Pollution Prevention and Control Technologies for Plating
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.
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