Auxiliary materials used to reduce filter cake resistance or filter medium resistance. It is usually a hard powder or fibrous solid. Filter aids are only used for filtration operations aimed at obtaining clean filtrate. There are two types of filter cake formed by filtration: one type has considerable rigidity and its structure does not change with the increase of operating pressure, which is called incompressible filter cake; The structure of another type of filter cake changes with the increase of operating pressure, resulting in a reduction in the flow path of the filtrate in the filter cake, and a sharp increase in the flow resistance of the filtrate. This is called a compressible filter cake.