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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.6 ION TRANSFER
4.6.3 Applications and Restrictions
Exhibit 4-20 shows the two basic configurations used for application of ion transfer technologies for bath maintenance. Configuration IT-1 shows the use of the less expensive in-tank porous pot. This unit does not have its own rectifier, but is powered by the plating tank rectifier (alternatively, PS 118 connects to their electrocleaner rectifier). Therefore, its use is restricted to the time period when the plating tank rectifier is energized. The unit must be removed from the tank during idle periods, otherwise the contaminates will migrate from the catholyte back into the bath. One of the survey respondents indicated that they remove the unit from the tank whenever it is idle for more than 8 hours (PS 118).
Configuration IT-2 shows an external unit that is powered by its own rectifier. Either the multi-pot porous pot unit or the external polyfluorocarbon membrane unit would be used in this manner. Plating solution is pumped into the tank containing the cells, circulated and returned by gravity to the plating bath.
Although the ion transfer technology has its roots in non-chromium applications, and one reference indicated that other applications are possible (ref. 371), no non-chromium applications were found during the course of the NCMS project. The specific applications that were identified were all for chromic acid solutions except for one application to caustic chromium strip. The chromic acid applications include hard chromium plating (single and dual catalyst), decorative chromium plating (single and dual catalyst), chromic acid anodizing and chromic acid etch.
Two important restrictions should be noted for the ion transfer technology. First, this technology should be considered as a bath maintenance method and not a means of quickly rejuvenating a spent bath. Chromic acid baths that are laden with dissolved metal contaminants will take months to correct with ion transfer and a significant volume of chromium waste can be generated in the process. The correct application of this technology is as a continuous maintenance method that is first applied before the bath is overly contaminated. Second, the ion transfer technology is not practical as a bath maintenance method where the desired tramp metal contamination level (excluding consideration of Cr+3) is less than 4 g/l. To reach a lower point would require frequent changes of the catholyte solution, resulting in a very high waste volume.
Exhibit 4-21 shows the basic configuration (IT-3) used for application of ion transfer for chromium recovery from rinse water. The IT-3 configuration takes advantage of the significant amount of surface evaporation that normally occurs in chromium plating baths operating between 115¡F and 140¡F. By using countercurrent recycle of rinse water through multiple-rinse tanks and a chromium recovery unit on the final rinse, the chromium plating process becomes a closed-loop operation. Chromic acid is removed from the rinse water and concentrated into the ion transfer system modules where it can reach a concentration of approximately 150 g/l. The recovered chromic acid is periodically drained from the ion transfer system modules and transferred to the plating bath (ref. 39).
Exhibit 4-22 presents vendor supplied data that shows the performance of an ion transfer unit (APS-3, see specifications in Section 4.6.4) applied to a decorative chromic acid plating bath (ref. i3 file). During the data collection period, the unit was operated at 110 amps and 6.5 volts. The data indicate there is an average removal rate of 385 grams of cations (copper, nickel and zinc) per day (0.85 lbs/day) when the bath (6,018 l) is treated from a concentration of 12.2 g/l to 10.6 g/l.
The PPS1 or the PPS2 can also be used as a chromium recovery device when placed into a caustic strip solution and reverse polarity is applied. Chromic acid will accumulate within the ceramic pot and is periodically removed and transferred to the plating bath. One of the survey respondents used the equipment in this way and reported that it was the best application for this equipment (i.e., they also tried using the PPS1 as a bath maintenance technique) (ref. PS 006).