Metal Finishing Guide Book


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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, carbon 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 canister 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, followed by some settling before transfer clarification can be achieved. Continuous Purification A separate purification chamber holding bulk granular carbon, a carbon canister, 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 carbon 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 replaceable filters that employ this type of media. They may include an outer layer, which serves as a prefilter, and an inner layer, which serves as a trap filter. These handy cartridges are ideal for small filter chambers because of the ease and convenience of quickly replacing a conventional depth tube with the carbon tube when necessary. They may also be used with submersible filter systems, but in this case the flow rate could be greatly reduced. Carbon Canister Granular carbon may be used in ready-to-use chambers, each with a number of canisters holding up to 10 lb of granular carbon, and placed in line to the tank. A built-in trap filter eliminates migration of the carbon. Prefiltration ahead of the purification chamber will prevent solids from coating the surface of the carbon in the canister, assuring maximum adsorbency. The carbon in the canister can be replaced when its adsorption capacity has been reached. This method of separate purification offers the most flexibility. Any portion or all of the filtrate can be treated as needed by means of a bypass valve after the filter. 644

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