Abstract: A rotary kiln includes a cylindrical kiln body supported for rotation about a central axis inclined with respect to a horizontal plane, a cylindrical combustion chamber formed inside the kiln body and extending along the central axis, and a plurality of cylindrical calcination chambers formed inside the kiln body so as to lie parallel with and surround the combustion chamber. The combustion chamber has an outlet at its lower end from which residue produced by combustion of fuel inside the combustion chamber drops for further combustion and/or discharge.