Dehydration process of gypsum in waste sulfuric acid from the sulfate manufacturing process of titanium oxide was studied.
When Calcium sulfate dihydrate changed into unhydrite, two different dehydration processes were observed according to the experimental conditions. In one process, dihydrate directly changed into unhydrite and in the other, hemihydrate appeared as the intermediate. By the optical microscope examination, it was observed that the crystals of both hemihydrate and unhydrite were formed through solution mechanism.
When the crystal seeds of unhydrite were added, the induction period of the reaction decreased with increasing amount of the crystal seeds. In the case where the crystal seeds were fine, different kinds of dehydration process was observed.
