Metal Finishing Guide Book


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Fig. 2. Single-stage atmospheric evaporation schematic. impurity accumulation, complete return of drag-out necessitates purification/maintenance operations or may increase the frequency of those already practiced. Since virtually every such operation creates loss of bath this is again an offsetting consideration to any recovery that is being gained. A proper analysis of the optimum scheme should include all losses from the operation and the impact the recovery of drag-out will have on other sources of loss. Evaporation Evaporation is the oldest and most broadly applied of the separation technologies and has an extensive operating history. In the surface-finishing industry, evaporative recovery is classified as a concentrate and return technology and its track record and benefits are well demonstrated. Evaporation is routinely used for point source separation and recovery of plating baths and their associated rinsewaters for recycle to the finishing system. Evaporation is also being used successfully to minimize liquid discharges from manufacturing plants by concentrating certain pretreated wastewaters, or brines, for haul-away and disposal while recovering additional process water for recycle to the process. Compared to other separation methods, evaporation is more energy intensive; however, it is the only recovery technology that can treat plating rinsewaters to separate the solvent (water) from the dissolved chemicals and concentrate the remaining solution back to, or even beyond, bath strength. To minimize energy consumption recovery rinsewater volume can be minimized by the application of counter-current rinse hydraulics. On the positive side, evaporation is a straightforward, rugged, reliable, broadly applicable, and widely practiced recovery technique. Materials of construction are available for virtually any process bath. 644

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