Nickel Plating Bath Recovery
by Vacuum Evaporation

Nickel baths routinely recovered by vacuum evaporation include: bright nickel, sulfur-free semi-bright nickel, Watts nickel, high-chloride nickel, Woods nickel, and specialized microporous nickel.

Most nickel sulfamate baths can also be recovered for reuse. In some sulfamate baths extra sulfate may be generated during the evaporation process, in which case the drag-out can still be concentrated for its nickel metal value.

Operating Conditions

The QVF Nickel Evaporator is constructed of borosilicate glass, PTFE, and refractory metal, all fully Compatible with nickel solutions. Operating temperature is in the 160-170 ^oF (71-77 ^oC) range at a vacuum of 18-20" (457-508 mm) Hg.

Water distillate will be produced having less than 5 ppm of nickel. Since the concentrate will exit the evaporator at approximately 160-170 ^oF, it may be necessary to provide a small concentrate holding tank to allow the solution to cool before returning it to the plating tank.

However, since the volume of concentrate discharged at any one time is small (10-40 gallons), the recovered concentrate can frequently be discharged directly to the plating tank with no appreciable effect on the temperature of the plating solution, or to the plating quality.

Purification

No special purification techniques or equipment are necessary for successful nickel bath recovery. All that is required is the careful attention to accepted standard practice of nickel-bath maintenance:

• Promptly remove all dropped parts.
• Regularly filter nickel plating baths (except particle nickel baths), and treat with carbon as required.
• Take special care to prevent introducing contaminants from cleaners, or from poorly maintained racks.
• Where metallic contamination is a chronic problem, such as with small zinc castings, use of a continuous electrolytic purification, or "dummying," cell is recommended.

Concentration Limits

The concentration limit of most plating baths in an evaporator is the saturation-concentration, or "salting-out," point of the constituents, or that concentration beyond which precipitation occurs.

In a nickel plating bath, boric acid is generally the least soluble constituent and is maintained very near its saturation concentration in most nickel baths. Therefore, with evaporative recovery. the normal plating bath concentration is the practical concentration limit for most nickel plating solutions.

Since boric acid is slightly more soluble at higher temperatures, slightly higher concentrations can be handled by he evaporator. However, take care to ensure that the concentrate is removed quickly and not allowed to cool and possibly crystallize while inside the evaporator.

Fortunately, since the evaporation rate from the bath surface of most nickel solutions is relatively high because of their elevated operating temperature, it is rarely necessary to concentrate dragged-out nickel bath even up to bath concentration.

Special Considerations

As noted earlier, the QVF Vacuum Evaporator is fully compatible with a wide range of nickel plating processes and formulations.

However, sulfur-free semi-bright nickel systems, as used widely in the automotive industry, do pose a limitation. In such a system, the drag-out of bright nickel generally equals the drag-in of semi-bright nickel into the bright nickel solution. This results in a relatively stable concentration of bright nickel solution and a gradually diluted semi-bright nickel solution.

Thus, it would be desirable to recover the drag-out from the bright nickel and return it to the semi-bright nickel solution. However, this cannot be done because of the chemical incompatibility of the brightener systems. Drag out from the bright nickel bath can, of course, be concentrated and recovered for the nickel value.

Occasionally, some difficulties may be encountered with chrome coverage on plated nickel parts where nickel recovery is practiced. This is particularly true if transfer times are slow and/or many rinse stations have been installed prior to the chrome plating tank.

This problem, which is essentially passivation of the nickel surface caused by rinse water that is actually too pure, can be easily rectified by acidifying the rinse tank closest to the plating tank. Adding a small amount of sulfuric or hydrochloric acid once a day is usually sufficient. Some trial and error may be necessary to determine the optimum addition rate.

This Process Profile supersedes all previous issues.

QVF Process Systems pursues a policy of continuous product improvement. We therefore reserve the right to alter any product or process as described and illustrated.