Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 249 of 331

Side-Draft Systems In smaller booths, the most common method of sludge removal has historically been skimming. Some portion of most solvent-based paints will usually float if un- treated; caustic-based treatments will typically result in partial float/partial sink on a continuous basis, especially when a flotation aid is used. Many users of small booths were, therefore, accustomed to continual skimming of floating material from their systems. With the advent of paint-dispersing polymer treatments, continuous manual skimming is unnecessary. Elimination of this process reduces much of the daily la- bor and its associated costs. In side-draft systems, use of a polymer paint detacki- fier normally keeps paint in a suspended, dispersed state, allowing for flocculation and flotation on a batch (pe- riodic) basis. Manual skim- ming, with screens or rakes, is still possible at this point. Manual skimming has the next-to-lowest capital cost (the lowest being passive settling, which will be discussed in de- tail in the downdraft section) but is also labor intensive. The next level of sophisti- cation in side-draft sludge re- moval would be the use of semiautomatic or automatic Fig. 1. Tank-side weir for removing floating sludge. equipment to remove the floating waste. One way of reducing labor and elimi- nating manual skimming in batch flocculation clean outs would be to use a wet- vacuum filtration system. This basically consists of an industrial wet-vacuum head on a steel drum containing a burlap (or other coarse filter cloth) bag. The floating sludge (and some water) is vacuumed from the top of the booth tank. The paint sludge should collect in the bag, while the water is drained (or pumped) from the bottom of the drum back into the booth. This method can also be used for sludge settling out on the booth tank bottom, although the settled sludge must be completely detackified. Another method for removing periodically produced float- ing sludge is the use of a tank-side weir (see Fig. 1) In essence, a small weir is welded onto the side of the booth tank, allowing floating material to overflow from the booth and be pumped to a filtering tank (or other system) for dewatering. Side-draft booths can also be equipped for automatic continuous removal of float- ing sludge, using equipment generically referred to as a consolidator (see Fig. 2). This type of system pumps water from the booth into a separate tank. As the water is pumped in, a flocculating polymer is injected into the water, causing the detack- ified paint sludge to float to the top where it is skimmed off by a continuously mov- ing blade. The clean water is cycled back into the booth. Paint sludge can also be re- moved continuously without flocculation/floating using filtration methods. The simplest filtration equipment consists of filter beds utilizing paper or cloth media. These systems allow the solid material to settle out on the filter media,with the wa- ter draining to some collection unit where it can be returned to the booth. Although this type of system has low labor and capital requirements, it is often very cumber- some, which can be a problem since space around a painting area is usually at a pre- mium. Gravity filtration systems are also slow and restricted as to through put vol- ume, which makes them suitable for only low levels of water or sludge to be processed. 248

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