Robust Multi-objective Optimization of Rare Earth Element Chromatography
Abstract: Rare earth elements comprise the metallic elements known as lanthanides as well as scandium and yttrium. They are extensively used in modern technological industries and are considered as strategic commodities in many countries. Rare earth element minerals with varying compositions are found at deposits throughout the world, though most of the global REE supply comes from only a few sources. The current industry standard is to employ liquid-liquid extraction methods to separate the elements and upgrade them to suitable purity levels for commercial applications. Chromatography has historically mainly been used as a final purification method, but it is developing to become an alternative separation method with benefits such as achieving higher purity levels, reducing the number of separation steps and utilizing less extractants compared to liquid-liquid extraction. This study is intended as a contribution to the work of developing chromatography as a rare earth element separation method, and focuses on optimization of chromatographic separation on a preparative scale. This has been done through experimental work, and to a large extent by applying optimization methods in conjunction with experimentally validated mathematical chromatography models.In the experimental optimization work, an overloaded one-step separation of the rare earth elements samarium, europiumand gadolinium was accomplished through preparative ion-exchange high-performance liquid chromatographywith an bis(2-ethylhexyl) phosphoric acid impregnated column and nitric acid as eluent. The main focus was to optimize the productivity rate, subject to a yield requirement of 80% and a purity requirement of 99% for each element, by varying the flow rate and batch load size. The optimal productivity rate was found to be 1.32 kg samarium/(m3 column,h), 0.38 kg europium/(m3 column,h) and 0.81 kg gadolinium/(m3 column,h).The model based optimizations have involved the separation of europium from a mixture of the middle rare earth elements samarium, europium and gadolinium as well as the separation of thulium from a heavy rare earth element mixture containing most of the elements. The results from the thulium batch separation showed that a productivity ranging between 0.1-0.45 kg/(m3 column,h) for yields between 73-99% can be expected under a purity constraint of 99%. The findings from the europium batch separation optimization were used to provide with a general strategy for achieving desirable operation points, resulting in a productivity ranging between 0.61−0.75 kg europium/(m3 column,h) and a pool concentration between 0.52−0.79 kg europium/m3, while maintaining a purity above 99% and never falling below an 80% yield for the target component. In addition to this, a comparative study indicated that the performance of the batch separations can be improved by employing continuous multicolumn countercurrent solvent gradient purification chromatography due to its nature of being a continuous process and its ability to lower the solvent consumption through internal recycling.Finally, the impact of process disturbances was investigated for the europium batch separation process in conjunction with a robust optimization study. The results from the robust optimization were used to chart the required operation point changes for keeping the amount of failed batches at an acceptable level when a certain level of process disturbance was introduced. It was found that the process is very sensitive towards disturbances and a productivity loss in the range of 10-20% can be expected when accounting for robustness.
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