Solvoleaching Process of Metal Oxides Using Acidic Organic Extractants

The applicability of solvoleaching process for treating various metal oxides (MOs) using acidic extractant, di-(2-ethylhexyl)phosphoric acid/D2EHPA or neodecanoic acid/Versatic 10, was investigated. Solvoleaching of MOs was found to be in accordance with the values of standard Gibbs energy of reaction between H+ and MOs (ΔG0298K), standard Gibbs energy of formation (ΔGf0) and lattice energy (U). Exceptions happened to MOs like Cu2O, iron oxides (FeO, Fe3O4, Fe2O3), Co3O4, MnO2, V2O5 and NiO, which could be explained by the proposed solvoleaching mechanism using D2EHPA. Addition of a small amount of water could substantially increase solvoleaching yields and kinetics. Metallic copper could serve as a reducing agent (creating coupled reduction–oxidation/redox reactions with the MOs) to improve the metal solvoleaching and prevent oxidation of D2EHPA molecules, whilst graphite acted as a catalyst for the redox reactions. Versatic-10 showed similar solvoleaching behavior, albeit with a lower solvoleaching capacity than D2EHPA. Recovery of metals from loaded D2EHPA was accomplished by stripping with a pH-controlled aqueous solution or by precipitation-stripping with oxalic acid to produce final products. In particular, Fe stripping could only occur using HCl, necessitating a selective stripping procedure. Recyclability of D2EHPA was evaluated by performing five solvoleaching-stripping cycles.