Rare earth elements extracted from waste

HOUSTON, TX – Rare-earth elements are hard to obtain and difficult to recycle, but a flash of intuition led Rice University scientists to a possible solution.

Chemist James Tour’s Rice Lab Reports successfully extracted valuable rare earth elements (REE) from waste at yields high enough to solve manufacturers’ problems while increasing their profits.

The lab’s Joule flash heating process, introduced several years ago to produce graphene from any solid carbon source, has now been applied to three sources of rare earth elements – coal fly ash , bauxite residue and e-waste – to recover rare earth metals, which have magnetic and electronic properties essential to modern electronics and green technologies.

The researchers say their process is more environmentally friendly by using far less energy and by turning the acid stream often used to recover elements into a trickle.

The study appears in Scientists progress.

Rare earth elements are not really rare. One of them, cerium, is more abundant than copper, and all of them 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.

“The United States used to mine rare earth elements, but you also get a lot of radioactive elements,” Tour said. “You’re not allowed to re-inject the water, and it has to be disposed of, which is expensive and problematic. On the day the United States eliminated all rare earth mining, foreign sources increased their price tenfold.

So there are plenty of incentives to recycle what has already been mined, he said. Much of this is piled up or buried in fly ash, the byproduct of coal-fired power plants. “We have mountains of it,” he said. “The residue from burning coal is oxides of silicon, aluminum, iron and calcium that form glass around the trace minerals, making them very difficult to extract.” Bauxite residue, sometimes called red mud, is the toxic byproduct of aluminum production, while e-waste comes from obsolete devices like computers and smart phones.

While industrial extraction of this waste usually involves leaching with strong acid, a time-consuming and non-green process, Rice Lab heats fly ash and other materials (combined with carbon black to improve conductivity) at about 3,000 degrees Celsius (5,432 degrees Fahrenheit) in one second. The process transforms the waste into highly soluble “activated rare earth species”.

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

In experiments conducted by postdoctoral researcher and lead author Bing Deng, researchers found that coal fly ash (CFA) heated by Joule flash more than doubled the yield of most rare-earth elements using a highly acidic mild compared to the leaching of untreated CFA in strong acids.

“The strategy is general for various wastes,” Bing said. “We proved that REE recovery efficiencies were improved from coal fly ash, bauxite tailings and e-waste by the same activation process.”

The generality of the process makes it particularly promising, Bing said, because millions of tons of bauxite residue and e-waste are also produced each year.

“The Department of Energy has determined that this is a critical need that must be addressed,” Tour said. “Our process tells the country that we are no longer dependent on environmentally damaging mining or foreign sources for rare earth elements.”

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

Co-authors of the study are graduate students Xin Wang and Zhe Wang, alumnus Duy Xuan Luong, undergraduate Robert Carter, and civil and environmental engineering professor Mason Tomson. Tour holds the TT and WF Chao Chair in Chemistry as well as a professor of computer science, materials science and nano-engineering.

– This press release originally appeared on the Rice University website

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