In ancient China, filtration technology was applied to production. In 200 BC, paper made from plant fibers was used. In 105 AD, Cai Lun improved the papermaking law. In the papermaking process, he smashed plant fiber pulp onto dense bamboo curtains. The water is filtered through the slit of the bamboo curtain, and a thin layer of wet pulp is left on the bamboo curtain surface, and then dried into a paper.

Most of the earliest filtration was gravity filtration, and later the filtration was used to increase the filtration speed, and vacuum filtration appeared. The drum vacuum filter invented in the early 20th century achieved continuous filtration operation. Thereafter, various types of continuous filters have appeared one after another. Intermittently operated filters (such as plate and frame filter presses, etc.) have been developed for automated operation, and the filtration area is increasing. In order to obtain a filter residue having a low moisture content, a mechanically pressed filter has been developed.

Principle of action

Separating the container into upper and lower chambers with a filter medium constitutes a simple filter. The suspension is added to the upper chamber and enters the lower chamber through the filter medium under pressure to form a filtrate, and the solid particles are trapped on the surface of the filter medium to form a filter residue (or filter cake). During the filtration process, the filter residue layer on the surface area of ​​the filter medium is gradually thickened, and the resistance of the liquid passing through the filter residue layer is increased, and the filtration speed is reduced. When the filter chamber is filled with filter residue or the filtration speed is too small, the filtration is stopped, the filter residue is removed, and the filter medium is regenerated to complete a filtration cycle.

The liquid must pass through the filter cake layer and the filter medium to overcome the resistance, so there must be a pressure difference on both sides of the filter medium, which is the driving force for the filtration. Increasing the pressure difference can accelerate the filtration, but the particles deformed after being pressed are likely to block the pores of the filter medium when the pressure is large, and the filtration is slowed down.

The suspension is filtered by three methods: filter layer filtration, depth filtration and sieve filtration. 1 Filter layer filtration: In the initial stage of filtration, the filter medium can only retain large solid particles, and the small particles pass through the filter medium with the filtrate. After the formation of the initial filter cake layer, the filter residue layer plays a major role in the filtration, at which time both large and small particles are trapped, such as filtration by a plate and frame filter press. 2Deep filtration: The filter medium is thicker, the suspension contains less solid particles, and the particles are smaller than the pores of the filter medium. When filtering, the particles are adsorbed in the pores after entering, such as filtration of a porous plastic tube filter or a sand filter. 3 Screening: The solid particles intercepted by the filtration are larger than the pores of the filter medium, and the inside of the filter medium does not adsorb solid particles, for example, the rotary filter screen filters out coarse impurities in the sewage. In the actual filtration process, the three methods often occur simultaneously or sequentially.

Filter speed

When the solid particles in the suspension are large and the particle size is uniform, the pores of the filtered filter residue layer are relatively smooth, and the filtrate passes through the filter residue layer at a relatively high speed. The use of a coagulant to aggregate fine particles into larger agglomerates helps to increase the filtration speed.

For the suspension with fast sedimentation rate of solid particles, the filter applied in the upper part of the filter medium is applied so that the filtration direction is consistent with the direction of gravity, and the coarse particles are first settled, which can reduce the clogging of the filter medium and the filter residue layer; in the difficult-to-filter suspension ( If the solid particles such as diatomaceous earth and expanded perlite are mixed into the colloid, the filter residue layer can be loosened; when the viscosity of the filtrate is large, the suspension can be heated to lower the viscosity. These measures can speed up the filtration.

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