Coal refuse contains elements and compounds that have been separated from cleaned coals. Some of those elements are of environmental concern, which is why coal refuse (especially slurry ponds) are regulated. The refuse, however, also contains small amounts of potentially useful elements. Several research investigations have examined the elemental contents of coal (e.g., Hower and others, 1999, 2016, 2020a; Valian and others, 2021), coal mine and preparation refuse (e.g., Luttrell and others, 2016, 2019; Li and Zhang, 2022), and coal-fired power plant combustion refuse (fly ash) (e.g., Hower and others, 2015; 2017, 2020b, 2022; Hower and Groppo, 2021; Hood and others, 2017) and found they contain Rare Earth Elements (REEs).

Rare Earth Elements are a group of elements considered critical minerals, because we rely on them for a wide range of industries and products we use every day; including magnets, electronics, optics, and metal alloys (USGS, 2014; Bradley and others, 2017; NETL, 2021a). Currently many critical minerals, including REEs, are mostly sourced from other countries. For that reason, research has been undertaken to find potential domestic sources of REEs and other critical minerals; one of which may be coal refuse.

• In 2014, he Kentucky Geological Survey (KGS) and University of Kentucky Center for Applied Energy Research (CAER) began working with Dr. Rick Honaker (University of Kentucky Department of Mining Engineering) to evaluate the amounts of REEs in coal mine and preparation refuse in Kentucky, southern West Virginia, and southwestern Virginia. Since then, KGS has been analyzing coal and coal refuse samples from across the state for REEs. Dr. Honaker's group has examined a variety of coal wastes for their REE content (e.g. Zhang and others, 2020) and developed a small-scale REE-processing facility with funding from the U.S. Department of Energy in western Kentucky to examine methods for extracting REEs from coal mine refuse (Honaker and others, 2018; Zhang and Honaker, 2020).
• See also Dr. Honaker (2018) YouTube video on REE extraction.

From 2021 to 2024, KGS worked with other state geological surveys and research groups through regional CORE-CM projects sponsored by the U.S. Department of Energy. CORE-CM stands for Carbon Ore, Rare Earth, and Critical Minerals. These U.S. Department of Energy projects aim to characterize a variety of possible domestic sources of Critical Minerals, including REEs, from across the United States (NETL, 2021b). KGS has been involved with an Appalachian Basin project (eastern Kentucky) and an Illinois Basin (including western Kentucky) project, which are investigating the potential for REEs in coal mine refuse, coal preparation refuse, and coal combustion refuse. Although the amounts of REEs in these different types of coal refuse are usually less than are found in commercial igneous rocks (which are the source of most of the world’s REEs), characterizing and understanding the range of REE (and other critical mineral) concentrations in coals and coal refuse materials provides the data to: (a) determine if they constitute a potential future domestic source or at what price they might constitute a future domestic resource; (b) potentially drive innovation and improvement of technologies that allow for more economic extraction of REEs (or other critical minerals) from coal refuse; and/or (c) extraction of REEs during or after reclamation of coal refuse.