Abstract: A cyclone comprising: an upper cylindrical portion which opens into the wider end of a lower frusto-conical portion, with the longitudinal axes of said upper and lower portions aligned and substantially vertical; a primary air inlet into the cyclone arranged such that the inlet air is substantially tangential to the circumference of the cyclone; an exhaust outlet at or adjacent the top of the cylindrical portion; a control valve associated with said exhaust outlet and capable of partially or completely shutting off said exhaust outlet; a secondary air inlet associated with the narrow end of the frusto-conical portion and provided with an airflow stabilizing device which is adapted to admit a stream of air substantially along or spirally around the longitudinal axis of the cyclone; means for removing processed product from the cyclone; means for moving said airflow stabilizing device into and out of the narrow end of the frusto-conical portion during product processing and/or between product processing.