Address: Canadian Center for Pollution Prevention, 100 Charlotte Street, Sarnia, Ontario N7T 4R2
Country: Canada
Phone: 1 (519) 337-3423
Fax: 1 (519) 337-3486
E-mail: c2p2@sarnia.com
Past position(s): Manufacturing Manager, ICI Nitrogen Products
Education: BSc, Cape Town
Areas of interest: Pollution prevention strategies, information and technologies
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:
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:
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:
Environmental planning for specific wastes
Specific waste streams, usually spent plating solutions, can be reduced by:
Several methods of reducing drag-out include:
Drag-out recovery of chemicals uses a variety of technologies. These include:
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:
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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