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Penn research simplifies recycling of rare-earth magnets

19 June 2015

Researchers at the University of Pennsylvania have developed a process that could enable the efficient recycling of two rare earth materials: neodymium and dysprosium.

Size differences between the neodymium and dysprosium ions mean that ligand-neodymium complexes bind with one another, while their dysprosium counterparts do not (photo: University of Pennsylvania)

Despite their ubiquity in consumer electronics, rare-earth metals are, as their name suggests, hard to come by. Mining and purifying them is an expensive, labour-intensive and ecologically devastating process.

Researchers at the University of Pennsylvania have now pioneered a process that could enable the efficient recycling of two of these metals, neodymium and dysprosium. These elements comprise the small, powerful magnets that are found in many high-tech devices.

In contrast to the massive and energy-intensive industrial process currently used to separate rare earths, the Penn team's method works nearly instantaneously at room temperature and uses standard laboratory equipment.

Obtaining neodymium and dysprosium from used electronics rather than the ground would increase their supply at a fraction of the financial, human and environmental cost.

Neodymium magnets perform over a range of temperatures but these thermal qualities are achieved by mixing neodymium with other elements, including the rare-earth metal dysprosium, in different ratios. Because those ratios differ based on the application the magnet is being used for, the two metals need to be separated and remixed before they can be reused.

Currently, whether purifying the neodymium and dysprosium out of minerals or out of an old power tool motor, the same costly and energy-intensive process is used. The technique, known as liquid-liquid extraction, involves dissolving the composite material and chemically filtering the elements apart. The process is repeated thousands of times to get useful purities of the rare-earth metals, and so it must be conducted on an industrial scale.

Rather than this liquid-liquid method, the team, led by Penn's Eric Schelter, has devised a way to separate the two metals.

They designed a way to separate the two metals by selectively dissolving the neodymium in a solution and leaving behind the dysprosium as a solid. This quick and easy method  equal mixtures of the metals to be separated into samples that are 95 percent pure - and in a matter of minutes.

Starting with the two elements as a mixed powder, a metal-binding molecule known as a ligand is applied. The type of ligand the research team designed has three branches, which converge on the metal atoms and hold them in the aperture between their tips. Because of neodymium's slightly larger size, the tips don't get as close together as they do around dysprosium atoms.

"The difference in size between the two ions is not that significant, which is why this separation problem is difficult," says Schelter."But it's enough to cause that aperture to open up more for neodymium. And, because it is more open, one ligand-neodymium complex can combine with another, and that really changes its solubility."

The combination of the two neodymium complexes, known as a dimer, encapsulates the neodymium ions, enabling them to dissolve in solvents like benzene or toluene. The dysprosium complexes do not dissolve, enabling the two metals to be easily separated. Once apart, an acid bath can strip the ligand off both metals, enabling it to be recycled as well.

"If you have the right ligand, you can do this separation in five minutes, whereas the liquid-liquid extraction method takes weeks," Schelter adds.

Further modification of the ligand could enable other rare earths in technology products, such as compact fluorescent light bulbs, to be recycled this way.


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