New synthetic [LREE (LREE = La, Ce, Pr, Sm), Pb]-phosphate phases

Search for inexpensive and efficient methods of critical raw materials recovery is of great importance across the world due to growing demand for green technologies. Formation and detailed characterization of new Pb- and Light Rare Earth Elements (LREE)-containing phosphates, compared to already described Pb- or LREE-containing phosphates, was described in this work. These phases were precipitated at experimental conditions similar to these used in a newly proposed coprecipitation route for REE recovery from aqueous solutions. The formation of La, Ce, Pr and Sm phosphates from aqueous solutions proceeded differently in the presence of Pb than in its absence. No rhabdophane group minerals, (REE,Ca,Th)(PO₄)·nH₂O were formed, which were the product of crystallization in the absence of Pb, as evidenced by the PXRD analysis of the control LREE phosphates. Instead, a new, distinct phase was formed, which is neither a ‘phosphoschultenite’, PbHPO₄, with La, Ce, Pr or Sm substitution nor a rhabdophane with Pb substitution. This showed that PbHPO₄ structure did not accept isomorphic substitutions of LREE elements and rhabdophane structures are reluctant to accept Pb substitutions. At the same time, the formation of a hitherto unknown crystalline phase was found to be a mixed (LREE,Pb)-phosphate. A lower pH caused higher crystallinity of phases, as confirmed by SEM and PXRD. FTIR spectroscopy showed the hydrous nature of the obtained phases, which was additionally confirmed by thermal analysis. Decreasing pH of the reaction solution resulted in a higher crystalline water content. Moreover, La-bearing phases contained more chemically bound water than other phases. A combined EDS analysis and ICP-OES led to the chemical composition of new Pb phases with La, Ce, Pr and Sm that can be expressed as La₂Pb₃(PO₄)₄·3.5H₂O, Ce₂Pb₃(PO₄)₄·3.3H₂O, Pr₂Pb₃(PO₄)₄·3.1H₂O and Sm₂Pb₃(PO₄)₄·3.3H₂O, respectively. These results give a better understanding of potential novel recovery pathways of REE from phosphate mineral sources or wastes.