Can coal mines be developed to extract rare earths?

Deposits of designated critical minerals needed to transition the world’s energy systems away from fossil fuels may ironically lie parallel to coal deposits mined to produce the fossil fuel most contributory to climate change.

Now, research conducted by the University of Utah has documented elevated concentrations of an important subgroup of critical minerals known as rare earth elements (REEs) in active mines on the edge of the Uinta Coal Belt in Colorado and Utah. The work will be published in the journal Frontiers in Earth Science.

These results raise the possibility that these mines could see a secondary resource stream in the form of metals used in renewable energy and numerous other high-tech applications, according to study co-author Lauren Birgenheier, associate professor of geology and geophysics.

“The model is: If you are already moving stones, could you then move a few more stones to gain resources for the energy transition?” said Birgenheier. “In these areas, we find that the rare earth elements are concentrated in fine-grained shale units, the muddy shale layers, that lie above and below the coal seams.”

This research was conducted in collaboration with the Utah Geological Survey and the Colorado Geological Survey as part of the Carbon Ore, Rare Earth and Critical Minerals (CORE-CM) project.

Although these metals are critical to U.S. manufacturing, particularly for high-end technologies, they are largely sourced from overseas.

“When we talk about them as ‘critical minerals,’ a lot of the criticality is related to the supply chain and processing,” said Michael Free, professor of metallurgical engineering. “This project is about looking for alternative, unconventional domestic sources for these materials.”

The connection between coal and REE deposits is well documented elsewhere, but little data has been collected or analyzed for the coal fields in Utah and Colorado.

“The goal of this Phase 1 project was to collect additional data to determine whether this was 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 sort of by-product or value-add to the coal mining industry?”

The researchers analyzed 3,500 samples from ten mines, four mine dumps, seven stratigraphically complete drill cores and even some coal ash piles near power plants.

“The coal itself is not enriched in rare earths,” Vanden Berg said. “Coal mining doesn’t create a byproduct, but could a company mining the coal layer remove a few feet of the ground at the same time? Could they take off a few feet of ceiling? Could there be potential there? The data has led us in this direction.”

The team used two different methods to measure the rare earth content, expressed in parts per million, or ppm, in the samples. One was a handheld device for quick measurements in the field, and the other used inductively coupled plasma mass spectrometry (ICP-MS) in a laboratory on campus.

“We primarily use this portable X-ray fluorescence device, an analytical gun that we hold up to the rock for two minutes, and it only gives us five or six of the 17 rare earth elements,” Birgenheier said. When the samples had concentrations greater than 200 ppm, they performed a more comprehensive analysis using the more expensive mass spectrometry equipment.

The Department of Energy has set 300 ppm as the minimum concentration for rare earth mining to be potentially economically viable. However, for the study, the researchers considered concentrations above 200 ppm to be “REE-enriched.”

The study found the highest prevalence of such concentrations in formations of siltstone and shale adjacent to coal, while sandstone and the coal itself were largely free of rare earths.

The team has analyzed 11,000 samples so far, far more than were used in the published study. The next steps include determining how much rare earth ore is present. This will likely be done in collaboration with colleagues at the University of Wyoming and the New Mexico Institute of Mining and Technology.