Trivalent chromium plating has become more and more important in recent years. Many platers have come to realize that the operational advantages of trivalent chromium plating, combined with substantially lower wastewater treatment and air scrubbing costs make trivalent chromium an economic winner when compared to traditional hexavalent chromium plating baths.
In the past, however, one major limitation to the success of trivalent chromium plating has been the bath's sensitivity to relatively low levels of ionic, metal contaminants. The contaminants that have caused the most concern are copper, zinc and nickel. Copper and zinc have been reported to cause discoloration and poor coverage when present at levels of 20 ppm or more, while nickel causes problems at concentrations above 50 ppm. When contaminant metal concentrations reach unacceptable levels, the plating bath must be treated to restore proper bath performance.
In most applications, bath treatment has been accomplished using chemical precipitation or dummying. Unfortunately, both these procedures have serious limitations. The chemical precipitation process is messy and labor intensive, while dummying is restricted to copper and zinc only. In addition (and often more importantly) chemical precipitation or dummying must be done off-line so the bath must be taken out of production for several hours while treated.
What is needed for successful trivalent chromium plating bath operation is a simple, cost-effective process that can remove copper, nickel and zinc from the bath without interrupting bath operation.
Ion Exchange Development. In 1989, Bio-Recovery Systems, Inc., Artesia, New Mexico, began an investigation of the applicability of ion exchange technology for the treatment of trivalent chromium plating baths. The objectives for the project were as follows:
- Identify an ion exchange resin capable of removing copper, nickel and zinc from trivalent chromium plating baths without affecting the concentration of trivalent chromium;
- Develop a suitable regeneration scheme for the resin; and
- Demonstrate the effectiveness of the resin over repeated cycles of the operation.
The goal was to develop a simple, easy-to-use, cost-effective process for removing contaminant metal ions from trivalent chromium plating baths without interrupting bath operation.
Samples of trivalent chromium plating bath chemistry were obtained from a major plating bath supplier for evaluation. Small amounts of copper, zinc and nickel were added, and the bath samples were passed through columns of ion exchange resin to determine whether copper, zinc and nickel ions could be removed from the bath.
Many different ion exchange resins were tested. Some were found completely ineffective for metal ion removal while others were able to remove only a single contaminant metal. In the end, only one resin was identified that was capable of effectively removing all three contaminant metals without removing chromium from the bath.
Having identified a resin capable of removing copper, zinc and nickel, a regeneration procedure for the resin was needed. The amount of metal an ion exchange resin is capable of retaining is called its binding capacity. Once the resin's binding capacity has been reached, the resin must be regenerated. During regeneration, metal ions are stripped off the resin and the resin is put back into the proper form for reuse. After testing several regeneration procedures, it was found that good regeneration could be obtained using dilute solutions of sulfuric acid and ammonia.
Having developed the proper regeneration procedure, the resin was run through several cycles of loading and regeneration to make sure the process was viable on a long-term basis.
Application. The ion exchange technology described in this paper has been used in a number of locations to remove contaminant metal ions from trivalent chromium plating baths. A small ion exchange treatment unit is installed near the bath. Bath solution is pumped directly from the plating tank, through the treatment unit and returned directly to the plating tank. Treatment takes place while the bath is in operation. No shutdown time is required. The unit is operated either continuously or intermittently, depending on the rate of contaminant metal build-up.
A properly sized treatment unit requires regeneration about every week to two weeks. Regeneration typically takes about two hours and can be scheduled during a time that is most convenient for the operator. Regeneration can also be performed while the bath is in operation. Once again, no shutdown time is required.
Efficient and cost-effective treatment of trivalent chromium plating baths for the removal of contaminant metal ions is readily accomplished using ion exchange technology. The benefits gained include the following:
- Copper, zinc and nickel ions are all removed from the bath simultaneously.
- Treatment can be done during bath operation (no shutdown time required).
- Treatment can be done continuously or intermittently depending on need.
- The treatment units are small and usually can be located right at the plating tank.
- Treatment is simple, consisting of pumping solution from the tank, through the treatment unit and back to the tank.
Regeneration of the ion exchange resin takes about two hours and occurs about every week to two weeks for a properly sized treatment unit.