M. Baghbanzadeh, S. Mosadeghsedghi, K. Volchek, M.E. Sauber
Natural Resources Canada,
Canada
Keywords: electrodialysis, electrochemical separation, rare earth elements separation
Summary:
At the industrial scale, rare earth elements (REE) are mainly separated by the solvent extraction technique (SX), which is a widely accepted, effective but complicated process with a large number of stages. In addition, organic solvents are used in SX which leads to the generation of hazardous wastes, A promising alternative separation technique to SX could be electrodialysis (ED), an electrochemically driven membrane separation technology that has widely been used at a commercial scale for separation and concentration applications. ED has clear advantages over conventional SX, including no need to organic solvents, no hazardous waste production, and simplicity and modularity of the process. Furthermore, being coupled with renewable sources of energy, ED can be considered a green process. In this study, the feasibility of combining ED and SX for the separation of REE was investigated. EquilibriumSX, a simulation software developed by Laval University was used to simulate the number of stages and specifications of the streams in the SX process (Figure 1). A lab-scale ED system with a microflow cell consisting of five pairs of Neosepta cation and anion exchange membranes in an alternating configuration was utilized. Since REE have identical size and valence electrons, ED alone would not be effective in separating them. To enhance the selectively of separation, chelating agents, such as DCTA and HEDTA were used in this study. Three scenarios were considered to investigate the effectiveness of ED in the simulated SX circuit: 1) at the upstream of SX, as an alternative to the SX-1 to separate light REE (LREE) from medium REE (MREE) and heavy REE (HREE), 2) to separate MREE from HREE (replace SX-5 with ED), and 3) to separate LREE (Lanthanum from Nonindium and Praseodymium-replace SX-9 with ED). The results showed that using ED as an alternative to SX-1 (Scenario 1) did not result in good REE separation as the separation factors fall in the range of 1–2. Replacing SX-5 with ED to separate MREE from HREE (scenario 2) also resulted in poor REE separation factors, as indicated in. On the other hand, using ED at the downstream of SX-1 to separate LREE (Scenario 3) resulted in high separation factors of about 10 times higher than those obtained in scenarios 1 and 2. It was observed that the REE separation factors were up to 50% higher in the presence of HEDTA compared with those in the presence of DCTA. Given the positive outcome in replacing SX-9 with ED, further attempts were made to improve the performance of the integrated ED/SX, by using multi-stage ED. In comparison to the single-stage ED, REE separation factors were significantly increased by using multi-stage ED. Using a three-stage ED in the presence of HEDTA, high separation factors of 59.4, 162.3, and 623.2 were achieved for the La-Pr, La-Nd and La-Sm, respectively, corresponding to 920%, 2,100%, and 6,100% increase compared with the separation factors obtained by the single-stage ED. Results of this study suggest that chelation-assisted ED can be incorporated into the SX circuit to augment REE separation.