The principle of vacuum filtration
The vacuum filtration is a driving force that generates a negative pressure at the filtrate outlet as a filter. This filter is divided into two types: intermittent operation and continuous operation. The intermittently operated vacuum filtration filters various concentrations of the suspension, and the continuously operated vacuum filtration is suitable for filtering thick suspensions containing more solid particles.
Intermittently operated filters have been developed for automated operation and the filtration area is growing. In order to obtain a filter residue having a low moisture content, a mechanically pressed filter has been developed. Separating the container into upper and lower chambers with a filter medium constitutes a simple filter press. 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 slag layer filtration, deep filtration and sieve filtration. The filter residue layer filtration means that the initial filter residue layer is formed after the initial stage of filtration. Thereafter, the filter residue layer plays a major role in the filtration. At this time, both large and small particles are trapped; the deep filtration means that the filter medium is thick and the suspension contains The solid particles are less, and the particles are smaller than the pores of the filter medium. When filtering, the particles are adsorbed in the pores after being filtered; the sieve filter is the solid particles trapped by the filter are larger than the pores of the filter medium, and the inside of the filter medium does not adsorb the solid particles. Filtration methods, such as a tumble filter screen, filter out coarse impurities in the sewage.
In the actual filtration process, the three methods often occur simultaneously or sequentially. The processing capacity of the filter depends on the filtration 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 process.