High concentrations of REEs occurring near coal could improve domestic supply.

"The model is if you're already moving rock, could you move a little more rock for resources towards energy transition?" said co-author Lauren Birgenheier regarding a study of rare earth elements (REEs) found in conjunction with coal-producing regional mines across the Uinta coal belt of Colorado and Utah.

This research seeking out alternative sources of rare earths was conducted in partnership with the Utah Geological Survey and Colorado Geological Survey as part of the U.S. Department of Energy's Carbon Ore, Rare Earth and Critical Minerals, or CORE-CM, project.

In Utah, the researchers studied samples from the active Skyline, Gentry, Emery and Sufco mines; recently idled Dugout and Lila Canyon mines in the Book Cliffs; and the historic Star Point and Beaver Creek No. 8 mines.

The Colorado mines included in the study were the Deserado and West Elk.

Not so rare

REEs are used in solar panels, TV and phone screens, and magnets in everything from medical devices to wind turbines, superconductors, phosphors, catalysts, lasers, and luminescent materials. They are also used in rechargeable batteries, large and small.

"There's a lot of strategic kinds of things with some of these elements that are critical, that are used in high-end electronic devices and satellite technologies and defense applications. These kind of elements perform much better than the more common elements that we're familiar with," said co-author Michael Free, a University of Utah professor of metallurgical engineering.

The association between coal and rare earth deposits has already been documented, but turning analysis toward Utah and Colorado's coal fields has the added benefit of increasing U.S. potential for a domestic supply.

The U.S. averages 8,300 metric tons of rare-earth oxides needed per year, according to the U.S. Geological Survey. The Mountain Pass mine in California's Mojave Desert is the nation's largest producer of REEs, with most of its output processed overseas.

"When we talk about them as 'critical minerals,' a lot of the criticality is related to the supply chain and the processing," said Free. "This project is designed around looking at some alternative unconventional domestic sources for these materials."

"The supply here is not very established in some cases. It was established to some extent, but then it got shipped overseas because we didn't want to do the sourcing here. We didn't want to open up new mines here," Free said. "So that leaves us vulnerable for a lot of these higher-end technologies and the clean-energy technologies that we're trying to get more into."

Bringing REEs back home

Michael Vanden Berg, courtesy of the University of Utah

Lauren Birgenheier taking samples from a mine waste pile.

For the study, researchers analyzed 3,500 samples from numerous mines, waste piles, and even power plant coal ash.

Since then, the team has analyzed more than three times what was used in the original publication. The team worked directly with mine operators to collect rock samples, examine coal seam outcrops and processed waste, as well as drill cores, both archived and recently drilled.

"In those areas, we're finding that the rare earth elements are concentrated in fine-grain shale units, the muddy shales that are above and below the coal seams," Birgenheier concluded.

The results demonstrate that coal mines could take advantage of a secondary resource stream in the form of these critical tech metals used in high-tech applications rather than sourcing them largely from overseas.

"The goal of this phase-one project was to collect additional data to try and understand whether this was something worth pursuing in the West," said study co-author Michael Vanden Berg, energy and minerals program manager at the Utah Geological Survey. "Is there rare earth element enrichment in these rocks that could provide some kind of byproduct or value added to the coal mining industry?"

"The coal itself is not enriched in rare earth elements," Vanden Berg said. "There's not going to be a byproduct from mining the coal, but for a company mining the coal seam, could they take a couple of feet off the floor at the same time? Could they take a couple of feet off the ceiling? Could there be potential there? That's the direction that the data led us."

The Department of Energy has set 300 parts per million as the minimum concentration to consider rare earth mining to be economically viable, but the study's researchers designated 200 ppm to be sufficiently REE enriched for their purposes. Analysis uncovered significant concentrations in coal-adjacent formations of siltstone and shale, while the coal itself was mostly devoid of rare earths.

The next phase will include determining how much rare earth ore is present, which is likely to be done with the help of the University of Wyoming and New Mexico Institute of Mining and Technology. New findings will form the basis for a grant request for an additional $9.4 million in federal funding to continue the research.