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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.2 Development and Commercialization
Membrane electrolysis is the most recent technology applied to the maintenance or purification of plating baths. Although the membrane electro-transport phenomenon was observed in the 1800's, it did not achieve any commercial application until durable, ion-selective membranes were developed in the 1950's. The first applications were for desalination of brackish water using electrodialysis. In the mid 1970's, the first attempts in the U.S. were made to use electrodialysis to reclaim plating chemicals from rinse solutions. The earliest commercial success was the recovery of nickel. The implementation of membrane electrolysis for bath purification proceeded less quickly due to operational problems with membrane fouling and electrode maintenance. Early work on bath maintenance was performed by the U.S. Bureau of Mines for rejuvenation of spent chromic acid pickling solution in the late 1970's. The Bureau built an industrial scale unit using Du PontÕs Nafion¨ ion exchange membrane and a sulfuric acid catholyte and tested it at several plating shops in the mid-west in 1979 and 1980. A commercial unit (COPS), based on the Bureau of Mines concept and design was developed and applied to a sodium dichromate/sulfuric acid pickle for brass forgings and a chromic/sulfuric acid copper strip solution (following carburization) (ref. 389). The use of membrane electrolysis for plating bath maintenance did not accelerate until the mid-1980s when a second generation of technology, also using the Nafion® membrane, was introduced for the purification of hard chromium plating baths (ref. 370). That system (Ionsep) uses a patented approach employing a caustic catholyte solution, rather than sulfuric acid, to precipitate the tramp metals as they cross the membrane. Also, during the early 1980's, commercial efforts were also directed at an ion transfer technology using a non-selective polyfluorocarbon membrane (CatNapper) (see Section 4.6). A French company manufactured a membrane electrolysis unit that was marketed in the U.S. in the mid- to late-1980's (marketed by Pentol Incorporated USA). That system was previously applied to chromic acid anodizing baths by aerospace firms in France (ref. 390). Other plating bath and pickling solution applications of membrane electrolysis including non-chromium applications, have since been commercially proven. Increased commercial activity in membrane electrolysis was spurred by cost increases for chemicals and waste disposal and environmental laws such as the requirements in the Resource Conservation and Recovery Act (RCRA) for waste minimization. These considerations have urged plating shops to search for methods of reducing the disposal frequency of process solutions.