Filters are divided into gravity filter, vacuum filter and pressurized filter according to different methods of obtaining filtration driving force.

1. Gravity

With the help of the gravity and potential difference of the suspension, the pressure formed on the filter medium is used as the driving force of filtration, which is generally intermittent operation, such as sand filter.

2. Vacuum

A negative pressure is formed at the filtrate outlet as the driving force of filtration. The operating vacuum pressure of industrial vacuum filter is (2.5 ~ 8.0) × 105 PA. The temperature of suspension during filtration shall be lower than the vaporization temperature of filtrate under operating vacuum. The filter is divided into intermittent operation and continuous operation. The intermittently operated vacuum filter can filter suspensions of various concentrations. The vacuum filter fills the container shell with suspension. The filter blades wrapped with filter cloth on both sides are immersed in the suspension, and the inner cavity of the filter blade is connected with the vacuum system. After the filtrate is sucked out, it is led out by the conduit, and the filter residue accumulated on the surface of the filter blade is cleaned after shutdown. The continuous operation vacuum filter is suitable for filtering thick suspension containing more solid particles. The working principles of drum vacuum filter, inner filter surface drum vacuum filter, disc vacuum filter and Tipper vacuum filter are similar. The whole filter surface is divided into a plurality of separated filter chambers, and each rotary filter chamber is connected with each fixed pipe in sequence through the distribution valve to suck out the filtrate and washing liquid in the filter chamber or send compressed air. Each filter chamber rotates one circle to complete the whole process of filtration operation, and the operations of multiple filter chambers are connected to form continuous filtration. The structure of belt vacuum filter is similar to that of belt conveyor. It has a continuous filter belt, which is suitable for easily filtered suspension. The belt vacuum filter, the inner filter surface drum vacuum filter and the tipper vacuum filter are fed on the upper part of the filter medium (filter cloth), which is especially suitable for the suspension with high solid particle density and fast sedimentation.

3. Pressurization

It takes the pressure applied at the suspension inlet or the mechanical pressing force applied to wet materials as the filtering driving force. It is suitable for suspension requiring large filtering pressure difference. It is also divided into intermittent operation and continuous operation. Intermittently operated tubular pressure filter and pressurized filter are used for filtration of low concentration suspension. The filter is also an intermittent pressurized filter, which is divided into plate frame type, box type and vertical type, and is widely used. The oil filter is a plate and frame filter press unit equipped with oil pump, vacuum evaporator and other accessories. The continuously operated drum pressure filter and disc pressure filter perform pressure filtration in a closed shell, and their structure is similar to that of drum vacuum filter and disc vacuum filter. Due to its complex structure, it is rarely used.

The commonly used pressure filtration is to filter by reducing the volume of the filter chamber or applying mechanical pressing force. The moisture content of the filter residue is low, which is suitable for the suspension with high solid particle concentration. In the belt press filter, the wet filter residue preliminarily dewatered by gravity or vacuum moves between the two filter belts, and then is extruded and dewatered by rollers. The screw press filter has a cylinder with pores, in which there are rotating screws. The depth of the spiral groove is different. The materials are added from the deep groove end to the shallow groove end. The space of the filter chamber is gradually reduced, the materials are pressed, the filtrate is discharged from the pores of the cylinder, and the filter residue is discharged from the small end