Metastability of Gypsum Dihydrate in the System CaSO₄-H₃PO₄-H₂O

Abstract

The metastability of CaSO₄·2H₂O in 24.63-49.93 (P₂O₅%) H₃PO₄ solutions was studied by a wet DTA method. On the basis of the measured values of reaction temperature and induction period for dehydration of CaSO₄·2H₂O, the relation between H₃PO₄ concentration Z (P₂O₅%) and upper limit temperature Y (°C) for metastability of CaSO₄· 2H₂O in the case of 1 minute induction period for dehydration was given as follows :
Y=-4.314e^0.04601Z+112.4
From above results, it was confirmed that the metastable zone for CaSO₄·2H₂O in H₃PO₄ aqueous solution existed in the wide range of higher temperature than those reported by Nordengren, Zdanovskii and Murakami on metastable equilibrium for CaSO₄·2H₂O = α-CaSO₄·1/2H₂O in H₃PO₄ aqueous solution.