Patent ID: 7588634

Claim:
A process of manufacturing a product comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate comprising the steps of: feeding a 50-75 wt. % gypsum slurry to a first reactor, the slurry comprising calcium sulfate dihydrate and water; calcining the slurry in the reactor at conditions sufficient to form a partially calcined slurry comprising water, calcium sulfate dihydrate and alpha calcium sulfate hemihydrate, wherein the slurry is held in the first reactor at conditions for calcining the gypsum to convert 50 to 90% of the gypsum to alpha calcium sulfate hemihydrate, wherein at least one crystal modifier is added to the calcium sulfate dihydrate and water before said calcining in the first reactor, wherein the first reactor is operating at a pressure of 15 to 100 psig during the calcining in the first reactor, wherein the residence time of the slurry in the first reactor ranges from 2 to 30 minutes during the calcining in the first reactor; dewatering the partially calcined slurry to form a water stream and dewatered solids comprising the calcium sulfate dihydrate and alpha calcium sulfate hemihydrate; feeding the dewatered solids to a second reactor; and calcining the dewatered solids in the second reactor to convert at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate, wherein the calcining of the dewatered solids occurs in the second reactor operated at atmospheric pressure and a temperature of from 150 to 1000° F.