Metal Finishing Guide Book


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Fig. 1. Higher clarification is achieved by, increasing the number of tank turnovers per hour. readily determined. For instance, a 100 gal/min (gpm) effluent containing 100 ppm TSS (100 mg/L) will generate 5 lb of solids per hour, as calculated below: 100 gpm 3.79 L/gal 100 mg/L 60 min/hr (1000 mg/g 454 g/lb) = 5 lb/hr (2.3 kg/hr) Therefore, the filter must have sufficient capacity to hold approximately 40 lb of solids/8 hr of operation. A horizontal gravity filter would be the most cost efficient 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 ultimate level of acceptable quality. This brings up an important consideration. Contamination by organic compounds, inorganic salts, wetting agents, and oils is not removed by filtration, but by adsorption on activated carbon. Some plating solutions, such as bright nickel baths, generate organic byproducts during plating. It cannot be assumed that both types of contamination increase at the same rate. A batch treatment, therefore, may eventually become necessary, either because of insoluble or soluble impurities. 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 solution 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 677

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