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 (ΔG⁰_298K), standard Gibbs energy of formation (ΔG_f⁰) 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.