Anionic substitution effects (F, Cl, Br, I, OH) on cerium incorporation within the pyromorphite-type lead phosphate apatite Pb5(PO4)3X framework

Rare earth elements (REE) constitute essential critical raw materials, driven by their role in the ongoing evolution of high-technology and green energy sectors. Growing demand is driving the search for new sources and recovery technologies. A new technology for recovering REE via coprecipitation with lead apatite - pyromorphite Pb₅(PO₄)₃Cl - has recently been proposed [1]. Different sources of REE with varying chemical compositions may contain anions that compete with Cl in the pyromorphite structure [2, 3] and potentially affect REE recovery. The aim of this study was to investigate the effect of F, Br, I, and OH, compared to Cl, on the incorporation of Ce (as a proxy for other rare earth elements) into the structure of pyromorphites.

Synthesis of pyromorphite analogs consisted of slow mixing of two solutions containing cations and anions in a reaction chamber, under atmospheric pressure, ambient temperature around 21°C and at pH=3, with vigorous stirring, and leaving the obtained suspensions for 48 hours for maturing. Ce-free phases were also synthesized as controls.

The precipitate comprises primarily appropriate anionic variety of crystalline pyromorphite: fluor-, chlor-, brom-, and hydroxyl-pyromorphite. Neither in the control experiment nor in the experiment with Ce was iodine pyromorphite formed. The crystal lattice got changed to accommodate Ce in the structure. It was expressed as change in unit cell parameters – dimensions a were elongated and the c dimensions were shortened, compared to each control. The extent of Ce substitution was not very sensitive to the anion used, with content measured at 0.44 (F), 0.57 (Cl), 0.52 (Br), and 0.43 (OH) atoms per formula unit (apfu). The precipitates of pyromorphites containing Ce were accompanied by small amounts of an additional phase, Ce₂Pb₃(PO₄)₄·nH₂O [4]. 

For the first time, anionic substitution effects (F, Cl, Br, I, OH) on cerium incorporation in the pyromorphite-type lead phosphate apatite Pb₅(PO₄)₃X has been studied. These will enable future optimization of REE recovery technologies from mineral materials of varying chemical composition, both qualitatively and quantitatively. Further extensive research is necessary to fully understand the details of REE substitutions in lead-apatites.

This research was funded by Polish National Science Center research grant no. 2021/43/O/ST10/01282. 

[1] Sordyl, J., Staszel, K., Leś, M. & Manecki, M. Removal of REE and Th from solution by co-precipitation with Pb-phosphates. Applied Geochemistry 158, 105780 (2023).

[2] Pan, Y. & Fleet, M. E. Compositions of the Apatite-Group Minerals: Substitution Mechanisms and Controlling Factors. Reviews in Mineralogy and Geochemistry 48, 13–49 (2002).

[3] Hopwood, J. D. et al. The Identification and Synthesis of Lead Apatite Minerals Formed in Lead Water Pipes. Journal of Chemistry 2016, 9074062 (2016).

[4] Staszel, K. et al. New synthetic [LREE (LREE = La, Ce, Pr, Sm), Pb]-phosphate phases. Mineralogia 54, 58–68 (2023).