Thermodynamic model for methanesulphonic acid recovery by tri-n-butyl phosphate

A unified, semi-empirical thermodynamic model for liquid-liquid equilibria was developed within the OLI Mixed-Solvent Electrolyte (MSE) framework to describe the solvent extraction (SX) of methanesulphonic acid (MSA) by tri-n-butyl phosphate (TBP) in aliphatic diluents, based on experimental data obtained for this study. The model accounts for non-ideality in both phases and captures the key TBP protonation and molecular MSA extraction mechanisms. The model reproduces MSA Gibbs free energy of transfers, extraction efficiencies, organic-phase density, volume change, and water uptake across 0.1-9.9 mol L^-1 MSA, 50-100 wt% TBP, and 24-77 °C, and captures exothermic behaviour (standard state enthalpy of transfer is -20.6 ± 0.9 kJ mol^-1). Validation with data on MSA extraction from aqueous nickel(ii) methanesulphonate solutions demonstrates robustness under realistic conditions. Process simulations of counter-current mixer-settlers predict that an overall MSA recovery of 94% could be achieved from a 1.5 mol L^-1 solution of Ni(CH₃SO₃)₂ with 1 mol L^-1 excess MSA at realistic process conditions, enabling the recovery and recycling of MSA. This validated model provides a practical, predictive basis for designing and optimising MSA recovery in industrial hydrometallurgical flowsheets. Comparison of the experimental MSA extraction data with literature data on other strong acids reveals the following order of extraction efficiency by TBP: HNO₃ > HCl ≈ MSA > H₂SO₄.