Possibilities and challenges for ionic liquids in hydrometallurgy
Introduction
In recent years, ionic liquids (IL) have become very popular in the scientific community due to their excellent properties: high solvating power, design solvent, high selectivity in separation processes, and negligible vapor pressure, among other characteristic that have led ILs to be applied in many separation processes like extractive distillation [1], [2], liquid–liquid extraction of organics [3], [4], liquid–liquid phase microextraction [5], solid–liquid extraction [6], [7], membrane based separations [8], [9], etc.
The solvent extraction process (SX) is another liquid–liquid based separation technology where ILs have been extensively applied for the recovery, separation and purification of metal ions, commonly present in aqueous solutions. This operation unit belongs to the hydrometallurgical way of recovering metal ions from ores. The process starts with the leaching of the mineral ores, where an acid is commonly used as the leaching agent [10], [11], [12], [13], releasing metal ions into an aqueous solution from which one or several of those specific ions require to be selectively and energy-efficiently recovered by means of SX. In this separation technology the aqueous phase is put in contact with a water-immiscible phase composed of an extractant dissolved in a diluent which is commonly of an organic nature. However, these organic compounds exhibit high volatility and toxicity, and furthermore, normally the SX process is carried out at temperatures around 40–70 °C, causing a fast loss of solvent, air pollution, and safety concerns. Besides that, these solvents bring health issues to workers. Kerosene is one of the most widely used diluents in SX, and one of the kerosene constituents is benzene, which is known as a carcinogen of the blood and its components [14].
Therefore, ILs have been proposed as a green alternative to replace organic solvents due mainly to their negligible vapour pressure, avoiding solvent loss by volatilization. At present there are many studies available in the literature where ILs have been successfully applied to the SX of copper [15], [16], [17], [18], lithium [19], [20], lanthanides [21], [22], [23], gold [24], [25], and other valuable metal ions from aqueous solutions [26], [27], [28], [29].
However, ILs not only have been proposed due to their low vapour pressure, but also due to the improvements in the extraction performance over molecular solvents. ILs have shown better distribution ratios [30] as well as higher selectivity [31] than organic solvents, representing a very attractive alternative for future industrial applications. These latter advantages bring a reduction in the separation stages and also a decrease in the volume of diluent involved in the SX process, along with the possibility of recycling. However, in spite of these advantages, the application of ILs at the pilot or industrial scale is very rare [32] so the challenges and opportunities for ILs to be applied industrially need to be reviewed.
Therefore, this article presents a comprehensive review of the use of ILs as a green alternative in the hydrometallurgical process for the recovery of valuable metal ions, starting with the leaching process, reviewing the different uses of ILs in SX, analyzing the alternative of replacing aqueous electrolytes with ILs in the electrowinning (EW) process, and giving information on their toxicity and biodegradability in case of the incorrect handling of these solvents. In the literature there are several reviews on the SX of rare earth metals [33] and rare earths using ionic liquids [34], [35], [36], and the separation of lanthanides and actinides [37]. However, these reports are focused on particular metal ions or processes within the hydrometallurgical recovery of metals. A recent review by Smirnova and Pletnev (2019) [38] presents a detailed discussion of SX with new ionic liquids. However, this review is not focused on the features of ILs that provide a better performance than organic solvents nor on extractant and diluent effects. Also, no review is provided either on leaching with ILs or possible environmental problems of ILs when applied in SX. Finally, an extensive analysis on ILs as diluents was carried out by Dietz (2006) [39], and the present work may be considered an updated revision. However, this work also includes other aspects of hydrometallurgy necessary to show the performance and capabilities of ILs that were not mentioned in the work of Dietz. Therefore, this review covers the complete hydrometallurgical process from leaching to EW using ILs. As far as we can tell, no such review has been published yet.
Finally, this review is intended to provide insights for the SX process design considering the increasing interest in urban mining that requires a leaching and SX process for the recovery of valuable elements from electronic waste. Also, this review will show the capabilities of ILs to recover and purify metal ions from refractory ores to avoid the pyrometallurgical path, which is water- and energy-consuming.
Section snippets
Ionic liquids as leaching agents
The first step in the hydrometallurgical way of treating mineral ores is leaching, where the use of ILs as leaching agents is a novel use of these green reagents. Whitehead et al. (2004) [40] reported the first description of ores leached with ionic liquids, the selective leaching of gold and silver from ores with an aqueous phase containing 1-butyl-3-methyl-imidazolium hydrogen sulfate [bmim][HSO4], thiourea, and ferric sulfate. The performance of extraction of gold and silver from ore was
Ionic liquids used as diluents in solvent extraction
After the leaching of the mineral ores, the recovery, separation and purification are preferably carried out by SX. Currently, there is large number of studies that can be found using ILs as diluents in the SX process (See Table 1), showing the capabilities of the ILs to dilute varied extractants and participate in the extraction of different metals. The extraction of molybdenum and rhenium with di(ethylhexyl) phosphoric acid (D2EHPA) using the IL 1-octyl-3-methylimidazolium
Ionic liquids in electrowinning
The final step in the hydrometallurgical process is the electrowinning (EW) of metal ions which are electrodeposited on a cathode, commonly reduced in the metallic form, thanks to a current that passes through an electrolyte coming from an inert anode.
In this sense, the use of ILs as electrolytes for electrodeposition of metal ions brings advantages over aqueous electrolytes because of their high electrical conductivity [88], [89], [90], [91], [92], wide electrochemical windows [93], [94], [95]
Environmental issues
Due to the negligible vapor pressure, hydrophobicity, and solvent capacity of ILs, they are a very attractive alternative to organic solvents widely used in hydrometallurgy. In view of their properties, it is unlikely that ILs will pollute the air, and it is more common for ILs to relate to the environment with soils and water either in a controlled manner or accidentally in industrial use [114], [115], [116].
From an environmental perspective, ionic liquids are not only used in hydrometallurgy
Challenges and possibilities
Besides the many published works on leaching, SX, and EW with ILs with successful results and many possibilities to be finally apply in industry, it is still necessary to answer some questions before a possible industrial application. The first question to solve is the viscosity of ILs. In general, in industry one or two equilibrium stages are required to achieve high extraction percentages when using ILs either as diluents or extractants. However, in a continuous operation, mass transfer
Conclusions
In this review, the application, challenges, and possibilities of ILs in the hydrometallurgical processes of recovering metal ions was discussed. This review covered the leaching process, solvent extraction, and electrowinning, where ILs have been successfully applied.
It was stated that ILs can be better leaching agents than the commonly used acids and can be recovered to be used in many leaching cycles. Hydrophobic ILs have been proposed as leaching agents to avoid mixing ILs with water.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Project PAI 79140047, FONDECYT Project 11150417 from CONICYT Chile, and DICYT Proyecto Asociativo 091711QM_DAS from the Universidad de Santiago de Chile are gratefully acknowledged.
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