The Rice lab of chemist James Tour has successfully extracted valuable rare earth elements (REE) from waste at yields high enough to resolve issues for manufacturers while boosting their profits. The lab’s flash Joule heating process, introduced several years ago to produce graphene from any solid carbon source (earlier post), has now been applied to three sources of rare earth elements—coal fly ash, bauxite residue and electronic waste—to recover rare earth metals.

The researchers say their process is kinder to the environment by using far less energy and turning the stream of acid often used to recover the elements into a trickle. An open-acess paper on their study appears in Science Advances.

Here, we report an ultrafast electrothermal process (~3000°C, ~1 s) based on flash Joule heating (FJH) for activating wastes to improve REE extractability. FJH thermally degrades or reduces the hard-to-dissolve REE species to components with high thermodynamic solubility, leading to ~2× increase in leachability and high recovery yields using diluted acid (e.g., 0.1 M HCl). The activation strategy is feasible for various wastes including coal fly ash, bauxite residue, and electronic waste. The rapid FJH process is energy-efficient with a low electrical energy consumption of 600 kWh ton⁻¹.

—Deng et al.

Rare earth elements aren’t actually rare. One of them, cerium, is more abundant than copper, and all are more abundant than gold. But these 15 lanthanide elements, along with yttrium and scandium, are widely distributed and difficult to extract from mined materials.

Microscopic glass spheres found in coal fly ash contain rare earth elements that could be recycled rather than buried in landfills, according to Rice University scientists. Their flash Joule heating process has been adapted to recover the elements. Courtesy of the Tour Group

The US used to mine rare earth elements, but you get a lot of radioactive elements as well. You’re not allowed to reinject the water, and it has to be disposed of, which is expensive and problematic. On the day the US did away with all rare earth mining, the foreign sources raised their price tenfold.

—James Tour

There’s plenty of incentive to recycle what’s been mined already, Tour said. Much of that is piled up or buried in fly ash, the byproduct of coal-fired power plants.

“We have mountains of it. The residue of burning coal is silicon, aluminum, iron and calcium oxides that form glass around the trace elements, making them very hard to extract.

—James Tour

Bauxite residue, sometimes called red mud, is the toxic byproduct of aluminum production, while electronic waste is from outdated devices such as computers and smart phones.

While industrial extraction from these wastes commonly involves leaching with strong acid, a time-consuming, non-green process, the Rice lab heats fly ash and other materials (combined with carbon black to enhance conductivity) to about 3,000 degrees Celsius (5,432 degrees Fahrenheit) in a second. The process turns the waste into highly soluble “activated REE species.”

Tour said treating fly ash by flash Joule heating “breaks the glass that encases these elements and converts REE phosphates to metal oxides that dissolve much more easily.” Industrial processes use a 15-molar concentration of nitric acid to extract the materials; the Rice process uses a much milder 0.1-molar concentration of hydrochloric acid that still yields more product.

In experiments led by postdoctoral researcher and lead author Bing Deng, the researchers found flash Joule heating coal fly ash (CFA) more than doubled the yield of most of the rare earth elements using very mild acid compared to leaching untreated CFA in strong acids.

The strategy is general for various wastes. We proved that the REE recovery yields were improved from coal fly ash, bauxite residue and electronic wastes by the same activation process.

—Bing Deng

The generality of the process makes it especially promising, Bing said, as millions of tons of bauxite residue and electronic waste are also produced every year.

Tour’s lab introduced flash Joule heating in 2020 to convert coal, petroleum coke and trash into graphene, the single-atom-thick form of carbon, a process now being commercialized. The lab has since adapted the process to convert plastic waste into graphene and to extract precious metals from electronic waste.

The Air Force Office of Scientific Research (FA9550-19-1-0296) and the Department of Energy (DE-FE0031794) supported the research.

Resources

• Bing Deng, Xin Wang, Duy Xuan Luong, Robert A. Carter, Zhe Wang, Mason B. Tomson, James M. Tour (2022) “Rare earth elements from waste” Science Advances doi: 10.1126/sciadv.abm3132