Simplifying recycling of rare-earth magnets
Rather than this liquid-liquid method, Schelter’s team has devised a way to separate the two metals.
“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. This quick and easy method has allowed us to separate equal mixtures of the metals into samples that are 95 percent pure.”
Their method can, in a matter of minutes, separate an equal mixture of the two elements into samples that are 95 percent pure.
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,” 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 release notes that 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.”
Future work will involve improving the stability of the ligand so it is less likely to fall off before the metals are separated.
“These results are encouraging,” Bogart said. “We feel that through slight adjustments to the system, the purity level could be increased even further.”
Further modification of the ligand could enable other rare earths in technology products, such as compact fluorescent light bulbs, to be recycled this way.
— Read more in Justin A. Bogart et al., “An Operationally Simple Method for Separating the Rare-Earth Elements Neodymium and Dysprosium,” Angewandte Chemie International Edition (26 May 2015) (DOI: 10.1002/anie.201501659)
