An apparatus for subdividing a mass of small particles into a plurality of sub streams of relatively variable size comprising two disc-shaped members which between them define an interspace, and at least one of which rotates at high speed. The mass of particles is introduced at the centrum of the interspace and slung outwardly by centrifugal force into the interspace with substantially uniform peripheral distribution about the circumference of the disc-shaped members and into a circumferential annular chamber which is subdivided into sections by means of peripherally displaceable collecting shields; the length of the arc shaped sections being adjustable relative to one another determine the relative magnitude of the sub stream caught in each section by its associated shield which stream is discharged through an opening formed in the circular outer wall of the respective section.