Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 7. Diffusion dialysis system schematic. The total electrical potential drop across an ME cell includes the concentra- tion polarization and the electrical potential required to overcome the electrical resistance of the cell itself. This resistance is caused by the friction between ions, membranes, and water during transfer from one solution to another, all of which results in an irreversible energy dissipation in the form of heat. Because of the heat generated, the total energy required in practice is significantly higher than the theoretical minimum energy required. The energy necessary to remove metals from a solution is directly proportional to the total current flowing through the cell and the voltage drop between the two electrodes. The electric current required to remove metals from a solution is direct- ly proportional to the number of ions transferred through the ion exchange mem- brane from the anolyte to the catholyte. The electrical energy required in ME is directly proportional to the quantity of metal (cations) that must be removed from a certain volume of anolyte to achieve the desired product quality. Energy consumption is also a function of the electrical resistance of a cell pair. The electrical resistance of a cell pair is a function of the individual resistances of the membrane and the solution in the cell. Furthermore, because the resistance of the solution is directly proportional to its ionic concentration, the overall resis- tance of a cell is usually determined by the resistance of the weaker electrolyte. Figure 6 is a schematic of the ME cell. ME can be utilized to remove metal impurities from process baths, such as etch and stripping baths, as well as conversion coating, chemical milling, and sealing solutions. An effective membrane surface area between anolyte or process solu- tion and catholyte of 0.07 m2 or 0.75 ft2 allows a maximum amperage of 60 to 100 A for process solution purification. This membrane electrolysis process does not only remove metals from process solutions but also helps to maintain these solu- tions at certain activity levels. When applied for the purification of a very corrosive solution that can dissolve metal electrodes, a three-compartment ME system must be used. A center com- 551

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