Ionic Liquid Technology in Metal Refining: Dissolution of Metal Oxides and Separation by Solvent Extraction

Ionic liquids form a relatively new class of solvents entirely composed by ions that have specific properties such as low volatility, low flammability, large electrochemical window, broad liquidus range and a high thermal stability. These properties make ionic liquids very attractive for applications in a wide variety of fields. In the field of metal processing, ionic liquids have been used as extraction media in solvent extraction and promising results have already been reported. Nevertheless, there are some challenges for the use of ionic liquids as extraction phase. The general application of ionic liquids for liquid-liquid extraction processes is hampered because of the ion exchange problem where metal ions are exchanged with the ionic constitutes of the ionic liquid. This PhD thesis presents different approaches in an attempt to overcome the practical problems with ion exchange and offers a greener approach to solvent extraction by replacing volatile organic compounds with non-volatile ionic liquids; and it shows the separation between metals, in particular cobalt and nickel, by solvent extraction using ionic liquids as extraction media. Furthermore, the dissolution of metal oxides in ionic liquids is generally poor and so far there are only a few examples which reports the successful dissolution of metal oxides. In this thesis another approach for the processing of metal oxides in ionic liquids is presented and further, the metal species in solution and the reaction between selected metal oxides and ionic liquids are investigated. The first part presents the use of the ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) as extraction phase for the extraction and separation of metal ions. The selective extraction of cobalt from nickel to Cyphos IL 101 has been demonstrated. In the described process no additional organic diluents are added to the ionic liquid phase, which is a main advantage in comparison with the current state as no volatile odorous compounds are emitted offering thus a more sustainable approach to solvent extraction. Furthermore the practical issues accompanied with ion exchange are overruled in this case - the metals are extracted as anionic chlorocomplexes and because a chloride based ionic liquid is used, there is no net loss of the ionic liquid constitutes to the aqueous phase. The separation between cobalt and nickel shows a very high selectivity with a separation factor of about 50000. The properties of the extraction phase and the extraction parameters are discussed and compared with other phosphonium and ammonium ionic liquids. This batch ionic liquid extraction process has been transformed subsequently into a lab pilot-scale continuous process and the performance of this process proves to be competitive with currently applied industrial processes. This work is a significant step towards achieving a competitive industrial process using ionic liquids as extraction phase. The ionic nature of ionic liquids make them able to absorb water. In the second part advantage is taken of this behaviour to obtain an water saturated acidic solution of trihexyl(tetradecyl)phosphonium chloride used to dissolve metal oxides (CaO, MnO, Fe2O3, CoO, NiO, CuO, ZnO) directly in ionic liquid phase. This presents particular opportunities for bringing metal ions which cannot enter the ionic liquid phase via extraction into ionic liquid solution and it offers opportunities to further process these metals in ionic liquid phase. Additionally the selective back-extraction to aqueous phase of these dissolved metals is investigated. In the third part another approach for the use of ionic liquids in metal separation is applied, by which both the aqueous phase and the organic phase are replaced by two mutually immiscible ionic liquids. A proof of principle for the separation of metals by redistribution between two mutually immiscible ionic liquids is given and the process is applied for the cobalt and nickel mixture. In the process cobalt is selectively extracted from 1-ethyl-3-methylimidazolium chloride to trihexyl(tetradecyl)phosphonium bis-[2,4,4-trimethylpentyl]phosphinate. Processing steps such as washing, stripping and regeneration the ionic liquid phase are further discussed.The last part presents the dissolution of metal oxides in imidazolium ionic liquids. Although ionic liquids show in general a low solubility of metal oxides it is shown that metal oxides can be dissolved in imidazolium ionic liquids. Silver(I) carbene complexes are formed upon reaction of Ag2O with the imidazolium moieties which have a hydrogen atom in the C2 position. The presence of carbenes in the solution is detected by 13C NMR spectroscopy and the reactions were also monitored by Raman spectroscopy. The dissolution of other metal oxides, namely CuO, ZnO and NiO in imidazolium ionic liquids is studied and it is found that stable zinc(II) carbenes are formed in solution.

Publication year:2014

Accessibility:Open