(683f) Development of a Predictive Thermodynamics Model for Solvent Extraction of Rare Earth Elements

Rare Earth Elements (REEs) are found in various locations around the world, either as by-products or co-products at mining sites, wastewater, and mineral reserves. They are primarily considered “rare” due to the difficulties involved in their extraction process. These materials have found applications across different sectors and are considered critical for producing modern and high-tech devices [1]. Consequently, there has been a significant surge in demand for these elements. To prevent disruptions in the REE supply chain, there is a dire need to intensify efforts toward optimal, economical, and environmentally sustainable extraction methods from new domestic sources and recycling from end-of-life products. However, REEs occur together as a mixture and possess similar chemical properties, making it quite challenging to separate them selectively to acceptable purity.

One of the important steps of REE extraction is solvent extraction, where an organic solvent along with an extractant is used to preferentially extract the desired REEs from the aqueous solution to the organic solution. Based on the solubility of metal complexes, they can be separated into two immiscible liquid phases: typically, an aqueous phase from which cations are selectively extracted and an organic phase that contains a cation exchange ligand. This liquid-liquid extraction process has been identified as an effective and one of the most common methods for separating rare earth metals [2]. Thermodynamics of the underlying complexation process is complex; therefore, distribution or partition coefficients are often obtained from the experimental data, and models are developed to correlate them to operational variables such as pH and extractant dosage. However, these models are not predictive since the complex interactions among cations-anions and molecular species get lumped in the distribution/partition coefficient, so these models cannot be used for systems where the feed and product composition greatly vary compared to the experimental system. This work describes a model developed to predict the thermodynamic behavior of solvent extraction processes, which can be applied to various solvents and REEs across different operating conditions.

Several models have been developed for the thermodynamic properties of only aqueous electrolyte solutions using various thermodynamic models (i.e., Pitzer, MSE, and AeNRTL) [3] [4] [5]. In several instances, the organic phase is considered an ideal system [6], while models that account for the non-idealities in both phases are sparse in the literature. Also, in many existing approaches, the effect of speciation in the aqueous phase is neglected. The speciation model is developed based on the literature. Previous studies of similar systems operating within the conditions considered in this work suggest that and the conjugate base (Y^-) from acid dissociation, forms and complexes with being more pronounced [7]. Hence, and are considered in the model formulation.

The developed model has shown improved predictive capability compared to existing models. Using experimental data from a pilot scale plant for scenarios where DEHPA and a mixture of DEHPA and TBP were used as extractants and/or phase modifiers, thermodynamic parameters for species not reported in literature and databases were estimated for six REEs (Nd, Ce, Y, Sm, Gd, and Dy). Using estimated parameter values, distribution coefficient values were predicted and are comparable to experimental literature data. Beyond the working conditions of the available experimental data, the model predicts the extraction percentage with a reasonable degree of accuracy. With appropriate modifications, it is expected that this model can be readily extended to REE solvent extraction processes that involve other types of organic-phase solvents and extractants.

References

[1] Bauer, Diana, David Diamond, Jennifer Li, David Sandalow, Paul Telleen, and Brent Wanner, “US Department of Energy Critical Materials Strategy,” 01 2010. Accessed: Feb. 12, 2025. [Online]. Available:

https://www.osti.gov/servlets/purl/1000846-1gn61Q/#page=1.00&gsr=0

[2] C. K. Gupta and N. Krishnamurthy, Extractive metallurgy of rare earths. Boca Raton, Fla: CRC Press, 2005.

[3] Z.-C. Wang, M. He, J. Wang, and J.-L. Li, “Modeling of Aqueous 3–1 Rare Earth Electrolytes and Their Mixtures to Very High Concentrations,” J. Solut. Chem., vol. 35, no. 8, pp. 1137–1156, Oct. 2006, doi: 10.1007/s10953-006-9050-0.

[4] G. Das, M. M. Lencka, A. Eslamimanesh, A. Anderko, and R. E. Riman, “Rare-earth elements in aqueous chloride systems: Thermodynamic modeling of binary and multicomponent systems in wide concentration ranges,” Fluid Phase Equilibria, vol. 452, pp. 16–57, Nov. 2017, doi: 10.1016/j.fluid.2017.08.014.

[5] S. Bhaiya, “Modeling Rare Earth Elements (REE) with Association Electrolyte Nonrandom Two-Liquid Activity Coefficient Model,” 2022.

[6] S. Pavón, A. Fortuny, M. T. Coll, and A. M. Sastre, “Solvent extraction modeling of Ce/Eu/Y from chloride media using D2EHPA,” AIChE J., vol. 65, no. 8, p. e16627, 2019, doi: 10.1002/aic.16627.

[7] Y. Marcus, "Anion exchange of metal complexes—XV (1): Anion exchange and amine extraction of lanthanides and trivalent actinides from chloride solutions," Journal of Inorganic and Nuclear Chemistry, vol. 28, no. 1, pp. 209–219, 1966.