Abstract: Hot gas is introduced into a fluidised bed of granular material, for example gypsum, contained in a vessel 10 having a grinding device or mill 11 in its lower region, which may comprise grinding rollers 15 cooperating with a table 17 on the base of the vessel. The gas may be supplied through a tube 20 passing down through the vessel, while raw material is fed into the bed, preferably by a feed screw 34. The ground product is discharged from the bed by an overflow 28 which determines the level of the bed.

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.

Abstract: The present invention is directed to the treatment of gypsum plaster prepared from gypsum rock having a substantial content of soluble salts wherein the gypsum plaster is rendered more suitable for manufacturing processes such as the production of gypsum board by rapid and intimate mixing with a flow of cold water followed by rapid separation of the bulk of the water on a continuous filtration apparatus. A preferred form of an apparatus is a suction belt filter and it is preferred that the residence time of the wet plaster on the filter does not exceed 15 seconds. The total time elapsing between initial contact with water and recovery from the continuous filter preferably does not exceed 30 seconds. The salt content of the plaster is considerably reduced by this treatment without significant hydration occurring and the treatment is economic because cold water (up to 40.degree. C.) and no retarder or other additives are preferably employed.

Abstract: A method and apparatus for calcining calcium sulphate dihydrate in a calcination vessel in which the dihydrate is heated in the vessel to calcination temperature not only by heat applied to the exterior of the vessel but also by hot gas, especially hot gaseous combustion products, supplied into the interior of the mass of calcining material within the vessel. The supply of hot gas according to the invention can improve the production rate in both batch and continuous calcination operations without adversely affecting the product quality, by permitting increase in the heat input to the vessel without entailing the risk of vessel bottom burn-out.