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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.4 ION EXCHANGE
4.4.1 Overview
The use of ion exchange for bath maintenance is a relatively widespread practice; however the scope of applications is rather small. Each of the applications from the Users Survey, except one, involves the removal of cations from chromic acid solutions. The one exception is a bath maintenance application for a trivalent chromium plating solution. The Vendors Survey reveals the same two applications. Three vendors were identified that market ion exchange bath maintenance equipment. Each of these firms offers chromic acid maintenance equipment and one also sells a trivalent system. The literature indicates that other ion exchange bath maintenance applications have been attempted, but no commercial examples were found (e.g., nickel chloride strike treatment using selective ion exchange, ref. 46, and carbonate removal from cyanide plating baths). Within this text, acid sorption (also referred to as acid retardation) is presented in Section 4.5 as a separate technology from ion exchange. Although acid sorption is performed with similar equipment, as explained in Section 4.5, during the separation process ion exchange does not occur.
The results of the Users Survey show that 11 (or 3.5%) of the respondents have used ion exchange for chromic acid or trivalent chromium bath maintenance. The types of solutions treated by respondents include: hard chromium, decorative chromium (Cr+3), chromic acid anodize and chromic acid copper strip solution. Hard chromium is the most frequently treated solution, making up 64% of all ion exchange bath maintenance applications identified during the Users Survey. Of the 81 shops reporting the use of hard chromium plating, 7 (or 8.6%) have employed ion exchange for bath maintenance, making it the second most popular hard chromium bath maintenance technology among survey respondents. Ion transfer (e.g., porous pot) is the most frequently employed maintenance technology (see Section 4.6).
When used to purify plating baths, ion exchange (cation only) removes impurities that buildup in baths from drag-in, corrosion of parts, racks and anodes, reduced or decomposed bath chemicals, and other sources. These contaminants reduce the performance of the bath and eventually accumulate up to a concentration where the bath must be discarded. Also, contaminated baths cause platers to increase the concentration of plating chemicals so that they are able to maintain plating efficiency. This results in higher solution viscosity which in turn increases drag-out rates and overall chemical losses. Other negative aspects of operating contaminated baths include lower plating rates and higher electrical consumption (ref. 370).
A typical application for ion exchange bath purification is the removal of iron and trivalent chromium from hexavalent chromium plating solutions. Purification can be accomplished by directly treating the bath. However, in some cases, such as chromium plating, the concentrated bath has a detrimental impact on the resins, which shortens the life of the material. As an alternative method, the bath can be diluted, treated with ion exchange and reconcentrated. Reconcentration is unnecessary in instances where the surface evaporation rate of the plating bath provides sufficient headroom to return the treated solution. Also, if drag-out recovery is practiced, the drag-out, which is typically less concentrated than the bath, can be treated with ion exchange before it is returned to the bath. A sufficiently high drag-out rate is needed for this strategy to work. Even when chromic acid is diluted, it has a detrimental effect on ion exchange resin. The short resin life (6 to 12 months) and its replacement cost are simply accepted as part of the operating costs for this process.
Ion exchange competes with ion transfer (Section 4.6) and membrane electrolysis technologies (Section 4.7) as a chromic acid bath maintenance technology for tramp metal removal. Dummy plating (high current density electrolysis) is an alternative method for trivalent chromium oxidation, but it is ineffective for tramp metal removal. There is no clear choice between ion exchange, ion transfer and membrane electrolysis for tramp metal removal.
The porous pot type ion transfer technology is the least capital intensive technology (single tank models are less than $1,000), but it has a questionable role as a pollution prevention tool due to the high quantity of residual waste generated. In some cases, the porous pot is comparable to a "bleed and feed" method of tramp metal control. It is however, an effective method of trivalent chromium oxidation. The polyester membrane ion transfer technology may reduce residual waste quantities, however, there are insufficient data available to evaluate its performance.
Ion exchange can also generate a significant chromium waste volume. This process is unable to oxidize trivalent chromium to the desired hexavalent state, like ion transfer and membrane electrolysis. However, unlike ion transfer, it does not produce significant quantities of hexavalent chromium wastes. Also, trivalent chromium losses can be reduced by using selective resins and operating them to exhaustion (discussed in Section 4.4.3).
Membrane electrolysis, which perform both trivalent chromium oxidation and tramp metal removal, appears to be the best technology in terms the ratio of chromium residual volume generated to the volume of bath treated (ref. 370). However, some users of this technology indicate that it has significant O&M problems (see Section 4.7.7). Also, it is the most capital intensive method of the three technologies.
As such, the problem of chromic acid bath maintenance is still unresolved. One respondent to the Users Survey, who described this dilemma as "one of the biggest problems facing hard chrome platers," has purchased and operated an ion transfer unit (porous pot), an ion specific electrochemical membrane unit and ion exchange technology (PS 234). This respondent concluded that ion exchange was the best method, but indicated that it produced a high waste load (see complete comments by PS 234 in Section 4.6.6).
General background information on the ion exchange process and applications involving chemical recovery are presented in Section 3. End-of-pipe applications of ion exchange are discussed in Section 6.