Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 251 of 331

(much like a clothes drier set on end), which imparts the centrifugal force that "throws" the water out of the solids. A cake of solid material then builds up on the walls of the drum. In the simplest of these systems, the centrifuge (see Fig. 4) is al- lowed to operate until the drum is full, after which the drum is removed and man- ually emptied. The more elaborate systems will periodically go through a "cleaning" cycle where the solids will be automatically scraped from the drum and allowed to fall into a container. These sys- tems can produce sludge at a level of 85% solids or more (with some paints), as com- pared to the average of 40% to 60% solids from a hydro- cyclone. The major downside of this is, as previously men- tioned, the high equipment cost. A fully automatic system will cost upwards of $35,000, as compared to the $15,000 for a hydrocyclone. Since these centrifugal- type systems function by pulling solids directly from the booth water, it is vital to maintain uniformity of the water through agitation and circulation. To assure com- plete agitation, many of these centrifugal systems are pack- aged with booth agitation equipment, which may in- clude some type of tank bot- tom sprayers. Since poly- meric detackifiers tend to settle out in still water, the addition of bottom circulation may enhance the operation of the chemicals, not only with centrifugal systems but in all operations. Depending on the method chosen and its efficiency, the system water can be vir- tually free of contaminants after treatment. With an efficient method of solids removal, the water can be reused in the booth for sometime, conserving water usage and re- ducing disposal costs. The selection of the chemical treatment program and sludge removal system is dependent upon the type of paint, type of booth, and the money available. Virtually any level of water quality is achievable, given that capital is avail- able; for example, distillation equipment can be purchased for complete solvent re- moval from water and infrared drying systems are available to reduce paint sludge to a dry powder to minimize the cost of disposal. Most companies, however, do not have unlimited capital to spend on paint booths or the related products and equip- ment for water clean up. Using the information provided here and an understand- ing of the particular system in question, an end user should be able to pick the right chemical and mechanical means to minimize the water pollutants coming from the paint shop. Doing so will reduce overall operating costs, reduce water consumption, and help in conserving one of the most important natural resources. Fig. 4. Centrifuge system for sludge dewatering. 250

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