Lanthanides (Ln's) are the major components of rare earth elements, which are critical components of many high-valued products. The ions of Ln’s have the same valence and little differences in size and other physical and chemical properties. They cannot be separated using conventional ion exchange or chromatography processes. Multiple sequential and parallel solvent extraction steps and large amounts of solvents have been used for producing high-purity Ln’s, resulting in grave impacts on the environment.
The feasibility of ligand-assisted displacement chromatography using polymeric ion exchangers was demonstrated for Ln separation by Spedding and Powell in the 1950’s and 1960’s. Three lanthanides were recovered with relatively high purity (~99%) and with yields from 83 to 93%. However, each separation run took more than three weeks, resulting in very low adsorbent productivity. More importantly, the complex mechanisms in this system were not well understood, and no systematic design/optimization method or scaling rules have been reported.
In this study, intrinsic adsorption, reaction, and mass transfer parameters for three lanthanides were estimated from frontal experiments. Rate model simulations were developed and verified using frontal and ligand-assisted displacement chromatography data. A verified model and model parameters were used to optimize the displacement process. Experimental results show the average yield of high-purity (99%) products is more than 97%, and sorbent productivity is an order of magnitude higher than that of Spedding and Powell.
Dynamic simulation studies show that the mechanisms and the criteria for separation in ligand-assisted displacement processes are quite different from those of conventional displacement chromatography. Displacement and migration of solute bands is driven by the reactions of the solutes with the ligand in the mobile phase. The complexation reactions help the solute with higher affinity for the ligand migrate downstream from a solute with a lower affinity for the ligand. If the column is sufficiently long, the different solutes eventually are separated to form pure bands with sharp boundaries. Both boundaries migrate at the same speed, resulting in a fully developed displacement train. The migration speed of the train is controlled by the interstitial velocity, the ligand concentration, the complexation equilibrium constant of the ligand with the presaturant, and the adsorption equilibrium constant of the presaturant with the sorbent. The solute with the highest affinity for the ligand will elute first. By contrast, in conventional displacement chromatography, the solute with the lowest affinity for the sorbent elutes first.
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