First evidence of strong REE compatibility with pyromorphite Pb5(PO4)3Cl

Abstract

Constant development of high and green technologies makes rare earth elements (REE) more viable than ever. Relative scarcity of REE and lack of proper substitutes increase the demand for inexpensive and efficient methods of their recovery. Some new methods and technologies have been already proposed but their effectiveness leaves much to be desired. Most recently, precipitation in form of lead apatite has been proven to provide with very high levels of REE removal from aqueous solutions.Substitution of REE in apatites has been broadly studied for calcium–phosphate apatite specimens, while lead–phosphates have been usually omitted. Recent studies suggested that precipitation of Pb and individual REE, induced by the presence of phosphates and Cl ions, results in near complete removal of cations from solutions in the form of REE-containing pyromorphite Pb5(PO4)3Cl. The goal of this study was to optimize the procedure. Pyromorphite (Pym) was precipitated from a solution of phosphoric acid (1:1) containing REE (each element at the concentration of 15 ppm) by addition of NaCl and Pb(NO3)2 (powder). The addition was conducted 5 times with very small amount of Pb relative to phosphates: 1/200, 1/100, 1/50, 1/20 and 1/10 of Pb needed for complete reaction with phosphoric acid. The amount of Cl was used in 1.5 times excess with respect to stoichiometric Pb. Each time solids and solutions were separated and sampled for the analysis.The synthesis was carried out under atmospheric pressure, at an ambient temperature of about 21°C and at pH=3. Powder X-ray diffraction (XRPD) was used to identify the obtained phases, scanning electron microscopy (SEM) to examine the morphology of the crystals, energy-dispersive X-ray spectroscopy (EDS) for analysis of the elemental composition of solids, while solutions were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-OES).Most REE are removed already at the first, lowest dose of Pb: more than 99% of La, Ce, Pr, Nd, Sm, Eu, and Gd; between 90 and 96% of Tb, Dy, Ho, and Er; between 80 and 50% of Tm, Yb, and Lu; 46% of Sc, 86% of Y, and 98% of Th disappear from the solution. After the third addition, REE are removed completely (except for Sc, which required 5 amendments). All these elements were removed from solution by precipitation of REE-containing Pym. An admixture of "phosphoschultenite" PbHPO4 also appears in the precipitate, which most likely does not contain REE (or contains much less than Pym).These preliminary results indicate strong affinity of REE with Pym structure. In contrast to previous findings (Sordyl et al., 2023), a fractionation of REE was observed. Further analyses are in demand for better understanding of the mechanism of these processes which will allow for optimization of industrial applications.This research was funded by National Science Centre research grant no. 2021/43/O/ST10/01282.