Metal Finishing Guide Book

2012-2013

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Fig. 7. Skimmer, pump, and prefilter with carbon or free oil separator. to settle in a cone-shaped tank. The supernatant liquid drains into a head box, which directs the flow across the filter medium (paper or plastic) supported by a motor-driven conveyor belt. The liquid passes through the disposable fabric by gravity flow into a receiving tank below. When the pores of the media become clogged, the liquid level rises and a float switch activates the belt drive. Fresh media is fed over the tank and filtration is continuous. The cake on the fabric is allowed to drain before it is dumped into the sludge box. Gravity drain or an immersion pump empties the filtered water from the tank. Cycling and indexing of the filter are automatic. The occasional replacement of the filter fabric roll is the only labor required. The sediment in the bottom of the cone can also be dewatered periodically by filtration on the fabric. Other systems feature pressure or vacuum filtration. The sludge cake contains from 5 to 35% solids, depending upon the equipment and type of cake. Cakes can be further treated by air evaporation or with heat for dry disposal. The filtrate can be discharged to the sewer if it meets local effluent regulations or can be recycled through the system. The performance of the unit can be improved greatly by the addition of coagulants and flocculating agents, such as polyelectrolytes, which increase the amount of solids, particle size, and settling rate. The flow rate is approximately 1 gpm/ft2 with 90 to 95% solids retention; with coarse filter media, flow rates increase up to 10 gpm/ft2. Filter aid can also be precoated to improve retention. The filter media is available in porosities of 1 to 125 ��m and rolls 500 yd long. Carbon-impregnated paper is used for purification and removal of organic contaminants. The unit must be sized properly for each application to operate efficiently and with a minimum media cost. Steel, coated, stainless steel, or plastic units are available for corrosive solutions. BATCH AND CONTINUOUS ACTIVATED-CARBON PURIFICATION Virtually all plating solutions and some cleaners or rinses at some time will require purification via the adsorption of impurities on activated carbon. Those solutions that contain wetting agents require the most carbon; when oil is introduced into the bath, the carbon is dispersed throughout the solution and clings to the parts, causing peeling or spotty work. Solutions that do not contain wetting agents have a tendency to float oil to one corner, depending on the recirculation set up by the pump, and in this case the oil may be removed with a skimmer or coalescer (see Fig. 7). 686

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