A new process developed by American researchers could enable efficient recycling of rare-earth metals, possibly making the electronics industry more sustainable.
The method proposed by a research team from the University of Pennsylvania focuses on two types of rare earth metals commonly used as magnets in various electronic devices.
Those metals are neodymium and dysprosium, both hard to find in nature and only extractable by complex and environmentally damaging methods.
Surprisingly, the new method is rather simple, works nearly instantaneously, at room temperature and requires only standard laboratory equipment.
Instead of using natural ores, it recycles neodymium and dysprosium from discarded devices. The researchers believe it would thus be considerably cheaper as well as environmentally friendlier than conventional extraction techniques.
"Neodymium magnets can't be beaten in terms of their properties," said Eric J. Schelter, the lead author of the study. "They give you the strongest amount of magnetism for the smallest amount of stuff and can perform at a range of temperatures."
Dysprosium is mixed into neodymium magnets to improve their thermal qualities. For reuse, it is important to separate those two again as different ratios are needed for different applications.
"It's, in principle, easier to get the neodymium and dysprosium out of technology than it is to go back and mine more of the minerals they are originally found in," Schelter said. "Those minerals have five elements to separate, whereas the neodymium magnet in a wind turbine generator only has two."
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.
"When we started," Bogart said, "our goal was to make rare earth separations simpler and more efficient and we have made strides towards just that. We have designed a way to separate the two metals by selectively dissolving the neodymium in a solution and leaving behind the dysprosium as a solid.”
The researchers started with the two elements as a mixed powder to which a metal-binding molecule known as a ligand was 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," Schelter said, "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 said. "A potential magnet recycler probably doesn't have the capital to invest in an entire liquid-liquid separations plant, so having a chemical technology that can instantaneously separate these elements enables smaller scale recyclers to get value out of their materials."
The method was described in the latest issue of the journal Angewandte Chemie.