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Recycling rare earth elements from the ash left over from coal burning is cheaper, easier and more environmentally friendly than digging them out of mines
A microscope image of tiny glass spheres in coal fly ash that contain rare earth elements
Rare earth elements that are vital to smartphones and electric vehicles can now be extracted from coal waste instead of being dug out of the ground.
Neodymium, europium, terbium and other rare earth metals that were once barely heard of are now commonplace in phone touchscreens, electric vehicle motors, wind turbines and other modern technologies due to their useful magnetic and electronic properties. Mining them is expensive and inefficient, since large areas of land must be dug up to extract small amounts.
James Tour at Rice University in Houston, Texas, and his colleagues have come up with a way to recycle these metals from fly ash, a fine black powder that is left over when coal is burned in power plants.
They developed a technique called flash joule heating that involves packing the ash into a quartz tube and running a large electrical current through it for 1 second to heat it to 3000°C.
This rapid heating breaks open microscopic glass spheres in the ash that contain rare earth metals. It also converts the metals from phosphate to oxide forms that are easier to separate out using mild acids.
Each tonne of fly ash contains only about half a kilogram of rare earth elements, but because we have “literal mountains” of the ash left over from decades of coal burning, the total amount of rare earth elements we could extract is vast, says Tour.
The researchers have also shown that the same process can be used to recover rare earth metals from “red mud” – the waste generated during aluminium production – as well as from discarded consumer electronics after they have been ground up into a powder.
The flash heating process also releases toxic heavy metals from these materials, but they can be easily trapped as they come out, says Tour. The rest of the ash that is left over could potentially be mixed with concrete in order to recycle it, he says.
The technology has now been licensed to a company called Universal Matter that will seek to scale it up and extract commercial quantities of rare earth elements.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abm3132
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