Metal Finishing Guide Book


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employed during the entire lifespan of the cartridge. This is due, in part, to the higher head pressures of pumps employed without chancing the rupture of a cartridge. Since all of the dirt is retained on and in the cartridge, the cartridge filter can be turned off and on at will, unless the cartridges are precoated. Cartridges are changed with very little maintenance expense and no solution loss; however, simplicity of use is perhaps the most predominant single factor in their selection. Precoat Filters Precoated filters consist of a membrane (leaf, sleeve, or screen) such as paper, cloth, ceramic, sintered metal, wire mesh, or wound cartridges. These membranes support the diatomite or fibrous-type filter aid, which has been mixed in a slurry of water or plating solution and picked up by the membrane openings. The dirt is retained on the outer surface of the cake. When the pressure has increased and the flow rate has decreased to a point where filtration is no longer efficient, the dirt and cake are washed from the membrane. Paper membranes are discarded and replaced. The ability to obtain long runs is dependent upon proper selection of the foundation media, coupled with a coarser-than-usual nonfibrous-type filter aid (to be used where possible). Periodic (daily, if necessary) additions of small quantities of filter aid should be made to lengthen the cycle between servicing. The dirt-holding capacity of this type of filter is usually measured in square feet of filter surface. (If the standard 2.5 x 10-in. long cartridge is used, its outer surface when precoated would be equivalent to about 0.50 to 0.67 ft2 of area.) Flow rate and dirt-holding capacity of the various precoated membranes or cartridges would be about equal. Before precoating, the operator should know or determine the filtration area to be covered. The amount of filter aid used depends on its type and on the solution being filtered. Generally, 0.5 to 2 oz/ft2 of filter is sufficient. The manufacturer's recommendations for type and amount of filter aid should be followed if optimum results are to be obtained. A slurry of filter aid and plating solution or water is mixed in a separate container or in a slurry tank, which may be an integral part of the filtration system. The slurry is then caused to flow through the filter media and create a filter cake. Usual flow rates range from 0.5 to 2 gpm/ft2 of filter surface. A lower flow rate improves particle retention and smaller particles will be removed. It should be pointed out that, although there may be a wide range in flow rate, the range of selectivity of particles being removed is between 0.5 and 5 μm, which is the most significant difference between precoat and depth-type cartridges and offers a wider choice of porosity. Buildup of cake should be gradual, and recirculation should continue until the solution runs clear. Cake should be dispersed uniformly across the media before the plating solution is allowed to flow across the filter. A slurry tank piped and valved into the filtration system becomes a convenient and versatile piece of equipment. The slurry may be prepared with plating solution, rather than water, to avoid diluting critical mixtures. Via valving, the solution is drawn into the slurry tank for sampling, preparation of slurry, and chemical additions. Similarly, the solution is returned to the plating tank. This method eliminates the necessity of transfer hoses between tanks, and the subsequent risk of loosening the cake or losing pump prime. The integral slurry tank is also a convenient storage for backwash water. 641

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