Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 551 of 707

Fig. 6. Membrane electrolysis system schematic. cessfully used to recover gold, silver, nickel, and tin electrolytes as well as select- ed acids and rinsewater. An interesting feature of this technology is that a bright nickel electroplating bath can be circulated at a slow rate through the unit, thus providing a contin- uous removal of organic impurities, essentially eliminating the need for batch purification with its associated major losses of nickel metal. Membrane electrolysis (ME) is a membrane process driven by an electrolytic potential. It is mainly used to remove metallic impurities from plating, anodiz- ing, etching, stripping, and other metal-finishing process solutions. This tech- nology utilizes a diaphragm or an ion exchange membrane and an electrical poten- tial applied across the diaphragm or membrane. Compared to electrodialysis, most membrane electrolysis systems utilize only a single membrane or diaphragm posi- tioned between two electrodes. Membrane Electrolysis The use of ion exchange membranes is advantageous because higher ion transfer rates can be achieved in comparison to inorganic- or organic-based diaphragms. Ion exchange membranes are ion permeable and selective, permit- ting ions of a given electrical charge to pass through. Cation exchange membranes allow only cations, such as copper or aluminum, to pass through. Similarly, anion exchange membranes allow only anions, such as sulfates or chlorides, to pass through. The efficiency of ME depends on the migration rate of ions through the ion exchange membranes. The energy required is the sum of two terms: (1) the elec- trical energy required to transfer the ionic components from one solution through the membrane into another solution, and (2) the energy required to pump the solutions through the unit. Electrochemical reactions at the electrodes are other energy-consuming processes, but the energy consumed for electrode reactions is generally less than 1.0% of the total energy used for ion transfer. 550

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