Metal Finishing Guide Book

2012-2013

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Fig. 5. Various types of replaceable tips. so that all of the solution passes through the filter in a certain period of time, such as 1 hr or 1 min. Note the small amount of solution that is filtered in 5 min if a rate of one turnover per hour is used (Fig. 4) as compared with the amount that would pass through at a rate of ten turnovers per hour (assume a 100-gallon solution): At one turnover per hour, 1 100 gal/60min = 1.6 gpm 5 min = 8 galfiltered At ten turnovers per hour, 10 100 gal/60 min = 16.6 gpm 5 min = 83 filtered The point here is that if nearly the entire solution is turned over every 5 min, then the plating bath will exhibit a high degree of clarity and purity. The net result should be fewer rejects caused by occlusion of particulate matter in the deposit. In modern electroplating, no area that can result in improved quality should be overlooked. The plater can use the principles of high tank turnover and solution velocity to his advantage in his quest for zero rejects. During recent years the flow rate through the filter, or tank turnover as it is referred to, has increased to two or three per hour or higher for most plating solutions (see Table I). This means that 1,000 gallons require a flow rate of at least 2,000 to 3,000 gallons per hour (7.6-11.5 m3/hr); however, platers should recognize the need and employ turnovers of 10 or even 20 times per hour when all solids must be removed (see Fig. 1). Alkaline solutions may require even higher flow rates for more effective solids removal by recirculation. Depending on the filter medium and its retention efficiency, flow rates in the range of 0.5 to 2 gpm (2 to 8 Lpm) per square foot of filter surface area are obtainable. Although 5 gpm per 10-in. (25-cm) cartridge is permissible, flow rates under 1.5 gpm per cartridge offer better economy. In fact, at a given flow rate with a cartridge filter, servicing, cartridge cleaning, or replacement can be reduced significantly by increasing the size of the filter. For example, if the size of the filter was multiplied by four the annual amount of filter cartridges consumed would be cut in half and the filter itself would operate unattended for at least four times as long before cartridge cleaning or replacement was necessary. This is an important consideration to reduce media consumption. It has also been found that the effective life of surface filters may often be tripled by doubling the surface. By increasing the dirt-holding capacity and reducing the frequency of filter servicing and replacement, the cost of filtration 680

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