Surface Treatment And Plating

The major source of hazardous metals from chromating and other chemical conversion processes, as well as from electro- and electroless plating, is generally the process baths themselves. Spent cleaners and etchants also can contain large quantities of metal removed from workpieces.

The most basic type of source reduction is to eliminate the need for dangerous process baths. This can be accomplished by developing products that do not require plating, by using alternative, less dangerous process chemicals, or by eliminating the need for some of the chemicals. One way to do this is through use of abrasive rather than chemical cleaning and stripping methods. It is difficult to find replacements for electroplated products, although there have been some substitutions, such as replacing chromed metal automobile trim with plastic trim. Also, modern buffing and polishing techniques can sometimes eliminate the need for surface treatment. It is more realistic at present, however, to find more environmentally benign process chemicals that can perform the same function as the highly hazardous ones. For instance, cadmium plating can be replaced in many situations with far less dangerous zinc plating, while carcinogenic hexavalent chromium plating solutions can, if processes are carefully controlled, be replaced with trivalent chromium solutions.

Many waste streams are generated when process solutions are discarded due to impurities buildup. The waste generation rate can be lowered through lengthening solution life. This can be accomplished by impurity removal through filtration and ion exchange, as well as through reduction in the quantity of impurities introduced into the process solution.

By minimizing the drag-out carried on workpieces from process baths into the rinses, the amount of metal-bearing process chemicals reaching effluent streams can be reduced. Closed-loop rinsing systems route process chemicals that do get dragged out back into the plating bath, rather than into the effluent. Rinsing operations can also be made more efficient through the use of multiple rinse tanks, fog and spray rinses, automatic flow controls and agitation systems.

Source reduction methodologies also apply to the design of treatment systems, which generally precipitate toxic metals as hydroxides or carbonates, resulting in sludges that require further treatment or disposal. Changes in precipitating agents can result in less sludge generation. For example, sodium hydroxide can be substituted for lime for precipitation of chromium. Segregating wastes and treating them separately also reduces sludge volumes. Ferric sulfide, for instance, is useful in treating chelated metal wastes, but can produce unnecessary sludge if allowed into other waste streams.

Many process and rinse solutions can be recycled in some way if operators and plant engineers fully understand the chemistry of their waste streams. Rinse solutions too contaminated for their original purpose can often be used as rinses elsewhere. Metals can be recovered from spent process solutions and wastewater using technologies such as reverse osmosis, ion exchange, electrolytic recovery, and evaporation.

Treatment methodologies for process solution wastes include pH adjustment, flocculation, precipitation of metals, dewatering and sludge drying. Chelated metals require special treatment to break the complexes. Chromium, cyanide, electroless plating, and printed circuit board wastes often need to be segregated from other streams in order to reduce waste volumes and avoid the chance that some complexed metals may escape the treatment system

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