The Role of Additives in Metal Extraction in Oil/Water Systems

Nonaqueous Solvent Extraction for Enhanced Metal Separations: Concept, Systems, and Mechanisms

Efficient and sustainable separation of metals is gaining increasing attention, because of the essential roles of many metals in sustainable technologies for a climate-neutral society, such as rare earths in permanent magnets and cobalt, nickel, and manganese in the cathode materials of lithium-ion batteries. The separation and purification of metals by conventional solvent extraction (SX) systems, which consist of an organic phase and an aqueous phase, has limitations. By replacing the aqueous phase with other polar solvents, either polar molecular organic solvents or ionic solvents, nonaqueous solvent extraction (NASX) largely expands the scope of SX, since differences in solvation of metal ions lead to different distribution behaviors. This Review emphasizes enhanced metal extraction and remarkable metal separations observed in NASX systems and discusses the effects of polar solvents on the extraction mechanisms according to the type of polar solvents and the type of extractants. Furthermore, the considerable effects of the addition of water and complexing agents on metal separations in terms of metal ion solvation and speciation are highlighted. Efforts to integrate NASX into metallurgical flowsheets and to develop closed-loop solvometallurgical processes are also discussed. This Review aims to construct a framework of NASX on which many more studies on this topic, both fundamental and applied, can be built.

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

Several molecular polar solvents have been used as solvents of the more polar phase in the solvent extraction (SX) of metals. However, the use of hydrophilic ionic liquids (ILs) as solvents has seldomly been explored for this application. Here, the hydrophilic IL ethylammonium nitrate (EAN), has been utilized as a polar solvent in SX of transition metal nitrates by tri-n-butyl phosphate (TBP). It was found that the extraction from EAN is considerably stronger than that from a range of molecular polar solvents. The main species of Co(II) and Fe(III) in EAN are likely [Co(NO3)4]2- and [Fe(NO3)4]-, respectively. The extracted species are likely Fe(TBP)3(NO3)3 and a mixture of Co(TBP)2(NO3)2 and Co(TBP)3(NO3)2. The addition of H2O or LiCl to EAN reduces the extraction because the metal cations coordinate to water molecules and chloride ions stronger than to nitrate ions. This study highlights the potential of using hydrophilic ILs to enhance SX of metals.

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

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.

Enhancing Metal Separations by Liquid-Liquid Extraction Using Polar Solvents

The less polar phase of liquid–liquid extraction systems has been studied extensively for improving metal separations; however, the role of the more polar phase has been overlooked for far too long. Herein, we investigate the extraction of metals from a variety of polar solvents and demonstrate that, the influence of polar solvents on metal extraction is so significant that extraction of many metals can be largely tuned, and the metal separations can be significantly enhanced by selecting suitable polar solvents. Furthermore, a mechanism on how the polar solvents affect metal extraction is proposed based on comprehensive characterizations. The method of using suitable polar solvents in liquid–liquid extraction paves a new and versatile way to enhance metal separations.