Abstract: This application discloses a separator for separating granular materials according to specific weight including an air permeable reciprocatory deck for supporting a bed of the granular material thereon and defining an elongate discharge opening therefrom through which a certain separated fraction of the granular material is to be discharged after separation; a fan forcing air upwardly through the air permeable deck and the bed of granular material carried on the deck to fluidize the bed and cause the granular material in the bed to separate according to the specific weight thereof as the deck is reciprocated; and a pneumatic pressure operated gate selectively covering the discharge opening so as to selectively control the flow of the granular material therethrough in response to the pneumatic pressure in said bottom air plenum with the gate selectively varying the effective length and height of the opening in response to the pneumatic pressure below the deck.