Eutectic freeze crystallization for recovery of NiSO4 and CoSO4 hydrates from sulfate solutions

Abstract

In this study, eutectic freeze crystallization (EFC) was investigated to recover NiSO4 and CoSO4 hydrates from aqueous and dilute sulfuric acid solutions of metal sulfates. Binary phase diagrams were established using a combination of thermodynamic modeling and experimental data. The mixed-solvent electrolyte (MSE) model was employed to model solid–liquid phase equilibria. The predicted binary phase diagrams from the model were in good agreement with the experimental results. Experimental eutectic temperatures and eutectic metal sulfate concentrations for the NiSO4-H2O and CoSO4-H2O systems are −3.3 °C and 20.8 wt% and −2.9 °C and 19.3 wt%, respectively. For NiSO4-H2SO4-H2O and CoSO4-H2SO4-H2O systems, the eutectic temperature and eutectic metal sulfate concentration decrease with increasing H2SO4 concentration. Batch experiments were performed to study the EFC of different sulfate solutions, including 25- wt% NiSO4 in H2O, 20- wt% NiSO4 in 0.5 mol/kg H2SO4, 25- wt% CoSO4 in H2O, and 20- wt% CoSO4 in 0.5 mol/kg H2SO4. The results show that controlling the supersaturation allows high-quality ice and salt crystals to be recovered as separate phases under eutectic conditions, with the crystalline salts in the form of heptahydrates. This study shows that EFC can be a promising alternative to evaporative crystallization for recovering NiSO4 and CoSO4 hydrates from sulfate solutions.