Precoat pressure

Typical particle size and feed concentration range 0.1-40 pm and 1% w/w. 

Precoat filtration can be incorporated within a wide range of pressure filters including leaf, multi-element and plate and frame types (see Section 1.4.2). Up to 700 g m of precoat is typically filtered onto the filter medium prior to introduction of the feed suspension. The feed, which may also contain a significant addition of filter aid to improve cake permeability, is filtered until the filtrate flow rate is sufficiently low to warrant cake discharge in the normal way. It is not economical to recover the feed solids from the precoat, and it follows that washing of the solids is not practised. Moreover, the filter aid tends to abrade the pumps used to promote the filtration. Precoat pressure filtration is most often used for the removal of finer particles from dilute suspension where other potential processes would require too high an investment. 

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These filters are used on slurries with small amounts of solids, usually less than 0.1%, and generally do not form any visible cake. Solids to be removed are usually very small panicles that may be trapped on the surface of the filter medium or within the medium. This type of filter is usually used in a polishing application where excellent quality liquids are needed as in food or beverage, pharmaceutical, and electronic processing operations. The most common clarifying filters are disk and plate presses, cartridge filters, precoat pressure filters, deep bed filters, and membrane filters. 

If the vacuum tests are not successfiil because the rate is too slow or, if the tests are successful but the equipment for vacuum filtration would be too large or too costly, or simply because filtration is not achieved, then one has to look at four possible alternatives. These are a bottom feed vacuum filter with precoating flocculating the feed or adding body feed, either for bottom feed or top feed vacuum filters or pressure filtration in all its ramifications, either with or without flocculation or body feed. In certain cases one can even look at precoating pressure filters. In this respect, of course, pressure filters can equally well be centrifugal filters, or compression filters. 

Precoat, pressure leaf type [Pg.245]

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