Method for the preparation of fibrous insoluble calcium sulfate anhydrite

A process for the formation of fibrous water insoluble calcium sulfate anhydrite, the fibers thereof having an overall average length in the range of from 30 to 70 microns, individual maximum lengths not exceeding 180 microns, and aspect ratios in the range of from 10:1 to 100:1;

• which comprises:
• a. forming an aqueous suspension of gypsum, said suspension being not greater than 0.30 molar in gypsum, and said gypsum prior to dispersion being in the form of particles at least 90% of which are minus 4 mesh in size;
• and
• b. converting the gypsum in said suspension to said fibrous calcium sulfate hemihydrate by reaction in a pressure vessel in the presence of saturated steam at a temperature in the range of from 120.degree. C to 155.degree. C for a period of 0.5 to 2.5 hours, thereby forming fibers of calcium sulfate hemihydrate having the aforesaid dimensions;
• and
• c. thereafter calcining said fibers of calcium sulfate hemihydrate at a temperature in the range of 500.degree. C to 750.degree. C for a period of 0.5 to 3.0 hours to convert said fibers of calcium sulfate hemihydrate to fibers of water insoluble calcium anhydrite having substantially the same crystalline shape and dimensions.