Separation of rare earth elements by synergistic solvent extraction with phosphonium-based ionic liquids using a <i>β</i>-diketone extractant and a neutral ligand
Masahiko Matsumiya, Daiki Nomizu, Yusuke Tsuchida, Yuji Sasaki
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: MH2O93+aq+3HLIL+2SIL⇔ML3S2H2OIL+3H+aq+8H2OaqM:Tb or Dy,HL:Htta or Hbfa,S:TOPO MH2O83+aq+2HLIL+2SIL⇔ML2S2H2O2+IL+2H+aq+6H2Oaq(M:ProrNd,HL:HttaorHbfa,S:TOPO)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.