Can coal mines be tapped for rare earth elements?

Deposits of designated critical minerals needed to transition the world’s energy systems away from fossil fuels may, ironically, be co-located with coal deposits mined to produce the fossil fuel most implicated in climate change.

Now, research led by the University of Utah has documented elevated concentrations of an important subset of critical minerals known as rare earth elements, or REEs, in active mines along the Uinta coal belt of Colorado and Utah. The work has been published in the magazine Frontiers in Earth Sciences.

These findings open the possibility that these mines could see a secondary resource flow in the form of metals used in renewable energy and a host of other high-tech applications, said co-author Lauren Birgenheier, associate professor of geology and geophysics.

“The model is that if you’re already moving rock, can you move some more rock for raw materials for the energy transition?” Birgenheier said. “In those areas, we find that the rare earth elements are concentrated in fine-grained shale units, the muddy shale that lies above and below the coal seams.”

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

While these metals are critical to U.S. manufacturing, especially in high-end technologies, they largely come from abroad.

“When we talk about them as ‘critical minerals,’ a lot of the criticism has to do with the supply chain and their processing,” said Michael Free, professor of metallurgical engineering. “This project is designed around looking at some alternative, unconventional domestic sources for these materials.”

The link between coal and REE deposits has been well documented elsewhere, but little data had previously been collected or analyzed in the coal fields of Utah and Colorado.

“The goal of this phase one project was to collect additional data to try to understand whether this was worth pursuing in the West,” said 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 added value to the coal mining industry?”

Researchers analyzed 3,500 samples from 10 mines, four mine waste piles, seven stratigraphically complete cores and even some coal ash piles near power plants.

“The coal itself is not enriched with rare earth elements,” Vanden Berg said. “There won’t be a byproduct of mining coal, but for a company that’s mining the coal seam, could they take a few feet off the floor at the same time? Could they take a few feet off the ceiling? Could there be potential? That’s the direction the data has taken us.”

The team used two different methods to record the levels of rare earth metals, expressed in parts per million, or ppm, in the samples. One was a handheld device for rapid measurements in the field, the other used inductively coupled plasma mass spectrometry, or ICP-MS, in an on-campus laboratory.

“We usually use this portable X-ray fluorescent machine, an analysis gun that we hold against the rock for two minutes, and it only gives us five or six of the seventeen rare earth elements,” Birgenheier said. If samples showed concentrations above 200 ppm, they performed a more complete 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. But for the study, researchers considered concentrations above 200 ppm to be “REE-enriched.”

The study found the highest prevalence of such concentrations in coal-adjacent formations of siltstone and shale, while sandstone and the coal itself were mostly devoid of rare earth metals.

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