Binary mixtures of the ionic liquid betainium bis(trifluoromethylsulfonyl)imide and water show an upper critical solution temperature. This solvent system has been used to extract metal ions by phase-transition extraction, using zwitterionic betaine as extractant. The system is efficient for the extraction of trivalent rare-earth, indium and gallium ions. This new type of metal extraction system avoids problems associated with the use of viscous ionic liquids, namely, the difficulty of intense mixing of the aqueous and ionic liquid phases by stirring.

Several fundamental extraction parameters such as the kinetics and loading were studied for a new type of metal solvent extraction system with ionic liquids. The binary mixture of the ionic liquid betainium bis(trifluoromethylsulfonyl)imide and water shows thermomorphic behavior with an upper critical solution temperature (UCST), which can be used to avoid the slower mass transfer due to the generally higher viscosity of ionic liquids. A less viscous homogeneous phase and mixing on a molecular scale are obtained when the mixture is heated up above 55 °C. The influence of the temperature, the heating and cooling times, were studied for the extraction of neodymium(III) with betaine. A plausible and equal extraction mechanism is proposed in bis(trifluoromethylsulfonyl)imide, nitrate, and chloride media. After stripping of the metals from the ionic liquid phase, a higher recovery of the ionic liquid was obtained by salting-out of the ionic liquid fraction lost by dissolution in the aqueous phase. The change of the upper critical solution temperature by the addition of HCl or betaine was investigated. In addition, the viscosity was measured below and above the UCST as a function of the temperature.

The ionic liquid betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf 2 N] was used for the extraction of scandium from aqueous solutions. The influence of several extraction parameters on the extraction efficiency was investigated, including the initial metal concentration, phase ratio, and pH. The extraction kinetics was examined, and a comparison was made between conventional liquid−liquid extraction and homogeneous liquid−liquid extraction (HLLE). The stoichiometry of the extracted scandium complex was determined with slope analysis. Scandium(III) is extracted as a complex with zwitterionic betaine in a 1:3 stoichiometry, with three bis(trifluoromethylsulfonyl)imide counterions. Upon extraction of scandium(III), proton exchange occurs and three protons are transferred to the aqueous phase. Scandium is an important minor element present in bauxite residue (red mud), the waste product that results from the industrial production of alumina by the Bayer process. To evaluate the suitability of [Hbet][Tf 2 N] for the selective recovery of scandium(III) from red mud leachates, the extraction of other metals present in the leachates (La(III), Ce(III), Nd(III), Dy(III), Y(III), Fe(III), Al(III), Ti(IV), Ca(II), Na(I)) was considered. It was shown that the trivalent lanthanide ions, yttrium(III) and the major elements aluminum(III), titanium(IV), calcium(II), and sodium(I), are all poorly extracted, which is advantageous for the selective recovery of scandium(III) from red mud. Iron(III) showed an extraction behavior similar to that of scandium(III). Scandium recovery was examined from a multielement rare-earth solution. Scandium could be separated from the other rare-earth elements by extraction with [Hbet][Tf 2 N] and subsequent scrubbing of the loaded ionic liquid phase to remove coextracted metal ions. The extracted scandium was recovered from the ionic liquid phase by using back-extraction with hydrochloric acid or precipitation stripping with oxalic acid. ■ INTRODUCTIONScandium is generally considered to be part of the group of the rare-earth elements since both scandium and the other rareearth elements have a stable trivalent oxidation state. However, the chemical properties of scandium differ significantly from those of the other rare-earth elements. The ionic radius of scandium(III) is much smaller (0.745 Å for sixfold coordination) than any of the trivalent lanthanide ions. As a result, complexes of scandium(III) show higher stability constants than the corresponding lanthanide(III) complexes. 1,2 The main application of scandium is aluminum−scandium alloy. 3 Scandium-containing aluminum shows superior mechanical and physical properties compared to other high-strength alloys, such as higher strength, better corrosion resistance, and less hot cracking in welds. 4 Other important applications of scandium include scandia-stabilized zirconia for solid oxide fuel cells (SOFC), analytical standards, high-intensity metal halide lamps, laser crystals, and oil-well tracers (radioactive isotope 46 Sc). 3,5 The number ...

The metal extraction mechanism of basic extractants is typically described as an anion exchange process, but this mechanism does not correctly explain all observations. This paper introduces a novel model for the extraction of metals by basic extractants from chloride media supported by experimental data on methyltrioctylammonium chloride and Aliquat 336 chloride systems. This model relies on the hypothesis that the metal species least stabilized in the aqueous phase by hydration (i.e., the metal species with the lowest charge density) is extracted more efficiently than the more water stabilized species (i.e., species with higher charge densities). Once it is transferred to the organic phase, the extracted species can undergo further Lewis acid−base adduct formation reactions with the chloride anions available in the organic phase to form negatively charged chloro complexes, which than associate with the organic cations. Salting-out agents influence the extraction, most likely by decreasing the concentration of free water molecules, which destabilizes the metal complex in the aqueous phase. The evidence provided includes (1) the link between extraction and transition-metal speciation, (2) the trend in extraction efficiency as a function of the concentration of different salting-out agents, and (3) the behavior of HCl in the extraction system. The proposed extraction model better explains the experimental observations in comparison to the anion exchange model and allows the prediction of optimal conditions for metal extractions and separations a priori, by selecting the most suitable salting-out agent and its concentration.

Neodymium and dysprosium can be separated using a new extraction system based on a deep-eutectic solvent and Cyanex® 923 diluted in toluene.

The ionic liquid choline bis(trifluoromethylsulfonyl)imide, [Chol][Tf2N], was used for the extraction of neodymium(III), in combination with choline hexafluoroacetylacetonate, [Chol][hfac], as the extractant. The binary mixture of [Chol][Tf2N] and water shows temperature-dependent phase behavior, with an upper critical solution temperature of 72 °C. A novel extraction technique, homogeneous liquid-liquid extraction (HLLE), was applied to this solvent system. HLLE is based on the use of thermomorphic solvent mixtures and has the advantage of forming a homogeneous phase during mixing. Extraction is not kinetically hindered by an interface and the extraction equilibrium is reached faster than in the case of heterogeneous mixing in conventional solvent extraction. Several extraction parameters were studied for the extraction of neodymium(III) with [Chol][hfac]: temperature, pH, extractant concentration and loading of the ionic liquid phase. A speciation study was performed to determine the stoichiometry of the extracted neodymium(III) complex and a plausible extraction mechanism is proposed. Neodymium is extracted as a tetrakis hexafluoroacetylacetonate complex with one choline cation as counter ion. The crystal structure of the extracted complex showed the presence of a coordination bond between the choline counter ion and the neodymium(III) center, resulting in a coordination number of nine. The stripping of the loaded neodymium and the influence of acid and extractant concentrations on the phase behavior of the [Chol][Tf2N]-H2O system were investigated.

Bauxite residue (red mud) is a waste product of the alumina refining in the Bayer process and contains significant concentrations of critical metals, including scandium. Greek bauxite residue contains exploitable levels of scandium and is thus considered a suitable source for its production. A process was developed to recover scandium from Greek bauxite residue using a combination of sulfation-roasting-leaching and solvent extraction with the hydrophobic ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N]. Sulfation-roasting-leaching was the preferred leaching technique to dissolve scandium from bauxite residue because of the low acid consumption, good selectivity towards scandium and low co-dissolution of the major elements. The scandium concentration in the leachate was increased by applying multistage leaching, during which the obtained leachate is contacted multiple times with freshly roasted material. In a next step, scandium was selectively extracted from the obtained leachate with [Hbet][Tf2N]. To improve the separation between scandium and iron, Fe(III) was reduced to Fe(II) by addition of ascorbic acid to the sulfate leachate prior to extraction. The phase ratio and pH of the extraction were optimized to achieve high extraction and the concentration of scandium in the ionic liquid phase. Co-extracted metal ions were scrubbed from the loaded ionic liquid phase by HCl and the purified scandium was removed by stripping with H2SO4. Scandium was recovered from the strip solution by precipitation together with sodium. Finally, the entire process was performed on lab scale as a proof-of-principle.

Lanthanides are extracted to basic nitrate-based extractants, like trihexyl(tetradecyl)phosphonium nitrate, as pentanitrato lanthanide complexes.