Metal Finishing Guide Book

2013

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Fig. 6. Automatic disposable fabric filtration system for neutralized solid/liquid waste separation. Precoat Backwash Filters These filters operate the same as, and have the same functional purpose as, ordinary filters with the further advantage that they can be cleaned quickly by reversing the flow through the filter media. Backwashing the filter aid and dirt away makes the media available for prompt repeat precoating. The basic advantage is that the filter chamber need not be opened each time the filter requires cleaning. Finer grades of filter aid may be precoated on top of the coarse filter aid when fine powdered carbon is to be used continuously. Here again, periodic (daily, if necessary) additions of small quantities of filter aid should be made to lengthen the cycle between backwashing. The media may be cleaned automatically with sluicing or using other devices. Iron hydroxide sludges can be dissolved by circulating dilute hydrochloric acid from the slurry tank; additional manual cleaning may also be required occasionally. Some disadvantages of precoat and backwashing are the possible loss of solution, increased waste treatment loading, and the possibility of migration of filter aid and carbon into the plating tank. The use of rinse water for backflushing will reduce waste treatment loading; however, if evaporation is used to control dragout, this may interfere with evaporator operation and the economies achieved by using this equipment. Sand Filters Using sand as the filter media, the pump and filter operate like a precoat surface filter and backwash like a precoat without the need of additional aid to achieve fine particle retention. Performance can be acceptable based on recirculation turnover rates, with the basic disadvantage coming from a smaller surface area, which increases the need for frequent backwashing and resulting solution loss to maintain the desired flow rate (turnover required). Horizontal Fabric and Screen Filters These filters are especially well suited for the continuous dewatering of hydrated metal sludges resulting from the neutralization of plating wastewater prior to 642

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