A THERMODYNAMIC MODEL FOR EXTRACTION OF RARE EARTH ELEMENTS (REE) FROM PHOSPHATE-BEARING RESOURCES

Rare earth elements (REEs) are energy-critical elements. At the present time, most of the world’s REEs’ production come from two types of ore deposits in China. One type is carbonatite-hosted ore deposits represented by the Bayan Obo deposit, which are enriched with light rare earth elements (LREEs). The other type is ion-adsortion clay-type deposits, which are enriched with heavy rare earth elements (HREEs). In USA, there are abundant sedimentary phosphorites that have appreciably high REE concentrations, and they are the potential resources for extraction of REEs. The extraction of REEs from phosphate rocks employs the following acids: H₂SO₄, HCl, HNO₃, and H₃PO₄, or the binary combination of these acids. The processes using H₃PO₄ as the primary leaching agent seem to be currently favored, as anion impurity is not introduced, and REEs are enriched in leaching solutions.

In this study, we develop a thermodynamic model to describe interactions of phosporous species with REE and impurities in phosphate rocks. As calcium is the dominant impurity, the decomposition of phosphate rocks in reaction with H₃PO₄, H₂SO₄, HCl, and HNO₃, will result in the following solution systems: (1) H₃PO₄—Ca(H₂PO₄)₂, (2) H₃PO₄—H₂SO₄—Ca(H₂PO₄)₂, (3) H₃PO₄—HCl—Ca(H₂PO₄)₂, and (4) H₃PO₄—HNO₃—Ca(H₂PO₄)₂, respectively. Therefore, the interactions of REEs with phosphorous species in the above solution systems are systematically modelled. In the modeling, we use the Pitzer equations for activity coefficient calculations. In this presentation, we will focus on the interactions of REEs with phosphorous species in the first solution system. In addition, as apatite [Ca₅(PO₄)₃(Cl, F, OH)] is an important phase for REEs and a waste-form for actinides in high level nuclear waste disposal, our experimental and modeling studies also include apatite.

Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This research is funded by an LDRD project (Project Number 222400), and the SFWST programs administered by the Office of Nuclear Energy (NE) of the U.S. Department of Energy. SAND2023-06802A.