Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 605 of 707

Fig. 6. Automatic disposable fabric filtration system for neutralized solid/liquid waste separation. filter media and create a filter cake. Usual flow rates range from 0.5 to 2 gpm/ft2 of filter surface. A lower flow rate improves particle retention and smaller particles will be removed. It should be pointed out that, although there may be a wide range in flow rate, the range of selectivity of particles being removed is between 0.5 and 5 ┬Ám, which is the most significant difference between precoat and depth-type cartridges and offers a wider choice of porosity. Buildup of cake should be gradual, and recirculation should continue until the solution runs clear. Cake should be dispersed uniformly across the media before the plating solution is allowed to flow across the filter. A slurry tank piped and valved into the filtration system becomes a convenient and versatile piece of equipment. The slurry may be prepared with plating solution, rather than water, to avoid diluting critical mixtures. Via valving, the solution is drawn into the slurry tank for sampling, preparation of slurry, and chemical additions. Similarly, the solution is returned to the plating tank. This method eliminates the necessity of transfer hoses between tanks, and the subsequent risk of loosening the cake or losing pump prime. The integral slurry tank is also a convenient storage for backwash water. 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 advan- tage 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 (dai- ly, if necessary) additions of small quantities of filter aid should be made to length- en the cycle between backwashing. The media may be cleaned automatically with sluicing or using other devices. Iron hydroxide sludges can be dissolved by 604

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