Separation of rare earth elements by synergistic solvent extraction with phosphonium-based ionic liquids using a β-diketone extractant and a neutral ligand

ABSTRACT

We investigated the solvent extraction of four rare earth (RE) elements (Pr, Nd, Tb, and Dy) from Nd-Fe-B magnets using mixtures of 1-(2-thienyl)-4,4,4,-trifluoro-1,3-butanedione (Htta) or 4,4,4-trifluoro-1-phenyl-1,3-butanedione (Hbfa) chelating extractants and tri-n-octylphosphine oxide (TOPO) neutral ligand in phosphonium-based ionic liquids. Based on the results of slope analysis data, the following equations were proposed for the RE extraction:

$\begin{array}{c}{\left[\mathrm{M}{{\left({\mathrm{H}}_2\mathrm{O}\right)}_9}^{3+}\right]}_{\mathrm{a}\mathrm{q}}+3{\left[\mathrm{H}\mathrm{L}\right]}_{\mathrm{I}\mathrm{L}}+2{\left[\mathrm{S}\right]}_{\mathrm{I}\mathrm{L}}\iff {\left[\mathrm{M}{\mathrm{L}}_3{\mathrm{S}}_2\left({\mathrm{H}}_2\mathrm{O}\right)\right]}_{\mathrm{I}\mathrm{L}}+3{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{a}\mathrm{q}}+8{\left[{\mathrm{H}}_2\mathrm{O}\right]}_{\mathrm{a}\mathrm{q}}\\ \left(\mathrm{M}:\mathrm{T}\mathrm{b}\mathrm{o}\mathrm{r}\mathrm{D}\mathrm{y},\mathrm{H}\mathrm{L}:\mathrm{H}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{o}\mathrm{r}\mathrm{H}\mathrm{b}\mathrm{f}\mathrm{a},\mathrm{S}:\mathrm{T}\mathrm{O}\mathrm{P}\mathrm{O}\right)\end{array}$

$\begin{array}{rl}& {\left[\mathrm{M}{{\left({\mathrm{H}}_2\mathrm{O}\right)}_8}^{3+}\right]}_{\mathrm{a}\mathrm{q}}+2{\left[\mathrm{H}\mathrm{L}\right]}_{\mathrm{I}\mathrm{L}}+2{\left[\mathrm{S}\right]}_{\mathrm{I}\mathrm{L}}\iff {\left[\mathrm{M}{\mathrm{L}}_2{\mathrm{S}}_2{{\left({\mathrm{H}}_2\mathrm{O}\right)}_2}^{+}\right]}_{\mathrm{I}\mathrm{L}}+2{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{a}\mathrm{q}}+6{\left[{\mathrm{H}}_2\mathrm{O}\right]}_{\mathrm{a}\mathrm{q}}\\ & (\mathrm{M}:\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{r}\mathrm{N}\mathrm{d},\mathrm{H}\mathrm{L}:\mathrm{H}\mathrm{t}\mathrm{t}\mathrm{a}\mathrm{o}\mathrm{r}\mathrm{H}\mathrm{b}\mathrm{f}\mathrm{a},\mathrm{S}:\mathrm{T}\mathrm{O}\mathrm{P}\mathrm{O})\end{array}$

A synergistic effect was observed for the extraction of the RE elements with the combination of extractant and neutral ligand. The values of the equilibrium constant and separation factor were also evaluated in this study. The results reveal that the [NTf₂]⁻ anion did not participate in the extraction mechanism of the RE elements, and the Raman spectroscopic analysis of the extracted phase also supported the proposed stoichiometry of the extracted complex. Furthermore, the separation of Tb(III) and Dy(III) from other RE(III) components was investigated through a series of hydrometallurgical processes, which consisted of leaching, removal of [Fe(OH)_x]^3-x, and solvent extraction. Finally, the separation of Tb(III) and Dy(III) from other RE(III) components was performed with seven extraction cycles using the HL + TOPO/[P₂₂₂₅][NTf₂] system.

KEYWORDS:

• Ionic liquids
• rare earth
• solvent extraction
• synergetic effect

Additional information

Funding

This research was partially supported by the Grant-in-Aid for Scientific Research (No. 18H03404) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.