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Filter Press - A Closer Look





The Filter Press is a machine for the solid/liquid separation. It is based on the simple principle Romans used to squeeze oil and wine.  Modern technology has allowed to apply the filter press to several new, precious, and current industries. In these industries, the filtration of the semi-liquid material is of great importance, both for the disposal of the sludge and the recovering of water and possible precious material.

The filter press can be applied to many fields, thanks to its simplicity, versatility and efficiency. It is also known as plate filter, deriving its name from the filtrating element, or as chamber or diaphragm/membrane filter press, which gives a more specific and complete definition of the filter plates’ typology.



The filter press is mainly used as a batch pressure filter with fixed volumes. In this case, the machine is designed to process a specific quantity of solids per cycle, while the batch term means that it is necessary to stop the cycle to discharge the filtrated cakes before starting a new one. The pressure is produced by the feeding pump during the separation process.

Another application of the filter press is for “cleaning”, that is to say removing the small quantities of solids from a flow. When used for cleaning the filter is not designed on the amount of solids it can process, but on the maximum surface that can be filtered and the maximum hydraulic output. The cakes produced by the filter press are not dry and when the output drops below acceptable levels the cycle is ended.

When a variable volume is needed instead, special filtrating plates called diaphragm or membrane plates must be installed in the filter press. These plates have a flexible draining surface which creates a sac, the diaphragm, once the plate is completely sealed. Then it can be inflated to further squeeze the cake and obtain better results in terms of residual moisture or dehydration uniformity between one cycle and another, besides reducing cycle time drastically.



Essentially, the filter press has two main components:
   – The framework
   – The filter pack

They are the same for every kind of filter press, no matter the design, or whether it is an over-beam or side-beam one.  The framework has the main function of keeping the filter pack together in spite of the internal pressure developed during the filtration process. The terminology may slightly vary from one manufacturer to another, but the different components remain more or less the same:
  – Head
  – Mobile plate;
  – Hydraulic cylinder;
  – Manifold

Everything is kept together by the side-beam or the over-beam.  The filter pack is where the actual solid/liquid separation happens. It consists of a series of filtrating elements (the plates whose number is variable) which create one chamber for each pair of plates.  The surface of the chamber walls has small projecting cylinders which are covered by a permeable cloth. These cylinders form drainage flows for the filtrated liquid and there are holes on every corner of the plates which connect the drainage surface to the discharging outlet.   When the plates are held together in a unique pack, the corner outlets form individual manifolds which connect the drainage surface to the external pipes. The central hole from which the sludge is injected (less frequently there are feeding holes in the corners) creates a manifold which connects to the chambers of the filter pack.



The filter presses is closed by the hydraulic piston/cylinder and the sealed chambers form between the plates, where the sludge is pumped into by the feeding pump.  The sludge is injected in the filter press through the inlet manifold in the head. While every chamber fills up, the liquid flows through the cloths, the drainage surface and the outlets and exits through the corner eyes by gravity.

The primary function of the filtrating element is to provide a permeable structure for supporting the cake during its forming. At the beginning, some solids may pass through the cloths, causing a light turbidity of the filtrate. Then the bigger particles of the sludge start to reduce the dimension of the cloth holes gradually. Smaller particles start to fill the reduced holes up and, consequently, the filtration of the cake. Once a layer of particles is 1 or 2 mm thick, it carries out the function of separating finer particles, then the cake grows thicker while producing a filtration whose turbidity level is very low.



The pressure which pumps the sludge (usually 100 psi, even though it can reach 900psi, from 7 to 60 bars) is obtained from a feeding pump – a positive displacement one or a centrifugal one.  Thanks to drainage by gravity obtained on the filtrate side, it is created a pressure difference between the feeding pressure and the discharging along the filter pack and  the cake, whose thickness is increasing. It is this pressure difference, not just the feeding pump pressure, which permits the filtration.

The solid element contained in the sludge go towards the cake forming area, where the difference is smaller, and the results are cakes that form in a uniform way above the drainage surface and on both walls of the chamber.  The feeding cycle continues until the filtrated cakes forming on the walls of the chambers have reached the center of it, filling the filter press with solid material; then the filtration process ends. At a later stage, the hydraulic closing of the filter press is loosened, the filtrating elements separate allowing the discharging of the cakes, usually by gravity, among specific containment walls or directly in the truck ready for the transportation.