Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins

Abstract

Chromatographic separation of rare earth elements (REE) as anionic complexes with chelating aminopolycarboxylate ligand methylglycine N,N-diacetate (MGDA) was studied experimentally. A synthetic mixture of La, Nd, and Eu were used to model a REE mixture obtained from processed secondary sources such as phosphogypsum (PG). In the REE extraction from PG, the REEs can be recovered with MGDA to obtain the REE–MGDA mixture. Three strong base anion exchange resins (Dowex 1X8, IRA-402, and IRA-410) were used as the separation materials. Successful separation of the REEs by elution with dilute HNO3 and HCl was attributed to differences in the stabilities of the REE–MGDA complexes. The pKa values of the complexes were determined by titration to be 3.81, 3.25, and 2.96 for La, Nd, and Eu, respectively. Fractionation of the ternary La-Nd-Eu mixture (with a 1:1:1 mole ratio) were studied. La was recovered at approximately 80% purity and 80% yield, but strong trade-offs between the yield and the purity of Nd and Eu must be made. Chromatographic separation was found to be an efficient process option, considering its simplicity and the recovery of several product fractions. The initial process design offers a promising starting point for investigating more advanced process configurations for the efficient recovery of pure REE from phosphogypsum.