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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.4 ION EXCHANGE
4.4.3 Applications
The primary application of ion exchange, when used as a bath maintenance technology, is the removal of cations from chromic acid baths. Of the eleven respondents to the NCMS/NSMF Users Survey that employed ion exchange for bath maintenance, ten or their applications were for chromic acid baths and one for a trivalent chromium bath. The chromic acid bath applications included seven hard chromium, two chromic acid anodize and one chromic acid copper strip solution.
Exhibit 4-8 shows two typical applications for this technology. In IXBM-1 the ion exchange process is applied directly to the process solution and in IXBM-2 the process is applied to the return flow from the drag-out recovery tank. IXBM-1 is applicable to both hexavalent chromium and trivalent chromium baths. In order for IXBM-2 to be effective, the drag-out rate would have to be sufficiently high such that the percentage of the chromic acid bath treated over a given time period is sufficient to keep bath contaminants at a tolerable level. The necessary drag-out rate depends heavily on the rate of contaminant buildup. Typically, a drag-out rate equal to one tank volume per year will be sufficient for most hard chrome plating processes to operate with the IXBM-2 configuration. Anodizing processes and chromium plating processes with high contamination rates may require even greater drag-out rates to use this configuration. When the drag-out rate is insufficient (this will be evidenced by a steady buildup in contaminant concentration), the plater can bleed the bath into the drag-out tank. However, this practice may overly contaminate the rinse system and result in poor rinsing.
Several respondents to the Users Survey combined the use of ion exchange with evaporative recovery operations for chromic acid recovery. In each of these cases, ion exchange was applied to the recovered chromic acid solution before it was concentrated by the evaporator and returned to the plating tank (PS 018, PS 082, PS 125).
Resin selectivity is an important factor with the chromic acid bath maintenance process because it reduces trivalent chromium losses. The practice of separating ions of the same charge is referred to as selective ion exchange (SIE) (ref. 46). SIE takes advantage of the preference that ion exchange resins exhibit for some ions over others to selectively separate different ions that have the same charge and would normally be removed by the same resin. Essentially, all resins exhibit selectivity. However, some resins are more selective than others and the ordering of selectivity among metal species varies among resins. When employed for chromic acid purification, the ion exchange sites of the resin exchange hydrogen for the tramp metals and Cr+3. When most of the hydrogen is displaced from the resin sites, the resin column shows its preference by displacing Cr+3 that has been attached to the resin with the preferred tramp metals. The Cr+3 goes back into solution and is returned to the bath along with the chromic acid. The process, if run to exhaustion (i.e., the point where the percentages of ions held by ion exchange sites are in proportion to the preference of the resin), will result in a high removal of the preferred tramp metals and a low removal of Cr+3 (ref. 384). If Cr+3 buildup presents a plating quality problem with a given bath, electrolysis or dummy plating (Section 4.2.2.3) can be employed.
Sulfuric acid is used to regenerate the cation exchange columns for hexavalent chromium bath applications and sulfuric acid and ammonium hydroxide (NH4OH) are used for trivalent chromium bath applications. One respondent to the Users Survey regenerates their cation bed with hydrochloric acid for a hard chrome bath application (PS 198). Although this shop has not reported any O&M problems with their plating bath, one would be concerned with chloride contamination of the bath, which is known to promote burning, poor coverage, pitting, gray deposits, reduced efficiency and etching (ref. 369). Ammonium hydroxide is used with the trivalent application to remove copper, which is tightly held by the resin and is not sufficiently removed by the acid. The ammonium hydroxide step is not usually performed during each regeneration cycle, but rather as needed to remove the copper (e.g., every two to four times purification is performed).
Because ion exchange is a separation technique that is dependent on the general chemistry of the process solution and electrochemical differences between the metal species (i.e., plated metal vs the tramp metal), it is not applicable to the maintenance of many plating solutions. This is due to the fact that, most frequently, the metal being plated and the contaminants are both cations (e.g., nickel baths). Although different cations can be separated by selective ion exchange, it is a more difficult and less efficient process than the separation of cations and anions. In this regard, chromic acid baths are a good candidate for ion exchange since the desired species of chromium (i.e., hexavalent chromium) is an anion and the tramp metals (including trivalent chromium) are cations. Cyanide-metal complexes in cyanide plating solutions (e.g., copper cyanide) also have the opposite charge of tramp metals. However, if ion exchange bath maintenance was attempted, the cyanide-metal complex would be destroyed by the acidic conditions in the cation resin bed causing the plated metal to be removed from solution along with the tramp metals (see Section 3.4.3). Also, for these applications, ion exchange would have to compete with low current density electrolysis (dummy plating), which is an inexpensive and effective method of controlling tramp metals for many non-chromium baths.
The literature indicates that selective ion exchange (see Section 4.4.4) has been investigated as a potential solution maintenance technology for acid etch and nickel strike baths (ref. 384). The results of the investigation showed that the SIE process was impractical for these applications because the resin could not be adequately regenerated. Additionally, like the cyanide-metal baths, these two baths are treatable by dummy plating, making the SIE application less significant.
Tinker Air Force Base (Oklahoma City, OK) is conducting research, through their contractor Science Applications Incorporated (SAIC), to evaluate the applicability of ion exchange to the maintenance of cadmium stripper solution (ammonium nitrate). Early project results are very promising.