Rare earth elements are waiting to be discovered in waste

Rare earth elements are a group of 17 metal oxides that are difficult to acquire and recycle. They are widely used in the manufacturing of various electronic devices. A group of researchers at Rice University recently made a groundbreaking discovery that could revolutionize rare mineral extraction.

Professor James Tour reported that his team has recovered valuable rare earth elements (REE) from garbage at yields high enough to help manufacturers address problems while increasing profitability.

The lab’s flash Joule heating process, which was first used to produce graphene from any solid carbon source several years ago, is now being used to retrieve rare metals that have magnetic and electronic properties from three sources: coal fly ash, bauxite residue, and electronic waste. These metals are critical to modern electronics and green technologies.

The researchers claim that this method is better for the environment because it uses significantly less energy and reduces the amount of acid necessary to recover the elements to a trickle.

Rare earth elements are not all that uncommon. Cerium, for example, is more plentiful than copper, and they all outnumber gold. However, together with yttrium and scandium, these 15 lanthanide elements are widely scattered and tough to extract from mined minerals.

“The U.S. 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 U.S. did away with all rare earth mining, the foreign sources raised their price tenfold.” explained Professor Tour.

There’s plenty of incentive to recycle what’s previously been mined, said Tour. Much of it is heaped up or buried in fly ash, a coal-fired power plant waste. Bauxite residue, sometimes known as red mud, is a harmful byproduct of aluminium manufacture, whereas electronic waste comes from old computers and phones.

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

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, whereas industrial extraction from these wastes typically involves leaching with strong acid, a time-consuming and non-green process. The waste is converted into highly soluble “activated REE species” during the process.

“Breaking the glass that encases these elements and converting REE phosphates to metal oxides that dissolve much more easily,” Tour said of treating fly ash with flash Joule heating. To extract the components, industrial techniques utilize a 15-molar concentration of nitric acid. The Rice method uses a much softer 0.1-molar concentration of hydrochloric acid, which nonetheless provides more output.

The researchers discovered that flash Joule heating coal fly ash (CFA) more than doubled the production of most rare earth elements when compared to leaching untreated CFA in strong acids in studies headed by postdoctoral researcher and primary author Bing Deng.

“The method is general for various wastes.By using the same activation procedure, we were able to enhance REE recovery yields from coal fly ash, bauxite residue, and electronic wastes,” said Bing.

The process’s generality makes it particularly promising, according to Bing, because millions of tons of bauxite residue and electronic trash are created each year.

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

In 2020, Tour’s lab pioneered flash Joule heating to convert coal, petroleum coke, and waste into graphene, a single-atom-thick form of carbon that is now commercialized. Since then, the group has improved the procedure to recover precious metals from electronic waste and turn plastic waste into graphene.

The research was supported by the Air Force Office of Scientific Research and the Department of Energy. The study is published in the journal Science Advances.

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By Ashikha Raoof, Earth.com Staff Writer