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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.8 DIFFUSION DIALYSIS
4.8.1 Overview
Diffusion dialysis is an ion exchange membrane technology that competes with acid sorption (Section 4.5) as a purification/recovery method for acids that have become contaminated with metals (e.g., pickling, anodizing, stripping, etching, and passivation baths). This technology has been commercialized for only a few years, which is reflected by the fact that none of the respondents to the Users Survey indicated that they have employed diffusion dialysis.
The diffusion dialysis process separates acid from its metal contaminants via an acid concentration gradient between two solution compartments (contaminated acid and deionized water) that are divided by an anion exchange membrane. Acid is diffused across the membrane into the DI water whereas metals are blocked due to their charge and the selectivity of the membrane. A key difference between diffusion dialysis and other membrane technologies such as electrodialysis or reverse osmosis is that diffusion dialysis does not employ an electrical potential or pressure across the membrane. Rather, the transport of acid is caused by the difference in acid concentration on either side of the membrane. As such, the energy requirements for this technology are low.
Exhibit 4-34 describes the diffusion dialysis process. The process uses ion exchange membranes which are assembled in a membrane stack. The membrane separates two liquids: (1) acid contaminated with metal and (2) deionized water. The physical laws of diffusion and electroneutrality cause material in high concentration to move to an area of low concentration without an imbalance of electrical charge. Because of the presence of the anion membrane, the metals in the concentrated solution are unable to pass from the concentrate to the DI water. However, anions in the concentrate (e.g., chlorides, sulfates, nitrates, phosphates) are permitted passage. Also, hydrogen ions, although positively charged, diffuse along with the disassociated acid (anions). The passage of hydrogen, which is key to the success of this process, is due to the small size of the hydrogen molecules and their mobility. The passage of the positively charged hydrogen ions satisfies the law of electroneutrality, preventing an imbalance of ionic charge on either side of the membrane (ref. 192, 336).
Diffusion dialysis, like other membrane technologies, is not 100 percent efficient; not all of the acid will be recovered and some leakage of metal will occur. In the laboratory, the process has yielded acid recovery efficiencies as high as 99% with 98% metal removal. In the manufacturing environment, the practical limits are 80% to 95% acid recovery with 60% to 90% of the metal contaminants removed. Also, the recovered acid may be of insufficient concentration to permit direct reuse. In such cases, vacuum evaporation may be needed to increase its concentration (ref. 338), although the economics of a concentration step are questionable. One source indicates, based on 1.5 years of experience with diffusion dialysis, that it is more efficient and economical than acid sorption for certain applications (e.g., recovery of mixed acid pickling baths) (ref. 336).
A related predecessor to this technology, Donnan dialysis, was a popular research topic for the metal finishing industry in the late 1970Õs (ref. 39). Donnan dialysis is more often associated with metal recovery applications (e.g., nickel and copper) using cation membranes than with acid bath maintenance. However, anion membrane applications of Donnan dialysis, including metal recovery from cyanide solutions are also discussed in the literature (ref. 39, 380). The recovery of nickel from rinse water was one of the primary targets of this technology. This was accomplished by separating nickel bearing rinse water from a low pH solution using a cation membrane. Nickel ions from the rinse water would replace hydrogen ions in the cation membrane matrix and pass from the nickel rinse water to the low pH solution while the hydrogen ions passed into the rinse water. Simultaneously, hydrogen ions must be replaced in the low pH solution from an outside source to maintain electroneutrality and the driving force for ion transfer. To maintain electroneutrality, two hydrogen ions (2H+) must move from the low pH solution to the rinse water when one nickel ion (Ni++) is transferred. Anions present in either compartment are restricted from migration due to the use of the cation membrane (ref. 39). No commercial metal finishing applications of Donnan dialysis are known to exist.