Abstract: A multiple hearth furnace is constructed from a plurality of cylindrical heating chamber modules, connected in a vertical configuration. Each module contains a hollow hearth, which forms a floor in the chamber, the hearth having an open portion approximately centered in the chamber floor. A rotating central shaft is disposed vertically through open portions in the hearths and is provided with projections for distributing material to be heated across the hearths. Heating of the modules is accomplished by passing heated gases through the hollow hearths. Material to be heated is introduced into the uppermost module, is distributed across the hearth, and is discharged into successively lower modules, after which material exits the furnace. Gases which form during the heating can be removed separately from gases used to heat the furnace.