Highlights
- University of Utah study found elevated concentrations of rare earth elements in coal-adjacent rock formations in Utah and Colorado mines.
- Researchers analyzed 3,500 samples from 10 mines.
- Identified potential secondary resource stream for critical minerals used in renewable energy technologies.
- The Department of Energy-funded project explores domestic alternatives for sourcing rare earth elements, currently largely imported from overseas.
Does it turn out that the same coal mines dependent on today for a carbon economy could be essential for access to the rare earth minerals necessary to help the transition to a greener, decarbonized economy? Research led by the University of Utah earlier in the spring of this year documented elevated concentrations of a key subset of those critical minerals, known as rare earth elements (REEs) in active mines rimming the Uinta coal belt of Colorado and Utah. The U-led study was published last month (opens in a new tab).) in the journal Frontiers in Earth Science.
The study results published earlier in the year show promise. The University of Utah published an article (opens in a new tab) on the topic earlier this year. The team targeted the coal-producing region stretching from Utah’s Wasatch Plateau east across the Book Cliffs deep into Colorado. Researchers analyzed 3,500 samples from 10 mines, four mine waste piles, seven stratigraphically complete cores, and even some coal ash pilesnear power plants.
The study included Utah’s 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 studied were the Deserado and West Elk.
Could these mines emerge as a secondary resource stream in the form of metals used in renewable energy and numerous other high-tech applications? That is the case according to study co-author Lauren Birgenheier (opens in a new tab), an associate professor of geology and geophysics.
“The model is if you’re already moving rock, could you move a little more rock for resources towards energy transition?” Birgenheier said. “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.”
The Project—Funded by CORE-CM
Utah Geological Survey (opens in a new tab) and Colorado Geological Survey (opens in a new tab) were partners in this research, part of the Department of Energy-funded Carbon Ore, Rare Earth and Critical Minerals (opens in a new tab) project, or CORE-CM. The new findings will form the basis for a grant request of an additional $9.4 million in federal funding to continue the research.
While these metals are crucial for U.S. manufacturing, especially in 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 the processing,” said Michael Free (opens in a new tab), a professor metallurgical engineering and the principal investigator on the DOE grant. “This project is designed around looking at some alternative unconventional domestic sources for these materials.”
Brief Background
On average the U.S. uses 8,300 metric tons of rare-earth oxides a year, according to the U.S. Geological Survey. The Mountain Pass mine (opens in a new tab) in California’s Mojave Desert is the nation’s largest producer of rare earth elements, but most of its output is sent overseas for processing.
“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.
The association between coal and REE deposits has been well documented elsewhere, but little data had been previously analyzed relevant to Utah and Colorado’s once busy coal fields, which have fallen on hard times as domestic demand for coal has shriveled. Among a longer-term decline, however, remaining active coal mines in Utah and Colorado report that they can’t mine fast enough in recent years to meet demand and high coal prices.
Lauren Birgenheier, University of Utah
“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 (opens in a new tab), 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?”
Analyzing rock samples by the thousands
“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 feet of the floor at the same time? Could they take a couple feet of the ceiling? Could there be potential there? That’s the direction that the data led us.”
To gather samples, the team worked directly with mine operators and examined coal seam outcrops and processing waste piles. In some cases, they analyzed drilling cores, both archived cores and recently drilled ones at the mines. The team entered Utah mines to collect rock samples from the underground ramps that connect coal seams.
Researchers deployed two different methods to record levels of REE’s present, expressed in parts per million, or ppm, in the samples. One was a hand-held device for quick readings in the field, the other used Inductively Coupled Plasma-Mass Spectrometry, or ICP-MS, in the on-campus lab overseen by Fernandez.
“We’re mostly using this portable X-ray fluorescence device, which is an analysis gun that we hold to the rock for two minutes, and it only gives us five or six of the 17 rare earth elements,” Birgenheier said. If samples showed concentrations higher than 200 parts per million, or ppm, they ran a more complete analysis using the mass spectrometry equipment on campus.
Findings
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, researchers deemed concentrations greater than 200 ppm to be considered “REE enriched.”
The authors of this study report finding 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 earths.
The team has analyzed 11,000 samples to date, far more than were used in the published study.
“We still have results that are ongoing and papers that’ll be coming out soon,” Birgenheier said. “We’re writing a proposal now for phase two. We can’t make resource volume estimates yet because we don’t have that data. This next phase will push us towards answering, ‘how do we actually calculate a volume of rare earths in these deposits?’”
Origins of the REE: Two Hypotheses
The study did not identify the geological process that enriched the coal-adjacent formations, but Birgenheier has a few theories. Many of the Utah coal-bearing formations were deposited during the Cretaceous period that ended 66 million years ago, a time when the western U.S. was volcanically active.
“There are two models. One is maybe volcanic ash brought rare earths into ancient peat bogs,” she said. “The other is there’s evidence that terrestrial organic material in the peat bog actually takes in heavy rare earths.”
Then, through time, heat and burial, the peat bogs enriched in rare earths became Utah and Colorado coal deposits.
“We think rare earths were in the coals and have migrated into the adjacent mudstones or siltstones above and below the coals,” Birgenheier explained, “probably through a process called diagenesis, basically any fluid movement that happens in the rock after it was deposited.”
Team members included graduate students Haley Coe, the lead author, and Diego Fernandez, a research professor who runs the lab that tested samples.
Next Steps
The researchers will determine how much rare earth ore is present, likely to be done with colleagues at the University of Wyoming and New Mexico Institute of Mining and Technology (opens in a new tab).
Daniel
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