A pretreatment mixing and stirring device includes a cylindrical housing having a top plate, a bottom plate arranged to oppose to the top plate, a side plate arranged between the top plate and the bottom plate, and a mixing and stirring region where a calcined gypsum and an additive are mixed and stirred in an interior surrounded by the top plate, the bottom plate, and the side plate, a disk-shaped rotating plate arranged inside the housing, a rotating drive shaft, penetrating the top or bottom plate, and coupled to the rotating plate, a calcined gypsum supply port, arranged in the top plate, and supplying the calcined gypsum to the mixing and stirring region, and an additive supply port, arranged in at least one of the top and side plates, and supplying the additive to the mixing and stirring region.

A process for the calcination of raw gypsum in a heatable extruder, wherein the raw gypsum is feed into the extruder and heated to a temperature of from 150 C. to 350 C. to thereby convert the raw gypsum to calcined gypsum with a content of calcium hemihydrate of at least 50 wt.-%. By using a heatable extruder for the calcination, calcined gypsum with a high content of calcium sulphate hemihydrate and low residual amounts of the respective dihydrate and anhydrite can be obtained. The present invention further concerns a system, which is adapted to such preparation of calcined gypsum and includes an appropriate heatable extruder and the use of a heatable extruder for the calcination of raw gypsum.

An extruder, preferably for calcining and extruding raw gypsum, including a calcination zone and a mixing zone, wherein the extruder is optionally configured to capture and/or retain water liberated in the calcination zone, apparatuses and processes employing such extruders and respective uses.