Metal Finishing Guide Book

2012-2013

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Vacuum filtration, such as that done using diatomaceous earth filters, is effective on completely detackified materials, but can add to the overall volume of waste produced due to the contributions of the disposable media itself. Centrifugal methods of sludge removal/dewatering are somewhat more expensive to purchase and install than skimming or filtration equipment but can Fig. 2. Consolidator for continuous removal of sludge make up the difference from side-draft booths by floating sludge. in cost with their performance. The two most commonly encountered centrifugal separator types are the hydrocyclone and the centrifuge. Hydrocyclones (see Fig. 3) are basically solids-concentrating devices. Liquid (in this case, paint-booth water) enters the hydrocyclone under pressure and spins around the inside surface of the cone. This spinning imparts an increased force of gravity to the liquid, which in turn causes the heavier solid particles to be pulled outward (by the centrifugal force) to the walls of the cone. The opening at the bottom releases part of the pressure, which causes the lighter liquid to be pulled back upward through the cone exit in a vortex much like a tornado (hence the ���cyclone��� part of the name). The solids (and some water) exit at the bottom. Because of the fact that some water does exit along with the paint sludge solids, hydrocyclones by themselves are not efficient dewatering devices. Typically the sludge and water exiting the cone drains into a drum or container where it is further expected to separate due to gravity. Several of these systems have been designed with secondary filtration systems (such as filter belts) to further dewater the sludge. These systems are effective if the paint remains fully detackified at all times. Any tacky paint entering the system can cause problems and plugging of both the cone and the belt. Centrifuges work on the same principle as the hydrocyclone except that, instead of the water spinning through the cone, the Fig. 3. Principle of hydrocyclone water is pumped into a spinning drum separators. 706

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