Filter Press: a closer look

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Filter Press Machine: a closer look

The filter press machine is 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 sectors. In these sectors, 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 machine 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.

What are the main applications of the filter press machine?

The filter press machine 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.

How does the filtration process work?

First of all, 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 machine 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.

What is the filtration principle?

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.

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