An alkali metal oxysulfur compound is converted to a corresponding alkali metal sulfide by reaction with a solid carbonaceous material. Reaction takes place during the heating of an oxysulfur compound and carbonaceous material which results in a melt being formed. The reactants are subjected to centrifugal force and are forced to the periphery of a reaction zone where they form a downwardly flowing melt in which the reduction of the oxysulfur compound to the sulfide continues. Product is recovered from a melt exiting the reaction zone. In a preferred embodiment, one or more of the reactants enter the reaction zone tangentially entrained in a carrier gas.