Reusable Ionic Liquid Enables Extraction of Rare Earth Elements

Researchers from Georgia Tech’s School of Civil and Environmental Engineering have discovered a way to extract rare-earth elements— essential ingredients for nearly all modern electronics—from the ash left behind at coal-burning power plants using a non-toxic ionic liquid.

In a paper published in ACS’s Environmental Science and Technology, the Georgia Tech researchers showed that by applying an ionic liquid directly to solid coal fly ash, rare-earth elements can be successfully removed in a safe process that creates little waste. The study is co-led by Ching-Hua Huang, a professor of environmental engineering and recent Ph.D. graduate Laura Stoy. A third co-author, Victoria Diaz, is an undergraduate student who joined the lab as part of Georgia Tech’s Summer Undergraduate Research in Engineering/ Sciences (S.U.R.E.) program.

Rare-earth elements (REEs) are a set of 17 elements that are utilized to make everything from permanent magnets in windmills to LED screens for computers and smart phones. While rare-earth elements aren’t as scarce as their name implies, only a few locations around the globe have deposits large enough to mine directly. Many of these reserves are in politically sensitive locations, resulting in global supply chain tensions.

Finding New Sources

Geopolitical tensions are exacerbating the scarcity of REEs, which are critical across many industries, such as renewable energy, national security and consumer technology. To ensure their supply, researchers are looking at alternative resources for REE recovery.

The U.S. Department of Energy is funding efforts to recover REEs from coal and coal combustion residuals. A promising source is coal fly ash, which is a byproduct from coalfired power plants. This waste produced by burning coal for power can have concentrations of REEs similar to those found in raw ores.

Stoy said one of the exciting aspects of their research is finding a safe way to utilize the 130 million tons of coal fly ash that are produced in the United States every year. While some coal fly ash can be reused in concrete and construction products, most of it ends up sitting in landfills. Some of these landfills aren’t lined, making surrounding communities susceptible to dangerous elements like mercury, chromium, and lead that may leach out and contaminate nearby water supplies.

Ionic Liquids Could be the Key Scientists know that coal fly ash contains high concentrations of REEs, but current methods of extracting these precious materials are hazardous and require several purification steps to obtain a usable product.

Huang and her research team have discovered a new method that utilizes an ionic liquid that is reusable and environmentally benign. Ionic liquids have only gained traction for use in metallurgy over the last decade, and this research represents the first time that ionic liquids have been directly applied to solid coal ash.

“This methodology is a significant paradigm shift away from the digestion of coal ash by strong acids or bases typical in other

REE recovery methods,” Huang said. “This new approach is safer industrially, and is more environmentally sustainable with the recyclability of the ionic liquid.”

The ionic liquid the researchers used, betainium bis(trifluoromethylsulfonyl) imide or [Hbet][Tf2N], selectively dissolves rare-earth oxides over other metal oxides. This ionic liquid is also unique because it dissolves into water when heated and then separates into two phases when cooled.

The researchers tested this ionic liquid on two types of coal fly ash. Class F ashes have a minimum of 70 percent by mass of primary oxides such as silicon oxides, aluminum, and iron oxides. Class C ashes contain less of these oxides, but also tend to have more calcium.

The researchers pretreated Class F coal fly ash with an alkaline solution and dried it, a step not needed for less recalcitrant Class C fly ash. Then, they heated ash suspended in water with [Hbet] [Tf2N], creating a single liquid phase. When cooled, the solutions separated. The ionic liquid extracted more than 77 percent of the rare-earth elements from freshly produced ash, and 97 percent from weathered ash that had spent years in a storage pond.

The researchers say that this low-waste approach produces solution rich REEs, with limited impurities, and could be used to recycle precious materials from the abundance of coal fly ash held in storage ponds.