Autoclave gypsum

Alpha-hemihydrate gypsum plaster (autoclave gypsum)... 

To convert calcium sulfate dihydiate to a-hemihydrate liquid water must be present, and the system mnst have a temperature that is higher than that of boihng water at atmospheric pressure. To meet these conditions the reaction must take place under antoclave conditions at a water vapor partial pressure exceeding 760 torr (=101.3 kPa). A temperature of about 120-140 °C is typically employed under large-scale production conditions. The product obtained in this way is nsually called autoclave gypsum. 

Guillini A process for making gypsum from the waste product from the Wet Process for making phosphoric acid. The waste is heated with water in an autoclave this removes impurities and converts the calcium sulfate dihydrate to the hemi-hydrate. 

Another method, first reported in the 1950s. has lower water demand. The dihydrate is heated in a water solution containing a metallic salt such as CuCIi at pressures not exceeding atmospheric. A third melhod, developed in 1967, prepares very low water-demand a -hemihydraie by autoclaving powdered gypsum in a slurry. A crystal-modifying substance such as succinic acid or malic acid is added to the slurry in the autoclave to produce large squat crystals. 

CaCOi to rranove the excess acid as gypsum, and to breed some elements. Indium and gallium are concentrated in the gypsum from the second neutralization stage. The neutralized solution is fed to autoclaves with oxygen for iron removal as hematite. The solution after iron removal, containing 70 g/1 of free acid, is recycled to leach calcine. 

Ztirz, A. et al. (1991) Autoclave-free formation of a-hemihydrate gypsum. Journal of the American Ceramic Society 74, 1117-1124. 

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