In apparatus for retorting oil shale, oil sands and similar materials wherein finely divided solids residue is used as a heat carrier and is heated in a vertical pneumatic conveyor-burner, mixed with fresh finely divided solid feed, product distillation vapors are removed from the mixture, and the solid distillation residue is returned to the conveyor-burner, the improvement whereby the propellant gas is introduced axially to the bottom of the conveyor-burner and the cool heat carrier is introduced thereinto from a concentric annular chamber thereabout through slits or openings in the propellant gas conduit by means of a flow of a control gas. Other specific improvements of portions of the retorting apparatus and solids circulating system are examplified and claimed, especially a screw conveyor-mixer retorting chamber.