The waste left over from spent fossil fuel may contain a treasure trove of rare-earth elements worth billions of dollars.
In a 2024 paper, geologists calculated that the waste ash from coal burnt in fossil fuel power plants may contain as much as US$165 billion worth of rare earth elements – and up to $97 billion may be feasibly extractable.
Working to recover these elements, says a team led by geoscientists at the University of Texas at Austin, could provide the US with a homegrown source of these critical elements without the hard work of mining or dependence on imports, which currently supply the majority of the country's rare-earth materials.
"This really exemplifies the 'trash to treasure' mantra," says geologist Bridget Scanlon of the Jackson School of Geosciences. "We're basically trying to close the cycle and use waste and recover resources in the waste, while at the same time reducing environmental impacts."
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Rare earth elements, or REEs, are a group of 17 elements consisting of the 15 lanthanides on the periodic table, plus yttrium and scandium. These elements are classified as "critical minerals" in the US, and they're crucial to hundreds of industries and technologies, including batteries, wind turbines, electric cars, and smartphones.
The US doesn't have much in the way of REEs reserves of its own. Instead, it depends almost entirely on imports, with the majority – around 70 percent – coming from China.
However, recent research suggests there may be an untapped deposit of REEs hiding in plain sight: The 52 billion tons of coal-ash waste produced by US power plants since the 1950s.
Here's how it works. Coal – ancient, fossilized plant matter – is not pure, but takes up trace amounts of other materials as it forms and hardens over eons. Those trace amounts are so small that they're not worth the effort of extraction; they certainly wouldn't exceed the value of the coal as a fossil fuel resource.
But when coal is burned, something interesting happens. The combustible parts of coal, such as carbon, hydrogen, and sulfur, burn away as gases, leaving behind the non-combustible parts. That includes clay minerals, quartz, and REEs.
Because most of the coal's mass vanishes as smoke, the concentration of REEs in the glassy ash left behind is 4 to 10 times higher than in unburnt coal.
To be clear, these concentrations are much lower than commercial ores. However, refining coal ash doesn't require more extractive mining – it uses what has already been unearthed – so it would be a worthwhile endeavor.
Co-led by Scanlon and geologist Robert Reedy, the team compiled decades of data on ash composition, extraction efficiency, and waste storage locations across the US. They estimate 11 million tons of REEs may lie in accessible coal-ash deposits between 1985 and 2021 – nearly eight times the amount in current US reserves.
The total theoretical value of the 15 lanthanide REEs in all US coal ash is about $56 billion. Depending on the site, 30-70 percent of that coal ash is accessible, reducing the total value in accessible ash to $14 billion.
The lanthanides that can feasibly be extracted from that accessible coal ash are worth up to $8.4 billion, the researchers determined.
However, adding yttrium and scandium to the mix skyrockets these figures to a total overall value of $165 billion, of which $97 billion is recoverable, according to the paper.
This is largely theoretical; extraction methods are still under investigation. But it's a valuable enough treasure trove that scientists and the US government are taking the possibilities seriously.
"In addition to enhancing energy security in the US through development of REE resources, the economic value of producing these REEs could help offset the costs of remediation of unlined ash landfills or ponds in environmentally vulnerable areas," the researchers conclude.
"The potential for REE resource development from coal ash should be evaluated globally in nations where coal ash is available."
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There are many other means of potentially sourcing rare earth elements, but many of them may also pose difficulties in extraction.
In recent years, scientists have suggested that volcanoes could be a fertile source of the valuable minerals.
"Extinct iron-rich volcanoes are often mined for iron ore," geologist Michael Anenburg from Australian National University explained at The Conversation.
"Our results indicate existing mines at such locations can potentially be modified to produce rare earths as well."
Another source is plant life. In research published only this month, scientists discovered a fern with an incredible superpower hidden in its tissues: the ability to collect and store rare earth elements from metallic soils, naturally extracting the resources from the ground.
"Rare earth elements are critical metals for clean energy and high-tech applications, yet their supply faces environmental and geopolitical challenges," Chinese Academy of Sciences geoscientist Liuqing He and colleagues explained in their paper.
"Phytomining, a green strategy using hyperaccumulator plants to extract metals from soil, offers potential for sustainable REE supply but remains underexplored."
While scientists explore the opportunities, there's no doubt a lot is riding on how the world will choose to extract these valuable elements.
The coal-ash research was published in November 2024 in the International Journal of Coal Science & Technology.