The present invention provides a method for the thermal treatment of material in a rotating conical chamber (4) placed in a housing (1). According to the invention material, brought into the rotating chamber (4) and ejected due to the conical shape, is returned to the chamber (4) for further thermal treatment, without leaving the housing (1). By applying the method of the invention, the thermal efficiency is increased. In addition, wear of the means required for returning material that is to be treated again into the housing (1), is avoided. The invention also relates to two installations for putting the method into practice.