GB1268396A - Anhydrous calcium sulphate and calcium sulphate semihydrate - Google Patents

Abstract

1,268,396. Calcium sulphate. PROGIL, and CIMENTS LAFARGE. 12 June, 1969 [14 June, 1968], No. 29959/69. Heading C1A. A process for the continuous manufacture of calcium sulphate #-semihydrate or anhydrous calcium sulphate from crude synthetic gypsum comprises (a) subjecting the gypsum to a series of successive water washes by passing a sludge of the crude gypsum in water through a series of water cyclones, the underflow from each cyclone before the last being mixed with more water and passed to the next cyclone; recycling at least part of the overflow from the last cyclone; and filtering the underflow from the last cyclone (to obtain calcium sulphate dihydrate) with the proviso that at some step before filtering a basic compound, e.g. slaked line is added to neutralize the gypsum; and (b) dehydrating the dihydrate of (a) at 120-160‹ C. to obtain the #-semihydrate, or (c) to obtain the anhydride forming granules from a mixture of the dihydrate obtained in (a), calcium sulphate #-semihydrate and water, and calcining the granules at 400‹ C. or more. The #-semihydrate of (c) may be that obtained in (b), and preferably the granules are formed from 40- 10% by weight of the semihydrate and 60-90% of the dihydrate, with water. Calcination of the granules at 400-600‹ C. yields an anhydride suitable for use in construction plasters, or calcination at 600-900‹ C. yields an anhydride suitable for use in normal or rapid setting cements. Step (a) may be conducted at ambient temperature and preferably there are two sets of cyclones in the series (i.e. two wash stages) and the filtrate is recycled to the last cyclone.