Boosting the extraction of rare earth elements from chloride medium by novel carboxylic acid based ionic liquids

Abstract

Two novel carboxylic acid based ionic liquids (ILs), benzyltributylammonium myristic acetate ([N444Bn][MA]) and benzyltributylammonium dodecanedioic acetate ([N444Bn]2[DDA]), have been rationally designed and successfully synthesized for the extraction of yttrium (denoted as Y(III)) from a chloride aqueous solution and for the separation of mixed rare earth elements (REEs). Comparison studies of the extraction performances of [N444Bn][MA] and [N444Bn]2[DDA] were carefully investigated under different conditions (e.g., equilibrium time, solution acidity, salting-out agent concentration, and REEs concentration). The results indicated that the extraction efficiency of Y(III) reached 87.18% after 2 min using [N444Bn]2[DDA], whereas it required 10 min to reach an extraction efficiency of 47.37% using [N444Bn][MA], mainly due to the larger electronegativity of the incorporated dicarboxylates in the anions of [N444Bn]2[DDA], which enhanced its coordination with Y(III) compared to [N444Bn][MA]. However, [N444Bn][MA] illustrated better selective separation behavior than [N444Bn]2[DDA] in mixed REEs solutions (including La, Nd, Eu, Ho, Yb and Y), and the selective factors of Yb and La were 16.21 and 3.30, respectively. Furthermore, the ion association mechanism, evolved from the slope analysis and FT-IR results, contributed to avoiding the saponification procedures and resultant saponification wastewater during the extraction process. In addition, the back-extraction experiments indicated that ~97% Y(III) can be stripped from [N444Bn]2[DDA] and [N444Bn][MA] using 1.0 M and 0.4 M HCl, respectively, demonstrating that these ILs can be further used in large-scale applications for the separation and recovery of REEs.