Abstract: A mixture of botanical fines and sand is separated by a porous separating medium horizontally disposed within an enclosed chamber in which an upflowing stream of gas carries out the separation. The porous separating medium is comprised of a pair of opposed rotatively movable juxtaposed porous members with means for rotatively adjusting the porous members so as to change the flow through area in the porous separating medium. As a gas flows up through the porous separating medium, only the sand passes down through the porous medium and the gas entrains the botanical fines portion of the mixture above the porous members removing the botanical fines with the gas stream. Control of the velocity of gas up through the porous medium as well as the prevention of botanical fines from flowing with the sand down through the porous medium is determined by the adjustments of the rotatively mounted porous members.