Julia Sordyl: Experimental immobilization of REE, Th and U by precipitation of pyromorphite [Pb5(PO4)3Cl]

Abstract

The globally increasing demand for rare earth elements (REE) opens up a vast field for the exploration of new methods for recovery of these metals from various sources. Extensive Research & Development is being conducted for each of the processing steps, i.e. (1) REE leaching, (2) REE immobilization, and (3) separation of individual REE as well as the accompanying thorium (Th) and uranium (U). Innovative and sustainable ideas for each of the steps are necessary to effectively face the global challenge of the continuous supply of these valuable metals.

This thesis seeks to explore new method of recovering REE, Th and U from solutions. The basic idea was that precipitation of pyromorphite [Pb5(PO4)3Cl] could efficiently removes metals from solution and immobilize them in a stable, crystalline phase. The main objectives were to quantify the REE, Th and U removal from aqueous solutions upon precipitation of pyromorphite and to determine the structural position of incorporated metals in its structure. Controlled laboratory experiments were followed by quantitative analyses of solutions and characterization of solids.

The results confirmed that pyromorphite precipitation under ambient conditions is an effective (>99%) method for removing REE, Th and U(VI) from solutions. Initial Pb concentration and pH of the synthesis affect the form of the resulting precipitate. There are no differences in the level of removal from solution of individual lanthanides. The latter preferentially substitute in the pyromorphite structure at the M1 position together with charge-balancing ions. In contrast, due to charge differences, U(VI) substitutes for Pb on the basis of the following substitution: (UO2)2+ ↔ Pb2+. Scandium and thorium tend to form a separate phase(s), but their removability from solution is equally high.

Altogether, in the scope of this thesis, a promising method for efficient recovery of REE from solutions was discovered and tested in laboratory scale. Moreover, the substitutions of REE, Th and U(VI) in pyromorphite, a member of the apatite supergroup, have not previously been studied in depth. Therefore, this research and the obtained results have contributed to fill this gap in the context of widely reported mineralogical and structural studies of apatite supergroup members.