Abstract: Produce air flow longitudinally through circular inlet passage toward frusto-conical chamber which expands to a cylindrical chamber. Produce air flow tangentially of cylindrical chamber, as by blower, to produce spiral flow therein and in a larger, coaxial annular chamber with a central restrictor having a projection extending into cylindrical chamber. Material fed into inlet passage, as through hopper or at open inlet end, which may be bell shaped. Comminuted material discharged from annular chamber through tangential duct, in which air flow is produced by another blower. Still another blower may discharge at acute angle longitudinally of inlet passage. Restrictor may be hemispherical or conical and also adjustable forwardly or rearwardly. Projection of restrictor may be cylindrical or cylindrical with rounded front end and also adjustable forwardly or rearwardly.