Pollution Prevention and Control Technologies for Plating
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).
Next Section|Main Table of Contents|Section 4