CHARACTERIZATION OF ASSOCIATION MICROSTRUCTURES IN HYDROMETALLURGICAL NICKEL EXTRACTION BY DI(2-ETHYLHEXYL) PHOSPHORIC-ACID

The association microstructures which form in the hydrometallurgical extraction of nickel from nitrate media of ionic strength 0.25 M by di(2-ethylhexyl)phosphoric acid (HDEHP) in n-heptane were characterized under constant high loading conditions. Photon correlation spectroscopy, fluorescence polarization, small-angle neutron scattering. FT-IR spectroscopy, and various analytical techniques were employed to investigate the physicochemical nature of the molecular aggregates in the nonpolar organic phase. This study shows that the metal-extractant aggregates are cylindrical reversed micelles which contain between five and six water molecules per HDEHP monomer. Furthermore, it appears that the metal-extractant aggregates are mixed reversed micelles consisting of predominately nickel-HDEHP and to a lesser extent NaDEHP. The water in the metal-extractant reversed micelles exists mainly in three different states, namely, free water and bound water in the aqueous core of the aggregates, and trapped water in the interfacial layer among the hydrocarbon chains of the extractant molecules near the micellar core. The important role of reversed micelles in the liquid/liquid extraction of metal ions by acidic organophosphorus extractants is also briefly discussed. © 1992.