Enhancing Metal Separations Using Hydrophilic Ionic Liquids and Analogues as Complexing Agents in the More Polar Phase of Liquid–Liquid Extraction Systems

Abstract

The separation of metals by liquid–liquid extraction largely relies on the affinity of metals to the extractants, which normally reside in the organic (less polar) phase because of their high hydrophobicity. Following a different route, using aminopoly(carboxylic acid)s (e.g., EDTA) as complexing agents in the aqueous (more polar) phase was found to enhance metal separations by selectively complexing metal cations. In this study, we demonstrate that, hydrophilic ionic liquids and analogues in the more polar phase could also selectively complex with metal cations and hence enhance metal separations. As an example, Cyanex 923 (a mixture of trialkyl phosphine oxides) dissolved in p-cymene extracts CoCl2 more efficiently than SmCl3 from a chloride ethylene glycol (EG) solution. However, when tetraethylammonium chloride is added into the EG solution, CoCl2 is selectively held back (only 1.2% extraction at 3.0 M tetraethylammonium chloride), whereas the extraction of SmCl3 is unaffected (89.9% extraction), leading to reversed metal separation with a separation factor of Sm(III)/Co(II) > 700. The same principle is applicable to a range of hydrophilic ionic liquids, which can be used as complexing agents in the more polar phase to enhance the separations of various metal mixtures by liquid–liquid extraction.

Coordinating Ability of Anions, Solvents, Amino Acids, and Gases towards Alkaline and Alkaline‐Earth Elements, Transition Metals, and Lanthanides

“…The classification by Chauvin of some of the anions in ionic liquids as highly coordinating or weakly coordinating are in excellent agreement with the ordering of their coordinating ability indices, and the wide choice of anions presented in Table should allow researchers to select alternative anions with stronger or weaker coordinating abilities than those currently used in ionic liquids, as a given application may require. For instance, Li et al . found that the different extraction properties of Co II and Sm III salts with different ionic liquids was not affected by their cation, but could be attributed to the different coordinating abilities of the anions towards transition metals and lanthanides.…”

“…For instance, Li etal. [93] found that the different extraction properties of Co II and Sm III salts with different ionic liquidsw as not affected by their cation, but could be attributed to the different coordinating abilities of the anions towards transition metals and lanthanides. Althoughm ost anionsu sed in ionic liquids show no significant (eco)toxicological effects, somefluorinated anions do presenta strong effect, and their biodegradability is another aspect to consider when selectinga na nion.…”

Ethylammonium nitrate enhances the extraction of transition metal nitrates by tri‐n‐butyl phosphate (TBP)

“…9 Recently, a series of non-aqueous SX systems have been investigated and several enhanced separations have been obtained. [10][11][12][13][14][15] The substitution of water by polar organic solvents changes the solvation of metal cations in the polar solvent, and the relative solubility of the complexes between the two immiscible phases also affects the extraction equilibrium. 15 The extraction of metals from polar organic solvents can be significantly altered by changing polar solvents.…”

“…The use of polar solvents instead of water in the extraction of metals is called non‐aqueous solvent extraction (NASX) 9 . Recently, a series of non‐aqueous SX systems have been investigated and several enhanced separations have been obtained 10‐15 . The substitution of water by polar organic solvents changes the solvation of metal cations in the polar solvent, and the relative solubility of the complexes between the two immiscible phases also affects the extraction equilibrium 15 .…”

Non-aqueous solvent extraction of indium from an ethylene glycol feed solution by the ionic liquid Cyphos IL 101: speciation study and continuous counter-current process in mixer–settlers

“…This observations can be explored for the development of more efficient solvent extraction processes, as illustrated in recent papers. [38][39][40][41][42] In this paper, an efficient non-aqueous solvent extraction process for separation of indium and zinc from an ethylene glycol (EG) feed solution is described. The quaternary phosphonium ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) and the quaternary ammonium ionic liquid Aliquat 336 (a mixture of tri-n-octylmethyl and tri-ndecylmethylammonium chloride with the former dominating) are used as extractants due to their commercial availability.…”