Thermodynamic evaluation using the law of mass action under consideration of the activity coefficients in the system NdCl₃-HCl (or NaOH)-H₂O-DEHPA-kerosene

For the recovery of neodymium, an important rare earth metal, solvent extraction using DEHPA as extractant is a possible process for winning and recycling. A preceding study by the authors has provided extensive experimental data of the system neodymium-chloride-hydrochloric acid (or sodium hydroxide)-water-di-(2-ethylhexyl)phosphoric acid (DEHPA)-kerosene. It was found that the description of the reaction Nd³⁺ + 3 (DEHPA)₂ <=> Nd(DEHP·DEHPA)₃ + 3 H⁺ by an ideal mass action law is only partly satisfactory. This article investigates the contribution of several parameters to non-ideality. On this basis, expressions for activity coefficients of neodymium in the aqueous phase as well as DEHPA and neodymium in the organic phase are derived. The resulting equations are shown to represent the system with considerably better accuracy than previously possible.