Metal Finishing Guide Book


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Fig. 1. This drawing represents a complete waste treatment strategy for electrocoating systems. Waste streams from cleaning, phosphating, chrome sealing, and coating operations are initially segregated for individualized treatment before combining pH adjustment and flocculation for solids removal. Segregated paint treatment stages are shown in the box at the upper right. Solutions containing paint wastes are first pumped to the E-coat waste tank, a storage container dedicated to that purpose. Because the volume is low, they are treated on a batch basis in the paint detackification tank. When cathodic paint is present, a detackification polymer is dissolved into the solution and the pH is raised to approximately 9.0. The paint destabilizes and becomes a small curdlike substance that is no longer tacky. If anodic paint is treated, the pH is lowered to 4.5 after addition of the detackification polymer. The paint will again destabilize and form a curd. The detackified paint curd is separated from the liquid portion of the paint by a dedicated filter press or some other filtering device. The filter cake can be reduced in volume by further dewatering or evaporation. In any case, it must be disposed of properly as it may bea RCRA hazardous waste under federal code 40 CFR 261.31, FO19. Liquid remaining after the filter press step is sent to the equalization tank where it is mixed with waste solutions from pretreatment processes before further treatment in a common waste stream including pH adjustment and clarification. During tank cleaning operations, the solution in the electrocoating tank or any of the post rinses are pumped to the E-coat waste tank for storage. The remaining paint wastes cleaned out the tank are flushed directly to the paint detackification tank for treatment. The contents of the E-coat waste tank can then be returned to the tank from which they originated. Proper handling of waste products created by the cleaning, pretreatment, and painting processes is an extremely important part of the entire electrocoating equation. Waste treatment strategies must focus on removing impurities from the system while retaining paint chemistry. Although small in volume, paint wastes are registered substances, which must be segregated and removed from the waste stream prior to proper disposal of the solids and pH-adjusted discharge of treated liquid components. 671

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