As chemists scramble to find ways to reclaim valuable metals from industrial waste and discarded electronics, one team has found a solution that sounds a little like magic: Zap the trash with flashes of electric heat.
Rare earth elements (REEs) present an environmental paradox. On one hand, these dozen or so metals, such as yttrium and neodymium, are vital components of wind turbines and solar panels, and cheap sources of REEs could give those green technologies a huge boost. On the other, mining REEs causes billions of dollars of environmental damage each year. Because the elements occur in low concentrations, mining companies have to chew through tons upon tons of ore, stripping and gutting landscapes. Moreover, REEs are often mixed with radioactive elements, and extracting them creates low-level nuclear waste.
Old electronics and other industrial waste, in contrast, are rich in REEs. But existing recycling methods are inefficient and expensive, and require corrosive chemicals such as concentrated hydrochloric acid. For every environmental problem that REEs could solve, they seemingly introduce two more.
The new process could help break that logjam. Today in Science Advances, a team led by organic chemist James Tour of Rice University reports using pulses of electrical heat to make it easier to extract REEs from industrial waste. The technique is roughly twice as efficient as current methods and uses far more benign chemicals.
“It’s a very interesting approach,” says Amir Sheikhi, a chemical engineer at Pennsylvania State University, University Park, who studies REE extraction. “With a pretty short, high-temperature treatment … these rare earth elements are set free.”
Tour’s team tested its process on fly ash, a powdery gray byproduct of burning coal that contains concentrated levels of the REEs originally present in the coal. The researchers mixed the ash with carbon black to improve electrical conductivity, and then placed the mixture in clear quartz tubes 1 to 2 centimeters wide and 5 to 8 centimeters long. Capacitors on the ends of the tubes sent a pulse of current through, causing the tube to flash yellow-white and produce a tiny puff of smoke. The temperature of the mixed powders spiked to 3000°C within 1 second, then rapidly cooled.
That spike of heat does two things. When coal is burned as fuel, microscopic bits of glass form inside and trap REEs, making them hard to extract. But the bursts of electric heat shock and shatter the glass, freeing the rare earths. Flash heating also induces chemical changes: Phosphates of REEs transform into REE oxides, which are more soluble and easily extractable.
China’s rare earth dominance
Rare earth elements are vital components of green technologies. Other countries have complained that China uses its market dominance to drive up prices by limiting exports.
As a result of these changes, Tour’s group can use less corrosive solutions to extract the REEs. Tour’s team gets by with concentrations of hydrochloric acid 120 times lower than current extraction methods, and still manages to extract twice as much. “It’s so dilute,” Tour says, “that—well, I wouldn’t do it, but I think you could drink [it].”
In addition to fly ash, Tour’s team has extracted REEs from so-called red mud—a byproduct of making aluminum—and from electronics. In the latter case, the team gutted an old laptop and ground its circuit board into powder to experiment with.
Many government officials are keen on grabbing REEs from waste, rather than mining them, for economic as well as environmental reasons. China has long dominated the international REE market, and Japan, the European Union, and the United States have complained to the World Trade Organization that China uses its nearmonopoly to curtail exports and drive up prices. (Japan has since explored measures such as dredging up REE-rich mud from the deep ocean floor, which isn’t exactly ecofriendly.) Relying on a foreign supplier for REEs puts countries “at an economic disadvantage if not a natural security disadvantage,” says Steven Winston, an independent chemical engineer and former vice president of Idaho National Laboratory who has studied mining waste. Flash heating of waste might open an alternate supply.
But hurdles remain. After the REEs are extracted, they need to be separated into individual elements for different applications. That “is still a big challenge,” says Heileen Hsu-Kim, an environmental engineer at Duke University who studies REE extraction. Companies usually use organic solvents such as kerosene, which themselves cause environmental problems or are difficult to recycle. To address such concerns, Sheikhi’s team fashioned biodegradable cellulose into filaments sporting “hairs” with functional groups that selectively bond to and capture neodymium, a vital component in the magnets in wind turbines.
Moreover, Tour’s process would need to be massively scaled up to make a difference. Sheikhi points out that “typically, high-temperature processes are expensive.” But Tour’s team argues that because flash heating is speedy, costs are low, just $12 per ton of fly ash. As for scaling up, the team previously developed a flash heating process to transform old tires and plastics into graphene, and a spinoff company has already scaled that process up using larger flash heaters.
If Tour’s method does work out, there’s plenty of industrial waste to have at. Every year, humankind produces 40 million tons of electronic waste, 150 million tons of red mud, and 750 million tons of coal fly ash, much of it piled in giant mounds. Considering that burning coal helped create our current environmental mess, it would be fitting if the spark for green technologies could be extracted from its waste. “We don’t need any more coal to be burned for this [recycling] process to work,” Tour says. “We have sufficient mountains of this forever.”