Water activity-based design of a single-stage CSTR reactive crystallization process for producing super-azeotropic HCl and well grown metastable α-calcium sulfate hemihydrate crystals from CaCl2 solution

Abstract

The regeneration of strong (≥6mol/L) hydrochloric acid (HCl) is an important but capital and energy intensive unit operation in chloride metallurgical facilities. This paper discusses a novel low temperature process capable of producing super-azeotropic hydrochloric acid via the reactive crystallization of calcium sulfate α-hemihydrate, a valuable by-product, in a continuous stirred-tank reactor (CSTR). The design of the process was enabled via model-based water activity calculations that allowed for selecting process conditions promoting the metastability of calcium sulfate α-hemihydrate in strong HCl solutions, while preventing the formation of the undesirable calcium sulfate anhydrite phase. The CSTR crystallizer was operated in the range from 13°C to 60°C by reacting concentrated CaCl2 feed solution (up to 5mol/L) with concentrated H2SO4 (up to 18mol/L). Super-azeotropic HCl (up to 9.5mol/L, ~30wt.%) and calcium sulfate α-hemihydrate (α-HH) was produced at 60°C. Calcium sulfate dihydrate (DH) could also be produced but with lower strength HCl (sub-azeotropic ≤6mol/L) and at ≤40°C. By maintaining a low CaCl2 concentration (0.1–0.4mol/L) the growth of metastable α-HH crystals (30–60μm crystal size) was promoted due to a favorable supersaturation regime. Finally, it was determined that the super-azeotropic HCl producing crystallization process can be carried out in a thermally autogenous mode at 60°C taking advantage of the exothermic character of the reaction.