Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 599 of 707

Fig. 3. Typical flow versus pressure curve. Q represents the maximum open pumping against no restriction, whereas P represents the pressure that the pump can develop at zero flow. A might indicate the pressure drop across a depth type media or a bare support membrane, whereas points B and C indicate the reduction in flow caused by the addition of filter aid and carbon, respectively. FREQUENCY OF FILTRATION AND PURIFICATION Since it is desirable to plate with a solution as free of suspended solids as possi- ble, the quickest way to achieve clarification is by transfer pumping all of the solu- tion from one tank, through a filter, to another tank (batch treatment); howev- er, to maintain both clarity and uniform deposit quality, continuous recirculation through a filter is most effective. Although continuous filtration is more desir- able, 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 pos- sible 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 con- tinued 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 neces- sary. In practice, contaminants are not introduced at a steady rate; for instance, most are introduced with the parts to be plated and, therefore, at the moment of immersion the degree of contamination is sharply increased until it is again reduced by the action of the filters. It then increases again when more parts are put into the tank for plating. Figure 2 indicates the reduction in flow caused by the dirt buildup in the filter on a day-to-day basis, where one week's filtration would be effected before service of the filter becomes necessary. This reduction in flow rate could also have been representative of a longer time interval between filter 598

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