Patent ID: 9458025
Filing Date: 2016-10-04
CPC Classification: C01F,C01P,C04B,Y10T

Claim Text:
1. A process for modifying dihydrate gypsum, comprising: a calcination step of subjecting the dihydrate gypsum as a starting raw material to calcination so as to convert the dihydrate gypsum into hemihydrate gypsum; a recrystallization step of forming a first slurry comprising the hemihydrate gypsum, which has been obtained in the calcination step, contained as solid matter therein, hydrating the hemihydrate gypsum, and recrystallizing the hemihydrate gypsum in the first slurry in the presence of at least one material selected from the group consisting of a surfactant and a defoaming agent so as to modify the hemihydrate gypsum into recrystallized dihydrate gypsum having a higher brightness compared with the dihydrate gypsum as the starting raw material, thereby forming a second slurry; and then an impurity separation step of removing the impurities comprising the darkness components, which provide darkness to the modified dihydrate gypsum, from the second slurry comprising the recrystallized dihydrate gypsum by a water-elutriation method, so as to form the modified dihydrate gypsum in a form of a third slurry, wherein a solid content of the first slurry is in a range from 10 to 50 mass %, and the water-elutriation method separates and eliminates the impurities from the second slurry by using a difference of a sedimentation rate between crystals in the modified dihydrate gypsum and the impurities, and discharging supernatant water containing the impurities.