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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.8 DIFFUSION DIALYSIS
4.8.6 Performance Experience and Operational and Maintenance Problems
As indicated previously, none of the respondents to the Users Survey indicated that they have employed this technology and therefore no firsthand performance experience is available. Various literature sources have reported on the efficiency of the diffusion dialysis process (ref. 192, 252, 336). These articles and conference papers, written by equipment manufacturers and suppliers, indicate that 80 to 95 percent of the initial acid can be recovered with 60 to 95 percent of the contaminant metal removed (i.e., a residual metal content of 5 to 40 percent of original concentration remains in the recovered acid).
Vendors of the technology suggest that the extremely low pressure feed of diffusion dialysis puts minimal stress on the stack components and therefore, increases the longevity of the units. However, as with any membrane technology, there is concern over membrane fouling and deterioration. Membrane fouling is most frequently caused by the presence of solids in the acid feed solution. It is reported (ref. 252, 336) that prefiltering will effectively eliminate the potential fouling problem. With prefiltering, membrane cleaning is required approximately two times per year (ref. 252).
Organic substances (e.g., oil and grease) can also cause membrane fouling. This problem can be overcome using a pretreatment of the raw acid solution with activated carbon.
Membrane life is reported to be three to seven years (ref. 252, 336). Membrane life will be shortened when the technology is used with hydrogen peroxide, chromic acid, and nitric acid with concentrations over 20 percent, as well as temperatures over 45C and organic substances such as oil and grease, solvents or cleaners (ref. 336).