Abstract: In an apparatus for heating and mixing a particulate material composition, an elongated drum is supported for rotation on a frame and hot gases are supplied to the drum at an output end of the drum and directed toward an input end. An outer shell encircles the drum adjacent the output end and defines an outer mixing chamber between the drum and the shell. A first particulate material is introduced into the drum adjacent the input end, is conveyed toward the output end, and is diverted into the mixing chamber. A second particulate material is introduced into the outer mixing chamber and is heated to an output temperature during mixing with the first material. Thereafter, the mixed materials are delivered from the outer mixing chamber and from the apparatus.