Phase transformation kinetics of calcium sulfate phases in strong CaCl2HCl solutions

Hydrochloric acid is a key reagent in many leaching operations. A main aspect of process economics is the ability to efficiently recover the hydrochloric acid. This paper discusses phase transformation kinetic aspects relating to a continuous, three stage HCl regeneration process based on the reaction of CaCl2 solution, which is a by-product of the mineral leaching process, with sulfuric acid. The influence of process temperature (40 °C to 90 °C), solution composition (0.25 to 4.97 mol/L CaCl2, 0 to 9.43 mol/L HCl) and presence of 5 mol% calcium sulfate anhydrite (AH) seed on the induction time and speed of transformation of the metastable phases calcium sulfate dihydrate (DH) and calcium sulfate α-hemihydrate (α-HH) to anhydrite (AH) were investigated. The selected reaction environment reflects steady state conditions of the three stages of the HCl regeneration process. The results show the lifetime of metastable phases (DH, α-HH) to decrease with increased temperature and increased HCl concentration. Furthermore, it was found that the presence of AH seed shortens the induction time preceding the onset of transformation to this phase. The obtained phase quantity vs. time relationships suggest a dissolution–precipitation mechanism for the DH to α-HH transformation and a topotactic nucleation and growth mechanism for the α-HH to AH transformation. The obtained results allow the definition of temperature, concentration and time conditions to facilitate the production of the valuable DH and α-HH phases, while avoiding the formation of fibrous AH, which renders solid–liquid separation non-operational.