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Minerals 2022, 12, 190 2 of 12 biodiversity of the environment and human health [7], not to mention the fact that water is extremely scarce in places where lithium is found [8]. In this sense, solvent extraction (SX) could be proposed as an alternative to the con- ventional process in order to avoid time-consuming processing and to considerably reduce water consumption. However, an important drawback is that the direct extraction of lithium from high concentration magnesium brines, which share similar physicochemical properties [9], would require a highly selective SX process. To increase the separation factor between these two metals, the use of ionic liquids (ILs) appears to be a promising alternative. ILs are molten salts at room temperature, which have favorable physicochemical properties that make them suitable solvents for SX processes [10–12], for example, negligible vapor pressure. Additionally, ILs can increase the distribution ratios and selectivity of Li+/Mg2+ (αLi+/Mg2+ ) when compared to conventional solvents [13]. Furthermore, the use of a neutral donor (e.g., tributhyl phosphate (TBP); trioctyl phosphine oxide (TOPO) and triisobutyl phosphate (TIBP)) in combination with ILs could enhance extraction [14–17] or selectivity [18]. Shi et al. used two different non-fluorinated ILs with the anion bis(2-ethylhexyl)- phosphate and different alkyl length in the ammonium ([N4444]+ and [N8888]+), achieving high extraction towards lithium over sodium, potassium, rubidium, and cesium [19]. In addition,Lietal.wereabletoseparatelithiumfrommagnesium(αMg2+/Li+ =21.8)usinga binary extractant composed of Aliquat 336 and Versatic Acid 10 [A336][V10], in which the extracting phase can be stripped and regenerated by water [20]. These two works show the feasibility of using direct SX without precipitation and concentration steps, which could improve the efficiency of lithium extraction from brines by SX. In turn, ILs have many possible combinations of cations and anions that can be used to increase extraction and/or selectivity [21–23]. However, studying each of these combinations would be time consuming and require high experimental costs. In this sense, a quick tool for exploring these combinations is COSMO-RS (Conductor-like Screening Model for Real Solvent) [24], which has succeeded in the prediction of metal extraction with conventional organics solvents [25–27] and also with ILs as diluents [28]. For instance, Zhao et al. used COSMO-RS to predict the extraction of LiCl in different ILs [23]. However, the main problem of this approach is that lithium will be not extracted as a salt but the extraction mechanism would include the exchange of chloride anions for an anion of the IL. In fact, ILs are known to participate in the extraction of metal [12,29,30], including the transfer of the IL ions from the organic phase to the aqueous phase. Therefore, to obtain good trends between experimental and theoretical results, the extraction stoichiometry and all the local thermodynamic equilibrium of the species involved in the complexation should be considered [28]. Therefore, the aim of this work is to establish a theoretical SX model to predict selective extraction of lithium over sodium and magnesium, which are present in brines, using dif- ferent ILs as extractants by means of COSMO-RS. To this end, selectivity will be calculated based on the distribution constant of all species involved in extraction stoichiometry and free energy of complexation in the aqueous phase. Theoretical results will be compared with experimental data of extraction percentages and selectivity from lithium recovery. Finally, an IL that allows for increasing selectivity over the reported values from previous works is proposed. 2. Model Development and Computation Procedure 2.1. Predictive Method for Selectivity Calculation According to our previous work [28], COSMO-RS is capable of calculating local equilibrium and the constants from the stoichiometry in the SX process. This allows for obtaining the theoretical selectivity between Li+ and the other metals (Na+ and Mg2+) present in brines, considering an extracting phase composed of a synergistic agent and different hydrophobic ILs as extractants diluted in conventional organic solvents. In thisPDF Image | Ionic Liquids for the Selective Solvent Extraction of Lithium
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