Coal ash waste stream might become feedstock for extracting much-needed domestic rare earth elements.

The U.S. has relied almost entirely on imports for its supply of rare earth elements (REEs) – nearly 75% of which comes from China, whose geopolitical tensions with the West have put several critical mineral supply chains onto shaky ground. Meanwhile, at a special landfill in Shrewsbury, Massachusetts, researchers have discovered massive potential for this suite of elements hidden in the chalky remnants of coal that has been burned for fuel.

Coal ash has been piling up across the U.S. for decades, and has a surprisingly complex table of contents that are at the same time toxic and valuable, including arsenic, boron, cadmium, chromium, lead, radium, and selenium. Now it appears this national coal ash waste stream could contain enough rare earths to quickly and significantly bolster domestic supply without the long wait for decades of new, expensive mine development.

"This really exemplifies the 'trash to treasure' mantra," said Bridget Scanlon, a research professor at UT's Bureau of Economic Geology at the Jackson School of Geosciences and co-lead author of a study on rare earth reserves in U.S. coal ash. "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."

Researchers have calculated that U.S. coal ash could contain as much as 11 million tons of rare earths – more than six times the amount in today's domestic reserves. The study, published in the International Journal of Coal Science & Technology, is the first to tally up national coal ash resources. It includes the estimate that $8.4 billion worth of rare earth elements could be extracted from accessible sources.

Companies are required to follow strict disposal procedures or divert coal ash to beneficial reuse, recycling it as building material or structural fill – used in concrete, roads, and foundations. More than half of coal ash produced each year is reused, with more currently sitting unused in landfills. Though the percentages of REEs in coal ash are relatively low compared to geological deposits, the ready availability of huge caches nationwide is an attractive resource, notes study co-author Davin Bagdonas, a research scientist at the University of Wyoming.

Several technologies are being developed with the potential to efficiently extract rare earths from the roughly 1.87 billion tons of coal ash that has been accumulating in landfills, ponds, and offsite storage produced since 1985.

Coal ash contains different percentages of REEs depending on where it originates from, and every location presents its own unique difficulties in extraction – ashes tested from Appalachian Basin coal averaged 431 milligrams of rare earths per kilogram, the highest amounts, but only 30% 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%.

"The idea of getting rare earth elements out of tailings just makes a lot of sense," said Chris Young, chief strategy officer at Element U.S., a company that extracts critical minerals from mineral and metallic waste. "The challenge is to convert that common-sense approach to an economic approach."

Element U.S. plans to move its analytical lab and pilot equipment to utilize the mineral expertise at The University of Texas at Austin and offer critical mineral experience to students interested in critical mineral research and careers.

"We're excited about building that relationship with The University of Texas around mineral processing and mineral separation," Young said.

The U.S. Department of Energy is also applying the study's methodology to conduct its own separate assessment of national coal ash resources.

"This kind of broad reconnaissance-level analysis has never been done," Scanlon said. "It provides a foundation for others to go into more detail."