Characterization of a metal-extracting water-poor microemulsion

Solvent extraction is the key separation method in hydrometallurgy. With proper conditioning of such a system, the transfer of metal-species from an aqueous phase into an organic phase can be triggered, either the enrich a desired metal species or to remove undesired impurities. On a laboratory scale, many powerful extractants have been synthesized that enable to selectively and efficiently select any desired metal species. However, application on an industrial scale is yet out of grasp, as fundamental principles of solvent extraction formulation are yet poorly understood. One key problem that is encountered by engineers is the formation of undesired phases (e.g. liquid crystals, emulsification and third-phase formation) and no model is available that allows to predict the occurrence of these phases. This work is dedicated to elaborate the macroscopic phase behaviour of solvent extraction systems by systematic screening of the macroscopic properties of a reference model. Therefore, we use HDEHP as extractant molecule and its sodium salt represents the extractant engaged in complex formation and how solvent penetration plays a crucial role. Establishing a phase prism, where the Z-axis represents ratio between HDEHP and NaDEHP, we are able to employ different cuts, known from surfactant science. These allow to deduce the properties of the complexes (also referred to as reverse micelles) on a nanoscopic level. The major revelation is that two different types of phase separation have been identified: an emulsification failure, where the interior solvent (water, that is co-solubilized inside the reverse micelles) is rejected from the extracting organic phase. Second, a liquid-gas type of phase separation, where the exterior solvent (the organic diluent) is repelled, resulting in two organic phases, where the heavy one carries all the extractants and the light one is purely the organic diluent. This second type needs to be avoided in solvent extraction formulation. In the second part of this work and based on the phase diagrams determined in the first part, the conductivity profile is extensively screened in monophasic regions. Three limits have been determined to play a major role on the conducting properties of a water-poor microemulsion: the critical aggregation concentration (CAC); the degree to which the head-groups are hydrated: transition from reverse micelles, where water is immobilized as it hydrates the extractant head-groups, towards swollen reverse micelles, exhibiting a liquid water-core; and the percolation threshold. In total, this gives 5 different regions which have dissimilar conducting properties