Abstract: A centrifuge for sifting granular material, ranging in particle size from a few microns to several hundred microns, for example, comprises an upwardly concave frustoconical disk rotatable about a horizontal axis and provided with a similarly shaped cover defining therewith a narrow annular gap of outwardly converging radial cross-section. Air, or some other gaseous or liquid fluid, is axially introduced from above into that gap together with the granular material to be centrifuged, the fluid stream imparting to the smaller particles an added acceleration making the velocities of all particles substantially equal at the disk periphery.