CHARACTERIZATION OF THE CONCENTRATIONS AND MODES OF OCCURRENCE OF CM-REE AND LITHIUM IN COAL-ASSOCIATED SEDIMENTS, AMD SLUDGE AND COAL PROCESSING WASTES OF THE SOUTHERN APPALACHIAN BASIN

The rising demand for critical minerals and rare earth elements (CM-REE) and their importance to the economy and national security of the United States have prompted the identification of domestic sources and the economic recovery of these strategic minerals to avert supply chain disruptions. Previous studies have shown that coal-associated sediments can be promising sources of CM-REE. Coal underclay, roof rock (predominantly shale) and wastes in the Southern Appalachian (SoApp) Basin have not been previously characterized for CM-REE contents. This study aims to examine the abundances and modes of occurrence of REE+Y+Sc (REE+) and Li in the coal-associated sediments and wastes of the SoApp Basin to evaluate their potential as feedstock materials for these important elements.

Samples of coal underclay (N=15), roof rock (N=13), AMD sludge (N=4), and waste rock (N=3) were collected from cores and mine sites in Alabama and Tennessee. The samples underwent complete digestion (ASTM D6357) followed by analysis of the digestate solutions for REE+ and Li concentrations using ICP-MS (EPA 200.8). The REE+ concentrations in underclay samples ranged from 187.3-334.4 ppm, with an average of 270.1 + 37.95 ppm. Roof rock REE+ concentrations ranged from 34.9-290.7 ppm, with an average of 236.4 + 65.74 ppm. Coal processing waste rock and AMD sludge REE+ concentrations averaged 163.6 + 111.3 and 103.2 + 75.06, respectively. Average Li concentrations were highest in coal processing wastes (224.6 + 190.7 ppm) and underclay samples (217.4 + 143.0 ppm), and lowest in AMD sludge samples (54.2 + 78.6). Four underclay samples recorded REE+ contents greater than 300 ppm, often cited as the minimum feedstock grade, whereas none of the other collected samples met this criterion.

Micro-XRF elemental mapping is being conducted on selected samples to gain better understanding of the distribution and extractability of CM-REE by determining their potential host mineral associations. The preliminary results of micro-XRF analysis indicate strong-to-moderate correlations between Ca-Sc-P as well as Fe-Co-S-Mn in an underclay sample and between Ca-P-Y in a waste rock sample, suggesting phosphatic mineral hosts for some REE in coal-associated sediments and wastes. SEM/EDX will be used to confirm CM-REE/trace mineral associations in these samples.