Metal Finishing Guide Book


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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. 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, 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 cleanings. Graphically, it indicates why platers may experience roughness at varying intervals in the plating filtration cycle. The amount of solids increases in the tank as the flow rate decreases to a level that may cause rejects. After the filter is serviced, the increased flow rate agitates any settled solids. Therefore, it is advisable to delay plating of parts until the contaminant level is again reduced by filtration to within tolerable limits. This phenomenon generally occurs in a still tank, since the dirt has more chance to settle. For this reason, when the solution is pumped into a treatment tank, sludge may be found on the bottom of the plating tank. Dirt in an air-agitated tank can settle any time after the air is shut off. If carbon and/or a filter aid is used in the filter during the continuous filtration cycle, it should be borne in mind that, as these solids are collected on the media, the pressure increases appreciably, reducing the initial flow rate by almost 25% and the overall volume pumped through the filter by as much as 50% before servicing is necessary (Fig. 3). Frequent laboratory checks will verify the amount of insolubles in the plating tank, which will tell whether a uniform degree of clarity is being maintained or whether it is increasing slowly toward the reject level. More frequent servicing of the existing filtration equipment will increase the total volume pumped and, in turn, maintain the lowest possible level of contamination and minimize the need for batch treatment. It is, therefore, necessary for the plater to determine the particle size to 636

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