Recovery of rare earth elements (REEs) from leach solutions by solvent extraction with predispersion

Date

2024-6-25

Author

Rahim, Muhammad Zubair

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Rare Earth Elements (REEs) are crucial in various modern industrial products, renewable energy systems, and catalysis. Due to their increasing demand and limited natural abundance, efficient extraction methods are imperative for ensuring a stable supply chain. Solvent extraction (SX) is typically considered the best commercial approach for extracting REEs because it can handle massive volumes of diluted pregnant liquors. Conventional SX equipment have some operational limitations, causing a decrease in the recovery of the product and process kinetics. These issues include insufficient mass transfer because of the poly-dispersed droplets and phase separation of product streams, thus requiring a huge settling chamber for mixer-settler, and back-mixing of feed solutions in spray columns, reducing the efficiency of mass transfer between the two immiscible streams. These problems have been addressed and overcome by the newly designed “solutions spray system.” In this new technique, the feed solutions were sprayed counter-currently in the form of mono-dispersed micro-sized droplets in the mixing column, where the mass transfer took place, and the emulsion produced was continuously sent to the settling column, where the two product phases were separated. The geometry of this settling column enhanced the quick separation in the aqueous raffinate solution and the loaded organic stream. The primary aim of this experimental study was to study the performance of this solutions spray system with the intent to enhance the recovery of the REEs and kinetics of SX operation. Variables like feed solution pH, DEHPA concentration, ionic strength, concentrations of REEs in the aqueous feed, type of feed solutions, and aqueous/organic (A/O) phase ratio were studied and evaluated the performance of this setup while extracting lanthanum (La), neodymium (Nd), and europium (Eu) from their synthesized chloride solutions. For feed solution having 1.0 g/L of each REO, the %E of La, Nd, and Eu was 95.03%, 99.36%, and 99.98%, respectively, at 2.0 pH, and 1.54 mol/L ionic strength of the solution while using 0.303 M DEHPA. Under these conditions, the distribution ratio for La, Nd, and Eu was 19.14, 155.25, and 4165, respectively, representing complete extraction in a single-stage operation. Moreover, the values of separation factors, SFEu/La = 6.69, SFEu/Nd = 5.33, and SFNd/La = 1.26 at 0.50 pH, showed the selective separation of these elements. Stripping of loaded DEHPA was performed with 2.0 – 6.0 M HCl, and 2.0 M HCl strip solution gave good results of REEs recovery. Furthermore, for aqueous feed representative of Beylikova leach solution having 6.0 g/L La2O3, 2.5 g/L Nd2O3, and 100 mg/L Eu2O3, the %E of La, Nd, and Eu was 70.32%, 85.31%, and 99.26%, respectively, under the same variable conditions, and 4.0 M HCl stripping solution showed better recoveries in this case. McCabe Thiele diagrams were plotted to see the number of theoretical stages in recovering REEs as a group and their selective separation from each other for extraction and stripping sections. Additionally, the extraction of these REEs was also observed from sulfate solutions, and it was found that %E was superior from the HCl solution compared to the H2SO4 solution, especially at lower pH values of feed solutions. Finally, the two immiscible phases were mixed with the mechanical mixer, and the ultrasound mixing, and the extraction results were compared to that of the solutions spray system. The percent extraction of REEs was superior in the case of the solutions spray system, followed by ultrasound mixing and mechanical mixing. In light of these findings, the solutions spray system seemed better in terms of recovery of REEs and time of mixing and separation, thus enhancing the performance of the SX operation.

Subject Keywords

Solvent extraction, Rare earth elements, DEHPA, Solutions spray system

URI

https://hdl.handle.net/11511/110392

Collections

Graduate School of Natural and Applied Sciences, Thesis