Lump process alpha gypsum

Claim 2

In a process for the production of alpha calcium sulfate hemihydrate crystals of the type wherein lump gypsum particles of at least about 1/2 inch size are calcined at steam pressures of 13 to 60 psi gauge in a state of quiescence and with an at least first lower pressure cycle and a second higher pressure cycle, dried and ground to obtain calcium sulfate hemihydrate having a low normal consistency, the improvement comprising: heating under the lower pressure cycle without a crystal habit modifier to convert at least a substantial but minor part of the gypsum to hemihydrate; intermediate the low pressure and high pressure cycles adding an aqueous solution of crystal-habit-modifier; and continuing calcination under steam pressure until substantially all of the gypsum particles are converted to hemihydrate crystals.

Claim 3

In a process for producing alpha calcium sulfate hemihydrate crystals of the type wherein lump gypsum particles of at least about 1/2 inch size are calcined under steam pressures of 13 to 60 psi gauge in a state of quiescence and with a first lower pressure cycle with an aqueous solution of crystal-habit-modifier and a second higher pressure cycle, drying and grinding the product to obtain calcium sulfate hemihydrate having low normal consistency, the improvement of adding crystal-habit modifier solution in a plurality of discontinuous portions comprising: applying a first portion of the crystal-habit-modifier solution to the surface of the gypsum particles before autoclave calcination; and intermediate the lower pressure and higher pressure cycles adding in at least one second distinct portion, the remainder of the crystal-habit-modifier solution; and continuing calcination under steam pressure until substantially all of the gypsum particles are converted to hemihydrate crystals.

Claim 4

In a process for producing alpha calcium sulfate hemihydrate crystals of the type wherein lump gypsum particles of at least about 1/2 inch size are calcined under steam pressures of 13 to 60 psi gauge in a state of quiescence and with a first lower pressure cycle with an aqueous solution of crystal-habit-modifier and a second higher pressure cycle, drying and grinding the product to obtain calcium sulfate hemihydrate having low normal consistency, the improvement of adding crystal-habit modifier solution in a plurality of discontinuous portions comprising: applying a first portion of the crystal-habit-modifier solution to the surface of the gypsum particles before autoclave calcination; and intermediate the lower pressure and higher pressure cycles adding in at least one second distinct portion, the remainder of the crystal-habit-modifier solution; and continuing calcination under steam pressure until substantially all of the gypsum particles are converted to hemihydrate crystals.

Claim 5

The process according to claim 1 or 2 in which the gypsum particles are of the "selenitic" variety.

Claim 6

The process of claim 2 in which about 1/2 of the crystal-habit-modifier solution is added to treat the gypsum particles before autoclave calcination.

Claim 7

The process of claim 1 or 2 wherein the crystal-habit-modifier is selected from the group consisting of alkali and alkaline earth metal salts of sodium, potassium, magnesium, calcium, cesium, copper, zinc and of ammonia; aliphatic acids containing a dicarboxylic acid radical and having in its molecule at least two carboxyl groups separated by two carbon atoms with a single bond between intervening carbon atoms; and mixtures thereof.

Claim 8

The process of claim 5 in which the crystal-habit-modifier includes a succinic acid radical.