A KINETIC-STUDY OF THE THERMAL DEHYDRATION OF CALCIUM SULFITE HEMIHYDRATE

An isothermal kinetic study of the dehydration of orthorhombic calcium sulphite hemihydrate is reported: CaSO3 . 1/2H2O(s) --> CaSO3(s) + 1/2H2O(g) Fractional reaction (alpha)-time curves exhibit the sigmoid shape characteristic of nucleation and growth reactions proceeding in the solid state. Yield-time data exhibit a very satisfactory obedience to the Avrami-Erofe'ev (n = 2) equation [-1n(1 - alpha)]1/2 = kt Rate studies were concerned with the temperature interval 573-673 K and the calculated activation energy was 173 8 kJ mol-1, showing the hydrate to be thermally relatively stable. The mechanism of dehydration is discussed and reference is made to the dehydration of calcium hydroxide with which it shows close similarities.