Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Electroplating Industry Description and Practices 500 liters per square meter of surface plated) but is usually high in heavy metals, including cad- Electroplating involves the deposition of a thin mium, chrome, lead, copper, zinc, and nickel, and protective layer (usually metallic) onto a pre- in cyanides, fluorides, and oil and grease, all of pared metal surface, using electrochemical pro- which are process dependent. Air emissions may cesses. The process involves pretreatment contain toxic organics such as trichloroethylene (cleaning, degreasing, and other preparation and trichloroethane. steps), plating, rinsing, passivating, and drying. Cleaning or changing of process tanks and The cleaning and pretreatment stages involve a treatment of wastewaters can generate substan- variety of solvents (often chlorinated hydrocar- tial quantities of wet sludges containing high lev- bons, whose use is discouraged) and surface- els of toxic organics or metals. stripping agents, including caustic soda and a range of strong acids, depending on the metal Pollution Prevention and Control surface to be plated. The use of halogenated hy- drocarbons for degreasing is not necessary, as Plating involves different combinations of a wide water-based systems are available. In the plat- variety of processes, and there are many oppor- ing process, the object to be plated is usually used tunities to improve on traditional practices in the as the cathode in an electrolytic bath. Plating so- industry. The improvements listed below should lutions are acid or alkaline and may contain be implemented where possible. complexing agents such as cyanides. Changes in Process Waste Characteristics • Replace cadmium with high-quality, corro- Any or all of the substances used in electroplat- sion-resistant zinc plating. Use cyanide-free ing (such as acidic solutions, toxic metals, sol- systems for zinc plating where appropriate. vents, and cyanides) can be found in the Where cadmium plating is necessary, use wastewater, either via rinsing of the product or bright chloride, high-alkaline baths, or other from spillage and dumping of process baths. The alternatives. Note, however, that use of some solvents and vapors from hot plating baths re- alternatives to cyanides may lead to the release sult in elevated levels of volatile organic com- of heavy metals and cause problems in waste- pounds (VOCs) and, in some cases, volatile metal water treatment. compounds, which may contain chromates. Ap- • Use trivalent chrome instead of hexavalent proximately 30% of the solvents and degreasing chrome; acceptance of the change in finish agents used can be released as VOCs when baths needs to be promoted. are not regenerated. • Give preference to water-based surface-cleaning The mixing of cyanide and acidic wastewaters agents, where feasible, instead of organic clean- can generate lethal hydrogen cyanide gas, and ing agents, some of which are considered toxic. this must be avoided. The overall wastewater • Regenerate acids and other process ingredients stream is typically extremely variable (1 liter to whenever feasible. 307 308 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES Reduction in Dragout and Wastage Target Pollution Loads • Minimize dragout through effective draining A key parameter is the water use in each pro- of bath solutions from the plated part, by, for cess. Systems should be designed to reduce wa- example, making drain holes in bucket-type ter use. Where electroplating is routinely pieces, if necessary. performed on objects with known surface area • Allow dripping time of at least 10 to 20 sec- in a production unit, water consumption of no onds before rinsing. more than 1.3 liters per square meter plated • Use fog spraying of parts while dripping. (l/m2) for rack plating and 10 l/m2 for drum plat- • Maintain the density, viscosity, and tempera- ing should be achieved. The recommended pol- ture of the baths to minimize dragout. lution prevention and control measures can • Place recovery tanks before the rinse tanks achieve the target levels listed below. (also yielding makeup for the process tanks). The recovery tank provides for static rinsing • Cadmium plating should be avoided. Where with high dragout recovery. there are no feasible alternatives, a maximum cadmium load in the waste of 0.3 grams for Minimizing Water Consumption every kilogram of cadmium processed is rec- in Rinsing Systems ommended. • At least 90% of the solvent emissions to air must be recovered by the use of an air It is possible to design rinsing systems to achieve pollution control system such as a carbon 50–99% reduction in traditional water usage. filter. Testing is required to determine the optimum • Ozone-depleting solvents such as chlorofluo- method for any specific process, but proven ap- rocarbons and trichloroethane are not to be proaches include: used in the process. • Agitation of rinse water or work pieces to in- crease rinsing efficiency Treatment Technologies • Multiple countercurrent rinses • Spray rinses (especially for barrel loads). Segregation of waste streams is essential because of the dangerous reactions that can occur. Strong Management of Process Solutions acid and caustic reactions can generate boiling and splashing of corrosive liquids; acids can re- • Recycle process baths after concentration and act with cyanides and generate lethal hydrogen filtration. Spent bath solutions should be sent cyanide gas. In addition, segregated streams that for recovery and regeneration of plating are concentrated are easier to treat. chemicals, not discharged into wastewater treatment units. Air Emissions • Recycle rinse waters (after filtration). • Regularly analyze and regenerate process so- Exhaust hoods and good ventilation systems pro- lutions to maximize useful life. tect the working environment, but the exhaust • Clean racks between baths to minimize con- streams should be treated to reduce VOCs and tamination. heavy metals to acceptable levels before venting • Cover degreasing baths containing chlorinated to the atmosphere. Acid mists and vapors should solvents when not in operation to reduce be scrubbed with water before venting. In some losses. Spent solvents should be sent to sol- cases, VOC levels of the vapors are reduced by vent recyclers and the residue from solvent use of carbon filters, which allow the reuse of recovery properly managed (e.g., blended with solvents, or by combustion (and energy recov- fuel and burned in a combustion unit with ery) after scrubbing, adsorption, or other treat- proper controls for toxic metals). ment methods. Electroplating 309 Liquid Effluents of proper sludge disposal are likely to become an increasing incentive for waste minimization. Cyanide destruction, flow equalization and neu- tralization, and metals removal are required, as Emissions Guidelines a minimum, for electroplating plants. Individual design is necessary to address the characteristics Emissions levels for the design and operation of of the specific plant, but there are a number of each project must be established through the en- common treatment steps. For small facilities, the vironmental assessment (EA) process on the ba- possibility of sharing a common wastewater sis of country legislation and the Pollution Prevention treatment plant should be considered. Cyanide and Abatement Handbook, as applied to local con- destruction must be carried out upstream of the ditions. The emissions levels selected must be other treatment processes. If hexavalent chrome justified in the EA and acceptable to the World (Cr+6) occurs in the wastewater, the wastewater Bank Group. is usually pretreated to reduce the chromium to The guidelines given below present emis- a trivalent form using a reducing agent, such as sions levels normally acceptable to the World a sulfide. Bank Group in making decisions regarding The main treatment processes are equalization, provision of World Bank Group assistance. Any pH adjustment for precipitation, flocculation, and deviations from these levels must be described sedimentation/filtration. The optimum pH for in the World Bank Group project documenta- metal precipitation is usually in the range 8.5– tion. The emissions levels given here can be 11, but this depends on the mixture of metals consistently achieved by well-designed, well- present. The presence of significant levels of oil operated, and well-maintained pollution con- and grease may affect the effectiveness of the trol systems. metal precipitation process; hence, the level of The guidelines are expressed as concentrations oil and grease affects the choice of treatment op- to facilitate monitoring. Dilution of air emissions tions and the treatment sequence. It is preferred or effluents to achieve these guidelines is un- that the degreasing baths be treated separately. acceptable. Flocculating agents are sometimes used to facili- All of the maximum levels should be achieved tate the filtration of suspended solids. Pilot test- for at least 95% of the time that the plant or unit ing and treatability studies may be necessary, and is operating, to be calculated as a proportion of final adjustment of pH and further polishing of annual operating hours. the effluent may be required. Modern wastewa- ter treatment systems use ion exchange, mem- Air Emissions brane filtration, and evaporation to reduce the release of toxics and the quantity of effluent A 90% recovery of the quantity of VOCs released that needs to be discharged.