Mining Waste for Rare Earth Elements
Researchers have devised a simple technique to recover treasured rare earth elements (REEs) from trash. Heating electronic and industrial waste to intensely hot temperatures with flashes of electricity can extract more than twice the amount of REEs from the material than has been possible with previous techniques.
REEs, a group of 17 elements, are critical ingredients for technologies that make our world run: smartphones, EV motors, and wind turbines, to name a few. But extracting them from the earth is dirty business, causing environmental damage and tons of waste, including low-level radioactive waste.
Then there are global supply-chain issues. Contrary to their name, rare earths are actually not that rare in Earth’s crust. China, however, has cornered the lion’s share of the market, mining over 70 percent of the world’s REEs and processing an even greater percentage. To circumvent that stronghold, automakers outside China are trying to make electric motors that don’t use rare-earth-containing permanent magnets. Meanwhile, Japan is trying to retrieve the metals from deep-sea mud, and the Metals Co. plans to mine metallic nodules from the ocean floor.
A more sustainable solution for obtaining rare earths is to recycle them from old electronics and waste like fly ash, a by-product of burning coal. Salvaging them is tricky, though. REEs don’t dissolve easily for separation from other materials. In fly ash, they are present in the form of phosphates that don’t readily break down, and the ash particles are often encased in a layer of glass that forms in the coal-burning furnace.
REE extraction methods currently in use rely on large amounts of caustic chemicals such as acids, and they are inefficient. “You need strong acids to pull them out,” says James Tour, a chemist at Rice University. “And strong acids can’t even get through glass to start leeching them out.”
Tour and his colleagues decided to try using the flash Joule heating process they originally developed in 2020 to make graphene from carbon sources. Last year, they reported using the technique to remove precious metals and toxic heavy metals from printed circuit boards. They’ve now decided to turn their attention to REE extraction. “We have the hammer; we’re looking for nails,” Tour says. “If we could take trash and get from it these elements that we normally strip the earth for, that would be really good.”
In a paper published in Science Advances, they report using flash Joule heating on coal fly ash, discarded printed circuit boards, and red mud, the term for the iron-rich slurry that is the by-product of aluminum production. These wastes contain two to three times as much of the five most critical REEs—yttrium, neodymium, europium, terbium, and dysprosium—as the amount found in some of the most concentrated ores in the world.
Zapping the waste materials with short, intense bursts of electricity rapidly heats them to about 3,000 °C. That’s enough to crack the glass layer around fly ash particles, and to convert the REE phosphates found in both fly ash and bauxite into oxides that dissolve easily in very mild acid for subsequent removal. In electronic waste, REEs are usually in the form of easy-to-dissolve REE metals or oxides, Tour says. But here’s the rub: They are embedded into the matrix materials in layers, making them still hard to remove. Flash Joule heating cracks and separates the matrix layers, making REE separation and removal much easier.
As a result, the technique is able to extract far more rare earths than the conventional method using strong acids. “The concentrated acids that were the gold standard weren’t getting all the REEs out of fly ash,” Tour says, “so we’re getting 150 to 200 percent yield compared to that past standard.” What’s more, the new method does not use much heat, and the energy it uses would cost US $12 per tonne of fly ash when used on large scale.
Tour says he and his colleagues’ new technique has caught the attention of people in the REE industry. And from them he has learned that this novel process will not be an end-all. “This does not solve everything,” he says. “You still get mixtures of REEs and there are standard methods for separating these, but those can be major cost points as well. So this gives a new tool to address the problem, but there are still other issues that have to be solved.”
Other researchers are also trying to crack the problem of extracting REEs from waste. Last week, a team from Lawrence Livermore National Laboratory and Pennsylvania State University announced that they are working with Western Rare Earths to develop a process that uses a recently discovered natural protein to extract rare earths. And the U.S. Department of Energy announced that it plans to build the first large-scale facility to extract REEs and other critical metals from mine waste.