Researchers at the University of Pennsylvania, Philadelphia, have reportedly developed a process that could enable the efficient recycling of two rare earth metals, neodymium and dysprosium. They 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 method enables separating equal mixtures into samples that are 95% 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,” says Prof Eric J. 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 such as 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,” Prof. Schelter says. “A potential magnet recycler probably does not 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 photo shows a rare-earth facility in China.
