Flash of Heat to Extract Rare Earth Elements from Waste

In a new study published in Science Advances, Rice University researchers have developed a method to extract rare earth elements (REEs) from fly ash using flash of heat. Coal fly ash may be a particularly valuable source of REEs. James Tour, an organic chemist at Rice University and author of the study, said, "Between the extracting chemicals and the toxic waste, REE mining turns out to be an environmental disaster."

Tour explained that the U.S. government stopped licensing domestic rare earth mines years ago due to environmental challenges. Meanwhile, China ramped up production in the 1990s and began to dominate the market. Over the past few years, the U.S. Department of Energy has been investing in research projects to find better ways to extract rare earth elements from alternative sources, such as acid mine drainage and fly ash.

Fly ash could be a particularly valuable source of REEs, Tour said, because the U.S. has "literal mountains" of such waste. Extracting REEs may be a way to reduce fly ash waste while obtaining them in a less destructive manner, but the key is to find a safe and economical way to obtain them.

Tour's team developed a method to reduce the chemicals needed for extraction by first heat-treating the fly ash powder. Tour explains that the REEs in fly ash are trapped in tiny glass particles, and it is difficult for the acid to break through these particles. But just as your cold windshield will break if you try to heat it too quickly, the glass particles of fly ash can break when subjected to sudden and intense thermal changes.

The electric current heats the fly ash to 3000°C in 1 second, breaking the glass so that the REE can be extracted fairly easily.

Flash of heat is a method Tour and his colleagues developed several years ago for producing graphene from a carbon source. "We just thought, let's give it a try with fly ash," Tour said. To test their technique, team members mixed fly ash with a material called carbon black to improve its electrical conductivity and placed the powder mixture into a small tube with capacitors at each end. A current heats the fly ash to 3,000°C in one second, causing the glass particles to shatter.

"After flash of heat, the rare earth elements were in a more accessible state and could be extracted with a mild acid treatment," Tour said. The researchers found that they were able to obtain almost twice the amount of product without thermal treatment

• < Prev
• Next >