A positive displacement volumetric discharge mechanism of the screw auger conveyor type. A series of radially disposed conical screw type augers and underlying retarder plates substantially close the lower end of an annular chamber of a vertical shaft kiln. The augers are rotated to convey solid materials from a bed of such materials in the kiln chamber radially outwardly to an annular discharge throat. The conical augers are formed so that the volume of material conveyed increases with the radial distance from the center of the kiln in order to maintain a constant and uniform downward flow of solid materials throughout out the cross section of the kiln.
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