Metal Finishing Guide Book


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Fig. 4. Reverse osmosis flow schematic. The energy demand of a single-stage vacuum evaporator is roughly 1,000 Btu/lb water evaporated, or roughly 9,000 Btu/gal of water evaporated (allowing for losses), the same as the theoretical energy requirement for atmospheric operation. Because a high percentage of drag-out is usually returned with either atmospheric or vacuum evaporation, impurity removal and management may be required. Such purification techniques are well established. In the case of chrome baths, and thanks to the fact that chromium is present as an anionic complex, cation exchange or electropurification systems can be easily applied in a separate hydraulic loop around the rinse system to remove and control any cationic impurities that may accumulate. For chromium etch systems, electrolytic reoxidation of trivalent chromium or electropurification, should be considered. In this application, electropurification will produce less discharge than would a cation exchanger by its associated reagent waste stream. Contaminant removal or purification techniques normally used with other baths, such as carbon filtration or dummying for nickel baths, membrane electrolysis for metal impurity control, or carbonate removal from cyanide baths, can continue to be applied to the process baths as required. Vacuum evaporation has been successfully and dependably used for many years to recover a wide variety of plating baths including such difficult chemistries as encountered in chromic acid plating and chromic/sulfuric acid etch baths. Associated rinsewaters are also recovered for reuse in the plating process. An application for vacuum evaporation of some increasing interest is brine concentration. In some localities, the discharge of pretreated metal-finishing effluent is being restricted because the effluent still has a high salt concentration. Salt is the unfortunate and unavoidable byproduct of chemical treatment of metalbearing wastewater. Usually, pretreated wastewater effluent is further processed by membrane systems to further separate and consolidate the mixed salt solution. The reject from this step can then be processed by any of several types of vacuum evaporator to concentrate the brine either to a level slightly below the limit of solubility of the salt mixture or slightly beyond to produce a concentrate discharge from which 648

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