Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Most filter media are rated according to the size of particles that they are capable of stopping. Such a rating is based on laboratory tests and expressed in micrometers. A coarse media would be 100 µm; a dense media would be 10, 5, or 1 µm. The number suggests that at an efficiency level of 85 to 99%, all such par- ticles would be stopped, whereas if the micrometer retention level is expressed in "absolute" ratings, 100% of the stated micrometer size and larger sizes would be removed. It further stands to reason that the coarser media will offer more solids- holding capacity, and the denser media will offer less solids-holding capacity. Next we discuss where these troublesome solids come from and how they can be most effectively removed. DIRT LOAD The "dirt" (impurities) in a working plating bath can come from drag-in, anodes, water, and airborne sources. For their efficient removal, the system must be designed for the amount and type of contaminants present in the plating tank; these vary for each installation. Even without prior operating experience, an estimate of the dirt load can be made by reviewing the cleaning and plating processes to select and size the equipment needed. A filter with insufficient dirt-holding capacity will require frequent cleaning or servicing. The rapid pressure buildup in the system as solids are retained increases the stress and wear of pump seals. By minimizing the dirt load, main- tenance of the filter and pump can be reduced considerably. Even after thorough cleaning and rinsing, some solids and contaminants cling to parts, racks, and bar- rels. Thus, they are dragged into the plating solution. The amount of drag-in con- tamination depends primarily on the type of parts, plating method (rack or barrel), cleaning efficiency and rinsing cycles. In most plating plants, the type and amount of parts being processed may vary considerably. For trouble-free operation, the filtration system should be designed for the heaviest work load and most difficult-to-clean parts. Drag-in contami- nation with barrels is high, due to incomplete draining of cleaners and difficul- ty in rinsing of loads. Filtration and purification on automatic barrel lines must be continuous, and equipment must be of sufficient size to minimize servicing and work interruption. The amount of drag-in can often be reduced by improving the pretreat- ment. With the conversion of many vapor degreasing processes to aqueous cleaning, proper maintenance of cleaners and electrocleaners is of greater importance, particularly with machined or buffed parts carrying oil and lubri- cants. Recirculation and coalescing with an overflow weir on cleaner tanks will effectively skim off oil and scum, which would quickly foul the filter medium and carbon. More effective descaling will minimize the dirt load. Several coun- tercurrent rinse tanks and a final spray rinse with clean water will also reduce the drag-in contamination. Due to the nature of the cleaning process, contamina- tion of the solution with organic soil (oil, wetting agents) and/or inorganic (metallic) compounds is sometimes unavoidable. These can generally be con- trolled by carbon treatment at the rinse tank before plating. Filterability depends on the nature, amount, and size of suspended parti- cles, which, in turn, are contingent upon the type and chemistry of the plating solution. Generally, alkaline solutions, such as cyanide baths, have slimy or flocculent difficult-to-filter insolubles, whereas most acid baths contain more grit- ty solids, which are relatively easy to filter even with a dense filter media. A 594

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