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 NiSO 4 –CoSO 4 –H 2 O 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 Co x Ni (2– x ) (SO 4 ) 2 ·12H 2 O forms in the system at 303.2, 333.2, and 363.2 K. The hydrated crystalline forms of NiSO 4 · n H 2 O and CoSO 4 · n H 2 O vary with temperature: CoSO 4 ·7H 2 O is observed at 303.2 K, while CoSO 4 ·H 2 O forms at 363.2 K; NiSO 4 ·7H 2 O occurs at 303.2 K, and NiSO 4 ·6H 2 O 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 NiSO 4 · n H 2 O 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.