a process for heat treating spent pot linings from hall - heroult electrolytic cells which comprise carbon and silico - aluminous materials impregnated with fluorides . the pot linings are ground to a particle size less than 5 mm , and formed into an intimate mixture with calcium sulfate or calcium sulfate dihydrate having a particle size less than 1 mm . the mixture is injected into the center of a vortex formed by a flow of circulating hot gas at the top part of a reactor at a temperature between 700 ° and 1100 ° c ., and extracted from the bottom part of the reaction at a temperature between 400 ° and 750 ° c . after a contact time of less than 2 seconds .