F2

Vanadium (V)

CF4

Titanium (Ti)

CF4

Tantulum (Ta)

CF4

Molybdenum (Mo)

CF4

Tungsten (W)

CF4

 

Another method that currently is being developed for etching is microwave downstream. It uses a high-power-density microwave discharge to produce metastable atoms with long lifetimes that etch material almost as if it were immersed in acid.

Physical etching processes are similar to sandblasting in that argon gas atoms are used to physically bombard the layer to be etched. A vacuum pump system is used to remove dislocated material. Reactive ion etching involves a combination of chemical and physical dry etching.

The sputtering process is one of ion impact and energy transfer. Sputter etching incorporates a sputtering system, where the wafer to be etched is attached to a negative electrode or target in a glow-discharge circuit. Material sputters from the wafer by bombardment with positive ions, usually argon, and results in the dislocation of the surface atoms. Power is provided by an RF source at 450 kHz frequency. An in-line vacuum system is used for pressure control and reactant removal.

Ion-beam etching and milling is a gentle etching process which uses a beam of low-energy ions. The ion-beam system consists of a source to generate the ion beam, a work chamber in which the etching or milling occurs, fixturing with a target plate for holding the wafers in the ion beam, a vacuum pump system, supporting electronics and instruments. The ion beam is extracted from an ionized gas (argon or argon/oxygen) or plasma, which is created by the electrical discharge. The discharge is obtained by applying a voltage between an electron-emitting hot-filament cathode and an anode cylinder located in the outer diameter of the discharge region.

Ion-beam milling is done in the low-energy range of ion bombardment, where only surface interactions occur. These ions, usually in the 500 to 1,000 eV range, strike a target and sputter off surface atoms by breaking the forces bonding the atom to its neighbour. Ion-beam etching is done in a slightly higher energy range, which involves a more dramatic dislocation of surface atoms.

Reactive ion etching (RIE) is a combination of physical sputtering and chemical reactive species etching at low pressures. RIE uses ion bombardment to achieve directional etching and also a chemically reactive gas, carbon tetrafluoride (CF4) or carbon tetrachloride (CCl4), to maintain good etched layer selectivity. A wafer is placed in a chamber with an atmosphere of chemically reactive gas compound at a low pressure of about 0.1 torr (1.3 x 10–4 atmosphere). An electrical discharge creates a plasma of reactive “free radicals” (ions) with an energy of a few hundred electron volts. The ions strike the wafer surface vertically, where they react to form volatile species that are removed by a low-pressure in-line vacuum system.

Dry etchers sometimes have a cleaning cycle that is used to remove deposits that accumulate on the inside of the reaction chambers. Parent compounds used for the cleaning cycle plasmas include nitrogen trifluoride (NF3), hexafluoroethane (C2F6) and octafluoropropane (C3F8).

These three gases used in the cleaning process, and many of the gases used in etching, are a cornerstone to an environmental issue facing the semiconductor industry which surfaced in the mid-1990s. Several of the highly fluorinated gases were identified as having significant global warming (or greenhouse effect) potential. (These gases are also referred to as PFCs, perfluorinated compounds.) The long atmospheric lifetime, high global warming potential and significant increased usage of PFCs like NF3, C2F6, C3F8, CF4, trifluoromethane (CHF3) and sulphur hexafluoride (SF6) had the semiconductor industry focus on ways to reduce their emissions.

Atmospheric emissions of PFCs from the semiconductor industry have been due to poor tool efficiency (many tools consumed only 10 to 40% of the gas used) and inadequate air emission abatement equipment. Wet scrubbers are not effective in removing PFCs, and tests on many combustion units found poor destruction efficiencies for some gases, especially CF4. Many of these combustion units broke down C2F6 and C3F8 into CF4. Also, the high cost of ownership for these abatement tools, their power demand, their release of other global warming gases and their combustion by-products of hazardous air pollutants indicated combustion abatement was not a suitable method for controlling PFC emissions.

Making process tools more efficient, identifying and developing more environmentally friendly alternatives to these dry etchant gases and recovery/recycling of the exhaust gases have been the environmental emphases associated with dry etchers.

The major occupational hygiene emphasis for dry etchers has been on potential exposures to maintenance personnel working on the reaction chambers, pumps and other associated equipment that may contain reaction product residues. The complexity of plasma metal etchers and the difficulty in characterizing the odours associated with their maintenance has made them the subject of many investigations.

The reaction products formed in plasma metal etchers are a complex mixture of chlorinated and fluorinated compounds. The maintenance of metal etchers often involves short-duration operations that generate strong odours. Hexachloroethane was found to be the major cause of odour in one type of aluminium etcher (Helb et al. 1983). In another, cyanogen chloride was the main problem: exposure levels were 11 times the 0.3 ppm occupational exposure limit (Baldwin 1985). In still other types of etchers, hydrogen chloride is associated with the odour; maximum exposure measured was 68 ppm (Baldwin, Rubin and Horowitz 1993). For additional information on the subject see Mueller and Kunesh (1989).

The complexity of the chemistries present in metal etcher exhausts has led researchers to develop experimental methods for investigating the toxicity of these mixtures (Bauer et al. 1992a). Application of these methods in rodent studies indicates certain of these chemical mixtures are suspected mutagens (Bauer et al. 1992b) and suspected reproductive toxins (Schmidt et al. 1995).

Because dry etchers operate as closed systems, chemical exposure to the operators of the equipment typically does not occur while the system is closed. One rare exception to this is when the purge cycle for older batch etchers is not long enough to adequately remove the etchant gases. Brief but irritating exposures to fluorine compounds that are below the detection limit for typical industrial hygiene monitoring procedures have been reported when the doors to these etchers are opened. Normally this can be corrected by simply increasing the length of the purge cycle prior to opening the etch chamber door.

The primary concern for operator exposure to RF energy comes during plasma etching and ashing (Cohen 1986; Jones 1988). Typically, the leakage of RF energy can be caused by:

  • misaligned doors
  • cracks and holes in the cabinets
  • metal tables and electrical cables acting as antennae due to improper grounding of the etcher
  • no attenuating screen in the viewing window of the etcher (Jones 1988; Horowitz 1992).

 

RF exposure can also occur during the maintenance of etchers, particularly if the equipment cabinet has been removed. An exposure of 12.9 mW/cm2 was found at the top of an older model plasma etcher with the cover removed for maintenance (Horowitz 1992). The actual RF radiation leakage in the area where the operator stands was typically less than 4.9 mW/cm2.

Doping

The formation of an electrical junction or boundary between p and n regions in a single crystal silicon wafer is the essential element for the functioning of all semiconductor devices. Junctions permit current to flow in one direction much more easily than in the other. They provide the basis for diode and transistor effects in all semiconductors. In an integrated circuit, a controlled number of elemental impurities or dopants, must be introduced into selected etched regions of the silicon substrate, or wafer. This can be done either by diffusion or ion implantation techniques. Regardless of the technique used, the same types or dopants are used for the production of semiconductor junctions. Table 5 identifies the main components used for doping, their physical state, electrical type (p or n) and the primary junction technique in use—diffusion or ion implantation.

Table 5. Junction formation dopants for diffusion and ion implantation

Element

Compound

Formula

State

Technique

n-type

Antimony

Antimony trioxide
Antimony trichloride

Sb2O3
SbCl3

Solid
Liquid

Diffusion
Diffusion

Arsenic

Arsenic trioxide
Arsenic trioxide
Arsine
Arsenic pentafluoride

As2O3
As2O3
AsH3
AsF5

Solid
Liquid
Gas
Gas

Diffusion
Diffusion—spin on
Diffusion and ion implantation
Ion implantation

Phosphorus

Phosphorus pentoxide
Phosphorus pentoxide
Phosphorus tribromide
Phosphorus trichloride
Phosphorus oxychloride
Phosphine
Phosphorus pentafluoride

P2O5
P2O5
PBr3
PCl3
POCl3
PH3
PF5

Solid
Liquid
Liquid
Liquid
Liquid
Gas
Gas

Diffusion
Diffusion—spin on
Diffusion
Diffusion
Diffusion
Ion implantation
Ion implantation

p-type

Boron

Boron nitride
Boron tribromide
Boron trioxide
Boron trioxide
Triethylborate
Silicon tetrabromide
Boron trichloride
Boron trifluoride
Diborane

BN
BBr3
B2O3
B2O3
B(COC2H5)3
SiBr4
BCl3
BF3
B2H6

Solid
Liquid
Solid
Liquid
Liquid
Liquid
Liquid
Gas
Gas

Diffusion
Diffusion
Diffusion
Diffusion—spin on
Diffusion—spin on
Diffusion
Diffusion ion implantation
Ion implantation
Ion implantation

 

Routine chemical exposures to operators of both diffusion furnaces and ion implanters are low—typically less that the detection limit of standard occupational hygiene sampling procedures. Chemical concerns with the process centre on the possibility of toxic gas releases.

As early as the 1970s, progressive semiconductor manufacturers began installing the first continuous gas-monitoring systems for flammable and toxic gases. The main focus of this monitoring was to detect accidental releases of the most toxic dopant gases with odour thresholds above their occupational exposure limits (e.g., arsine and diborane).

Most industrial hygiene air monitors in the semiconductor industry are used for flammable and toxic gas leak detection. However, some facilities are also using continuous monitoring systems to:

  • analyse exhaust duct (stack) emissions
  • quantify ambient air concentrations of volatile chemicals
  • identify and quantify odours in the fab areas.

 

The technologies most used in the semiconductor industry for this type of monitoring are colorimetric gas detection (e.g., MDA continuous gas detector), electrochemical sensors (e.g., sensydyne monitors) and Fourier transform infrared (e.g., Telos ACM) (Baldwin and Williams 1996).

Diffusion

Diffusion is a term used to describe the movement of dopants away from regions of high concentration at the source end of the diffusion furnace to regions of lower concentration within the silicon wafer. Diffusion is the most established method of junction formation.

This technique involves subjecting a wafer to a heated atmosphere within the diffusion furnace. The furnace contains the desired dopants in a vapour form and results in creating regions of doped electrical activity, either p or n. The most commonly used dopants are boron for p-type; and phosphorus (P), arsenic (As) or antimony (Sb) for n-type (see table 5).

Typically, wafers are stacked in a quartz carrier or boat and placed in the diffusion furnace. The diffusion furnace contains a long quartz tube and a mechanism for accurate temperature control. Temperature control is extremely important, as the rates of diffusion of the various silicon dopants are primarily a function of temperature. The temperatures in use range from 900 to 1,300 oC, depending on the specific dopant and process.

The heating of the silicon wafer to a high temperature allows the impurity atoms to diffuse slowly through the crystal structure. Impurities move more slowly through silicon dioxide than through the silicon itself, enabling the thin oxide pattern to serve as a mask and thereby permitting the dopant to enter silicon only where it is unprotected. After enough impurities have accumulated, the wafers are removed from the furnace and diffusion effectively ceases.

For maximum control, most diffusions are performed in two steps—predeposition and drive in. The predeposit, or diffusion with constant source, is the first step and takes place in a furnace in which the temperature is selected to achieve the best control of impurity amounts. The temperature determines the solubility of the dopant. After a comparatively short predeposit treatment, the wafer is physically moved to a second furnace, usually at a higher temperature, where a second heat treatment drives in the dopant to the desired depth of diffusion in the silicon wafer lattice.

The dopant sources used in the predeposit step are in three distinct chemical states: gas, liquid and solid. Table 5 identifies the various types of diffusion source dopants and their physical states.

Gases are generally supplied from compressed gas cylinders with pressure controls or regulators, shut-off valves and various purging attachments and are dispensed through small-diameter metal tubing.

Liquids are dispensed normally from bubblers, which saturate a carrier gas stream, usually nitrogen, with the liquid dopant vapours, as is described in the section on wet oxidation. Another form of liquid dispensing is through the use of the spin-on dopant apparatus. This entails putting a solid dopant in solution with a liquid solvent carrier, then dripping the solution on the wafer and spinning, in a manner similar to the application of photoresists.

Solid sources may be in the shape of a boron nitride wafer, which is sandwiched between two silicon wafers to be doped and then placed in a diffusion furnace. Also, the solid dopants, in powder or bead form, may be placed in a quartz bomb enclosure (arsenic trioxide), manually dumped in the source end of a diffusion tube or loaded in a separate source furnace in line with the main diffusion furnace.

In the absence of proper controls, arsenic exposures above 0.01 mg/m3 were reported during the cleaning of a deposition furnace (Wade et al. 1981) and during the cleaning of source housing chambers for solid-source ion implanters (McCarthy 1985; Baldwin, King and Scarpace 1988). These exposures occurred when no precautions were taken to limit the amount of dust in the air. However, when residues were kept wet during cleaning, exposures were reduced to far below the airborne exposure limit.

In the older diffusion technologies safety hazards exist during the removal, cleaning and installation of furnace tubes. The hazards include potential cuts from broken quartz ware and acid burns during the manual cleaning. In newer technologies these hazards are lessened by in situ tube cleaning that eliminates much of the manual handling.

Diffusion furnace operators experience the highest routine cleanroom exposure to extremely low-frequency electromagnetic fields (e.g., 50 to 60 hertz) in semiconductor manufacturing. Average exposures greater than 0.5 microteslas (5 milligauss) were reported during actual operation of the furnaces (Crawford et al. 1993). This study also noted that cleanroom personnel working in the vicinity of diffusion furnaces had average measured exposures that were noticeably higher than those of other cleanroom workers. This finding was consistent with point measurements reported by Rosenthal and Abdollahzadeh (1991), who found that diffusion furnaces produced proximity readings (5 cm or 2 inches away) as high as 10 to 15 microteslas, with the surrounding fields falling off more gradually with distance than other cleanroom equipment studied; even at 6 feet away from diffusion furnaces, the reported flux densities were 1.2 to 2 microteslas (Crawford et al. 1993). These emission levels are well below current health-based exposure limits set by the World Health Organization and those set by individual countries.

Ion implantation

Ion implantation is the newer method of introducing impurities elements at room temperature into silicon wafers for junction formation. Ionized dopant atoms (i.e., atoms stripped of one or more of their electrons) are accelerated to a high energy by passing them through a potential difference of tens of thousands of volts. At the end of their path, they strike the wafer and are embedded at various depths, depending on their mass and energy. As in conventional diffusion, a patterned oxide layer or a photoresist pattern selectively masks the wafer from the ions.

A typical ion implantation system consists of an ion source (gaseous dopant source, usually in small lecture bottles), analysis equipment, accelerator, focusing lens, neutral beam trap, scanner process chamber and a vacuum system (normally three separate sets of in-line roughing and oil-diffusion pumps). The stream of electrons is generated from a hot filament by resistance, an arc discharge or cold cathode electron beam.

Generally, after wafers are implanted, a high temperature annealing step (900 to 1,000°C) is performed by a laser beam anneal or pulsed annealing with an electron-beam source. The annealing process helps repair the damage to the exterior surface of the implanted wafer caused by the bombardment of dopant ions.

With the advent of a safe delivery system for arsine, phosphine and boron trifluoride gas cylinders used in ion implanters, the potential for catastrophic release of these gases has been greatly reduced. These small gas cylinders are filled with a compound to which the arsine, phosphine and boron trifluoride are adsorbed. The gases are pulled out of the cylinders by use of a vacuum.

Ion implanters are one of the most significant electrical hazards in the semiconductor industry. Even after power is off, a significant shock potential exists within the tool and must be dissipated prior to working inside the implanter. A careful review of maintenance operations and the electrical hazards is warranted for all newly installed equipment, but especially for ion implanters.

Exposures to hydrides (probably a mixture of arsine and phosphine) as high as 60 ppb have been found during ion implanter cryo-pump maintenance (Baldwin, Rubin and Horowitz 1993). Also, high concentrations of both arsine and phosphine can off-gas from contaminated implanter parts that are removed during preventive maintenance (Flipp, Hunsaker and Herring 1992).

Portable vacuum cleaners with high-efficiency particulate attenuator (HEPA) filters are used to clean arsenic-contaminated work surfaces in ion implantation areas. Exposures above 1,000 μg/m3 were measured when HEPA vacuums were improperly cleaned. HEPA vacuums, when discharging to the workspace, can also efficiently distribute the distinctive, hydride-like odour associated with ion implanter beam line cleaning (Baldwin, Rubin and Horowitz 1993).

While a concern, there have been no published reports of significant dopant gas exposures during oil changes of vacuum pumps used with dopants—possibly because this is usually done as a closed system. The lack of reported exposure may also be a result of low levels of off-gassing of hydrides from the used oil.

The result of a field study where 700 ml of used roughing pump oil from an ion implanter which used both arsine and phosphine was heated only showed detectable concentrations of airborne hydrides in the pump head space when the pump oil exceeded 70oC (Baldwin, King and Scarpace 1988). Since normal operating temperatures for mechanical roughing pumps are 60 to 80oC, this study did not indicate the potential for a significant exposure.

During ion implantation, x rays are formed incidental to the operation. Most implanters are designed with sufficient cabinet shielding (which includes lead sheeting strategically placed around the ion source housing and adjacent access doors) to maintain employee exposure below 2.5 microsieverts (0.25 millirems) per hour (Maletskos and Hanley 1983). However, an older model of implanters was found to have x-ray leakage above 20 microsieverts per hour (μSv/hr) at the unit’s surface (Baldwin, King and Scarpace 1988). These levels were reduced to less than 2.5 μSv/hr after additional lead shielding was installed. Another older model of ion implanter was found to have x-ray leakage around an access door (up to 15 μSv/hr) and at a viewport (up to 3 μSv/hr). Additional lead shielding was added to attenuate possible exposures (Baldwin, Rubin and Horowitz 1993).

In addition to x-ray exposures from ion implanters, the possibility of neutron formation has been postulated if the implanter is operated above 8 million electron volts (MeV) or deuterium gas is used as an ion source (Rogers 1994). However, typically implanters are designed to operate at well below 8 MeV, and deuterium is not commonly used in the industry (Baldwin and Williams 1996).

Chemical vapour deposition

Chemical vapour deposition (CVD) involves the layering of additional material on the silicon wafer surface. CVD units normally operate as a closed system resulting in little or no chemical exposure to the operators. However, brief hydrogen chloride exposure above 5 ppm can occur when certain CVD prescrubbers are cleaned (Baldwin and Stewart 1989). Two broad categories of deposition are in common use—epitaxial and the more general category of non-epitaxial CVD.

Epitaxial chemical vapour deposition

Epitaxial growth is rigidly controlled deposition of a thin single crystal film of a material which maintains the same crystal structure as the existing substrate wafer layer. It serves as a matrix for fabricating semiconductor components in subsequent diffusion processes. Most epitaxial films are grown on substrates of the same material, such as silicon on silicon, in a process referred to as homoepitaxy. Growing layers of different materials on a substrate, such as silicon on sapphire, is called heteroepitaxy IC device processing.

Three primary techniques are used to grow epitaxial layers: vapour phase, liquid phase and molecular beam. Liquid-phase and molecular-beam epitaxy are primarily used in the processing of III-V (e.g., GaAs) devices. These are discussed in the article “III-V semiconductor manufacturing”.

Vapour-phase epitaxy is used to grow a film by the CVD of molecules at a temperature of 900 to 1,300oC. Vapours containing the silicon and controlled amounts of p- or n-type dopants in a carrier gas (usually hydrogen) are passed over heated wafers to deposit doped layers of silicon. The process is generally performed at atmospheric pressure.

Table 6 identifies the four major types of vapour-phase epitaxy, parameters and the chemical reactions taking place.

Table 6. Major categories of silicon vapour-phase epitaxy

Parameters

Pressure

Atmospheric

Temperature

900–1300 °C

Silicon sources

Silane (SiH4), silicon tetrachloride (SiCl4), trichlorosilane (SiHCl3),
and dichlorosilane (SiH2Cl2)

Dopant gases

Arsine (AsH3), phosphine (PH3), diborane (B2H6)

Dopant gas concentration

≈100 ppm

Etchant gas

Hydrogen chloride (HCl)

Etchant gas concentration

≈1–4%

Carrier gases

Hydrogen (H2), nitrogen (N2)

Heating source

Radio frequency (RF) or infrared (IR)

Vapour-phase epitaxy types

Chemical reactions

Hydrogen reduction of silicon tetrachloride
(1,150–1,300 °C)

SiCl4 + 2H2 → Si + 4HCl

Pyrolytic decomposition of silane
(1,000–1,100 °C)

SiH4 → Si + 2H2

Hydrogen reduction of trichlorosilane

SiHCl3 + H2 → Si + 3HCl

Reduction of dichlorosilane

SiH2Cl2 → Si + 2HCl

 

The deposition sequence normally followed in an epitaxial process involves:

  • substrate cleaning—physical scrubbing, solvent degreasing, acid cleaning (sulphuric, nitric and hydrochloric, and hydrofluoric is a common sequence) and drying operation
  • wafer loading
  • heat up—nitrogen purging and heating to approximately 500 °C, then hydrogen gas is used and RF generators inductively heat wafers
  • hydrogen chloride (HCl) etch—usually 1 to 4% concentration of HCl is dispensed to the reactor chamber
  • deposition—silicon source and dopant gases are metered in and deposited on wafer surface
  • cool down—hydrogen gas switched to nitrogen again at 500°C
  • unloading.

 

Non-epitaxial chemical vapour deposition

Whereas epitaxial growth is a highly specific form of CVD where the deposited layer has the same crystalline structure orientation as the substrate layer, non-epitaxial CVD is the formation of a stable compound on a heated substrate by the thermal reaction or decomposition of gaseous compounds.

CVD can be used to deposit many materials, but in silicon semiconductor processing the materials generally encountered, in addition to epitaxial silicon, are:

  • polycrystalline silicon (poly Si)
  • silicon dioxide (SiO2—both doped and undoped; p-doped glass)
  • silicon nitride (Si3N4).

 

Each of these materials may be deposited in a variety of ways, and each has many applications.

Table 7 identifies the three major categories of CVD using operating temperature as a mechanism of differentiation.

Table 7. Major categories of silicon chemical vapour deposition (CVD)

Parameters

Pressure

Atmospheric (APCVD) or low pressure (LPCVD)

Temperature

500–1,100 °C

Silicon and nitride sources

Silane (SiH4), silicon tetrachloride (SiCl4), ammonia (NH3), nitrous oxide (N20)

Dopant sources

Arsine (AsH3), phosphine (PH3), diborane (B2H6)

Carrier gases

Nitrogen (N2), hydrogen (H2)

Heating source

Cold wall system—radio frequency (RF) or infrared (IR)
Hot wall system—thermal resistance

CVD type

Reaction

Carrier gas

Temperature

Medium temperature (≈ 600–1,100 °C)

Silicon nitride (Si3N4)

3SiH4 + 4 NH3 → Si3N4 + 12H2

H2

900–1,100 °C

Polysilicon (poly Si)

SiH4 + Heat → Si + 2H2

H2
N2

850–1,000 °C
600–700 °C

Silicon dioxide (SiO2)

SiH4 + 4CO2 → SiO2 + 4CO + 2H2O
2H2 + SiCl4 + CO2 → SiO2 + 4HCl *
SiH4 + CO→ SiO2 + 2H2 *

N2
H2
H2

500–900 °C
800–1,000 °C
600–900 °C

Low temperature (≈<600 C) Silox, Pyrox, Vapox and Nitrox**

Silicon dioxide (SiO2) or p-doped SiO2

     

Silox

SiH4 + 2O2 + Dopant → SiO2 + 2H2O

N2

200–500 °C

Pyrox

SiH4 + 2O2 + Dopant → SiO2 + 2H2O

N2

<600 °C

Vapox

SiH4 + 2O2 + Dopant → SiO2 + 2H2O

N2

<600 °C

Silicon nitride (Si3N4)

     

Nitrox

3SiH4 + 4NH3 (or N2O*) → Si3N4 + 12H2

N2

600–700 °C

Low temperature plasma enhanced (passivation) (<600°C)

Utilizing radio-frequency (RF) or
reactive sputtering

     

Silicon dioxide (SiO2)

SiH4 + 2O2 → SiO2 + 2H20

   

Silicon nitride (Si3N4)

3SiH4 + 4NH3 (or N2O*) → Si3N4 + 12H2

   

* Note: Reactions are not stoichiometrically balanced.

**Generic, proprietary or trademark names for CVD reactor systems

 

The following components are found in nearly all the types of CVD equipment:

  • reaction chamber
  • gas control section
  • time and sequence control
  • heat source for substrates
  • effluent handling.

 

Basically, the CVD process entails supplying controlled amounts of silicon or nitride source gases, in conjunction with nitrogen and/or hydrogen carrier gases, and a dopant gas if desired, for chemical reaction within the reactor chamber. Heat is applied to provide the necessary energy for the chemical reaction in addition to controlling the surface temperatures of the reactor and wafers. After the reaction is complete, the unreacted source gas plus the carrier gas are exhausted through the effluent handling system and vented to the atmosphere.

Passivation is a functional type of CVD. It involves the growth of a protective oxide layer on the surface of the silicon wafer, generally as the last fabrication step prior to non-fabrication processing. The layer provides electrical stability by isolating the integrated circuit’s surface from electrical and chemical conditions in the environment.

Metallization

After the devices have been fabricated in the silicon substrate, they must be connected together to perform circuit functions. This process is known as metallization. Metallization provides a means of wiring or interconnecting the uppermost layers of integrated circuits by depositing complex patterns of conductive materials, which route electrical energy within the circuits.

The broad process of metallization is differentiated according to the size and thickness of the layers of metals and other materials being deposited. These are:

  • thin film—approximate film thickness of one micron or less
  • thick film—approximate film thickness of 10 microns or greater
  • plating—film thicknesses are variable from thin to thick, but generally thick films.

 

The most common metals used for silicon semiconductor metallization are: aluminium, nickel, chromium or an alloy called nichrome, gold, germanium, copper, silver, titanium, tungsten, platinum and tantalum.

Thin or thick films may also be evaporated or deposited on various ceramic or glass substrates. Some examples of these substrates are: alumina (96% Al203), beryllia (99% BeO), borosilicate glass, pyroceram and quartz (SiO2).

Thin film

Thin film metallization is often applied through the use of a high-vacuum or partial-vacuum deposition or evaporation technique. The major types of high-vacuum evaporation are electron beam, flash and resistive, while partial-vacuum deposition is primarily done by sputtering.

To perform any type of thin film vacuum metallization, a system usually consists of the following basic components:

  • a chamber that can be evacuated to provide a sufficient vacuum for deposition
  • a vacuum pump (or pumps) to reduce ambient gases in the chamber
  • instrumentation for monitoring the vacuum level and other parameters
  • a method of depositing or evaporating the layers of metallizing material.

 

Electron-beam evaporation, frequently called E beam, uses a focused beamof electrons to heat the metallization material. A high-intensity beam of electrons is generated in a manner similar to that used in a television picture tube. A stream of electrons is accelerated through an electrical field of typically 5 to 10 kV and focused on the material to be evaporated. The focused beam of electrons melts the material contained in a water-cooled block with a large depression called a hearth. The melted material then vaporizes within the vacuum chamber and condenses on the cool wafers as well as on the entire chamber surface. Then standard photoresist, exposure, development and wet or dry etch operations are performed to delineate the intricate metallized circuitry.

Flash evaporation is another technique for the deposition of thin metallized films. This method is primarily used when a mixture of two materials (alloys) are to be simultaneously evaporated. Some examples of two component films are: nickel/chromium (Nichrome), chromium/silicon monoxide (SiO) and aluminium/silicon.

In flash evaporation, a ceramic bar is heated by thermal resistance and a continuously fed spool of wire, stream of pellets or vibrationally dispensed powder is brought in contact with the hot filament or bar. The vaporized metals then coat the interior chamber and wafer surfaces.

Resistive evaporation (also known as filament evaporation) is the simplest and least expensive form of deposition. The evaporation is accomplished by gradually increasing the current flowing through the filament to first melt the loops of material to be evaporated, thereby wetting the filament. Once the filament is wetted, the current through the filament is increased until evaporation occurs. The primary advantage of resistive evaporation is the wide variety of materials that can be evaporated.

Maintenance work is sometimes done on the inside surface of E-beam evaporator deposition chambers called bell jars. When the maintenance technicians have their heads inside the bell jars, significant exposures can occur. Removing the metal residues that deposit on the inside surface of bell jars may result in such exposures. For example, technician exposures far above the airborne exposure limit for silver were measured during residue removal from an evaporator used to deposit silver (Baldwin and Stewart 1989).

Cleaning bell jar residues with organic cleaning solvents can also result in high solvent exposure. Technician exposures to methanol above 250 ppm have occurred during this type of cleaning. This exposure can be eliminated by using water as the cleaning solvent instead of methanol (Baldwin and Stewart 1989).

The sputtering deposition process takes place in a low-pressure or partial-vacuum gas atmosphere, using either direct electric current (DC, or cathode sputtering) or RF voltages as a high-energy source. In sputtering, ions of argon inert gas are introduced into a vacuum chamber after a satisfactory vacuum level has been reached through the use of a roughing pump. An electric field is formed by applying a high voltage, typically 5,000 V, between two oppositely charged plates. This high-energy discharge ionizes the argon gas atoms and causes them to move and accelerate to one of the plates in the chamber called the target. When the argon ions strike the target made of the material to be deposited, they dislodge, or sputter, these atoms or molecules. The dislodged atoms of the metallization material are then deposited in a thin film on the silicon substrates which face the target.

RF leakage from the sides and backs on many older sputter units was found to exceed the occupational exposure limit (Baldwin and Stewart 1989). Most of the leakage was attributable to cracks in the cabinets caused by repeated removal of the maintenance panels. In newer models by the same manufacturer, panels with wire mesh along the seams prevent significant leakage. The older sputterers can be retrofitted with wire mesh or, alternatively, copper tape can be used to cover the seams to reduce the leakage.

Thick film

The structure and dimension of most thick films are not compatible with the metallization of silicon integrated circuits, primarily due to size constraints. Thick films are used mostly for metallization of hybrid electronic structures, such as in the manufacture of LCDs.

The silk-screening process is the dominant method of thick film application. Thick film materials typically used are palladium, silver, titanium dioxide and glass, gold-platinum and glass, gold-glass and silver-glass.

Resistive thick films are normally deposited and patterned on a ceramic substrate using silk-screening techniques. Cermet is a form of resistive thick film composed of a suspension of conductive metal particles in a ceramic matrix with an organic resin as filler. Typical cermet structures are composed of chromium, silver or lead oxide in a silicon monoxide or dioxide matrix.

Plating

Two basic types of plating techniques are used in forming metallic films on semiconductor substrates: electroplating and electroless plating.

In electroplating, the substrate to be plated is placed at the cathode, or negatively charged terminal, of the plating tank and immersed in an electrolytic solution. An electrode made of the metal to be plated serves as the anode, or positively charged terminal. When a direct current is passed through the solution, the positively charged metal ions, which dissolve into the solution from the anode, migrate and plate on the cathode (substrate). This method of plating is used for forming conductive films of gold or copper.

In electroless plating, the simultaneous reduction and oxidation of the metal to be plated is used in forming a free metal atom or molecule. Since this method does not require electrical conduction during the plating process, it can be used with insulating-type substrates. Nickel, copper and gold are the most common metals deposited in this manner.

Alloying/annealing

After the metallized interconnections have been deposited and etched, a final step of alloying and annealing may be performed. The alloying consists of placing the metallized substrates, usually with aluminium, in a low-temperature diffusion furnace to assure a low-resistance contact between the aluminium metal and silicon substrate. Finally, either during the alloy step or directly following it, the wafers are often exposed to a gas mixture containing hydrogen in a diffusion furnace at 400 to 500°C. The annealing step is designed to optimize and stabilize the characteristics of the device by combining the hydrogen with uncommitted atoms at or near the silicon-silicon dioxide interface.

Backlapping and backside metallization

There is also an optional metallization processing step called backlapping. The backside of the wafer may be lapped or ground down using a wet abrasive solution and pressure. A metal such as gold may be deposited on the back side of the wafer by sputtering. This makes attachment of the separated die to the package easier in the final assembly.

Assembly and testing

Non-fabrication processing, which includes external packaging, attachments, encapsulation, assembly and testing, is normally performed in separate production facilities and many times is done in Southeast Asian countries, where these labour-intensive jobs are less expensive to perform. In addition, ventilation requirements for process and particulate control are generally different (non-cleanroom) in the non-fabrication processing areas. These final steps in the manufacturing process involve operations that include soldering, degreasing, testing with chemicals and radiation sources, and trimming and marking with lasers.

Soldering during semiconductor manufacturing normally does not result in high lead exposures. To prevent thermal damage to the integrated circuit, the solder temperature is kept below the temperature where significant molten lead fume formation can occur (430°C). However, cleaning solder equipment by scraping or brushing of the lead-containing residues can result in lead exposures above 50 μg/m3 (Baldwin and Stewart 1989). Also, lead exposures of 200 μg/m3 have occurred when improper dross removal techniques are used during wave solder operations (Baldwin and Williams 1996).

One growing concern with solder operations is respiratory irritation and asthma due to exposure to the pyrolysis products of the solder fluxes, particularly during hand soldering or touch-up operations, where historically local exhaust ventilation has not been commonly used (unlike wave solder operations, which for the last few decades have typically been enclosed in exhausted cabinets) (Goh and Ng 1987). See the article “Printed circuit board and computer assembly” for more details.

Since colophony in the solder flux is a sensitizer, all exposures should be reduced to as low as possible, regardless of air sampling results. New soldering installations particularly should include local exhaust ventilation when soldering is to be performed for extended periods of time (e.g., greater than 2 hours).

Fumes from hand soldering will rise vertically on thermal currents, entering the employee’s breathing zone as the person leans over the point of soldering. Control usually is achieved by means of effective high velocity and low volume local exhaust ventilation at the solder tip.

Devices that return filtered air to the workplace may, if the filtration efficiency is inadequate, cause secondary pollution which can affect people in the workroom other than those soldering. Filtered air should not be returned to the workroom unless the amount of soldering is small and the room has good general dilution ventilation.

Wafer sort and test

After wafer fabrication is completed, each intrinsically finished wafer undergoes a wafer sort process where integrated circuitry on each specific die is electrically tested with computer-controlled probes. An individual wafer may contain from one hundred to many hundreds of separate dies or chips which must be tested. After the test results are finished, the dies are physically marked with an automatically dispensed one-component epoxy resin. Red and blue are used to identify and sort dies which do not meet the desired electrical specifications.

Die separation

With the devices or circuits on the wafer tested, marked and sorted, the individual dies on the wafer must be physically separated. A number of methods have been designed for separating the individual dies—diamond scribing, laser scribing and diamond wheel sawing.

Diamond scribing is the oldest method in use and involves drawing a precisely shaped diamond-imbedded tip across the wafer along the scribe line or “street” separating the individual dies on the wafer surface. The imperfection in the crystal structure caused by scribing allows the wafer to be bent and fractured along this line.

Laser scribing is a relatively recent die separation technique. A laser beam is generated by a pulsed, high-powered neodymium-yttrium laser. The beam generates a groove in the silicon wafer along the scribe lines. The groove serves as the line along which the wafer breaks.

A widely used method of die separation is wet sawing—cutting substrates along the street with a high-speed circular diamond saw. Sawing can either partially cut (scribe) or completely cut (dice) through the silicon substrate. A wet slurry of material removed from the street is generated by sawing.

Die attach and bonding

The individual die or chip must be attached to a carrier package and metal lead-frame. Carriers are typically made of an insulating material, either ceramic or plastic. Ceramic carrier materials are usually made of alumina (Al2O3), but can possibly consist of beryllia (BeO) or steatite (MgO-SiO2). Plastic carrier materials are either of the thermoplastic or thermosetting resin type.

The attachment of the individual die is generally accomplished by one of three distinct types of attachment: eutectic, preform and epoxy. Eutectic die attachment involves using an eutectic brazing alloy, such as gold-silicon. In this method, a layer of gold metal is predeposited on the backside of the die. By heating the package above the eutectic temperature (370°C for gold-silicon) and placing the die on it, a bond is formed between the die and package.

Preform bonding involves the use of a small piece of special composition material that will adhere to both the die and the package. A preform is placed on the die-attach area of a package and allowed to melt. The die is then scrubbed across the region until the die is attached, and then the package is cooled.

Epoxy bonding involves the use of an epoxy glue to attach the die to the package. A drop of epoxy is dispensed on the package and the die placed on top of it. The package may need to be baked at an elevated temperature to cure the epoxy properly.

Once the die is physically attached to the package, electrical connections must be provided between the integrated circuit and package leads. This is accomplished by using either thermocompression, ultrasonic or thermosonic bonding techniques to attach gold or aluminium wires between the contact areas on the silicon chip and the package leads.

Thermocompression bonding is often used with gold wire and involves heating the package to approximately 300oC and forming the bond between the wire and bonding pads using both heat and pressure. Two major types of thermocompression bonding are in use—ball bonding and wedge bonding. Ball bonding, which is used only with gold wire, feeds the wire through a capillary tube, compresses it, and then a hydrogen flame melts the wire. In addition, this forms a new ball on the end of the wire for the next bonding cycle. Wedge bonding involves a wedge-shaped bonding tool and a microscope used for positioning the silicon chip and package accurately over the bonding pad. The process is performed in an inert atmosphere.

Ultrasonic bonding uses a pulse of ultrasonic, high-frequency energy to provide a scrubbing action that forms a bond between the wire and the bonding pad. Ultrasonic bonding is primarily used with aluminium wire and is often preferred to thermocompression bonding, since it does not require the circuit chip to be heated during the bonding operation.

Thermosonic bonding is a recent technological change in gold wire bonding. It involves the use of a combination of ultrasonic and heat energies and requires less heat than thermocompression bonding.

Encapsulation

The primary purpose of encapsulation is to put an integrated circuit into a package which meets the electrical, thermal, chemical and physical requirements associated with the application of the integrated circuit.

The most widely used package types are the radial-lead type, the flat pack and the dual-in-line (DIP) package. The radial-lead type of packages are mostly made of Kovar, an alloy of iron, nickel and cobalt, with hard glass seals and Kovar leads. Flat packs use metal-lead frames, usually made of an aluminium alloy combined with ceramic, glass and metal components. Dual-in-line packages are generally the most common and often use ceramic or moulded plastics.

Moulded plastic semiconductor packages are primarily produced by two separate processes—transfer moulding and injection moulding. Transfer moulding is the predominant plastic encapsulation method. In this method, the chips are mounted on untrimmed lead frames and then batch loaded into moulds. Powdered or pellet forms of thermosetting plastic moulding compounds are melted in a heated pot and then forced (transferred) under pressure into the loaded moulds. The powdered or pellet form plastic moulding compound systems can be used on epoxy, silicone or silicone/epoxy resins. The system usually consists of a mixture of:

  • thermosetting resins—epoxy, silicone or silicone/epoxy
  • hardeners—epoxy novolacs and epoxy anhydrides
  • fillers—silica-fused or crystalline silicon dioxide (SiO2) and alumina (Al2O3), generally 50-70% by weight
  • fire retardant—antimony trioxide (Sb2O3) generally 1-5% by weight.

 

Injection moulding uses either a thermoplastic or thermosetting moulding compound which is heated to its melting point in a cylinder at a controlled temperature and forced under pressure through a nozzle into the mould. The resin solidifies rapidly, the mould is opened and the encapsulation package ejected. A wide variety of plastic compounds are used in injection moulding, with epoxy and polyphenylene sulphide (PPS) resins being the newest entries in semiconductor encapsulating.

The final packaging of the silicon semiconductor device is classified according to its resistance to leakage or ability to isolate the integrated circuit from its environment. These are differentiated as being hermetically (airtight) or non-hermetically sealed.

Leak testing and burn in

Leak testing is a procedure developed to test the actual sealing ability or hermetism of the packaged device. Two common forms of leak testing are in use: helium leak detection and radioactive tracer leak detection.

In helium leak detection, the completed packages are placed in an atmosphere of helium pressure for a period of time. Helium is able to penetrate through imperfections into the package. After removal from the helium pressurization chamber, the package is transferred to a mass-spectrometer chamber and tested for helium leaking out of imperfections in the package.

Radioactive tracer gas, usually krypton-85 (Kr-85), is substituted for helium in the second method, and the radioactive gas leaking out of the package is measured. Under normal conditions, personnel exposure from this process is less than 5 millisieverts (500 millirems) per year (Baldwin and Stewart 1989). Controls for these systems usually include:

  • isolation in rooms with access limited only to necessary personnel
  • posted radiation warning signs on the doors to the rooms containing Kr-85
  • continuous radiation monitors with alarms and auto shutdown/isolation
  • dedicated exhaust system and negative pressure room
  • monitoring exposures with personal dosimetry (e.g., radiation film badges)
  • regular maintenance of alarms and interlocks
  • regular checks for radioactive material leakage
  • safety training for operators and technicians
  • ensuring radiation exposures are kept as low as reasonably achievable (ALARA).

 

Also, materials that come in contact with Kr-85 (e.g., exposed ICs, used pump oil, valves and O-rings) are surveyed to ensure they do not emit excessive levels of radiation because of residual gas in them before they are removed from the controlled area. Leach-Marshal (1991) provides detailed information on exposures and controls from Kr-85 fine-leak detection systems.

Burn in is a temperature and electrical stressing operation to determine the reliability of the final packaged device. Devices are placed in a temperature-controlled oven for an extended period of time using either ambient atmosphere or an inert atmosphere of nitrogen. Temperatures range from 125°C to 200°C (150°C is an average), and time periods from a few hours to 1,000 hours (48 hours is an average).

Final test

For a final characterization of the packaged silicon semiconductor device’s performance, a final electrical test is performed. Because of the large number and the complexity of the tests required, a computer performs and evaluates the testing of numerous parameters important to the eventual functioning of the device.

Mark and pack

Physical identification of the final packaged device is accomplished by the use of a variety of marking systems. The two major categories of component marking are contact and non-contact printing. Contact printing typically incorporates a rotary offset technique using solvent-based inks. Non-contact printing, which transfers markings without physical contact, involves ink-jet head or toner printing using solvent-based inks or laser marking.

The solvents used as a carrier for the printing inks and as a pre-cleaner are typically composed of a mixture of alcohols (ethanol) and esters (ethyl acetate). Most of the component marking systems, other than laser marking, use inks which require an additional step for setting, or curing. These curing methods are air curing, heat curing (thermal or infrared) and ultraviolet curing. Ultraviolet-curing inks contain no solvents.

Laser marking systems utilize either a high-powered carbon dioxide (CO2) laser, or a high-powered neodymium:yttrium laser. These lasers are typically embedded in the equipment and have interlocked cabinets that enclose the beam path and the point where the beam contacts the target. This eliminates the laser beam hazard during normal operations, but there is a concern when the safety interlocks are defeated. The most common operation where it is necessary to remove the beam enclosures and defeat the interlocks is alignment of the laser beam.

During these maintenance operations, ideally the room containing the laser should be evacuated, except for necessary maintenance technicians, with the doors to the room locked and posted with appropriate laser safety signs. However, high-powered lasers used in semiconductor manufacturing are often located in large, open manufacturing areas, making it impractical to relocate non-maintenance personnel during maintenance. For these situations, a temporary control area is typically established. Normally these control areas consist of laser curtains or welding screens capable of withstanding direct contact with the laser beam. Entrance to the temporary control area is usually through a maze entry that is posted with a warning sign whenever the interlocks for the laser are defeated. Other safety precautions during beam alignment are similar to those required for the operation of an open-beamed high-powered laser (e.g., training, eye protection, written procedures and so on).

High-powered lasers are also one of the most significant electrical hazards in the semiconductor industry. Even after power is off, a significant shock potential exists within the tool and must be dissipated prior to working inside the cabinet.

Along with the beam hazard and electrical hazard, care should also be taken in performing maintenance on laser marking systems because of the potential for chemical contamination from the fire retardant antimony trioxide and beryllium (ceramic packages containing this compound will be labelled). Fumes can be created during the marking with the high-powered lasers and create residues on the equipment surfaces and fume extraction filters.

Degreasers have been used in the past to clean semiconductors before they are marked with identification codes. Solvent exposure above the applicable occupational airborne exposure limit can easily occur if an operator’s head is placed below the cooling coils that cause the vapours to recondense, as can happen when an operator attempts to retrieve dropped parts or when a technician cleans residue from the bottom of the unit (Baldwin and Stewart 1989). The use of degreasers has been greatly reduced in the semiconductor industry due to restrictions on the use of ozone-depleting substances such as chlorofluorocarbons and chlorinated solvents.

Failure analysis and quality assurance

Failure analysis and quality analysis laboratories typically perform various operations used to ensure the reliability of the devices. Some of the operations performed in these laboratories present the potential for employee exposure. These include:

  • marking tests utilizing various solvent and corrosive mixtures in heated beakers on hotplates. Local exhaust ventilation (LEV) in the form of a metal hood with adequate face velocities is needed to control fugitive emissions. Monoethanolamine solutions can result in exposures in excess of its airborne exposure limit (Baldwin and Williams 1996).
  • bubble/leak testing utilizing high molecular weight fluorocarbons (tradename Fluorinerts)
  • x-ray packaging units.

 

Cobalt-60 (up to 26,000 curies) is used in irradiators for testing the ability of ICs to withstand exposure to gamma radiation in military and space applications. Under normal conditions, personnel exposures from this operation are less than 5 millisieverts (500 millirems) per year (Baldwin and Stewart 1989). Controls for this somewhat specialized operation are similar to those utilized for Kr-85 fine-leak systems (e.g., isolated room, continuous radiation monitors, personnel exposure monitoring and so on).

Small “specific licence” alpha sources (e.g., micro- and millicuries of Americium-241) are used in the failure analysis process. These sources are covered by a thin protective coating called a window that allows alpha particles to be emitted from the source to test the integrated circuit’s ability to operate when bombarded by alpha particles. Typically the sources are periodically checked (e.g., semi-annually) for leakage of radioactive material that can occur if the protective window is damaged. Any detectable leakage usually triggers removal of the source and its shipment back to the manufacturer.

Cabinet x-ray systems are used to check the thickness of metal coatings and to identify defects (e.g., air bubbles in mould compound packages). While not a significant source of leakage, these units are typically checked on a periodic basis (e.g., annually) with a hand-held survey meter for x-ray leakage and inspected to ensure that door interlocks operate properly.

Shipping

Shipping is the endpoint of most silicon semiconductor device manufacturers’ involvement. Merchant semiconductor manufacturers sell their product to other end-product producers, while captive manufacturers use the devices for their own end products.

Health Study

Each process step uses a particular set of chemistries and tools that result in specific EHS concerns. In addition to concerns associated with specific process steps in silicon semiconductor device processing, an epidemiological study investigated health effects among employees of the semiconductor industry (Schenker et al. 1992). See also the discussion in the article “Health effects and disease patterns”.

The main conclusion of the study was that work in semiconductor fabrication facilities is associated with an increased rate of spontaneous abortion (SAB). In the historical component of the study, the number of pregnancies studied in fabrication and nonfabrication employees were approximately equal (447 and 444 respectively), but there were more spontaneous abortions in fabrication (n=67) than non-fabrication (n=46). When adjusted for various factors that could cause bias (age, ethnicity, smoking, stress, socio-economic status and pregnancy history) the relative risk (RR) for fabrication verses non-fabrication was 1.43 (95% confidence interval=0.95-2.09).

The researchers linked the increased SAB rate with exposure to certain ethylene-based glycol ethers (EGE) used in semiconductor manufacturing. The specific glycol ethers that were involved in the study and are suspected of causing adverse reproductive effects are:

  • 2-methoxyethanol (CAS 109-86-4)
  • 2-methoxyethyl acetate (CAS 110-49-6)
  • 2-ethoxyethyl acetate (CAS 111-15-9).

 

While not part of the study, two other glycol ethers used in the industry, 2-ethoxyethanol (CAS 110-80-5) and diethylene glycol dimethyl ether (CAS 111-96-6) have similar toxic effects and have been banned by some semiconductor manufacturers.

In addition to an increased SAB rate associated with exposure to certain glycol ethers, the study also concluded:

  • An inconsistent association existed for fluoride exposure (in etching) and SAB.
  • Self-reported stress was a strong independent risk factor for SAB among women working in the fabrication areas.
  • It took longer for women working in the fabrication area to get pregnant compared to women in non-fabrication areas.
  • An increase in respiratory symptoms (eye, nose and throat irritation and wheezing) was present for fabrication workers compared to non-fabrication workers.
  • Musculoskeletal symptoms of the distal upper extremity, such as hand, wrist, elbow and forearm pain, were associated with fabrication room work.
  • Dermatitis and hair loss (alopecia) were reported more frequently among fabrication workers than non-fabrication workers.

 

Equipment Review

The complexity of semiconductor manufacturing equipment, coupled with continuous advancements in the manufacturing processes, makes the pre-installation review of new process equipment important for minimizing EHS risks. Two equipment review processes help ensure that new semiconductor process equipment will have appropriate EHS controls: CE marking and Semiconductor Equipment and Materials International (SEMI) standards.

CE marking is a manufacturer’s declaration that the equipment so marked conforms to the requirements of all applicable Directives of the European Union (EU). For semiconductor manufacturing equipment, the Machinery Directive (MD), Electromagnetic Compatibility (EMC) Directive and Low Voltage Directive (LVD) are considered those directives most applicable.

In the case of the EMC Directive, the services of a competent body (organization officially authorized by an EU member state) need to be retained to define testing requirements and approve findings of the examination. The MD and LVD may be assessed by either the manufacturer or a notified body (organization officially authorized by an EU member state). Regardless of the path chosen (self assessment or third party) it is the importer of record who is responsible for the imported product being CE marked. They may use the third party or self assessment information as the basis for their belief that the equipment meets the requirements for the applicable directives, but, ultimately, they will prepare the declaration of conformity and affix the CE marking themselves.

Semiconductor Equipment and Materials International is an international trade association that represents semiconductor and flat panel display equipment and materials suppliers. Among its activities is the development of voluntary technical standards that are agreements between suppliers and customers aimed at improving product quality and reliability at a reasonable price and steady supply.

Two SEMI standards that specifically apply to EHS concerns for new equipment are SEMI S2 and SEMI S8. SEMI S2-93, Safety Guidelines for Semiconductor Manufacturing Equipment, is intended as a minimum set of performance-based EHS considerations for equipment used in semiconductor manufacturing. SEMI S8-95, Supplier Ergonomic Success Criteria User’s Guide, expands on the ergonomics section in SEMI S2.

Many semiconductor manufacturers require that new equipment be certified by a third party as meeting the requirements of SEMI S2. Guidelines for interpreting SEMI S2-93 and SEMI S8-95 are contained in a publication by the industry consortium SEMATECH (SEMATECH 1996). Additional information on SEMI is available on the worldwide web (http://www.semi.org).

Chemical Handling

Liquid dispensing

With automated chemical-dispensing systems becoming the rule, not the exception, the number of chemical burns to employees has decreased. However, proper safeguards need to be installed in these automated chemical-dispensing systems. These include:

  • leak detection and automatic shut-off at the bulk supply source and at junction boxes
  • double containment of lines if the chemical is considered a hazardous material
  • high-level sensors at endpoints (bath or tool vessel)
  • timed pump shut-off (allows only a specific quantity to be pumped to a location before it automatically shuts off).

Gas dispensing

Gas distribution safety has improved significantly over the years with the advent of new types of cylinder valves, restricted flow orifices incorporated into the cylinder, automated gas purge panels, high flow rate detection and shut-off and more sophisticated leak detection equipment. Because of its pyrophoric property and its wide use as a feed stock, silane gas represents the most significant explosion hazard within the industry. However, silane gas incidents have become more predictable with new research conducted by Factory Mutual and SEMATECH. With proper reduced-flow orifices (RFOs), delivery pressures and ventilation rates, most explosive incidents have been eliminated (SEMATECH 1995).

Several safety incidents have occurred in recent years due to an uncontrolled mixing of incompatible gases. Because of these incidents, semiconductor manufacturers often review gas line installations and tool gas boxes to ensure that improper mixing and/or back flow of gases cannot occur.

Chemical issues typically generate the greatest concerns in semiconductor manufacturing. However, most injuries and deaths within the industry result from non-chemical hazards.

Electrical Safety

There are numerous electrical hazards associated with equipment used in this industry. Safety interlocks play an important role in electrical safety, but these interlocks are often overridden by maintenance technicians. A significant amount of maintenance work is typically performed while equipment is still energized or only partially de-energized. The most significant electrical hazards are associated with ion implanters and laser power supplies. Even after power is off, a significant shock potential exists within the tool and must be dissipated prior to working inside the tool. The SEMI S2 review process in the United States and the CE mark in Europe have helped improve electrical safety for new equipment, but maintenance operations are not always adequately considered. A careful review of maintenance operations and the electrical hazards is needed for all newly installed equipment.

Second on the electrical hazard list is the set of equipment that generates RF energy during etching, sputtering and chamber cleaning processes. Proper shielding and grounding are needed to minimize the risk of RF burns.

These electrical hazards and the many tools not being powered down during maintenance operations require the maintenance technicians to employ other means to protect themselves, such as lockout/tagout procedures. Electrical hazards are not the only energy sources which are addressed with lockout/tagout. Other energy sources include pressurized lines, many containing hazardous gas or liquids, and pneumatic controls. Disconnections for controlling these energy sources need to be in a readily available location—within the fab (fabrication) or chase area where the employee will be working, rather than in inconvenient locations such as subfabs.

Ergonomics

The interface between the employee and the tool continues to cause injuries. Muscle strain and sprains are fairly common within the semiconductor industry, especially with the maintenance technician. The access to pumps, chamber covers and so on often is not well designed during manufacturing of the tool and during the placement of the tool in the fab. Pumps should be on wheels or placed in pull-out drawers or trays. Lifting devices need to be incorporated for many operations.

Simple wafer handling causes ergonomic hazards, especially in older facilities. Newer facilities typically have larger wafers and thus require more automated handling systems. Many of these wafer-handling systems are considered robotic devices, and the safety concerns with these systems must be accounted for when they are designed and installed (ANSI 1986).

Fire Safety

In addition to silane gas, which has already been addressed, hydrogen gas has the potential for being a significant fire hazard. However, it is better understood and the industry has not seen many major issues associated with hydrogen.

The most serious fire hazard now is associated with wet decks or etching baths. The typical plastic materials of construction (polyvinyl chloride, polypropylene and flame-resistant polypropylene) all have been involved in fab fires. The ignition source may be an etch or plating bath heater, the electrical controls mounted directly to the plastic or an adjacent tool. If a fire occurs with one of these plastic tools, particle contamination and corrosive combustion products spread throughout the fab. The economic loss is high due to the down time in the fab while the area and equipment are brought back to cleanroom standards. Often some expensive equipment cannot be adequately decontaminated, and new equipment must be purchased. Therefore, adequate fire prevention and fire protection are both critical.

Fire prevention can be addressed with different non-combustible building materials. Stainless steel is the preferred material of construction for these wet decks, but often the process will not “accept” a metal tool. Plastics with less fire/smoke potential exist, but have not yet been adequately tested to determine if they will be compatible with semiconductor manufacturing processes.

For fire protection, these tools must be protected by unobstructed sprinkler protection. The placement of HEPA filters above wet benches often blocks sprinkler heads. If this occurs, additional sprinkler heads are installed below the filters. Many companies also require that a fire detection and suppression system be installed inside the plenum cavities on these tools, where many fires start.

 

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Saturday, 02 April 2011 18:39

Liquid Crystal Displays

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Liquid crystal displays (LCDs) have been commercially available since the 1970s. They are commonly used in watches, calculators, radios and other products requiring indicators and three or four alphanumeric characters. Recent improvements in the liquid crystal materials allow large displays to be manufactured. While LCDs are only a small portion of the semiconductor industry, their importance has grown with their use in flat-panel displays for portable computers, very light laptop computers and dedicated word processors. The importance of LCDs is expected to continue to grow as they eventually replace the last vacuum tube commonly used in electronics—the cathode ray tube (CRT) (O’Mara 1993).

The manufacture of LCDs is a very specialized process. Industrial hygiene monitoring results indicate very low airborne contaminant levels for the various solvent exposures monitored (Wade et al. 1981). In general, the types and quantities of toxic, corrosive and flammable solid, liquid and gaseous chemicals and hazardous physical agents in use are limited in comparison with other types of semiconductor manufacturing.

Liquid crystal materials are rod-like molecules exemplified by the cyanobiphenyl molecules shown in figure 1. These molecules possess the property of rotating the direction of polarized light passing through. Although the molecules are transparent to visible light, a container of the liquid material appears milky or translucent instead of transparent. This occurs because the long axis of the molecules are aligned at random angles, so the light is scattered randomly. A liquid crystal display cell is arranged so that the molecules follow a specific alignment. This alignment can be changed with an external electric field, allowing the polarization of incoming light to be changed.

Figure 1. Basic liquid crystal polymer molecules

MIC030F1

In the manufacture of flat panel displays, two glass substrates are processed separately, then joined together. The front substrate is patterned to create a colour filter array. The rear glass substrate is patterned to form thin film transistors and the metal interconnect lines. These two plates are mated in the assembly process and, if necessary, sliced and separated into individual displays. Liquid crystal material is injected into a gap between the two glass plates. The displays are inspected and tested and a polarizer film is applied to each glass plate.

Numerous individual processes are required to manufacture flat panel displays. They require specialized equipment, materials and processes. Certain key processes are outlined below.

Glass Substrate Preparation

The glass substrate is an essential and expensive component of the display. Very tight control of the optical and mechanical properties of the material is required at every stage of the process, especially when heating is involved.

Glass fabrication

Two processes are used to make very thin glass with very precise dimensions and reproducible mechanical properties. The fusion process, developed by Corning, utilizes a glass feed rod that melts in a wedge-shaped trough and flows up and over the sides of the trough. Flowing down both sides of the trough, the molten glass joins into a single sheet at the bottom of the trough and can be drawn downward as a uniform sheet. The thickness of the sheet is controlled by the speed of drawing down the glass. Widths of up to almost 1 m can be obtained.

Other manufacturers of glass with the appropriate dimensions for LCD substrates use the float method of manufacturing. In this method, the molten glass is allowed to flow out onto a bed of molten tin. The glass does not dissolve or react with the metallic tin, but floats on the surface. This allows gravity to smooth the surface and allow both sides to become parallel. (See the chapter Glass, ceramics and related materials.)

A variety of substrate sizes are available extending to 450 × 550 mm and larger. Typical glass thickness for flat panel displays is 1.1 mm. Thinner glass is used for some smaller displays, such as pagers, telephones, games and so on.

Cutting, bevelling and polishing

Glass substrates are trimmed to size after the fusion or float process, typically to about 1 m on a side. Various mechanical operations follow the forming process, depending on the ultimate application of the material.

Since glass is brittle and easily chipped or cracked at the edges, these are typically bevelled, chamfered or otherwise treated to reduce chipping during handling. Thermal stresses at edge cracks accumulate during substrate processing and lead to breakage. Glass breakage is a significant problem during production. Besides the possibility of employee cuts and lacerations, it represents a yield loss, and glass fragments might remain in equipment, causing particulate contamination or scratching of other substrates.

Increased substrate size results in increased difficulties for glass polishing. Large substrates are mounted to carriers using wax or other adhesive and polished using a slurry of abrasive material. This polishing process must be followed by a thorough chemical cleaning to remove any remaining wax or other organic residue, as well as the metallic contaminants contained in the abrasive or polishing medium.

Cleaning

Cleaning processes are used for bare glass substrates and for substrates covered with organic films, such as colour filters, polyimide orientation films and so on. Also, substrates with semiconductor, insulator and metal films require cleaning at certain points within the fabrication process. As a minimum, cleaning is required prior to each masking step in colour filter or thin film transistor fabrication.

Most flat panel cleaning employs a combination of physical and chemical methods, with selective use of dry methods. After chemical etching or cleaning, substrates are usually dried using isopropyl alcohol. (See table 1.)

Table 1. Cleaning of flat panel displays

Physical cleaning

Dry cleaning

Chemical cleaning

Brush scrubbing

Ultraviolet ozone

Organic solvent*

Jet spray

Plasma (oxide)

Neutral detergent

Ultrasonic

Plasma (non-oxide)

 

Megasonic

Laser

Pure water

* Common organic solvents used in the chemical cleaning include: acetone, methanol, ethanol, n-propanol, xylene isomers, trichloroethylene, tetrachloroethylene.

Colour Filter Formation

Colour filter formation on the front glass substrate includes some of the glass finishing and preparation steps common to both the front and rear panels, including the bevelling and lapping processes. Operations such as patterning, coating and curing are performed repeatedly on the substrate. Many points of similarity with silicon wafer processing exist. Glass substrates are normally handled in track systems for cleaning and coating.

Colour filter patterning

Various materials and application methods are used to create colour filters for various flat panel display types. Either a dyestuff or a pigment can be used, and either one can be deposited and patterned in several ways. In one approach, gelatin is deposited and dyed in successive photolithographic operations, using proximity printing equipment and standard photoresists. In another, pigments dispersed in photoresist are employed. Other methods for forming colour filters include electrodeposition, etching and printing.

ITO Deposition

After colour filter formation, the final step is the sputter deposition of a transparent electrode material. This is indium-tin oxide (ITO), which is actually a mixture of the oxides In2O3 and SnO2. This material is the only one suitable for the transparent conductor application for LCDs. A thin ITO film is required on both sides of the display. Typically, ITO films are made using vacuum evaporation and sputtering.

Thin films of ITO are easy to etch with wet chemicals such as hydrochloric acid, but, as the pitch of the electrodes becomes smaller and features become finer, dry etching may be necessary to prevent undercutting of the lines due to overetching.

Thin Film Transistor Formation

Thin film transistor formation is very similar to the fabrication of an integrated circuit.

Thin film deposition

The substrates begin the fabrication process with a thin film application step. Thin films are deposited by CVD or physical vapour deposition (PVD). Plasma-enhanced CVD, also known as glow discharge, is used for amorphous silicon, silicon nitride and silicon dioxide.

Device patterning

Once the thin film has been deposited, a photoresist is applied and imaged to allow etching of the thin film to the appropriate dimensions. A sequence of thin films is deposited and etched, as with integrated circuit fabrication.

Orientation Film Application and Rubbing

On both the upper and bottom substrate, a thin polymer film is deposited for orientation of the liquid crystal molecules at the glass surface. This orientation film, perhaps 0.1 μm thick, may be a polyimide or other “hard” polymer material. After deposition and baking, it is rubbed with fabric in a specific direction, leaving barely detectable grooves in the surface. Rubbing can be done with a once through cloth on a belt, fed from a roller on one side, passing under a roller which contacts the substrate, onto a roller on the other side. The substrate moves underneath the cloth in the same direction as the cloth. Other methods include a travelling brush that moves across the substrate. The nap of the rubbing material is important. The grooves serve to aid the liquid crystal molecules to align at the substrate surface and to assume the proper tilt angle.

The orientation film can be deposited by spin coating or by printing. The printing method is more efficient in material usage; 70 to 80% of the polyimide is transferred from the printing roll to the substrate surface.

Assembly

Once the substrate rubbing step is completed, an automated assembly line sequence is begun, which consists of:

  • adhesive application (required for sealing the panels)
  • spacer application
  • location and optical alignment of one plate with respect to the other
  • exposure (heat or UV) to cure the adhesive and bond the two glass plates together.

 

Automated transport of both top and bottom plates occurs through the line. One plate receives the adhesive, and the second plate is introduced at the spacer applicator station.

Liquid Crystal Injection

In the case where more than one display has been constructed on the substrate, the displays are now separated by slicing. At this point, the liquid crystal material can be introduced into the gap between the substrates, making use of a hole left in the seal material. This entrance hole is then sealed and prepared for final inspection. Liquid crystal materials are often delivered as two or three component systems which are mixed at injection. Injection systems provide mixing and purging of the cell to avoid trapping bubbles during the filling process.

Inspection and Test

Inspection and functional testing are performed after assembly and liquid crystal injection. Most defects are related to particles (including point and line defects) and cell gap problems.

Polarizer Attachment

The final manufacturing step for the liquid crystal display itself is the application of the polarizer to the outside of each glass plate. Polarizer films are composite films which contain the pressure-sensitive adhesive layer needed to attach the polarizer to the glass. They are applied by automated machines which dispense the material from rolls or pre-cut sheets. The machines are variants of labelling machines developed for other industries. The polarizing film is attached to both sides of the display.

In some cases, a compensation film is applied prior to the polarizer. Compensation films are polymer films (e.g., polycarbonate and polymethyl methacrylate) that are stretched in one direction. This stretching changes the optical properties of the film.

A completed display will ordinarily have driver integrated circuits mounted on or near one of the glass substrates, usually the thin film transistor side.

Hazards

Glass breakage is a significant hazard in LCD manufacturing. Cuts and lacerations can occur. Exposure to chemicals used for cleaning is another concern.

 

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