H. Hsu-Kim, A. Middleton, B.C. Hedin
Keywords: rare earth elements, separations, waste recovery
Summary:Waste streams such as mine drainage, coal ash, and other combustion residuals have been proposed as a low grade resource of critical minerals such as rare earth elements (REE). These feedstocks are an attractive source of REE because of environmental benefits gained in converting wastes into valuable materials. However, major challenges exist in valorizing such wastes, including that waste disposal sites tend to be geographically dispersed and that such wastes comprise variable chemical composition that require customized and modular REE recovery approaches at each waste site . The presence of impurities in extracts of these waste are a critical barrier for efficient purification of REE. Here, we developed supported liquid membrane (SLM) technologies to concentrate REE (herein defined as the 14 stable lanthanides, yttrium and scandium) from wastes such as acid mine drainage (AMD) and coal fly ash. As described in our prior publication, the SLM approach is similar to solvent exchange processes except that the REE-chelating organic phase is embedded in a hydrophobic membrane. 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 potential advantage of the SLM process is that it requires much less volume of hazardous organic solvents than the conventional approach and could be implemented in a modular system. SLM-based separations were tested on AMD samples collected at 7 different sites in western Pennsylvania, and selected to represent the typical composition of REE-rich AMD sources. Separation fluxes of REE strongly depend on the composition of the feedstock matrix, including the pH and soluble iron and aluminum concentration in AMD samples and extracts of coal ash. For AMD samples that were allowed to age in the ambient environment prior to SLM separations, REE recovery rates were significantly decreased due to the oxidation of ferrous iron Fe(II) 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 establishes a framework for applying SLM for REE recovery from low-purity sources by establishing primary water quality parameters that influence separation flux and REE purity in the product. Such insights support a mechanistic understanding of critical metals separation by SLM and facilitate their application for complex and nontraditional feedstocks such as mine drainage and combustion waste residuals.