Researchers report successfully extracting valuable rare earth elements from waste at yields high enough to resolve issues for manufacturers while boosting their profits.

The Rice University lab of chemist James Tour introduced its flash Joule heating process several years ago to produce graphene from any solid carbon source. They’ve now applied the process to three sources of rare earth elements—coal fly ash, bauxite residue, and electronic waste—to recover rare earth metals, which have magnetic and electronic properties critical to modern electronics and green technologies.

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.

The study appears in Science Advances.

Rare earth elements (REE) 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.

“The US used to mine rare earth elements, but you get a lot of radioactive elements as well,” Tour says. “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.”

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

“We have mountains of it,” he says. “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.” Bauxite residue, sometimes called red mud, is the toxic byproduct of aluminum production, while electronic waste is from outdated devices like computers and smartphones.

While industrial extraction from these wastes commonly involves leaching with strong acid, a time-consuming, non-green process, the Tour 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 says 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; this 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,” Bing says. “We proved that the REE recovery yields were improved from coal fly ash, bauxite residue, and electronic wastes by the same activation process.”

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

“The Department of Energy has determined this is a critical need that has to be resolved,” Tour says. “Our process tells the country that we’re no longer dependent on environmentally detrimental mining or foreign sources for rare earth elements.”

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.

Tour is chair in chemistry as well as a professor of computer science and of materials science and nanoengineering.

The Air Force Office of Scientific Research and the Department of Energy supported the research.

Source: Rice University

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Team turns banana peels and other trash into ‘flash graphene’

A new process can turn bulk quantities of just about any carbon source into valuable graphene flakes.

A banana peel, turned into graphene, could help facilitate a massive reduction of the environmental impact of concrete and other building materials.

“This is a big deal,” says James Tour, chair in chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice University.

“The world throws out 30% to 40% of all food, because it goes bad, and plastic waste is of worldwide concern. We’ve already proven that any solid carbon-based matter, including mixed plastic waste and rubber tires, can be turned into graphene.”

Making graphene in a flash

As reported in Nature, flash graphene is made in 10 milliseconds by heating carbon-containing materials to 3,000 Kelvin (about 5,000 degrees Fahrenheit). The source material can be nearly anything with carbon content. Food waste, plastic waste, petroleum coke, coal, wood clippings, and biochar are prime candidates, Tour says.

“With the present commercial price of graphene being $67,000 to $200,000 per ton, the prospects for this process look superb,” he says.

Tour says a concentration of as little as 0.1% of flash graphene in the cement used to bind concrete could lessen its massive environmental impact by a third. Cement production reportedly emits as much as 8% of human-made carbon dioxide every year.

“By strengthening concrete with graphene, we could use less concrete for building, and it would cost less to manufacture and less to transport,” he says.

“Essentially, we’re trapping greenhouse gases like carbon dioxide and methane that waste food would have emitted in landfills. We are converting those carbons into graphene and adding that graphene to concrete, thereby lowering the amount of carbon dioxide generated in concrete manufacture. It’s a win-win environmental scenario using graphene.”

“Turning trash to treasure is key to the circular economy,” says co-corresponding author Rouzbeh Shahsavari, an adjunct assistant professor of civil and environmental engineering and of materials science and nanoengineering and president of C-Crete Technologies. “Here, graphene acts both as a 2D template and a reinforcing agent that controls cement hydration and subsequent strength development.”

In the past, Tour says, “graphene has been too expensive to use in these applications. The flash process will greatly lessen the price while it helps us better manage waste.”

“With our method, that carbon becomes fixed,” he says. “It will not enter the air again.”

The flash graphene process can convert that solid carbon into graphene for concrete, asphalt, buildings, cars, clothing, and more, Tour says.

Heating things up

Flash Joule heating for bulk graphene, which lead author Duy Luong, a graduate student, developed in the Tour lab, improves upon techniques like exfoliation from graphite and chemical vapor deposition on a metal foil that require much more effort and cost to produce just a little graphene.

Even better, the process produces “turbostratic” graphene, with misaligned layers that are easy to separate.

“A-B stacked graphene from other processes, like exfoliation of graphite, is very hard to pull apart,” Tour says. “The layers adhere strongly together. But turbostratic graphene is much easier to work with because the adhesion between layers is much lower. They just come apart in solution or upon blending in composites.

“That’s important, because now we can get each of these single-atomic layers to interact with a host composite,” he says.

The lab notes that used coffee grounds transformed into pristine single-layer sheets of graphene.

Bulk composites of graphene with plastic, metals, plywood, concrete, and other building materials would be a major market for flash graphene, according to the researchers, who are already testing graphene-enhanced concrete and plastic.

The flash process happens in a custom-designed reactor that heats material quickly and emits all noncarbon elements as gas.

“When this process is industrialized, elements like oxygen and nitrogen that exit the flash reactor can all be trapped as small molecules because they have value,” Tour says.

He says the flash process produces very little excess heat, channeling almost all of its energy into the target.

“You can put your finger right on the container a few seconds afterwards,” Tour says. “And keep in mind this is almost three times hotter than the chemical vapor deposition furnaces we formerly used to make graphene, but in the flash process the heat is concentrated in the carbon material and none in a surrounding reactor.

“All the excess energy comes out as light, in a very bright flash, and because there aren’t any solvents, it’s a super clean process,” he says.

Speeding up carbon’s evolution

Luong did not expect to find graphene when he fired up the first small-scale device to find new phases of material, beginning with a sample of carbon black.

“This started when I took a look at a Science paper talking about flash Joule heating to make phase-changing nanoparticles of metals,” he says. But Luong quickly realized the process produced nothing but high-quality graphene.

Atom-level simulations by coauthor Ksenia Bets confirmed that temperature is key to the material’s rapid formation.

“We essentially speed up the slow geological process by which carbon evolves into its ground state, graphite,” she says. “Greatly accelerated by a heat spike, it is also stopped at the right instant, at the graphene stage.

“It is amazing how state-of-the-art computer simulations, notoriously slow for observing such kinetics, reveal the details of high temperature-modulated atomic movements and transformation,” Bets says.

Tour hopes to produce a kilogram (2.2 pounds) a day of flash graphene within two years, starting with a project the Department of Energy recently funded to convert US-sourced coal.

“This could provide an outlet for coal in large scale by converting it inexpensively into a much-higher-value building material,” he says.

Additional coauthors are from Rice and C-Crete Technologies. The Air Force Office of Scientific Research and the National Science Foundation supported the research.

Source: Rice University

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