Reactive liquid-liquid test system ZnSO4/D2EHPA: Equilibrium, thermochemical, structural, and spectroscopic insights from experiments and density functional theory

The reactive liquid-liquid test system of the European Federation of Chemical Engineering (EFCE) ZnSO4/D2EHPA has been the subject of studies for decades. Although much information is known at a process engineering level, some is yet to be established from a fundamental chemical point of view. This is the case of the molecular structure of the zinc organocomplex formed at high loading conditions, which is still experimentally unproven due to the chemical similarities involved, as previous studies propose the formation of ZnR2 (breakage model) or (ZnR2)2 (agglomeration model) species. To gain further insight into such a classical hydrometallurgical system, two strategies were applied in this work using reported experimental measurements ranging from low to high loading conditions: equilibrium data fitting and computational chemistry by means of DFT calculations. Both analyses have pointed out that (ZnR2)2 is more favorable to be formed than ZnR2 species. The conclusions were supported by the revisited spectral, thermochemical, and equilibrium stoichiometry information. The bond and angle strain in the metal center seem to be among the main reasons behind the results. Moreover, the formation of D2EHPA dimers instead of monomers is crucial in order to guarantee reaction spontaneity.