Abstract
The search for a reliable U.S. domestic source of rare earth elements (REE) is necessary to support the demand of advanced energy applications (e.g., catalysts, electronics, magnets). Sedimentary deposits may be sources for selectively recovering REE and critical metals—specifically the interbedded seat rock, or underclay, that underlies or forms the floor of a coal seam. This material is often a major component of coal waste fines and refuse and thus readily available. This study examines several Appalachian Basin underclays associated with actively mined coal seams as potential feedstocks for the REE. Multimodal microanalytical electron microscopy (SEM, FIB-SEM, EMPA) synchrotron-based µXRF, and image processing techniques are coupled with detailed elemental and mineral data to classify the 2D and 3D petrophysical properties of the materials. The REE contents of Appalachian Basin underclays were measured from 235–399 ppm and predominantly observed as discrete REE-bearing minerals such as monazite and xenotime on the order of 10–100 µm in size. These REE-bearing minerals typically accounted for less than 1% of the scanned areas and volumes under SEM and FIB-SEM analysis, with the exception of regions enriched in crandallite. Synchrotron-based µXRF elemental maps further identified several REE deposition environments in different underclays, including micro-scale (10–100 µm) light REEs co-localizing with Ca and P, micro-scale heavy REEs with Fe, and large-scale light REEs (>200 µm) co-localizing with Sr, Ba, Ca and P.
Highlights
With the increased use of the rare earth elements (REE) in modern technologies across numerous sectors including energy, automobile, and defense, the criticality of securing reliable sources has moved to the forefront of many nations’ economic outlooks [1,2]
Following conventions defined in Seredin and Dai (2012) [21], the light REE (LREE: La, Ce, Pr, Nd, Sm), middle REE (MREE: Eu, Gd, Tb, Dy, Y), and heavy REE (HREE: Ho, Er, Tm, Yb, Lu) are reported
The Central Appalachian Basin underclays in this study had measured REE content ranging from 235–399 ppm
Summary
With the increased use of the rare earth elements (REE) in modern technologies across numerous sectors including energy, automobile, and defense, the criticality of securing reliable sources has moved to the forefront of many nations’ economic outlooks [1,2]. Minerals 2020, 10, 546 have been focused primarily on the ‘hard mineral’ occurrences of the REE, in the form of minerals such as bastnaesite, monazite, xenotime, and carbonatites (e.g., [1,5]). These mineral deposits often must be first subjected to significant processing of the ore to include crushing and grinding, calcining, roasting, flotation separation, and other beneficiation techniques [6]. The REE are extracted using strong inorganic acids such as sulfuric acid, H2 SO4 Such a process is often heavily capital- and energy-intensive and poses additional handling considerations for waste management and environmental protections
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