Development of the Air-Float Method of Gravity Separation

Abstract

Abstract The author gives the historical background and reviews the development of the air-float or pneumatic type of gravity separator used by the producers of such commodities as seed, beans, peas, peanuts, rice, wheat, coffee, cork, and coal. The principle of this type of separator is based on the fact that when particles of approximately the same size but of varying densities are fed over a porous table through which air is being blown at a velocity such that the lightest particles are held at the point of suspension, there is a tendency to stratify the mass with the heaviest particles nearest the table and the lightest particles farthest above the table. When in addition, the table is given a motion similar to that of a grasshopper conveyor, the heavy stock will climb the conveyor faster than the light stock, thus grading the particles from light to heavy all along the table. The first machine described by the author is an electrostatic separator which was designed to concentrate dry placer consisting chiefly of silica, iron oxide, and free gold. This first design was discarded because of low capacity, and a pneumatic jig was designed with an inclined conveyor belt of porous material across which wooden cleats were fastened. A chance inspection of the Wilfley table, a wet-process concentrator, led inventors Sutton, Steele and Steele to use a flat reciprocating conveyor instead of the belt conveyor in their pneumatic jig. A demand for increased capacity of the latter design, which has constantly been improved, has led to the steadily enlarging field for the application of the air-float gravity separator.