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.

Nickel (Ni) and cobalt (Co) have similar chemical properties and are found naturally together in many mineral deposits. Hydrometallurgical processing is predominant in the production of Ni and Co from primary sources. The basic processing steps include leaching, separation, purification, precipitation/crystallization of salt products, and production of metal or alloy by reduction. The sulfate system is most commonly used due to the low cost of sulfuric acid, leading to nickel(II) sulfate (NiSO4) and cobalt(II) sulfate (CoSO4) as formed salt products. NiSO4 and CoSO4 are important raw materials in the production of Ni-Mn-Co oxide or Ni-Co-Al oxide lithium-ion batteries (LiBs) and can also be used to produce other Ni and Co salts.

In hydrometallurgical processing, the separation of Ni and Co can be achieved by solvent extraction, ion exchange, membrane filtration, and electrochemical treatment, resulting in pure NiSO4 and CoSO4 solutions. The concentrations of Ni and Co ions in liquid streams from the separation unit are relatively high, and NiSO4 and CoSO4 phases are then recovered from their respective pure solutions by crystallization. This hydrometallurgical processing also applies to the recovery of Ni and Co from spent LiBs. In addition, sulfuric acid (H2SO4) is usually present in streams from the solvent extraction unit.