Total and potentially accessible coal ash reported by the electric power sector during 1985–2021. Credit: Reedy et al
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Even though the level of rare earth elements in coal ash is relatively low when compared with those mined from geological deposits, the fact that the ash is readily available in large quantities makes it an attractive resource, said co-author Davin Bagdonas, a research scientist at the University of Wyoming.

"There's huge volumes of this stuff all over the country," Bagdonas said. "And the upfront process of extracting the (mineral host) is already taken care of for us."

The researchers found that about 70% of the coal ash produced from 1985 to 2021—a total of about 1,873 million tons—is potentially recoverable, with the material stored in landfills, ponds and offsite storage areas. The rest of the coal ash has been sold and used by other industries, such as cement production and road construction.

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For example, ashes from Appalachian Basin coal contain the highest amounts of rare earth elements, with an average value of 431 milligrams per kilogram. But only 30% of the rare earth elements it contains can be extracted. In contrast, coal from the Powder River Basin has the lowest average value of rare earth elements at 264 milligrams per kilogram, but it has an extractability of about 70%.

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