Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 250 of 331

Vacuum filtration, such as that done using di- atomaceous earth fil- ters, is effective on com- pletely detackified ma- terials, but can add to the overall volume of waste produced due to the contributions of the disposable media itself. Centrifugal methods of sludge removal/de- watering are somewhat more expensive to pur- chase and install than skimming or filtration equipment but can make up the difference in cost with their per- formance. The two most commonly encountered centrifugal separator types are the hydrocyclone and the centrifuge. Hydrocyclones (see Fig. 3) are basically solids-con- centrating devices. Liquid (in this case, paint-booth water) enters the hydrocyclone un- der pressure and spins around the inside sur- face of the cone. This spinning imparts an in- creased 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 bot- tom releases part of the pressure, which caus- es 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 bot- tom. Because of the fact that some water does exit along with the paint sludge solids, hydro- cyclones by themselves are not efficient dewa- tering devices. Typically the sludge and water exiting the cone drains into a drum or con- tainer where it is further expected to separate due to gravity. Several of these systems have been designed with secondary filtration sys- tems (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. Fig. 2. Consolidator for continuous removal of sludge from side-draft booths by floating sludge. Centrifuges work on the same principle Fig. 3. Principle of hydrocyclone separators. as the hydrocyclone except that, instead of the water spinning through the cone, the water is pumped into a spinning drum 249

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