2022-11-04
Electric cars, wind turbines, and LED lighting all help keep the environment clean, but making them can be a dirty business. The high-performance magnets in motors and generators and the glowing phosphors in LEDs and flat screens all depend on substances called rare earth elements (REEs). And capturing REEs from the clay deposits in which many are found requires leaching agents that pollute soil and groundwater.
Now, a Chinese group has developed—and tested on tons of soil—an approach called electrokinetic mining that relies on electric currents to free the REEs, sharply reducing the need for polluting chemicals. The strategy, described this week in Nature Sustainability, could be “a game changer, providing that it is feasible at a large scale,” says Anouk Borst, a geologist at KU Leuven.
Despite their name, REEs are fairly abundant in Earth’s crust. It’s just hard to find deposits that are economical to mine. Heavy REEs—those with high atomic numbers, including dysprosium, yttrium, and terbium—are most commonly extracted from masses of clay formed through eons of weathering of igneous rocks such as granite. In these “ion-adsorption” deposits, the elements are adsorbed—or stuck—to the surface of clay particles. They are usually extracted by pumping large quantities of ammonium sulfate or a similar solution into the ground. The leachate pulls REEs from the clay and percolates down to bedrock, where it is collected for processing.
All that can contaminate water and soil, not to mention lay waste to large tracts of land. And roughly 80% of the world’s supply of heavy REEs come from ion-adsorption deposits in southern China and adjacent parts of Myanmar, where environmental regulations are poorly enforced and illegal mining is common. (The light REEs—cerium, neodymium, and praseodymium—are typically found in hard rock and are mined in a different way.)
Gaofeng Wang of the Chinese Academy of Sciences’s Guangzhou Institute of Geochemistry (GIG) and his colleagues thought electrokinetic technology might offer a cleaner alternative. The approach, in which electrodes on the top and bottom of a volume of soil induce an electric field, speeding the movement of the leaching agent and the ions it extracts, is already used in soil remediation and has been proposed for copper and gold mining. It has “the real potential to outperform traditional mining techniques in terms of efficiency, environmental impacts, and economics,” says Riccardo Sprocati, a specialist in the technology at the Technical University of Denmark.
The Chinese team started with a bench-top experiment, then scaled up to 20 kilograms of material, and finally moved to a field test at an actual ion-adsorption deposit, trying the technique on a 14-ton hunk of clay. The method extracted a higher percentage of the REEs more quickly than conventional leaching and needed less ammonium sulfate. It also left the soil cleaner and reduced contaminating elements in the leachate, which could simplify processing. The team calculates that the process could cut mining costs by about two-thirds.
Gareth Hatch, a rare earths expert at the Strategic Materials Advisory, a consulting firm in Manchester, England, notes that whether the technique can be scaled up “remains to be seen.” The group’s next test will include about 2000 tons of soil, where they will try “to optimize operating conditions,” says geochemist Hongping He, director of GIG and co-author on the paper. They will divide the site into sections to keep the electric current and voltage low enough to avoid harming surrounding soil, vegetation, or fauna, He says. And they have struck an agreement with a major rare earth supplier to test the process on an even larger scale.
“As with all mining methods, it will still impact the environment,” says Henning Prommer, an environmental engineer at the University of Western Australia, Perth, whose group has worked on applying electrokinetic mining to gold and copper. But, he says, “Given the crucial role that REEs play in our ambitions for a renewable energy infrastructure, any reduction in the environmental impact of mining is welcome.”