EXTRACTION OF RARE EARTH ELEMENTS FROM ACID MINE DRAINAGE BY SUPPORTED LIQUID MEMBRANE SEPARATIONS (Invited Presentation)

Waste streams such as acid mine drainage (AMD) are a potential low-grade resource of critical minerals such as rare earth elements (REE). Such wastes tend to be geographically dispersed and contain a variety of impurities that are barriers for efficient purification of REE (herein defined as the 14 stable lanthanides, yttrium and scandium). Thus, the recovery REE from mine drainage requires modular technologies that can be customized for each waste site. This presentation will discuss the recovery of REE from AMD fluids by supported liquid membrane (SLM) separations. The SLM approach is similar to solvent exchange processes except that the REE-chelating organic phase is embedded in a hydrophobic membrane, and this membrane acts as a cation exchange ‘filter’ to selectively partition REE ions from the feed solution on one side of the membrane into the acid stripping solution on the other side of the membrane. A major uncertainty in applying SLM separations for AMD wastes is the influence of feedstock chemical composition and metal-ligand competition for REE selectivity and recovery rate.

SLM-based separations were tested on AMD samples collected at 7 different sites in western Pennsylvania (USA), and selected to represent the typical composition of REE-rich AMD sources. Separation fluxes of REE strongly depended on feedstock composition variables, including the pH and soluble Fe, Ca, and Mn concentrations. For AMD samples that were allowed to age in ambient conditions prior to SLM separation, REE recovery rates were significantly decreased due to the oxidation of ferrous Fe(II) species and the formation of Fe(III) as a major interfering ion. Filtration of freshly collected AMD limited Fe(II) oxidation, enabling flexibility in feed stock storage time for AMD prior to REE separation. Altogether, this work highlights the need to consider the chemical composition of low-purity sources and relative selectivity of competing ions in the development of REE separation technologies.