Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 606 of 707

Fig. 7. Skimmer, pump, and prefilter with carbon or free oil separator. circulating dilute hydrochloric acid from the slurry tank; additional manual clean- ing 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 back- flushing will reduce waste treatment loading; however, if evaporation is used to control drag-out, 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 hydrat- ed metal sludges resulting from the neutralization of plating wastewater prior to sewer discharge. They are also effective in removing accumulated iron sludge from phosphating tanks. In one such system (Fig. 6), the waste containing 1 to 3% solids is first allowed to settle in a cone-shaped tank. The supernatant liquid drains into a head box, which directs the flow across the filter medium (paper or plastic) supported by a motor-driven conveyor belt. The liquid passes through the disposable fabric by gravity flow into a receiving tank below. When the pores of the media become clogged, the liquid level rises and a float switch activates the belt drive. Fresh media is fed over the tank and filtration is continuous. The cake on the fabric is allowed to drain before it is dumped into the sludge box. Gravity drain or an immersion pump empties the filtered water from the tank. Cycling and indexing of the filter are automatic. The occasional replacement of the filter fabric roll is the only labor required. The sediment in the bottom of the cone can also be dewa- tered periodically by filtration on the fabric. Other systems feature pressure or vac- uum filtration. The sludge cake contains from 5 to 35% solids, depending upon the equipment and type of cake. Cakes can be further treated by air evaporation 605

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