Apparatus for separating particulate material into relatively fine and relatively coarse particles wherein such particulate material and a gas stream are introduced independently of one another to a separating zone through which the gas stream flows at such volume and velocity as to entrain fine particles and convey them from the separating zone to grinding or other facilities and wherein the fineness of entrained particles may be adjusted by diverting a selected portion of the gas stream from the separating zone to a bypass passageway followed by recombining the diverted portion of such gas stream with the gas containing the entrained particles.