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

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

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.

Back

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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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