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A range of filter media can be used on drum filters, depending on the specific application. Most media have a cut-off point below which value particles pass through the filter. One way to achieve high filtration efficiencies at low particle sizes is to coat the filter medium with a fine powder – the precoat – to build up a thin cake on which the main filtration can then be undertaken without loss. A precoat layer will be used for treatment of suspensions with a very low solid content and/or where very fine solid particles have to be filtered, and an absolutely clear filtrate is required. Diatomaceous earth (diatomite or kieselguhr) and expanded perlite, together with some grades of cellulose, are the more common precoat materials used for this purpose.
A rotary vacuum drum precoat filter is a specially designed version of the vacuum drum designed to use a thick layer of precoat material to clarify liquid flows. Initially a slurry of the precoat material is prepared and filtered to produce a cake
of up to 100mm thickness on the filter cloth. Then the feed trough is drained and refilled with the suspension to be filtered, which is now filtered in the normal way through the precoat layer. The filtered cake can be washed if necessary, and is then
discharged by means of a full-width scraper blade. The blade removes the whole of the cake, together with a thin slice (about 0.5mm) of the precoat layer.
This process continues until the precoat has been reduced to its minimum acceptable thickness, whereupon the process cycle is repeated. Drainage of filtrate is provided through grids and drainage mats, which allows for removal of filtrate from the entire compartment. The cake, of course, comes away with some precoat, and the process is only possible if this is acceptable – which it usually is, because it is mostly used for clarification. This process is distinctive on account of the high filtrate quality achieved.