# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 4 Conclusions  
This study shows that MGDA-REE complexes can be chromatographically separated with SBA resin. The lower cross-linking density of the SBA resin enhances the affinity and separation; Type II dimethylethanolamine functional SBA resins perform better than Type I quaternary ammonium functional resins. Dilute strong mineral acid was shown to be a suitable eluent. HCl was chosen as the mobile phase because it does not have the unwanted interactions compared to the other alternatives (a strong interaction of HSO4\({}^{-}\) and SO4\({}^{2-}\) towards SBA resins and HNO3 induces nitration of the resin matrix). Separation of MGDA-REE complexes is based on the minor differences between the pKa values of the complexes. Since the desorption and overall separation efficiency was shown to be dependent on the pH of the system, the pH control must be precise. With 0.01 M HCl, good separation of MGDA-REE (REE = La, Nd, Eu) complexes with equivalent concentrations in the initial feed was obtained. The purity and yield for individual REEs can be carefully controlled by choosing a proper fractionation (\"cutting\") scheme. The main challenge is in recovering the middle-eluting Nd in sufficient purity and yield. The following purities (\(P\)) and yields (\(Y\)) were achieved in single column separation: La, \(P\) = 81% and \(Y\) = 79%; Nd, \(P\) = 57% and \(Y\) = 65%; and Eu, \(P\) = 76% and \(Y\) = 64%. Figure 12: Effect of fractionation cut times on purity and yield of each REE (Resin = IRA-410; T = 298 \(\pm\) 2 K; flowrate = 2.98 BV/h; Feed solution = #3; Feed volume = 0.025 BV; Acid concentration = 10 mM). The chemistry of the system was discussed and studied experimentally. It is concluded that the essential phenomena from a separation chemistry point of view are strong MGDA-REE complexation, protonation of MGDA (pH), and competition of chloride, dissociated free MGDA, and anionic MGDA-REE complexes for the functional groups in the resin. Separation could be further improved by utilizing gradient elution, adding multiple consecutive stages to the system, and/or utilizing advanced process schemes such as steady-state recycling chromatography. Overall, it was demonstrated that one can modify the mobile phase composition to improve the separation significantly. This enables the use of standard commercial anion exchange resins for such a challenging separation. Ion exchange with a properly modified mobile phase could be used for separation of REEs originating from an industrial resin-in-leach process where REEs are extracted from phosphogypsum and eluted from the resin with alkaline MGDA. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/met13030600/s1](https://www.mdpi.com/article/10.3390/met13030600/s1)]([https://www.mdpi.com/article/10.3390/met13030600/s1](https://www.mdpi.com/article/10.3390/met13030600/s1)), Figure S1. pH profile of the water loading step of the two-step process. The pH of pure deionized water used in loading is shown as reference. (T = 298 K \(\pm\) 2 K; flowrate = 2.98 BV/h; feed (#2) = 0.1 BV; eluent = deionized water). Figure S2. Small La leakage observed in the water loading step of the two-step process. Magnification of the a) picture from Figure 11. Figure S3. Mass spectrum of the pure MGDA-Na\({}_{3}\) feed. (Column = Waters Acquity Premier HSS T3; injection size = 10 \(\mu\)L; T (column) = 303 K; mobile phase = MeOH + 0.1% FA, H\({}_{2}\)O + 0.1% FA (60:40), flow rate = 0.2 mL/min; run time 3 + 1 min; sample = 20 \(\mu\)g/L MGDA (in H\({}_{2}\)O)). Figure S4. Relative amounts of free MGDA in different fractions from experiments with temperatures of 22 \({}^{\circ}\)C and 50 \({}^{\circ}\)C.  
Conceptualization, S.K., S.V. and T.S. ; methodology, S.V. and T.S. ; formal analysis, S.K. and S.V. ; investigation, S.K. ; resources, S.V. and T.S. ; data curation, S.K. ; writing--original draft preparation, S.K. ; writing--review and editing, S.V. and T.S. ; visualization, S.K. ; supervision, S.V. and T.S. ; project administration, S.V. and T.S.