Abstract: An apparatus for calcining a granular or powdered mineral, more specifically hydrated calcium sulphate, comprising a vessel the side wall or walls of which slope inwardly toward a bottom of small cross-sectional area, as compared with an upper region of the vessel, and provided with a downwardly extending heating tube connectable in an upper portion to a gas source and an opening in a lower portion into the interior of the vessel adjacent to the bottom thereof, the arrangement being such that, in use, hot gases issuing from the lower portion of the heating tube will mix and heat substantially all the contents of the vessel.