The processor is of the type incorporating horizontal, concentric, co-extensive inner and outer tubular members which rotate together. The processor is modified in the following respects:The front end of the inner tubular member is circumferentially corrugated and may be provided in the form of a plurality of parallel tubes arranged in a ring array, to thereby increase the shell area to promote heat transfer through the tube walls;Means are provided for interconnecting the tubular members, which means can accommodate differing rates of thermal expansion; anda rock recycle tube assembly is provided to recover oversize material leaving the corrugated portion of the inner tubular member and reject it from the processor.