The working principle of the vacuum filter

A vacuum filter is a device that achieves solid-liquid separation by utilizing vacuum negative pressure as the driving force. Under the action of vacuum negative pressure (0.04 - 0.07 MPa), the liquid in the suspension passes through the filter medium (filter cloth) and is removed, while the solid particles are retained by the medium, thereby achieving the separation of liquid and solid. In its structure, the filtration section is arranged along the horizontal length direction, enabling continuous completion of filtration, washing, drying, filter cloth regeneration, etc. The rubber belt filter has the advantages of high filtration efficiency, large production capacity, good washing effect, low moisture content of the filter cake, flexible operation, and low maintenance cost. The rubber belt filter can be widely applied in solid-liquid separation in fields such as metallurgy, mining, chemical industry, papermaking, food, pharmaceuticals, and environmental protection, especially in the gypsum dehydration aspect (FGD) of flue gas desulfurization.

The rubber belt filter can be widely applied in solid-liquid separation in fields such as metallurgy, mining, chemical industry, papermaking, food, pharmaceuticals, and environmental protection. It has excellent application in gypsum dehydration in the process of sulfur dioxide removal from flue gas (FGD).

The basic principle of vacuum filtration is as follows: Under the action of a vacuum negative pressure (0.04 - 0.07 MPa), the liquid in the suspension passes through the filter medium (filter cloth) and is removed, while the solid particles are retained by the medium. Thus, the separation of liquid and solid is achieved.

The vacuum filter applies the surface filtration mechanism. When the liquid in the suspension flows towards the filter medium, solid particles larger than or similar to the pore size of the filter medium will first form an initial layer on the surface of the medium in a bridging manner. The pore size of the filter medium is larger than its pore channels, thus retaining smaller particles. Therefore, the continuously deposited solid particles gradually form a filter cake with a certain thickness on the initial deposition layer.

The vacuum filter creates negative pressure at the outlet of the filtrate as the driving force for filtration. This type of filter can be divided into two types: intermittent operation and continuous operation. The intermittent operation vacuum filter can filter suspensions of various concentrations, while the continuous operation vacuum filter is suitable for filtering thick suspensions with a large amount of solid particles.

The intermittent operation filter has been developed due to its ability to achieve automated operation, and the filtration area has been increasing.

To obtain a filter residue with a low moisture content, mechanical pressing filters have been developed.

The filter press with a simple structure is formed by separating the container into upper and lower chambers using a filtering medium. The suspension is added to the upper chamber, and under the pressure effect, it passes through the filtering medium and enters the lower chamber to become the filtrate. The solid particles are retained on the surface of the filtering medium, forming a filter residue (or called a filter cake).

During the filtration process, the filter residue layer accumulated on the surface of the filtering medium gradually thickens, and the resistance of the liquid passing through the filter residue layer increases, resulting in a decrease in the filtration speed. When the filter chamber is filled with filter residue or the filtration speed is too low, the filtration is stopped, the filter residue is removed, the filtering medium is regenerated to complete one filtration cycle.

The liquid passing through the filter residue layer and the filtering medium must overcome resistance, so there must be a pressure difference on both sides of the filtering medium, which is the driving force for filtration. Increasing the pressure difference can accelerate filtration, but particles deformed under pressure are prone to堵塞 the pores of the filtering medium at a large pressure difference， and the filtration slows down instead.

There are three methods for filtering suspensions: filter cake layer filtration, deep filtration, and sieve filtration. Filter cake layer filtration refers to the formation of an initial filter cake layer after the initial stage of filtration. After that, the filter cake layer plays a major role in the filtration process, and both large and small particles are retained. Deep filtration means that the filter medium is thick, the suspended liquid contains fewer solid particles, and the particles are smaller than the pores of the filter medium. During filtration, the particles enter and are adsorbed in the pores of the filter. Sieve filtration is a filtering method where the retained solid particles are larger than the pores of the filter medium, and the filter medium does not adsorb solid particles. For example, a rotary drum filter can remove coarse impurities from wastewater through sieve filtration.

In actual filtration processes, these three methods often occur simultaneously or successively. The processing capacity of the filter depends on the filtration speed. When the solid particles in the suspended liquid are large and uniform in size, the pores of the filter cake layer are relatively unobstructed, and the speed of the filtrate passing through the filter cake layer is relatively high. Adding a coagulant to aggregate fine particles into larger lumps is beneficial for increasing the filtration speed.

For suspended liquids with fast solid particle sedimentation rates, using a filter machine with material feeding at the upper part of the filter medium, so that the filtration direction is consistent with the direction of gravity, the coarse particles will first settle, which can reduce the clogging of the filter medium and the filter cake layer; in difficult-to-filter suspended liquids (such as colloids), adding coarse solid particles such as diatomaceous earth and expanded perlite can make the filter cake layer more loose; when the viscosity of the filtrate is high, heating the suspended liquid can reduce the viscosity.