# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 3 Results and Discussion
### Process Performance  
A total of 21 separation experiments were performed in this study with the IRA-410 resin, using different feed solutions, feed volumes, and different conditions to study the separation of REEs as MGDA complexes. This publication presents data for 10 of those experiments in the figures and discusses the results. Data from the experiments not shown does not provide any new information, which could be helpful to explain the chemistry or performance of the proposed system. From the experiments that were conducted multiple times, the reproducibility of separation can be evaluated, which is good. This data is used to analyze process performance in this section, even though the experiments were not specifically designed to optimize the process.  
In ternary and multicomponent chromatographic systems, the recovery of the intermediate eluting compounds (here Nd) is always the greatest challenge. The high purity required for each metal in the corresponding product fraction leads to a trade-off between recovery yields and purities of the adjacent compounds. As observed in the chromatographic data presented above, Nd always overlaps with the adjacent La and Eu peaks. With suitable feed pH adjustments (see Section 3.2.3), the La can be separated from Nd and Eu with good purity and yield. The challenge with Eu fractions is their low concentration due to their broad elution profile. However, high purity levels can be achieved.  
Figure 12 gives an example of the tradeoffs between recovery yield and product purity for each metal. The first fractionation cut, \(\mathrm{Cut_{1}}\), refers to the end of the La product fraction (its beginning is obvious and is not considered). Since no to-be-recycled or waste fractions are considered here, \(\mathrm{Cut_{1}}\) is also the beginning of the Nd product fraction. The width of the Nd fraction equals \(\mathrm{Cut_{2}}\)-\(\mathrm{Cut_{1}}\), where \(\mathrm{Cut_{2}}\) is the beginning of the Eu product fraction. The end of the Eu fraction should be when it is completely eluted. With this fractionation strategy, the purity and yield of La and Eu can be presented as a single-parameter curve, whereas the data for Nd is presented as contour lines in a coordinate system spanned by the start of Nd fraction (\(\mathrm{Cut_{1}}\)) and its width (\(\mathrm{Cut_{2}}-\mathrm{Cut_{1}}\)).  
Figure 12 shows that setting \(\mathrm{Cut_{1}}\) = 0.84 BV results in 81% purity and 80% yield for La. Cutting a little later (\(\mathrm{Cut_{1}}\) = 1.1 BV) increases the La yield to 89%, but the purity decreases to 70%. This is, nevertheless, a good result for a single process step in a ternary REE mixture, considering that the feed purity is approximately 33% for each metal.  
The contour plots in Figure 12 show that the maximum yield and purity for Nd are not obtained with the same fractionation cut times. To obtain higher than 60% purity for Nd, Cut1 should occur between approximately 0.95 BV and 1.1 BV, and the fraction should not be wider than approximately 0.7 BV. In this range, the recovery yield is less than 50%. Decreasing the purity from 60% to 50% by collecting more Eu in the Nd fraction (\(\mathrm{Cut_{1}}\) = 0.95 BV, \(\mathrm{Cut_{2}}-\mathrm{Cut_{1}}\) = 1.5 BV) increases the Nd yield to 75%. This does, however, decrease the yield of Eu from 75% to 43%, while its purity increases from approximately 70% to 80%.  
In many of the experiments discussed above, a large amount of non-separated REE-MGDA complexes were eluted at the void volume of the column. This \"frontal peak\" could be collected separately and recycled back to the feed without a loss of yield or purity. The yields and purities of all REEs could be significantly improved by using the so-called steady-state recycling chromatography [45]. In this process, low-purity fractions are collected between the product fractions and recycled to the feed. Both recycling schemes would likely require adjustment of the pH to alkaline, thereby increasing the consumption of the acid.