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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.6 ION TRANSFER
4.6.1 Overview
The term ion transfer is used in this text to refer to a small group of technologies that are applied primarily to the maintenance of chromic acid baths, most notably hard and decorative chrome plating, chromic acid anodizing and chromic acid etch baths. Used for this purpose, ion transfer is a competing technology with ion exchange (cation) and membrane electrolysis. Ion transfer is also a chromium recovery technology, although it is used less frequently for this purpose. Competing technologies for chromium recovery include evaporation and ion exchange.
Among survey respondents, ion transfer was the most frequently used technology for chromic acid bath maintenance. Its popularity is due mainly to the commercialization of a particular low cost equipment line (Hard Chrome Plating Consultants PPS1 and PPS2 priced below $1,000). Of the 318 respondents to the Users Survey, 15 (or 5%) have employed ion transfer equipment. This technology is especially popular with hard chrome platers for the removal of dissolved iron and other tramp metals and the reoxidation of trivalent chromium. Of the 81 respondents that perform hard chrome plating, 10 (or 12%) have employed ion transfer equipment.
The basic ion transfer technology involves the use of a membrane, typically a porous ceramic pot or a polyfluorocarbon membrane (e.g., Teflon). The unit consists of an electrolytic cell with an anode and cathode (or sets of each) that are separated by the membrane. When energized, trivalent chromium present at the anode is oxidized to hexavalent chromium, and cations (e.g., dissolved iron) present in the anolyte migrate through the membrane into the cathode compartment. The catholyte is periodically discarded and the cathode cleaned of any deposits.
As a chromium recovery technology, ion transfer equipment has been applied by survey respondents to chromium strip solutions and rinse waters. The chromium strip application uses the same equipment (porous pot) as is used for bath maintenance, but it is operated with reverse polarity. A specially designed ion transfer system for chromium recovery from rinse water was introduced in the early 1980's. Although that particular equipment (ChromeNapper) is no longer manufactured, a similar device is presently available.