Synthesis and characterization of Pb–REE phosphate (REE₂Pb₃(PO₄)₄·nH₂O) for application in novel rare-earth element beneficiation methods

The development of inexpensive and efficient methods for the recovery of critical raw materials is of increasing importance within the European Union, driven by the growing demand for rare earth elements (REE) in green technologies and electronic devices, as well as the strategic need to reduce limited reliance on external suppliers and maintain competitiveness. Recently, formation of a Pb–REE phosphate phase was reported alongside pyromorphite as a product of REE beneficiation by coprecipitation with Pb-phosphates (Sordyl et al., 2023).

Building on earlier observations, this contribution presents the results of the synthesis of mixed Pb–REE phosphates: La₂Pb₃(PO₄)₄·3.5H₂O, Ce₂Pb₃(PO₄)₄·3.3H₂O, Pr₂Pb₃(PO₄)₄·3.1H₂O, and Sm₂Pb₃(PO₄)₄·3.3H₂O, following the protocol of Staszel et al. (2023). They precipitate from aqueous solutions (pH 2-3, REE:Pb molar ratio of 3:2, at ambient temperature, open to the air) as poorly crystalline granular aggregates composed of rounded nanoparticles. The combined analytical approach including chemical analysis and microanalysis, synchrotron pair distribution function (PDF) analysis, and Raman spectroscopy confirms the incorporation of La, Ce, Pr, and Sm into the Pb phosphate structure in the same way as in the phases precipitated and described by Staszel et al. (2023). The composition of the precipitated phases is in agreement with previous reports by Staszel et al. (2023). Structural constraints derived from PDF analysis indicate that precipitated Pb-REE phosphates are similar to the rhabdophane REE(PO₄)∙0.6H2O structure (space group P3₁21). This finding differs from the previously proposed orthorhombic crystal system (space group Cmmm) (Staszel et al. 2023).  Additional techniques, such as extended X-ray absorption fine structure (EXAFS) and small-angle X-ray scattering (SAXS), will be applied to further resolve the periodic structure and the local distortions.

Resolving the crystallographic framework is essential for improving our understanding of the crystal-chemical role and structural position of REE within Pb phosphate phases. A thorough characterization of these newly described phases is therefore critical for refining the coprecipitation protocol and evaluating its applicability to REE recovery from phosphate-rich mineral sources or mining wastes, such as those associated with iron oxide-apatite deposits in northern Sweden. This work will be complemented by future in situ experiments to better monitor the nucleation processes governing the competitive formation of pyromorphite versus Pb–REE phosphate phases, with the aim of optimizing the recovery pathway toward the formation of the most effective REE-bearing phase.

The project is supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) and Polish National Science Centre grant no. 2021/43/O/ST10/01282.

References

Sordyl, J., Staszel, K., Leś, M., & Manecki, M. (2023). Removal of REE and Th from solution by co-precipitation with Pb-phosphates. Applied Geochemistry, 158, 105780. https://doi.org/10.1016/j.apgeochem.2023.105780

Staszel, K., Jędras, A., Skalny, M., Dziewiątka, K., Urbański, K., Sordyl, J., Rybka, K., & Manecki, M. (2023). New synthetic [LREE (LREE = La, Ce, Pr, Sm), Pb]-phosphate phases. Mineralogia, 54(1), 58–68. https://doi.org/10.2478/mipo-2023-0006