Crystallization of alpha-calcium sulphate hemihydrate by aqueous reaction of calcium chloride with sulphuric acid

The crystallization of alpha-calcium sulphate hemihydrate with simultaneous regeneration of HCl by controlling the reaction of H 2SO4 (2.6M-8M) with concentrated CaCl2 (1-3.5M) solution at 80°C was investigated. The research included 3 studies: (a) study of the kinetics and mechanism of CaSO4 dehydrate (DH) conversion to hemihydrate (HH) and anhydrite (AH) in variable composition CaCl2 -HCl media; (b) reactive crystallization of alpha-hemihydrate in a semi-batch reactor configuration following the LaMer Diagram concept; and (c) the study of crystal growth and metal uptake by seeded crystallization with and without the presence of several metal chloride salts. According to the first study, AH is the only stable phase in the range of electrolyte concentration and temperature investigated with DH and HH being either metastable or unstable. In the absence of CaCl2 and at 3M HCl concentration or higher DH converts directly to AH. Maintenance of 0.3M or higher CaCl2 concentration was found to render hemihydrate metastable hence making its production feasible. Solubility estimation of three CaSO4 phases with the aid of OLI Stream Analyser software helped to explain the observed phase transition sequence. SEM characterization further revealed the transition from DH to HH to be governed by the topochemical mechanism. In the reactive crystallization study H2SO4 was the limiting reactant added to the CaCl2 solution up to 0.8 SO 4/Ca molar ratio. The nucleation and growth stages were successfully separated via supersaturation control achieved by regulated addition of the limiting reactant in multiple steps. This separation permitted the growth of the homogeneously nucleated crystals within the metastable zone to needle-shaped hexagonal crystals (acicular habit). Upon recycling (seeding) of the product further growth was achieved, producing crystals with 80 mum - volume based mean crystal diameter- or 100-120 mum length and 10-15 mum diameter. Growth of the seed crystals appeared to follow an agglomeration mechanism. However, in the presence of foreign metal cations (in particular divalent, Mg 2+, Fe2+, Ni2+ and trivalent, Al 3+, Fe3+) crystal growth was severely retarded via their apparent adsorption. Some of the metal cations, namely Na+, Ni2+, Al3+ were found to incorporate into the hemihydrate crystal structure to a variable extent from 0.3g/t for Na to 7g/t for Ni and 11g/t for Al. The chloride uptake was less than 10 ppm.