Deep eutectic solvents for separation and purification applications in critical metal metallurgy: Recent advances and perspectives

Abstract

Solvent extraction, a separation and purification technology, is crucial in critical metal metallurgy. Organic solvents commonly used in solvent extraction exhibit disadvantages, such as high volatility, high toxicity, and flammability, causing a spectrum of hazards to human health and environmental safety. Neoteric solvents have been recognized as potential alternatives to these harmful organic solvents. In the past two decades, several neoteric solvents have been proposed, including ionic liquids (ILs) and deep eutectic solvents (DESs). DESs have gradually become the focus of green solvents owing to several advantages, namely, low toxicity, degradability, and low cost. In this critical review, their classification, formation mechanisms, preparation methods, characterization technologies, and special physicochemical properties based on the most recent advancements in research have been systematically described. Subsequently, the major separation and purification applications of DESs in critical metal metallurgy were comprehensively summarized. Finally, future opportunities and challenges of DESs were explored in the current research area. In conclusion, this review provides valuable insights for improving our overall understanding of DESs, and it holds important potential for expanding separation and purification applications in critical metal metallurgy.

1. Introduction

Solvent extraction, an effective separation and purification technology, is crucial in critical metal metallurgy [1]. Organic solvents, which are primarily utilized for solvent extraction, are generally carbon-based molecules with high volatility, high toxicity, and flammability, causing a spectrum of hazards for human health and environmental safety. In the two previous decades, several neoteric solvents, particularly ionic liquids (ILs) and deep eutectic solvents (DESs), have been proposed. While this review does not focus on traditional ILs, a brief introduction to ILs and a comparison between ILs and DESs are crucial. At present, ILs have emerged as a highly researched area in the field of science [2].

ILs consist of organic cations and organic/inorganic anions that remain in liquid form even at temperatures below 10°C [3]. The most commonly used ILs can be divided into two categories. The first type is formed by combining eutectic mixtures of metal halides with organic salts. In this type of ILs, the mixture's eutectic composition creates bulky chloroaluminate or chlorozincate ions, which effectively reduce the lattice energy and freezing point of the mixture [4]. Another type of ILs contains discrete anions, such as PF6 or (CF3SO2)2N−. This class of ILs has the advantage of possessing large electrochemical windows, which enables electrodeposit of some noble metals [5]. ILs exhibit some unique physiochemical properties, including high polarity, non-flammability, excellent chemical stability, non-volatility, high thermal stability, and recyclability. Furthermore, these characteristics can be adjusted according to requirements by selecting different anions and/or cations; thus, they are also called “Task-specific ILs” [6]. Currently, ILs have completely changed some extraction processes, and they are extensively applied to solid-phase extraction, pressurized liquid extraction, liquid–liquid extraction, and novel liquid phase microextraction due to their high separation coefficients. However, the potential toxicities and non-biodegradability of ILs pose a risk to their broader applications as neoteric solvents with environmentally friendly features. The development of neoteric green reagents is essential to compensate for the shortcomings of ILs, serving as a viable alternative.

DESs, as neoteric green reagents, have gradually become the focus of green solvents due to a series of advantages. Fig. 1 displays the similarities and differences between DESs and ILs. DESs are generally composed of “hydrogen bond donor compounds” and “hydrogen bond acceptor compounds.” They can lead to the formation of a new eutectic phase through some intermolecular interactions. In addition, they have a significantly lower melting point than the individual components. The primary focus in the eutectic system is the melting point reduction. Fig. 2(a) depicts a phase diagram that represents the relationship between the melting temperature and composition of the eutectic system [7–8]. In the case of a eutectic mixture containing A and B, the eutectic point marks the lowest melting temperature. During the cooling process of a liquid system, eutectic mixtures do not undergo isothermal reactions between different components. Therefore, the melting point of eutectic mixtures is lower than that of individual components.