Metal Finishing Guide Book


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(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 Fig. 2. Consolidator for continuous removal of sludge opening at the bottom from side-draft booths by floating sludge. 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 water is pumped into a spinning drum (much like a clothes drier set on end), which imparts the centrifugal force that "throws" the water out of the solids. A cake of solid material then builds up on the walls of the drum. In the simplest of these systems, the centrifuge (see Fig. 4) is allowed to operate until the drum is full, after which the drum is removed and manually emptied. The more elaborate systems will periodically go through a "cleaning" cycle where the solids will be automatically scraped from the drum and allowed to fall into a container. These systems can produce Fig. 3. Principle of hydrocyclone sludge at a level of 85% solids or more (with separators. some paints), as compared to the average of 663

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