Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 257 of 331

systems and equipment ELECTROCOATING BY GORDON S. JOHNSON TTX ENVIRONMENTAL, STURGEON BAY, WIS. Because it is an immersion process, electrocoating employs the use of a large amount of water. Typically, pigment, resin, and additives make up only 10 to 15% of the contents of an electrocoating paint tank. Electrocoating is also an ex- tremely efficient coating process due to recycling of paint through ultrafiltration, with usage typically ranging from 95 to 99%; however, the small amount of paint, which may at some point elude deposition, must be extracted from the waste stream prior to discharge. Wastewater treatment strategies for electrocoating — like those for pretreat- ment stages — are based on three main considerations: (1) removing impurities from the process tanks; (2) retaining useful materials in the process tanks; (3) min- imizing the impurities for disposal. Waste streams from other sources, i.e., pretreatment stages, may not be initially compatible for treatment with electrocoating paint wastes. Problems arise often enough to warrant conservative strategies — the most basic of which is to sepa- rate the different wastes for differing methods of treatment before combining the resultant waste streams for common disposal (Fig. 1). Cleaning wastes are one category. Zinc and iron phosphate conversion coatings and their rinses are an- other. The chrome seal and the rinse(s) for that stage area third category. Final- ly, there are the E-coat paint wastes themselves. Treatment for pretreatment wastes has been examined elsewhere in this publica- tion. Here, we will consider treatment strategies specific to the electrocoating process. ELECTROCOATING WASTES As was previously stated, E-coating is an extremely efficient process in which 95% or more of the components (resin, pigment, and other additives) entered into the paint tank will eventually find themselves applied and cured on the product. Ex- cept in rare cases of catastrophic tank contamination, the amount of paint solids requiring waste treatment on a regular operating basis is typically very small. Solution from the paint tank continually undergoes ultrafiltration (UF) to pre- vent process contamination and produce final rinse makeup. Typically fronting the UF units are one or more bag filters of increasing filtering capabilities, which remove foreign particulates that have entered the paint bath. After ultrafiltration, the paint itself is returned to the tank while the permeate is pumped into the fi- nal post rinse. The post rinses counterflow back to the paint tank, returning excess paint to the electrocoating stage. Ultrafiltration is not considered a portion of the waste treatment system, al- though a small amount of paint will be removed from this closed loop when the bag filters are replaced. The bag filters are disposed of along with concentrated paint wastes. The main sources of paint waste are tears in anodes, tank cleaning and ul- trafilter cleaning operations, spillage, and final post rinse dumping in situa- tions where reverse osmosis (RO) water is used as tank makeup. 256 WASTEWATER TREATMENT FOR

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