Fractionation of REEs upon removal from solution by precipitation of lead apatite – pyromorphite (Pb,REE)5(PO4)3Cl

   Supply of technologically important rare earth elements (REEs) is of concern in Europe. Important European sources are associated with apatites and phosphate rocks. Due to high production, this is a potential resource, but technical and cost challenges hinder the commercial recovery of REEs. A new extraction approach exploring selective co-precipitation of REEs and Pb in the form of phosphates offers cheap and effective technology which can be included in the existing flow of ore processing.

   Most hydrometallurgical REE enrichment processes vary in efficiency for heavy and light REE. The aim of the present study was to verify whether this novel method of removing REEs from solution by co-precipitation with Pb-phosphates also has this drawback. Solution containing ca. 100 mg/L of Sc, Y, Th, and lanthanides (except Pm) was mixed with a solution containing Pb²⁺, PO₄^3-, and Cl^- to induce precipitation (pH between 2 and 4, ambient conditions). The initial and final solutions were analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES) for Pb and REE concentrations, while solids were filtered, dried, and analyzed with powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM).

   In all experiments, the formation of a precipitate composed mainly of crystalline pyromorphite (Pb,REE)₅(PO₄)₃Cl was found, accompanied in smaller amounts by a second, less crystalline phase. In the SEM images, pyromorphite is apparent as hexagonal rods and needles (micrometers in size) while the second phase forms Cl-free, loose aggregates of globular grains, tens of nanometers in size. The extent of REE substitution for Pb in the pyromorphite structure, determined in a separate study for La, is at the order of ca. 1 wt. % La₂O. At the experiment conditions, the charge difference between Pb²⁺ and REE³⁺ is compensated by Na⁺. Significant amounts of REEs are also precipitated in the form of Cl-free Pb-REE-phosphate, which constitutes an accompanying phase or a mixture of phases. At this stage of research, the structure and chemical composition of these phases could not be identified conclusively: the XRD pattern is obscured by pyromorphite, and the precipitate is too fine for regular microprobe analysis.

   The concentrations of metals in question were reduced in the solution very significantly. For initial concentrations in the 1-10 ppm range, they were completely removed from solution to concentrations below detection limits of 0.002 ppm (concentrations of Y and La dropped down to below 0.01 ppm). For initial concentrations of 80 ppm, only Sc and Th were removed completely while concentrations of Ce, Pr, Nd, Sm, Eu, Gd, and Tb were reduced by over 60%, these of Dy, Ho, Er, Tm, Yb, and Lu by ca. 50 % and concentrations of Y and La dropped down only by ca. 40%. This may indicate that although the fractionation of REEs is not systematic and not very significant, heavy REEs are removed from the solution slightly less effectively. However, the crystalline and heavy precipitate allows easy separation of the solution.

This research was partially funded by NCN research grants no. 2019/35/B/ST10/03379 and 2021/43/O/ST10/01282.