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Pollution Prevention and Control Technologies for Plating Operations
Section 5 - Substitute Technologies
5.4 CYANIDE USE REDUCTION/ELIMINATION
Cyanide, in the form of either sodium or potassium cyanide, has been a key component of plating solutions for many years. Some of the metals commonly plated from cyanide baths include: cadmium, zinc, copper, brass and precious metals. When used, cyanide forms a moderately stable complex with metal ions that permits the deposition of the metal under the influence of a suitable electrical potential.
Although cyanide-containing baths are eminently satisfactory as far as plating results go, pollution control poses a problem in terms of both compliance and economics (ref. 350). As a result, there has been a significant effort to find and implement cyanide-free plating processes since approximately 1975.
Survey respondents indicated concerns over the use and control of cyanide and expressed a desire to find alternative non-cyanide plating solutions. For example, 14 percent indicated that they currently have compliance difficulties with cyanide. Eleven percent indicated that improved cyanide plating alternatives or controls was a major environmental technology need for their company. Twenty-six percent of the respondents indicated that there exists a technology transfer insufficiency with respect to non-cyanide finishing. This was the highest percentage of response for any topic listed in the survey form (see summary of results in Section 1).
The earliest and most complete cyanide substitution that has taken place in the plating industry is the conversion from zinc cyanide plating to zinc chloride and zinc alkaline (non-cyanide) plating. Although some respondents indicated that they did not make this substitution until the 1990Õs, most respondents converted during the period of the late 1970Õs to mid-1980Õs. A total of 28 respondents (or approximately 17% of the zinc platers) successfully converted to zinc chloride, 18 respondents (or approximately 11% of the zinc platers) converted to zinc alkaline (non-cyanide) and 8 respondents (or approximately 5% of the zinc platers) converted to an unspecified zinc non-cyanide plating process (see Exhibit 5-4). Another 9 (or approximately 6% of the zinc platers) converted from a high cyanide to a low cyanide concentration bath. Perhaps even more impressive than the number of successful conversions is that only 2 respondents of all those that attempted conversions were unable to successfully make a substitution.
One significant drawback with regard to zinc cyanide plating substitution is that some shops found it necessary to install both zinc chloride and zinc alkaline (non-cyanide) baths to replace the single cyanide bath (PS 053, PS 059, PS 087, PS 204, PS 251, PS 288, PS 300). On a positive note, in addition to cyanide use reduction, some platers enjoyed production benefits from the substitution, including better and brighter plating.
In addition to zinc plating, a significant number of respondents have reduced or eliminated cyanide usage with copper plating and nickel stripping, although the success rates (i.e., ratios of successes to attempts) are not as high as for zinc plating. Other plating and finishing cyanide solutions for which substitutes were identified by respondents included cadmium, silver and brass plating and chromate conversion coatings. The number of successes was either limited or non-existent for these solutions.
In addition to the plating shops that indicated they have substituted a non-cyanide process for a cyanide process, there were 12 shops that simply eliminated approximately 22 cyanide plating processes in order to meet environmental regulations.