The driving force of filtration is the pressure difference between the two sides of the filter medium, which is called working pressure. The filtration process of the diaphragm filter press is as follows: suspension is introduced into each filter frame from the feed channel, and the filtrate passes through the filter medium layer under pressure, and flows to the filter surface of the filter plate. After the groove of the filter surface is concentrated, the filtrate is discharged from the outlet of the filter.
Diaphragm filter press usually uses filter cloth as filter medium, and other filter media can be used when there are special requirements, such as filter paper as filter medium in fine filtration process of petroleum industry. In the early stage of the filtration process, the fine solid particles in the suspension often pass through the filter cloth layer, so the filtrate is not very clear, but the larger particles are quickly intercepted by the filter cloth fiber, or stuck in the channel of the fiber gap, which makes the gap through the filter rapidly smaller. This process is called bridging phenomenon in filtration theory.
When cake layer is formed on the surface of filter cloth, the real filter medium is cake layer, so the application of diaphragm filter press is generally cake layer filtration. For the filtration of some suspensions with very low solid content, in order to obtain clear filtrate, the filter cloth can not be simply used as the filter medium. The commonly used method is to form a chemical stable and clean pre-coating layer on the surface of the filter cloth first, and use the pre-coating layer as the filter medium. This method is called pre-coating layer filtration.
The liquid content of filter cake is a technical index of special concern to the selection of diaphragm filter press. The cake layer on the surface of the filter cloth bears the extrusion of solid particles in the whole cake layer, so its compactness is greater than that of other parts, and the cake in contact with the suspension is the most loose. With the increase of cake thickness, the cake layers on both sides of the filter chamber are connected, and the liquid content of the cake in the middle part is the highest, which is in the semi-flow state.