Displacement washing of filter cakes containing porous particles is a challenging task, because mother liquor is contained in the inner pores of the particles as well as in the bulk material. Filter cakes consisting of silica gel particles with pore diameters of 100 nm were cleaned by displacement washing. The total load of the filter cake can be reduced to 1 %, though, the inner pores contain about 50 % of the mother liquor. Calculations were performed to estimate the diffusion time out of a single pore, which is useful to choose the washing apparatus most suitable.