Phase diagrams of CoSO4-H2O and CoSO4-H2SO4-H2O systems for CoSO4·nH2O (n = 6,7) recovery by cooling and eutectic freeze crystallization

This paper reports the solid-liquid phase equilibria of the CoSO₄-H₂O and CoSO₄-H₂SO₄-H₂O systems at low temperatures. Binary and ternary phase diagrams, including the stable solid phases CoSO₄·6H₂O and CoSO₄·7H₂O were established using experimental data and thermodynamic modeling applying the mixed-solvent electrolyte (MSE) model. The results showed that the addition of H₂SO₄ shifts the eutectic temperature and concentration to lower values for cobalt sulfate and ice crystallization. The trends obtained from the experimental data and the modeling are consistent for the binary CoSO₄-H₂O system with good agreement, but the ternary CoSO₄-H₂SO₄-H₂O system shows some deviations. In general, the MSE model is shown to be reliable for inferring and establishing the phase diagram of the low-temperature system. The phase diagrams are helpful for designing the pathways of cooling crystallization and eutectic freeze crystallization and assessing the performance of the low-temperature crystallization process in the production of CoSO₄ hydrates. In addition, some practical examples of cooling crystallization and eutectic freeze crystallization of CoSO₄ solutions are provided.