Semi-empirical thermodynamic model for solvent extraction of neodymium(III) nitrate by tri-n-butyl phosphate.

Abstract

The separation and purification of rare-earth elements (REEs) by solvent extraction (SX) is a complex process that requires strict process development and control. Semi-empirical thermodynamic models offer advantages over purely empirical approaches for predicting liquid-liquid equilibria, particularly in highly non-ideal systems. In this study, the OLI mixed solvent electrolyte (MSE) framework was used to develop a thermodynamic model for solvent extraction of Nd(III), achieving an error of ±6.74% on the extraction efficiency over the concentration ranges of 0.05-4.5 mol L-1 Nd(III), 0-7 mol L-1 HNO3, 0-5 mol L-1 NaNO3, and 30-100 vol% tri-n-butyl phosphate (TBP) in n-aliphatic diluents. The model incorporates detailed chemical speciation and accurately describes non-ideal behaviour in both concentrated electrolytes and non-aqueous solvents. Batch solvent extraction experiments covering a wide range of aqueous feed compositions that address gaps in the existing literature demonstrated that modelling Nd(III) extraction by considering only one metal complex, [Nd(NO3)3TBP3], gives a good agreement between experimental and calculated distribution data. Combined with infrared and UV-VIS absorption spectroscopy, these results indicate that nitric acid and salting-out agents do not alter the Nd(III) speciation during extraction. The model further showed a strong predictive capability when extrapolated to Nd(III)-HNO3-NaNO3 systems, demonstrating its general applicability within the investigated Nd(III)-TBP nitrate system.

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