The development of an extraction strategy based on EHEHP-type functional ionic liquid for heavy rare earth element separation

Highlights

• The EHEHP type functional ionic liquid is first studied for neighboring HREEs separation.

• Extractability of HEH[EHP] can be enhanced by preparing it as functional ionic liquid.

• EHEHP type functional ionic liquid contributes to avoid the saponification wastewater.

• HREE extracted by the EHEHP type functional ionic liquid can be fully stripped.

Abstract

To develop sustainable and efficient separation technology for heavy rare earth elements (HREEs), the extraction of Lu(III) and neighboring HREEs using EHEHP type acid–base coupling bifunctional ionic liquid (ABC-BIL) was studied for the first time. Selectivity and stripping performances of the ABC-BIL based system were investigated. Primary mechanism of the ABC-BIL based system was indicated to be ion-association. The EHEHP type ABC-BIL contributes to avoid the saponification wastewater from HEH[EHP]. Higher extractabilities of the ABC-BIL for HREEs, i.e., Ho(III), Y(III), Er(III), Tm(III), Yb(III), and Lu(III), are pronounced advantages of the IL based extracting system than traditional HEH[EHP] system.

Introduction

Rare earth elements (REEs) are critical materials in many cutting-edge technology products. The economic impact of the growing scarcity of REEs has been the reason for many countries to address their strategic positions on the criticality of REEs (Eliseeva and Bünzli, 2011). Ion-adsorption mineral is the important rare earth resource in China. Unlike bastnasite and monazite, ion-adsorption mineral is enriched in HREEs (Yang et al., 2013). According to the US DOE medium-term criticality matrices, the rare earth metals (dysprosium, neodymium, terbium, europium and yttrium), are assessed as most critical because of their applications in the clean energy technologies — including wind turbines, electric vehicles, photovoltaic cells and fluorescent lighting (Bauer et al., 2010). In the five REEs mentioned above, dysprosium, terbium, yttrium are heavy rare earth elements (HREEs). According to the statistics from United States Geological Survey (Long, 2011), the global yields of HREEs are far lower than those of light rare earth elements (LREEs). Therefore, HREEs are more important than LREEs since the supply risk and importance to clean energy. 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP], P507) is the most widely used extractant for individual REE separation in Chinese rare earth industry. Because of the well performance, HEH[EHP] has been applied for decades. However, some properties of HEH[EHP] for REEs separation are still unsatisfactory, such as the saponification wastewater (Wang et al., 2014). To develop efficient and environment-friendly method for HREEs separation, some separation technologies have been reported. Liao et al. (Liao et al., 2010) studied the mechanism of Cyanex272-HEH[EHP] impregnated resin for HREEs separation from hydrochloric acid solution using IR spectrometer, saturation method, and slope analysis. The HPLC method using acetic acid as novel eluent was explored for selective separation of yttrium from the HREEs (Kifle and Wibetoe, 2013). Abreu et al. (Abreu and Morais, 2014) investigated three organophosphorus acids (DEHPA, IONQUEST®801 and CYANEX®272), a mixture of DEHPA/TOPO (neutral ester) and three amines (ALAMINE®336, ALIQUAT®336 and PRIMENE®JM-T) in hydrochloric media and sulphuric media for HREEs separation.

Ionic liquids (ILs) are salts, they are generally liquid below 100 °C. Some applications of ILs at industrial scale have been established, e.g. aluminum plating in BASF, Difasol in Institut Français du Pétrole, paint additives in Degussa, hydraulic ionic liquid compressor in Linde, batteries in Pionics, and solar cells in G24i (Petkovic et al., 2011). IL-based extraction is a separation strategy that applies ILs instead of volatile organic compounds as diluents and/or extractants. The properties of ILs make them particularly suitable for solvent extraction, including their low volatility and combustibility, wide liquid range, thermal stability, adjustable functional group, high conductivity, and wide electrochemical window (Castillo et al., 2014, Sun et al., 2012a, Sun et al., 2012b, Coll et al., 2012, Tong et al., 2014). Sun et al., 2009, Sun et al., 2010 reported the inner synergistic effects between cations and anions of ABC-BILs for REEs extraction. The inner synergistic effect of ABC-BIL is a novel synergistic extraction. When the precursory extractants are prepared as ABC-BILs, their extractabilities can be increased due to the inner synergistic effect. Moreover, millions of tons of saponification wastewaters annually from acidic extractants in industrial REEs separation may be reduced by using the ABC-BILs. The achievement highlights considerable environmental value and economic value (Sun and Waters, 2014). Up to now, many interesting results concerning ABC-BILs have been reported. Wang et al., 2011 studied the extraction and separation of REEs using tricaprylmethylammonium sec-octylphenoxy acetate ([A366][CA-12]) and tricaprylmethylammonium sec-nonylphenoxy acetate ([A336][CA-100]) in chloride medium. Their results indicated that extractabilities of the bifunctional ILs were higher than those of conventional extractants, such as CA-12, CA-100, TBP, and P350. To develop efficient IL-based extraction systems, Sun et al., 2012a prepared ammonium- and phosphonium-based ILs that incorporating deprotonated di(2-ethylhexyl)phosphoric acid (HDEHP) as anion component. The anion-functionalized ILs resulted in significantly enhanced extractabilities and selectivities for REEs in TALSPEAK-like process. Rout et al., 2013 studied some ILs with bis(2-ethylhexyl)phosphate anion ([DEHP]⁻), i.e., [C₆mim][DEHP], [C₆mpyr][DEHP] and [N₄₄₄₄][DEHP] for Nd(III) extraction from nitric acid medium. The cations of ILs were indicated to have distinct influences on the extraction properties of extractants. It was possible to tune the extractability and selectivity of metal ion by a proper choice of IL cation. Castillo et al., 2014 removed Cu(II) from highly contaminated aqueous solutions (1000 mg/L Cu(II)) by quaternary ammonium type ionic liquid [A336/Cy272] in sulfate, chloride or mixed media, with extraction efficiencies up to 95%. In this paper, we first investigate the extraction and separation performances of EHEHP type ABC-BIL for neighboring HREEs.