ILLINOIS ABANDONED MINE LANDS: POSSIBILITIES FOR SOURCING CRITICAL MINERALS IN COAL MINING WASTE

Author

Ryan Bowman, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2025

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Lefticariu, Liliana

Abstract

“Illinois Abandoned Mine Lands (AMLs): Possibilities for Sourcing Critical Minerals (CMs) in Coal Mining Waste (CMW)” uses three related studies to address whether a) coal mining waste has the potential to become a resource for REEs, b) how to estimate the amount of resources at each site, and c) the impact of the weathering processes on the minerals in the waste. It provides information to inform those who are interested in repurposing Illinois coal mining waste as an alternate REE source. The first study, “A Model for Identifying Resources and Hazardous Elements at Coal Mine Lands (CMLs) in Illinois,” builds upon prior research and proposes a generalized model for identifying Abandoned Mine Land (AML) waste sites with the most potential as an alternate rare earth element including Y (REE) and CM resource. It hypothesized that the association patterns of concentration levels exhibited by REEs and CMs will differ based on the material type of CMW. Thirty-nine solid and 30 liquid coal mining waste samples, collected within the Illinois Basin, were analyzed at labs using inductively coupled plasma mass spectrometry (ICPS MS) and x-ray diffraction (XRD) for elemental geochemistry (26 elements), including REEs. The CMW materials were classified based on associations and correlations between REE contents and total organic carbon (TOC), used as a proxy for organic matter content, [Al + Si], used as a proxy for silicate content, and [Fe], used as a proxy for iron precipitate content. The first type of CMW, defined as high carbon refuse (HCR), represents materials that contain more than 40% wt. TOC, and [Al + Si] and [Fe] lower than 60%. The second type, medium carbon refuse (MCR), has between 10 wt% and 40 wt% TOC and [Al + Si] between 40 wt% and 60 wt% and [Fe] less than 10 wt%. The last category, the low carbon refuse (LCR) has TOC<10% wt%. Within LCR, two additional categories are identified; namely, the silicate-rich coal refuse (SiCR) characterized by [Fe]/([Al]+[Si])>2.0 and iron-rich coal refuse (FeCR) characterized by [Fe]/([Al]+[Si]) < 2.0. Further statistical analysis investigated the relationships between refuse types and total rare earth elements (TREE) and the relationships between specific elements, including hazardous elements (US EPA, 2024) and TREE. The results indicated that SiCR is most likely to contain the highest amounts of REEs, along with Ni and Cu. FeCR is most associated with As, while HCR and MCR are mostly associated with Pb. The CMW classification model was successfully applied to two additional data sets, one using additional weathered surface coal waste and another using samples from an active mine and coal preparation site.The second chapter, “Calculation of Potential Rare Earth Elements in Illinois Coal Mine Waste,” applies a method for estimating the TREE contents in CMW piles by replacing the variables used in the industry-standard methodology for estimating petroleum with measurements more appropriate for solid CMW (element concentration, volume, and density) when the TREE concentration is known. A bootstrap simulation approach was used to analyze the concentration data reported in chapter three. CMW pile volume was estimated using the Trapezoidal Method (Tearpock and Bischke, 2022), and density was defined as estimated by Rozanski et al. (2019). The total amount of REEs of the pile (hill) was calculated by multiplying the simulated concentrations of the pile, the estimated pile density, and the estimated pile volume, then converting the final units of measure. This study demonstrated that Illinois CMW piles are a potential resource for REEs and other CMs. Although this study focused on REEs, it has the potential to be used in approximating amounts of other elements of interest or concern. The third chapter, “The Fate of Phosphates in Coal Mining Waste: A Study of Rare Earth Element Mobility in Silicon-rich Coal Refuse,” investigated the effects of the duration of weathering -- short versus long -- had on CMW, building upon the research of Hicks et al. (2020). Monazite, a rare earth element (REE)-bearing phosphate, specifically neodymium (Nd)- and cerium (Ce)-bearing phosphates, were analyzed using X-ray Fluorescence spectroscopy (XRF) and X-ray Absorption Near Edge Structure (XANES) to examine the effects of weathering on the integrity of the monazite crystalline structure and the amount of TREE content available when in SiCR-CMW type waste. The results demonstrated that sites with a longer weathering duration had monazite with more weathered features, and younger coal piles had a higher TREE with a greater concentration of intact monazite crystals. While other processes could have contributed to these patterns, including the age of the coal seam, biological processes, and hydrothermal activity, the most likely reason is the duration of CMW weathering by AMD. The main limitations of this study are the small sample size and the lack of diversity in site locations. Although it has limited generalizability, this study is a solid step forward in expanding on the work of Hicks et al. (2020) regarding the effects of weathering on phosphate minerals with implications for REEs in weathering CMW.Based on these three interrelated studies, this dissertation provided evidence that CMW from Illinois coal shows promise as a potential alternate source of REEs and other CMs. It identified four types of CMW materials and demonstrated that SiCR is the most likely to contain high amounts of REEs. It provided a methodology for estimating the total amount of an element or mineral based on concentration levels; volume, as estimated by the trapezoidal method (Tearpock and Bischke, 2022); and density. Finally, it explored monazite, a REE-bearing mineral commonly found in CMW, to determine if weathering duration, based on the time from mine closure to the date of sampling, impacts the amount of REEs available for reprocessing the harvested mineral. For monazite, the more recently the coal mine was closed, the less CMW is weathered, thereby retaining more REEs within the crystalline structure. In practice, interested parties can use this information to more efficiently determine whether the site should be further investigated to assess the potential of reprocessing CMW to harvest REEs and to guide strategy selection for REE extraction.