Phase equilibria and thermodynamic properties of the nickel(ii) methanesulfonate–methanesulfonic acid–water system

Abstract

Methanesulfonic acid (MSA) is a promising reagent for hydrometallurgy, but the fundamental chemistry of methanesulfonate salts remains largely underexplored. Given the important role of nickel in cathode-active materials of lithium-ion batteries, the solubility of nickel(II) methanesulfonate and the influence of MSA on the solubility of this salt were studied. Solid–liquid equilibria of Ni(CH₃SO₃)₂–H₂O–MSA systems were investigated by the isothermal solubility method, followed by water activity and density measurements on the saturated aqueous solutions. Solid phases of different hydrates were formed and identified by thermogravimetric analysis and single-crystal X-ray diffraction, showing the presence of three hydrates: Ni(CH₃SO₃)₂·2H₂O, Ni(CH₃SO₃)₂·6H₂O and Ni(CH₃SO₃)₂·12H₂O. The solubility behavior of nickel(II) methanesulfonate in mixtures of MSA and H₂O shows a decrease in solubility with increasing acid concentration, reaching a minimum at 95 wt% MSA. Experimental data were obtained over a wide range of Ni(CH₃SO₃)₂ and MSA concentrations at different temperatures and used to build a thermodynamic model with the mixed-solvent-electrolyte (MSE) thermodynamic framework by OLI Systems. Thermodynamic properties of the different hydrates and interaction parameters for the aqueous phase were optimized.