Usg Corporation

Kirk S Moore

Michael L Cloud

A method and system for calcining gypsum to recover gypsum in a form consisting essentially of calcium sulfate anhydrite is disclosed. The method includes the steps of feeding the ground gypsum material into a first kettle and heating the ground gypsum in the first kettle to the first predetermined temperature. The first predetermined temperature is preferably below 400.degree. F. so that the gypsum will still contain a sufficient amount of chemically-combined water so that it will self-fluidize by release of water vapor so that it will flow through the apparatus. The ground gypsum is then overflowed through at least one subsequent stage and heated to a final predetermined temperature to produce an anhydrite product. To produce insoluble calcium anhydrite or dead burn material, the final predetermined temperature is greater than 900.degree. F. and preferably within the range of about 900.degree. F.-1300.degree. F. In the subsequent stages, the gypsum powder is heated and simultaneously fluidized with a fluidizing media, preferably air, so that the ground gypsum will flow through the apparatus. In the preferred embodiment, the ground gypsum is fluidized by a plurality of mixing blades having air injection pipes along their breaking edges for injecting air into the gypsum and a plurality of air injection nozzles disposed radially about a periphery of the kettles. The fluidization of the ground gypsum in the subsequent stages ensures that it will flow through the apparatus for production of an anhydrite product.

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Okeene, OK, United States of America

Kirk S Moore