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Can coal mines be tapped for rare earth elements?

New research documents elevated levels of these metals, needed for energy transition, above and below coal seams in Utah and Colorado.

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

Now, research led by the University of Utah has documented elevated concentrations of a key subset of critical minerals, known as rare earth elements, or REEs, in active mines rimming the Uinta coal belt of Colorado and Utah.

PHOTO CREDIT: Michael Vanden Berg, Utah Geological Survey

U geologist Lauren Birgenheier inspects samples at coal processing waste pile. Banner photo: Closeup of archived drilling cores used in the rare earth element research.

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These findings open 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, 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.”

A search for alternate sources of rare earths

This research was conducted in partnership with the Utah Geological Survey and Colorado Geological Survey as part of the Department of Energy-funded Carbon Ore, Rare Earth and Critical Minerals 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, 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."

The U-led study was published last month in the journal Frontiers in Earth Science. Team members included graduate students Haley Coe, the lead author, and Diego Fernandez, a research professor who runs the lab that tested samples.

What are rare earth elements?

Despite the moniker, rare earth elements (REEs) are not rare in Earth’s crust, but they are rarely found in concentrations high enough to make mining them economical  Nearly 90% of the global supply is processed in China, according to the Bipartisan Policy Center.

These metallic elements include the 15 within the lanthanide series as well as scandium (Sc) and yttrium (Y), all found in the third column of the Periodic Table.

These elements are usually found in their oxide forms. Because they exist in such low concentrations, these minerals are hard to separate from ores and from each other.

Rare earths hold special properties that make them essential ingredients in materials associated with high-tech applications.

“It's really rooted in the kinds of compounds that you can form with these rare elements or these critical minerals that make them attractive and more efficient,” said Michael Free, a University of Utah professor of metallurgical engineering.  “When you look at the rarer elements, neodymium (Nd) praseodymium (Pr) and dysprosium (Dy), they can be combined with other elements to form high-power magnets.”

Many lanthanide compounds are used in glass and catalysts, as well as magnets, superconductors, phosphors, lasers and luminescent materials. Rare earths also found in everyday technology, such televisions and smartphone screens, medical devices, auto- and fluid catalysts. Carbon-neutral energy technology, including wind turbines, solar panels, electric vehicles, rechargeable batteries and energy-efficient lighting, also require these elements.

“With turbine blades, for example, in a windmill to generate power, you want to use the higher powered magnets to make them more efficient. It basically helps us in some of this energy transition. It's about energy efficiency, it's about energy density for storage,” Free said. “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.”

The U.S. uses, on average, 8,300 metric tons of rare-earth oxides a year, according to the U.S. Geological Survey. The Mountain Pass mine 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.

PHOTO CREDIT: Lauren Birgenheier, University of Utah

U geology graduate student Haley Coe inspects drilling cores.

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“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 study 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 piles near 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.

Analyzing rock samples by the thousands

PHOTO CREDIT: Lauren Birgenheier, University of Utah

Michael Vanden Berg, geologist with the Utah Geological Survey, examines a coal outcrop near Utah's old Star Point mine.

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“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.

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 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 earths.

Lauren Birgenheier

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The team has analyzed 11,000 samples to date, far more than were used in the published study. The next steps include determining 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.

“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?’”

How did the elements get there?

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.”

Titled "Rare earth element enrichment in coal and coal-adjacent strata of the Uinta Region, Utah and Colorado," the study appeared on April 26 in Frontiers of Earth Science.