According to the process the grains are led to the surface or ducts of a rotary element farther in than the flange of the rotary element, thus the coarse grains are led to the flange of the rotary element in a counterflow to the sizing medium. In a housing on an impeller deflecting elements are arranged, the carrier medium and the mixture of material to be sized are fed into the housing by medium inlet tubes, and led out by coarse and fine fraction outlet tubes or nozzles, and one inlet disc is arranged above the impeller so that they form an annular assorting space which is divided by deflecting elements into segments, and a passage connected to the inlet tube of the sizing medium is formed between the mantle of the inlet disc and the inner wall of the housing at its upper end, where ducts leading into the assorting space are arranged in the inlet disc farther in than its flange, and which are connected to the carrier medium inlet tube.