Rare earth elements (REEs) are vital components of everyday life, found in electronics from laptops to solar panels. The United States currently sources most of its rare earth elements from mines controlled by China, but concerns over China’s dominance in the market—and the environmental cost of mining—have spurred the U.S. Department of Energy to invest millions of dollars in research into alternative sourcing options.
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In a new study published in Science Advances, researchers at Rice University presented one promising alternative to mining. They developed a method to extract REEs from an abundant toxic waste product called coal fly ash using a jolt of electricity. By subjecting coal fly ash to extreme heat, they freed up REEs from surrounding elements without the need for harsh, corrosive chemicals. And this “flash Joule heating” method could help extract REEs from other waste materials as well.
“This is a really novel way of improving rare earth element extraction,” said Laura Stoy, an environmental engineering researcher who recently completed a Ph.D. at the Georgia Institute of Technology specializing in rare earth elements. Stoy was not involved in the new research. “This method of shocking them into submission, in a way, is very cool.”
Tricky to Get, but Essential to Modern Life
Rare earth elements are composed of 17 metals (like neodymium and terbium, commonly used as magnets) that sit toward the bottom of the periodic table. They aren’t actually rare—all are more common than gold—but they tend to be mixed up together in Earth’s crust in low volumes, and extracting them traditionally requires harsh chemicals. REEs are also typically intermingled with their radioactive neighbors on the periodic table: uranium and thorium.
Between the extracting chemicals and the toxic waste, REE mining “turns out to be an environmental disaster,” said James Tour, an organic chemist at Rice University and an author of the study.
As a result of the environmental challenge, the U.S. government stopped issuing permits for domestic REE mines years ago, Tour explained. China, meanwhile, ramped up production in the 1990s and came to dominate the market. For the past several years, the U.S. Department of Energy has been investing in research projects to find better ways to extract REEs from alternative sources, like acid mine drainage and coal fly ash.
Coal fly ash could be a particularly valuable source of REEs because the United States has “literal mountains” of this waste material, Tour said. Extracting rare earth elements could be a way to both reduce fly ash waste and source REEs in a less destructive manner, but the trick is finding a way to access the elements safely and affordably.
From Waste to Treasure Trove in a Flash
Extracting rare earth elements from coal fly ash isn’t a straightforward task, as coal ash particles have already “been through hell” in the combustion chamber, Stoy explained. So far, the only way to pull REEs out of coal fly ash is with strong chemical treatments. But this process, in itself, produces toxic waste.
Tour’s team developed a way to reduce the chemicals needed for extraction by first treating the fly ash powder with heat. Rare earth elements in coal fly ash are trapped in microscopic glass particles that acids have a hard time breaking through, Tour explained. But just as your icy windshield will crack if you try to heat it up too fast, the ash’s glass particles will crack when subjected to a sudden and intense heat change.
Flash Joule heating is a method that Tour and his colleagues developed several years ago to produce graphene from a carbon source. “We just thought, let’s give it a try with fly ash,” Tour said. To test their technique, the team members mixed fly ash with a material called carbon black to improve its conductivity, and put the powder mixture into a small tube with capacitors on either end. A current heated the ash by 3,000℃ in 1 second, shattering the glass particles.
After the heat shock, the rare earth elements were in a more accessible state and could be extracted with an acid treatment so mild “you could practically drink [it],” Tour said. The researchers found that they were able to get nearly twice as much product as they could without the heat treatment. They tested the strategy on an industrial waste known as red mud as well, with similarly promising results. The process seems like it could be economically viable: It requires only about $12 of electricity per ton of waste product.
“This paper is a good step in the direction of using fewer chemicals,” Stoy said, but she cautioned that it would still produce some waste. “I would really like to see this become a closed loop,” she said.
—Rachel Fritts (@rachel_fritts), Science Writer