Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 598 of 707

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. cient for this dirt load and would operate automatically; however, if dryness of the retained solids is to be achieved, then a filter press would be recommended. Filtration and/or purification during nonproductive hours makes it possible to remove dirt at a time when no additional contaminants are being introduced into the tank, such as insolubles from anodes, chemical additions, plus that which would otherwise be dragged in from improper cleaning of the work. Again, individual tank operating characteristics and economics will determine the ulti- mate level of acceptable quality. This brings up an important consideration. Contamination by organic com- pounds, inorganic salts, wetting agents, and oils is not removed by filtration, but by adsorption on activated carbon. Some plating solutions, such as bright nick- el baths, generate organic byproducts during plating. It cannot be assumed that both types of contamination increase at the same rate. A batch treatment, there- fore, may eventually become necessary, either because of insoluble or soluble impu- rities. A check of clarity, flow rate, and work appearance and a Hull cell test will indicate the need for transfer filtration and/or carbon treatment. If analysis shows that the concentration of insolubles (in ppm) has increased, it would indicate that the solution is not being adequately filtered. Therefore, transfer pumping of the solution through the filter should be employed as the quickest way of getting all the solids out at once and returning the clean solu- tion to the plating tank. Soluble impurities can be detected by inspection of the work on a Hull cell panel. Pitting, poor adhesion, or spotty appearance indicates the need for fresh carbon. Here again, it may be desirable to completely batch treat the solution to restore it to good plating quality; however, since this necessitates shutting down the plating line and requires considerable labor, every effort should be made to maintain solution clarity and purity continuously, with- out having to resort to such batch treatment. 597

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