Abstract: A rotary kiln has a main cylindrical processing zone which is coupled downstream and upstream, respectively, to the charge-receiving and charge-withdrawal ports of the kiln through a pair of gas-tight spiral ducts. A reagent to be reacted with the charge is introduced coaxially into the main processing zone, and the spent reaction gases are coaxially removed therefrom. The reaction gas outlet conduit is provided with a hydraulic seal for maintaining a desired value of superatmospheric pressure in the main processing zone, and an additional gas conduit coaxial with the gas withdrawal conduit means is arranged to supply gaseous fuel to a plurality of heating tubes which are disposed in spaced relation through the walls of the main processing zone.