Communications of the ACM
Rethinking Rare Earth Elements
Anxiety about rare earth elements' supply-chain vulnerability heightened during the Covid-19 pandemic and the ongoing war in Ukraine.
Credit: Kiliii Yuyan/Bloomberg Businessweek
The 17 known rare earth elements (REEs) possess distinctive chemical and physical properties, such as unusual magnetic and optical characteristics, that make them extremely useful to us. They are vital components in both everyday and emerging technologies ranging from hard drives and smartphones to electric cars, cloud computing, and the Internet of Things.
Rare on account of their special characteristics and the challenges of mining and separating them, rather than their scarcity, consumers are often oblivious of their importance. Yet without REEs such as cerium, lanthanum, neodymium, and terbium, society as we know it would cease to function and the expansion of future-proofing technologies, like clean energy, would grind to a halt.
Demand for REEs is high and rising. According to a report by Fortune Business Insights, the global rare earth elements market is projected to grow from $2,831.0 million in 2021 to $5,520.2 million in 2028, increasing at a compound annual growth rate (CAGR( of 10.0%.
However, there are serious environmental and supply chain concerns around REEs, not least China's market dominance. Consumer and market data company Statista reported that in 2021, China had mined some 168,000 metric tons of rare-earth oxides, making China "the world's largest producer of rare earths by a large margin."
Anxiety about REE's supply-chain vulnerability heightened during the Covid-19 pandemic and the ongoing war in Ukraine, and action is being taken. In February 2021, for example, President Biden signed an executive order to review U.S. supply chains' reliance on foreign suppliers of REEs, and to support moves toward REE independence.
Today, researchers are finding ways of extracting REEs from unexpected domestic sources.
Tapping into slurry
"In the U.S. right now, we have zero processing capabilities, so it is a tremendous national security and economic stability issue," explained Lauren Greenlee, a chemical engineering professor at The Pennsylvania State University (Penn State).
Greenlee is principal investigator on an ambitious project to develop a sustainable process for recovering REEs from phosphogypsum, a by-product of fertilizer. The project—a collaboration with Case Western Reserve University and Clemson University—was recently awarded a four-year £571,658.00 grant by the National Science Foundation (NSF).
Mining and processing REEs can be hazardous, observed Greenlee; "The commercial process is very not environmentally friendly and it's very wasteful." Waste produced during processing accumulates radioactivity due to naturally occurring radionuclides in the ores, she said.
Historically, the U.S. did have REE processing capabilities, said Greenlee, but that changed with the introduction of legislation aimed at reducing harm to the environment, like the Clean Air Act and the Clean Water Act. "All of that led to the U.S. reducing, and then eventually completely eliminating, our processing capability and encouraging other countries, like China, to take on that capability."
It is difficult to find REEs in pure form; typically, deposits found contain mixtures of elements that need to be separated. This process is also problematic, Greenlee explained, because "It uses a lot of organic solvents, and it uses a lot of water. Both of those are sustainability issues: the solvents are a big waste issue and using a lot of water to process it is a challenge."
The goal of the project Greenlee is heading up is to develop a sustainable method of extracting and separating REEs that also results in the production of clean water. It starts with consideration of what is to be done with the processes' waste.
Phosphogypsum is the biggest waste stream in commercial phosphate fertilizer production, said Greenlee. "It's produced in approximately a 5:1 ratio in terms of the fertilizer itself. So, for every one unit of fertilizer, you get five units of this waste."
Currently, phosphogypsum is stored in a thick slurry form in what Greenlee describes as "ginormous" stacks. Radioactivity and separation challenges mean it is currently untapped and stored indefinitely. As the researchers note in their project abstract, "Today, an estimated more than 200,000 tons of rare earth elements are trapped in unprocessed phosphogypsum waste in Florida alone."
The new method being developed by the researchers will harvest REEs from the phosphogypsum using a multistage process based on engineered synthetic peptides, which will identify and separate the REEs via a specialized membrane. "If you can envision your water filter that you might have under your sink. It's going to be in that kind of form-factor where the peptide would be part of the filter material and would be capturing the rare earths."
Luckily, said Greenlee, techniques for synthesizing the needed peptides already exist, and they are manufacturable. The researchers are designing a novel, commercially scalable peptide sequence "by changing the sequence of the amino acids; by doing that, you can tune the binding selectivity to the rare earth elements."
Greenlee stresses that the project is in its early stages, and that it also will need to develop pre-treatment steps to release the REEs from the slurry. "Our goal with the technology that we're developing is to replace the solvent extraction process of separating rare earths— not only from the resource itself but from each other— in a way that would be more environmentally sustainable and efficient," she said.
From toxic waste to sustainable REEs
At Rice University in Houston, another group of researchers also have been investigating sources of REEs, including coal industry waste and recycled electronics.
In a recent paper published in Science Advances, Rice researchers Bing Deng, Xin Wang, Duy Xuan Luong, Robert A. Carter, Zhe Wang, and Mason B. Tomson demonstrate a method of extracting REEs from an abundant toxic waste product, coal fly ash. Project leader James Tour, a professor of chemistry, materials science, nanoengineering, and computer science at Rice, notes the historical context for the current crisis in REE supply: "When the U.S. closed down their business, China raised their price tenfold, so these are elements that are controlled particularly by China."
The Rice team's solution focuses on an abundant domestic source of REEs: coal fly ash, the inorganic residue that remains after burning coal. "We have mountains, literal mountains of fly ash in the United States from a hundred years of burning coal. Within that fly ash are rare earth elements," said Tour.
Right now, it is challenging to sustainably extract REEs from fly ash because metal oxides form glass structures around the REEs when the coal is burned, "You can get them out by using extremely strong acids and extremely strong bases, which are secondary waste streams in themselves."
A new method devised by the researchers improves REE extractability and sustainability through the use of an ultrafast electrothermal process based on flash joule heating, which Tour said does two key things. Firstly, because the glass heats and cools so quickly, it breaks, making the REEs accessible.
"Number two, it converts the rare earth element phosphates to rare earth element oxides and metals, which are very easy to dissolve. So, then we just use 0.1 molar hydrochloric acid, which is very dilute," he said.
The Rice team said it was able to leach out around twice as much REEs using Joule heating than could be accomplished with strong acid. The researchers also were optimistic about further global applications for the technology, including extracting REEs from bauxite residue (or red mud), a by-product of aluminum production, and even from existing tech.
They demonstrated their REE recovery technique on printed circuit boards, which had the additional benefit of releasing REEs that are already in separated form. "It's not just printed circuit boards," said Tour, "It is, for example, healthcare: X-ray scintillators have very high concentrations of rare earth elements."
They may be rare in name, but it is the abundance of untapped REEs in unexpected sources that looks likely to hold the key to their sustainability— and, in turn, the sustainability of vital technologies from cloud computing to clean energy.
Karen Emslie is a location-independent freelance journalist and essayist.
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