Abstract: The furnace is circular in plan and comprises an annular charge space charged through chutes in its top wall. Effluent gas offtakes are provided in the furnace outer wall and the level of the charge bed is maintained above the chute bottoms but below those offtakes. Hot gases generated externally of the furnace pass into it through an opening in its bottom wall or hearth and travel upwardly through the charge. Solids are discharged downwardly through the same opening in counterflow. A vertical axle is journalled centrally in an upper wall of the annular furnace chamber spaced above the opening in the hearth, and a plow affixed to the lower end of the axle rotates in an open space between the hearth of the annular furnace chamber and its inside wall, so scraping heated charge solids into the discharge opening. The axle is pressure sealed to the wall through which it passes, and that seal is the only rotating seal required for the furnace.