Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 5. Electrodialysis flow schematic. RO has seen limited application to nickel rinsewater. RO can separate and return clean nickel bath, but usually at too low a concentration for total return to the process bath. Also, with RO, boric acid is partially transported across the membrane requiring monitoring and make-up as required. Membrane performance decreases with operating time resulting in a decreased permeate flow rate (flux), which can be reasonably restored by periodic cleaning of the membrane. Over time, the membranes will likely require replacement due to damage from (1) hard water constituents; (2) fouling by organics; (3) gen- eral deterioration by acids or alkalis; (4) normal membrane compaction with use; and (5) destruction by oxidizing chemicals such as peroxides, hypochlorite, or chromic acid. Electrodialysis (ED) uses a "stack" of closely spaced ion exchange membranes through which ionic components of a solution are selectively transported. The driving force is a rectifier-generated voltage imposed on electrodes at the two ends of the stack. Ionic components are pulled out of a relatively dilute rinse stream (the first flowing rinse station) and accumulated in a highly concentrated stream, which can be either returned to the process, as shown in Figure 5, or oth- erwise recovered. The advantages of ED include low energy consumption, the ability to produce a highly concentrated stream for recovery, and the fact that only ionic materials are recovered, so that many undesirable impurities are retarded and rejected. On the negative side, ED is a membrane process, which requires clean feed, careful operation, and periodic maintenance to avoid damage to the stack, which is usu- ally reconditioned by the manufacturer when required. ED units can be suc- Electrodialysis 549

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