Elimination of Cadmium Cyanide Waste Treatment

It has long been a desire of the finishing industry to eliminate the potential liability of cadmium cyanide electrolytes for four reasons: Employee safety; Environmental concerns with the release of toxic substances; Regulatory compliance, avoidance of fines and legal action; and Costs associated with operating a cyanide destruct system such as chemical use, instrumentation maintenance and labor.

While several proprietary, non-cyanide cadmium plating products are available, the industry has generally not accepted these alternatives. The major concern has been that the resulting deposit will not meet the same performance criteria as a conventional cadmium cyanide deposit. Also, there is a belief that noncyanide cadmium plating solutions are inherently more difficult to control and are subject to much more variation.

Due to regulatory pressures, the predominant consideration was elimination of cyanide in the waste stream at Alert Plating, Sun Valley, California. Alert was willing to consider changing to a noncyanide cadmium plating process, even if it was more difficult for platers to use.

In May 1996, vendors were surveyed to identify available commercial products. After discussions with other shops and technical consultants, an acid-sulfate-type bath was chosen.

Initial testing in a two-liter beaker lasted about two weeks. The work was performed in the laboratory at Alert using samples of Kadizid, a proprietary brightener, from LeaRonal, Inc., Freeport, New York. Because the test results were encouraging, Alert began moving toward the final goal of cyanide replacement.

The next step was to confirm the lab tests. This was done by building a small (120 gal) production tank to plate parts daily. During the approximately four month trial period, the process performed exceptionally well on both rack and barrel plating. Only a few minor problems were encountered. Most of them were easily corrected by brightener additives. The brightener and cyanide-based deposits had similar appearances in the as-plated condition, but, after OD chromium plating, the color of the two finishes had noticeably different shades. This did not present a problem because the end user accepted both shades as long as the entire lot had the same finish.

Because of the success of the trial period, all 2,000 gal of cadmium cyanide plating solution were removed from the shop between August and October 1996. The cyanide destruct system has since been cleaned out and decommissioned. The new chemistry has been performing as expected for two shifts a day, five to six days a week. There have been no unexpected problems that could cause a loss of production time or an increase in the number of rejected or reworked parts.

Many of the anticipated difficulties or perceived shortcomings of a non-cyanide plating electrolyte did not materialize. The bath has excellent throwing power. The internal anodes used for certain deep parts are no longer necessary. The plating rate is faster than cyanide, and brightness is achieved more quickly. At the same current density and dwell time, the bath will deposit as much as 0.2 mil more cadmium. This represents a time savings of up to 10 minutes per load for an equivalent thickness of cadmium.

Bath maintenance is simple and easy. Wet analysis of acid and metal is required once a week. Both components are stable, needing only small adjustments. The metal has shown a tendency to climb slowly. However, it is not necessary to remove the anodes from the tank during normal off hours. Brightener additions are based on amp-hr readings. The consumption rate has been in line with the vendors guidelines of one gallon per 10,000 amp hr.

In the past nine months, one carbon filtration treatment was necessary to eliminate staining on the ID of some parts and to restore brightener response in the Hull cell. The string-wound filter tubes were replaced with carbon tubes for four hours to remove the contaminants.

The additive system is forgiving of over additions and chemical imbalances caused by improper feed ratios of brightener components. In one particular case, an excess of stabilizer was introduced into the tank. While this did cause an adverse effect, it did not halt production. Addition of brightener corrected the problem while production continued. Within three days the deposit was back to normal.

There were some unexpected difficulties and challenges encountered in the conversion. When plating directly, machined C1215 steel parts developed roughness. Preliminary testing indicated that with enough cleaning and stripping cycles the parts would eventually accept a smooth acid-cadmium deposit. A nickel or copper strike is also a possibility if the application would allow its use.

The effect of operating temperature was underestimated. Although the temperature never exceeded the maximum recommended in the operating guide, significant changes in the deposit brightness, uniformity, coverage and color (staining) occurred above 80F. Also, brightener consumption tripled, which led to the installation of a cooling coil.

There are a total of four additives for this process, making it somewhat difficult to interpret Hull cells and make necessary additions. In daily use, however, only one main component (Brightener KR) is needed. The remaining products are consumed primarily by dragout. Therefore they do not need to be added on a regular basis. The additives proved to be stable, predictable and easy to control despite potential unbalance.

The initial cost of equipment for this process can be high due to the zirconium anode baskets. It is possible to use slab anodes, but the surface area changes more as they corrode. With baskets, anode balls are added as the stacks shrink. In the long term, the savings from eliminating cyanide treatment will offset the equipment investment.