Equilibrium analysis of the reactive liquid–liquid test system ZnSO4/D2EHPA/n-heptane

This study presents an equilibrium model for the reactive liquid-liquid extraction system ZnSO4/di-2-ethylhexyl phosphoric acid (D2EHPA)/n-heptane, proposed as a standard by the European Federation of Chemical Engineering for testing extraction techniques under varying conditions. The objective was to develop a model valid across a wide concentration range of both extraction and stripping processes. The methodology involved assuming two simple consecutive reactions to describe the formation of two zinc complexes, with the relative proportions adapting as the system transitions from extraction to stripping. The model was solved using Newton’s method, and parameters were estimated through data fitting via the Hooke and Jeaves direct search method. Results indicated that the model successfully predicted aqueous concentrations with an average relative error below 11%, based on a simplified assumption of activity coefficients equal to 1.0. Although incorporating more accurate activity coefficients might enhance the model's precision, it does not validate more straightforward extraction mechanisms. This equilibrium analysis provides valuable insights for optimizing zinc recovery from complex mixtures.