Abstract: A wet mill for producing very finely ground end products with particles in the low micron range from feed particles sized up to about 300 microns has a cylindrical rotor closely inserted within a cylindrical vessel with a pumping space between the bottom of the rotor and the vessel and a narrow annular gap between the side walls of the rotor and vessel. The particles to be ground are introduced into the pumping space from which they are forcefully discharged by rotating vanes on the bottom of the rotor to pass upwardly through the grinding medium in the annular gap. The grinding medium is retained in the annular gap by the force of the pumping action and if thrown up may be caught on a screen over the outlet while the finer ground particles are discharged.