Separating rare earth metals from ash that is released during the combustion of coal is cheaper, easier and more environmentally friendly than extracting them from mines.
Rare earths needed for smartphones and electric cars can be obtained from coal waste, rather than from the ground.
Due to their useful magnetic and electronic properties, the metals are widely used. They are, for example, in touch screens of telephones, electric motors, wind turbines and other modern technologies. That while only recently few chemists were familiar with neodymium, europium, terbium and other rare earth metals. Mining these materials is expensive and inefficient: large amounts of land have to be excavated to extract small amounts.
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fly ash
chemist James Tour from Rice University in the United States and colleagues came up with a way to to recycle from ‘fly ash’. This is a fine black powder that is left over from the combustion of coal in power plants.
They developed a technique called flash joule heating† They package the ashes in a tube made of quartz – one of the most abundant minerals in the earth’s crust. They put that under high current for a second, which heats the whole thing up to 3000 degrees Celsius.
This rapid heating causes microscopic glass spheres to tear open in the ash, which contains rare earth metals. In addition, metal phosphides (metals bound to phosphorus) are converted to metal oxides (metals bound to oxygen). The metal oxides are easier to separate with weak acids.
Also other applications
After decades of burning coal, we have “literally mountains” of fly ash, Tour says. Therefore, the separation method could yield a large amount of rare earths from the ash, despite the fact that a tonne of fly ash contains about half a kilogram of rare earths.
The researchers also show that the same process can separate rare earth metals from electronic waste, and so-called ‘red mud’. The latter is the waste that arises during aluminum production.
The heating process also releases toxic heavy metals, but these can easily remain separated, says Tour. The rest of the ash that remains could still be useful in concrete, he says.
The technology is now in the name of a company called Universal Matter. That company will continue to develop the process to produce the metals on a commercial scale.