Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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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 coag- ulants 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 increase up to 10 gpm/ft2 with 90 to 95% solids retention; with coarse filter media, flow rates . Filter aid can also be precoated to improve retention. The filter media is available in porosities of 1 to 125 ┬Ám and rolls 500 yd long. Carbon-impregnated paper is used for purification and removal of organic con- taminants. The unit must be sized properly for each application to operate effi- ciently 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 solu- tions 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). The choice of purification method depends on the size of tank and amount of carbon required and also on other available auxiliary equipment. Generally, car- bon cartridges are used on small tanks (up to a few hundred gallons), and the bulk or canister type or the precoat method is used for the very largest tanks. The can- ister type is also used on the larger tanks supplemental to surface or depth-type cartridges or on certain automatic filters to supplement the amount of carbon. Batch Treatment The quality of the carbon is important and special sulfur-free grades are available. The average dosage is 10 lb of carbon to treat 500 to 1,000 gal of warm plating solution. At least sixty minutes contact time with agitation should be allowed, fol- lowed by some settling before transfer clarification can be achieved. Continuous Purification A separate purification chamber holding bulk granular carbon, a carbon can- ister, or cartridges offers the most flexibility in purification treatment. By means of bypass valving, the amount and rate of flow through the carbon can be regulated to achieve optimum adsorption of impurities without complete depletion of wetting agents and brighteners in the plating bath. It provides for uninterrupted production and fewer rejects. When necessary, the carbon can be changed without stopping filtration of the bath. Filtration should always precede carbon treatment, to prevent dirt particles from covering the car- bon surfaces. CONTINUOUS CARBON TREATMENT METHODS Carbon Cartridges Cartridges containing up to 8 oz of either powdered or granular carbon for every 10 in. of cartridge length are available and will fit most standard replace- able filters that employ this type of media. They may include an outer layer, which 606

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