Metal Finishing Guide Book


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Page 642 of 843

environmental controls FILTRATION AND PURIFICATION OF PLATING AND RELATED SOLUTIONS AND EFFLUENTS BY JACK H. BERG SERFILCO LTD., NORTHBROOK, ILL.; WWW.SERFILCO.COM This introduction reflects the response needed by platers for quality control, to meet just-in-time deliveries, and to achieve zero rejects. It also addresses the need for platers to continue to reduce solid waste after neutralization and employ filtration wherever possible to recycle or lengthen the service life of cleaners, etchants, and rinses. Filtration usually includes the use of carbon for undesirable organic impurity removal, which years ago also doubled as a filter media along with other forms of filter aids. Today's acceptance of granular carbon in many situations has lessened the need for powdered carbon and almost eliminated the weekly or monthly batch purification treatment. There are, however, some occasions when powdered carbon may be the only answer, and for that reason a separate piece of equipment held aside for such a need should be considered. Platers who appreciate the value of filtration must first understand that it is not as much an art as it is a science. The requirement of a science is to have an orderly body of facts, facts that can be correlated and anticipated results yielded. Although there has been some work done in this area over the last 5-10 years, platers must still rely on experience to a great extent. In the past, it has been suggested that the plater decide the level of quality sought and, using statistical quality control, determine if this goal has been achieved. It is further recommended that the plater needs to know the parts per million of contamination (solids) so that the necessary size or dirt-holding (solids) capacity of the filter could be established. The plater must also know the nature of the solids, which would be critical to success. Slimy, stringy, or oily contaminants blind a dense filter media surface quickly, whereas coarse, grainy, sandlike particles build a thick cake and still allow solution to pass, which provides for continued solid/liquid separation. By first assessing these factors, platers can ascertain what results can be achieved. For example, slimy solids would require more surface area, whereas gritty particles could get by with less area (i.e., less solids-holding capacity). However, all filter media are not manufactured in the same manner, for instance, filter paper, cloth, and plastic membranes provide a single junction to stop solids. Filter aids can enhance the ability of the filter media by creating a porous cake, which improves surface flow, but to really be successful a continuous mixing of filter aid and solids must be coordinated to maintain suitable porosity. Other types of filter media can provide the necessary junction to stop solids but are built in such a manner as to achieve results from a combination of surfaces or juncture points, which achieve the solids retention by impedance. Thus, it is possible for continuous solid/liquid separation to be maintained over a longer period of time. 631

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