The paper discusses the laws behind the filtering procedures of process liquids through porous materials. At metalwork finishing operations, the use of cutting fluids is of particular importance. During operation, liquids are continuously and intensively contaminated with solid metal parts. To restore the original properties, process fluids are cleaned of mechanical admixtures. The most widely used methods for purifying process liquids are those by filtration.
In this paper in the compliance with the method of the local averaging, the basic relations of surface physics and thermodynamics for the description of diffusion process of the liquid and gas phases in the inhomogeneous porous media have been considered. In line with the drainage and sorption properties of the liquid in the porous skeleton, the normalized function of distribution of the pore size for the effective radius has been defined. The relations for the determination of the intrinsic and relative permeability of phases in the solid skeleton have been proposed.