Abstract: A cyclone which includes an upper cylindrical portion opening into the wider end of a lower frustro-conical portion, with a primary air inlet such that the inlet air is substantially tangential to the circumference of the cyclone, and an exhaust outlet at or adjacent the top of the cylindrical portion; a control valve is associated with the exhaust outlet and can be used to partially or completely shut off the outlet; a secondary air inlet is associated with the narrow end of the frustro-conical portion and is provided with an air flow stabilising device adapted to admit a stream of air substantially along the longitudinal axis of the cyclone; also including means for withdrawing processed product from the frustro-conical portion.