Author ORCID Identifier

https://orcid.org/0009-0006-2874-6754

Semester

Summer

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Deniz Talan

Committee Member

Qingqing Huang

Committee Member

Deniz Tuncay

Abstract

Rare earth elements (REEs) and other critical minerals (CMs) are indispensable to modern clean energy, electronics, and defense sectors due to their unique physical and chemical properties. The increasing global demand for these elements—combined with concerns about supply chain disruptions and the environmental impact of conventional mining—has heightened the urgency to identify alternative, sustainable sources. Among various secondary resources, coal-derived wastes such as coal refuse, fly ash, bottom ash, and coal acid mine drainage sludge have emerged as promising feedstocks for REE and CM recovery within the United States.

In this study, a comprehensive mineralogical and geochemical assessment was performed on selected coal-derived materials sourced from diverse U.S. coal basins. A suite of advanced analytical techniques was utilized, including X-ray Diffraction (XRD) for crystalline phase identification, Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS) for microstructural and elemental mapping, Transmission Electron Microscopy (TEM) for nanoscale examination of mineral-hosting phases, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for quantitative determination of REE and CM concentrations. Furthermore, a sequential extraction protocol was applied to delineate the chemical speciation and extractability of REEs across operationally defined geochemical fractions.

The results reveal that all coal-derived samples contained significant concentrations of REEs and CMs, with measured total rare earth element (TREE) values ranging from approximately 323 to over 1,000 ppm depending on sample type and thermal treatment. Sequential extraction showed that the acid-soluble fraction accounted for the majority of extractable REEs in each material, indicating substantial potential for recovery via hydrometallurgical leaching. Carbonate-bound and organically associated REEs were also detected, suggesting some mobility under moderate chemical conditions. In contrast, the ion-exchangeable fraction consistently exhibited the lowest REE recoveries, implying limited surface adsorption or weak binding in these materials. Importantly, a strong correlation was observed between REE distribution and the presence of aluminosilicate phases—particularly in fly ash—supporting the conclusion that a substantial portion of REEs is structurally incorporated within amorphous or poorly crystalline silicate matrices formed during coal combustion. This finding underscores the necessity of aggressive chemical or thermal activation strategies for efficient REE mobilization. Critical elements such as lithium, cobalt, nickel, and aluminum were also identified at notable concentrations, particularly in fly ash and AMD sludge, underscoring the potential of coal by-products as multi-element resources.

Overall, the study demonstrates that coal-derived wastes represent viable secondary sources of critical elements, provided that their complex mineralogical contexts are fully understood and appropriately leveraged. By integrating mineralogical, microstructural, and chemical speciation analyses, this research provides a robust foundation for the development of sustainable recovery technologies, ultimately contributing to enhanced environmental stewardship and the resilience of the domestic REE and other critical mineral supply chain.

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