Coprecipitation of Ce with lead phosphates

Abstract

Current development of sustainable technologies creates a demand for new sources of Rare Earth Elements (REE). Recent studies suggest that coprecipitation in the form of Pb-phosphates is one of the most effective methods in REE removal from aqueous solutions. This work focuses on the experimental study of the mechanisms of Ce coprecipitation with Pb phosphates, in particular with lead apatite - pyromorphite (Pb₅(PO₄)₃Cl). Coprecipitation experiments were conducted at ambient conditions, at pH range 1–11, by mixing solutions containing high concentrations of Pb and Ce (∼66,700 ppm and ∼ 7600 ppm, respectively) with solutions containing stoichiometric concentrations of PO₄ and Cl. As a result of coprecipitation with Pb phosphates, the Ce concentration decreased significantly to <2 ppm. The main product of the coprecipitation was Ce-doped pyromorphite (PymCe). Removal of Ce was most effective under alkaline conditions, while performance was slightly worse under acidic conditions, due in part to the slightly higher solubility of the precipitate. This was compared with a series of control experiments in the absence of either Pb, Cl, or Ce. Precipitation in the absence of Pb resulted in the formation of fibrous rhabdophane-Ce. In the absence of Cl, various phases were formed depending on pH conditions, such as ‘phosphoschultenite’ (PbHPO₄), Pb-hopeite (PbPb₂(PO₄)₂·nH₂O), mixed Pb and Ce hydrous phosphates, and hydroxylpyromorphite (Pb₅(PO₄)₃OH). In the absence of Ce, pure microcrystalline pyromorphite precipitated. Coprecipitation of Ce with Pb phosphates in the presence of Cl is advantageous due to the recovery of almost all Ce from solution in the form of a micro-crystalline Pym-Ce that is a very stable, insoluble phase easily separated from suspension. The precipitation of Pym-Ce is homogenous over a wide range of pH, assuring consistency in the obtained phases regardless of starting conditions. Achieved high Ce removal levels may be crucial for the progress of the REE extraction industry, in terms of this method being applicable to other REE for their recovery from solutions, including apatite leachates.