Eric J. Schelter, assistant professor in the Chemistry Department at the University of Pennsylvania, speaks to AZoM.com about the recent development of a new way to chemically separate neodymium and dysprosium, the two key elements in NdFeB magnets.
AP: What are the associated issues with the mining of Rare Earth Elements (REE)?
ES: Rare Earth Elements typically occur in low-grade phosphate or carbonate deposits as mixtures of around 5–7 elements. Extracting REE requires ‘beneficiation’ of the mixtures of REE away from waste components in their ores as well as the radioactive thorium or uranium that often co-occur with them.
Once a solution concentrate of REE is obtained, separations chemistry by liquid-liquid extraction is performed on the mixtures to isolate the individual REE in high purities. Separations chemistry is all about expressing chemical differences in the two things you’d like to separate.
When it comes to REE, the differences exploited in liquid-liquid separations are the relative sizes of the cations. Because this size difference is so subtle across the series of REE, liquid-liquid extraction processes must be repeated many thousands of times resulting in a process that is highly solvent-, time- and energy intensive.
AP: Can you provide an overview of your recent paper entitled “An Operationally Simple Method for Separating the Rare-Earth Elements Neodymium and Dysprosium”?
ES: We and others have targeted mixtures of neodymium and dysprosium because those two are the chief REE components of NdFeB magnets. NdFeB magnets are the premier permanent magnetic material in the marketplace. We developed an organic compound that binds neodymium and dysprosium ions that have been dissolved in organic solvents.
Our organic compound, generally called a ‘ligand,’ grabs all of the rare earth ions in a solution to form metal-ligand ‘complexes’ in a 1:1 ratio. However, the manner in which the ligand encapsulates each type of ion is highly sensitive to the size of the ion. Larger dissolved REE ions, including neodymium, result in a more open structure, while dysprosium ions produce a closed one.
Beginning with both elements mixed as a powder, a metal-binding molecule known as a ligand is applied.
The open neodymium structure undergoes additional chemical changes, namely that two of the complexes can react to form a ‘dimer’ complex. In this way, the subtle size difference between the ions is magnified into significant changes in their chemical behaviour. We discovered the neodymium dimer was much more soluble in benzene and toluene than the dysprosium complex.
This observation allowed us to separate the neodymium and dysprosium complexes easily with simple solvent rinsing. Our separations method for this targeted pair of elements is much faster, easier and would produce less solvent waste on a comparable scale to liquid-liquid extraction.
AP: What are the current methods for separating these elements in electronic scrap?
ES: There is little difference between the separations processes currently used in mining and those required for recycling. In the recycling of NdFeB magnets, the magnets themselves must be removed from their mechanical or electronic components and their polymer or metal sheathing must be removed.
Dissolving the magnets and chemical treatment affords REE concentrates. There is actually a lot of creative work in other groups going on right now to isolate the neodymium/dysprosium mixtures away from the iron and boron.
The issue is that not all NdFeB magnets are created equal, they have different amounts of dysprosium added depending on their application. The REE concentrate mixtures must then be re-purified into the parent REE for reuse in new applications.
AP: With global supplies of Dysprosium dwindling, do you expect that this new method will help alleviate some of the issues with dramatically fluctuating prices that we have seen in recent years?
ES: The recent REE supply crisis was tied to reduction of materials export quotas from China. But the crisis sensitized people to the vulnerability of the REE supply chain. Life cycle analyses of dysprosium conducted in recent years have established that demand is expected to surpass supply in the next 10-15 years.
Binding increases the solubility of the neodymium over that of the dysprosium, allowing the former to be filtered off.
Dysprosium is an excellent target for recycling because the good sources we have enjoyed, namely the ion adsorption clays in southern China, are a finite and dwindling resource. My group and I believe the need for dysprosium recycling is a great opportunity to develop new chemistry that could potentially address the anticipated dysprosium supply problem.
AP: Do you believe there are alternatives to using dysprosium in NdFeB magnets, or at least ways to reduce the amount required?
ES: There is some great work going in the U.S. DOE Critical Materials Institute on replacing dysprosium with cerium, a much more abundant and easier to obtain REE. Others have focused on optimizing (minimizing) the amounts of dysprosium.
I certainly think it is possible to discover a suitable replacement for NdFeB but it’s going to take time. Neo magnets are pervasive in technology so the new materials will need to fit the bit in a similarly broad range of applications.
AP: Why are NdFeB magnets so desirable in so many applications?
ES: NdFeB magnets are amazing materials. There are few comparable ones that give you the same amount of magnetic power per amount of stuff. Many groups and agencies are working on replacing this material but currently there is no comparable one in terms of miniaturization.
AP: Will this new separation process be easy to scale up for large recycling facilities?
ES: In fact there is much to do to improve our system before we can answer this question. In general, leaching is a simpler chemical engineering process than liquid-liquid extraction. Our long-term goal is to discover and develop chemistry to equip recycling facilities with a simple toolkit that will allow them to generate value from REE containing wastes, namely pure REE, quickly and easily.
AP: What is the next stage of development in this process?
ES: There is much work to be done do on the new separations system. Our immediate goal is to assess its capability for separating mixtures of rare-earth elements across the series and our initial results are encouraging.
In particular we’d like to target mixtures of REE in phosphors used in compact fluorescent light bulbs. Recycling those materials would have the added benefit of contributing to removing mercury from the environment.
Beyond validating the capabilities of the current system we are focusing on improving its stability to make it more practical and refining the structure of our ligand to make it even more efficient.
About Eric J. Schelter
Eric J. Schelter is an assistant professor in the Chemistry Department at the University of Pennsylvania.
His research focuses on the chemistry of the lanthanides and actinides with applications in separations and catalysis.
Schelter was the recipient of a U.S. Department of Energy Early Career Research Program Award in 2011 and was named a Research Corporation for Science Advancement Cottrell Scholar in 2013
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