Solid–Liquid Equilibria (SLE) of the Aqueous Ternary System NiSO ₄ –CoSO ₄ –H ₂ O at 303.2, 333.2, and 363.2 K

To optimize leaching and crystallization for nickel–cobalt sulfate systems, the phase equilibria of the ternary system NiSO4–CoSO4–H2O were investigated at 303.2, 333.2, and 363.2 K using the isothermal dissolution equilibrium method combined with the Schreinemakers’ wet residue method, X-ray powder diffraction, and simultaneous thermal analysis. Results indicate that the solid solution CoxNi(2–x)(SO4)2·12H2O forms in the system at 303.2, 333.2, and 363.2 K. The hydrated crystalline forms of NiSO4·nH2O and CoSO4·nH2O vary with temperature: CoSO4·7H2O is observed at 303.2 K, while CoSO4·H2O forms at 363.2 K; NiSO4·7H2O occurs at 303.2 K, and NiSO4·6H2O is observed at 333.2 and 363.2 K. A comparison of the phase diagrams at these temperatures shows that cobalt sulfate exists only in the form of a solid solution, with no crystalline form of a single salt hydrate observed at 333.2 K, and the crystalline region of NiSO4·nH2O first expands and then contracts as the temperature increases. Based on the multitemperature phase diagrams, an isothermal evaporation–variable temperature crystallization process is proposed for the recovery of cobalt from nickel–cobalt mixed solutions.