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

Rare earth elements, including neodymium, are in widespread use today. They serve, for example, as alloying elements in magnesium (WE 43, WE 54, AE44, AE42), in permanent magnets (neodymium-iron-boron) or as luminescent materials. In addition, they are amongst the most important commodities in Europe. The demand for their use is growing and the recycling of these elements is indispensable. Their recycling potential can be increased through detailed scientific studies and to this end this article presents a thermodynamic evaluation of equilibrium data in the system neodymium-chloride-hydrochloric acid (or sodium hydroxide)-water-di-(2-ethylhexyl) phosphoric acid (DEHPA)-kerosene. Considering the relationship between the activity coefficients γ(Nd³⁺(aq)), γ(Nd(org)) and γ((DEHPA)₂), which arise from the law of mass action, the deviations of the experimental results from the ideal behaviour can be explained. From the calculation of γ(Nd³⁺(aq)) with the expanded Debye-Hückel equation and from literature data for γ((DEHPA)₂), indications arise for the development of the functions of the activity coefficients of DEHPA and neodymium in the organic phase.