Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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FILTRATION Filtration will greatly prolong bath life and allow for a more uniform cleaning process. Aqueous chemistries are designed to remove soils. Most chemistries are designed to emulsify or suspend soils in the wash bath. In time these soils will build up in the wash bath. For spray-type systems filtration should be inline between pump and spray headers. Filters should be constructed of stainless steel and sized to be, as a min- imum, 20% above pump volumetric flow rate. For bath-type systems filtration should be designed to exchange the total volume of the bath every three to five minutes. A tank bath of 200 gal should have, as a minimum, a 40 gpm pump and 50 gpm filter system. Filtration is designed to remove suspended finds, particulate, soils, dirt, etc. Good filtration will pre- vent redeposit of the soils on the parts. OIL REMOVAL Most chemistries are designed to split petroleum and tramp oil and cause the oil to float to the tank surface. Many types of oil removal systems are available and vary in price, complexity, and efficiency. Disk, belt, and decant systems are marginally effective. Coalescing and ultrafiltration are the most effective. Coalescing systems with surface skimmers or spargers are required for bath or agi- tating systems to ensure that the top surface of the water is free of oil prior to the removal of parts. Oil coalescers are normally freestanding units constructed of stainless steel. A coalescer is designed with perforated partitions, making it multiple tanks. The first area is a still tank. The second area is normally filled with polypropylene tubes. The polypropylene tubes are efficient in their ability to collect oil. The tubes attract oil globules to as small as 20 micron. When the oil globule builds and reaches a dimensional size of between 100 and 200 microns it breaks free from the polypropylene tube and floats to the surface of the tank where it is skimmed; therefore, the coalescer is effective in removing oil as small as 20 micron. The coalescor collects only oil and not water and chemistry. The coalescer then pumps clean water to a sparger pipe positioned to skim the top surface of the tank to an overflow weir. The overflow weir is connected to the coalescer and the system operates as a closed loop. Emulsified oils must be handled differently. Membrane or evaporation tech- nologies must be incorporated to deal with the emulsified oils in a solution bath. WASTE TREATMENT Alkaline chemistries and rinsewaters, by themselves, are normally not an environmental issue. Chemistries are designed to remove soils and oil from parts. If the soils are heavy- metal laden they will have to be treated accordingly. If petroleum-based oils are used, coalescing and other oil-extraction systems can readily remove these oils from baths prior to discharge. If emulsified oils are present in the bath these will have to be dealt with as reg- ulated by local EPA authorities. Either evaporation or membrane technologies, such as ultrafiltration, should be employed to remove emulsified oils prior to solu- tion discharge to drain. Normally rinsewaters are sufficiently free of heavy metals and emulsified oils and may be readily dischargable. 109

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