Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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tridges of 10 µm down to 1 µm will have a dirt-holding capacity of perhaps 3 oz to less than 0.5 oz. These figures merely indicate that the coarser cartridges have greater dirt-holding capacity, are more economical to use, and can be used longer before replacement. Also, as pointed out earlier, dirt loads vary from tank to tank, and cartridges should be selected according to the individual requirements. A dense cartridge having less dirt-holding capacity will load up more quickly, increasing the pres- sure differential and, therefore, reducing the flow (Fig. 5). Using coarser cartridges (greater than 30 µm on zinc, for example) that have greater dirt-holding capac- ity and a longer service life may make it possible to clarify the plating tank more quickly because of the high obtainable flow rate. This will be accom- plished at less cost. Usually two cartridges (three on zinc, tin, and cadmium) are recommended for each 100 gal of tank capacity. The pump should provide a pumping rate of at least 100 gph (two tank turnovers per hour) for each cartridge. Usually, a cartridge life of 6 weeks on nickel or 4 weeks on zinc can be expected, with some tanks running as long as 12 weeks; however, much depends upon dirt load, hours of plating, and so on. With cartridges, a higher dirt load can be retained in the filter chamber because of the coarseness of the filter media. Higher flow rates can usually be 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, sim- plicity 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 sup- port 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 foun- dation 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-hold- ing capacity of this type of filter is usually measured in square feet of filter sur- face. (If the standard 2.5 x 10-in. long cartridge is used, its outer surface when pre- coated 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 solu- tion being filtered. Generally, 0.5 to 2 oz/ft2 of filter is sufficient. The manufac- turer'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 inte- gral part of the filtration system. The slurry is then caused to flow through the 603

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