Metal Finishing Guide Book


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Fig. 2. Clean filter at point A will flow 4,800 gph and dirt removal is maximum. Flow rate has dropped to 2,000 gph at point B. Situation applied to a 2,000-galtank would represent a reduction in flow from almost 2.5 tank turnovers per hour to one tank turnover per hour during a time interval of one work week. If filter continued to operate without servicing, the rate of dirt removed would soon be less than the rate of dirt introduced into the system. The time interval during which the filter is performing effective filtration will be determined by job conditions. necessitates shutting down the plating line and requires considerable labor, every effort should be made to maintain solution clarity and purity continuously, without having to resort to such batch treatment. FREQUENCY OF FILTRATION AND PURIFICATION Since it is desirable to plate with a solution as free of suspended solids as possible, the quickest way to achieve clarification is by transfer pumping all of the solution from one tank, through a filter, to another tank (batch treatment); however, to maintain both clarity and uniform deposit quality, continuous recirculation through a filter is most effective. Although continuous filtration is more desirable, there are some plating installations that require only intermittent filtration, because relatively small amounts of solids are present. In other cases, it is necessary to filter and purify the bath continuously, even when not plating. A high flow rate is essential to bring the particles to the filter as quickly as possible and to prevent settling of dirt on parts being plated. Although plating in a solution completely free of solids would be best, this ideal can be approached only in the laboratory. Some contamination always exists, and must be accepted. Continuous filtration at a high flow rate can maintain a high level of product quality by keeping suspended solids to a minimum. As Figure 1 indicates, four to five complete tank turnovers effectively remove 97% of all filterable materials if no additional solids are introduced. Since, in many installations, the rate at which contamination is introduced is higher than the rate at which it is removed, the impurities and solids gradually increase with time unless filtration is continued even during nonplating periods. The greater the turnover rate, the longer the plating bath can be operated before the reject rate becomes too high and batch (transfer) filtration is necessary. In practice, contaminants are not introduced at a steady rate; for instance, 678

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