Filter presses are sometimes called “Plate-and-Frame Filters” which describes the style of filters developed from the 1800’s onwards. The majority of today’s filters are more correctly called as “Membrane Plate Filters”.
Many processes in the food, chemical or pharmaceutical industries make products that are liquid-solid suspensions or slurries. These mixtures are a little like a runny mud or milk-shake. The solids in them do not dissolve in the liquid but are carried along in it. Filter presses separate the solids from the liquids so that the useful part can be processed or packaged.
Filter presses generally work in a “batch” manner. They are loaded with slurry before completing a filtering cycle and producing a batch of solid filtered material, called the filter “cake”. The solid is removed, the press re-loaded with slurry and the batch cycle repeated.
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A filter press uses increased pressure to maximize the rate of filtration and produce a final solid with low water content. This is more efficient than filtration using a funnel and paper which utilizes the low pressure caused by the weight of liquid above the filter paper.
In another class of mechanical separations, placing a screen in the flow through which they cannot pass imposes virtually total restraint on the particles above a given size. The fluid, in this case, is subject to a force that moves it past the retained particles. This is called filtration. The particles suspended in the fluid, which will not pass through the apertures, are retained and build up into what is called a filter cake. Sometimes it is the fluid, the filtrate, that is the product, in other cases the filter cake.
The fine apertures necessary for filtration are provided by fabric filter cloths, by meshes and screens of plastics or metals, or by beds of solid particles. In some cases, a thin preliminary coat of cake, or of other fine particles, is put on the cloth prior to the main filtration process. This preliminary coating is put on in order to have sufficiently fine pores on the filter and it is known as a pre-coat.