Direct preparation of alpha-calcium sulfate hemihydrate from sulfuric acid

In this work, the crystallization of alpha-calcium sulfate hemihydrate in sulfuric acid solution and the correlation between its properties and preparation conditions by reaction of sulfuric acid with lime (CaO) have been thoroughly investigated. The research involved the study of conversion-dissolution of calcium sulfate dihydrate in H2SO4 solution, the measurement of solubilities, thermo-dynamic calculations and the preparation of alpha-CaSO4 ·0.5 H2O via different methods of reactive mixing of H2SO4 and CaO. It was found that the calcium sulfate solids can saturate the sulfuric acid solutions in only 5 minutes. The solubility of calcium sulfate hemihydrate in 0--3.0M H 2SO4 solution at 100°C was experimentally determined and thermodynamic calculations with the aid of FactSage and OLI have led to establishment of the phase diagram for the CaSO4-H2SO 4-H2O system. An operating window has been determined in terms of H2SO4 concentration, temperature and time within which alpha-hemihydrate can be produced by reaction of lime with H 2SO4. This window is defined as 0.6--1.1M H2SO 4 (steady-state concentrations), 98--105°C and 1 hour retention time. Dihydrate was found to form as intermediate phase quickly converting to hemihydrate. The kinetics of conversion depends on the acidity level.For the standard preparation procedure of adding lime into hot sulfuric acid, alpha-hemihydrate grows in the c-axis direction much more rapidly than in other directions ending in the form of fine needle crystals. Also, independent of the shape of the seed particles, the resultant crystals of hemihydrate are needle-shaped, which suggests a "dissolution-recrystallization" mechanism. Upon prolonged equilibration in their acid-preparation solution hemihydrate needle-shape crystals become fibrous and eventually convert to anhydrite. It is believed that uptake of SO42- instead of Ca2+ is the rate-determining step in the hemihydrate crystallization process. The hot SO42--rich environment rendered most of the additives (particularly organic) tried ineffective. Trivalent cations such as Fe3+ and Al3+, are the only ones found to modify the crystal morphology from needle-shape to small "grain" type morphology.Slow addition of H2SO4 solution to slaked lime - reverse procedure was found to favor the production of alpha-hemihydrate with column-shaped as opposed to needle-shaped crystal morphology within otherwise the same operating window, 0.6--1.1M H2SO4. Preliminary assessment of the properties of the alpha-hemihydrate materials synthesized in this work showed them to compare satisfactorily with other materials produced by conversion of dihydrate to hemihydrate