Rare Earth Recycling
Professor Eric Schelter’s remarks in advance of a recent presentation at the University of Michigan are below. The talk was entitled Emerging Directions in the Chemistry of the Rare Earth Elements: Sustainability and Bioinorganic Chemistry.
The chemistry of rare earth elements (La–Lu, Sc and Y) touches key issues in modern inorganic chemistry including metals criticality, sustainability, and the global carbon cycle. In this talk, I will present on two projects from my research group involving targeted separations chemistry for metals recycling and a new, natural bioinorganic chemistry for the rare earths. Rare earth elements are essential in renewable energies technologies including in permanent magnets for wind turbines, hybrid and electric vehicles, energy efficient lighting phosphors and many others. The rare earths are relatively abundant in the earth’s crust. But the conventional separations chemistry used to purify them proceeds through solvent extraction, a scalable but (stepwise) poorly efficient process that includes an unsustainable environmental burden. We have developed a hydroxylamine ligand, H3TriNOx, and its coordination chemistry. The RE(TriNOx) RE = La–Lu, Y series led to identification of an unusual solubility dependence of the resulting complexes, which afforded a new method for separating rare earth elements, especially for recycling. The TriNOx3– ligand also exhibits redox activity when coordinated to metal cations. We have recently determined that this redox activity can be exploited for metal separations based on variable rates of oxidation. Despite the relatively large abundances of early rare earth elements, it had been widely believed that these elements had no role in biology. Recently, however, a metalloenzyme was discovered that includes cerium, lanthanum and other elements in its active site. This methanol dehydrogenase, Xoxf, catalyzes the conversion of methanol to formaldehyde and, uniquely, to formate. We have developed a tethered quinolone quinone ligand that replicates the structure and function of the active site of the Xoxf metalloenzyme for the first time. We have used this compound to study the reactivity of alcohol dehydrogenation using a benzylic alcohol test substrate. Our results contribute to understanding about why Nature selects for lanthanides in this case and to understanding the reaction mechanism of the Xoxf methanol dehydrogenase.
Technical Details are below.
A new energy-efficient separation of rare earth elements could provide a new domestic source of critical materials.
Rare earth metals, including the lanthanide series, scandium, and yttrium, are critical components in permanent magnets, electric vehicles, smartphones, and more. The elements occur naturally as mixtures in ores and must be purified prior to use. However, the mining and separation of the mineral ore is challenging, in addition to being energy and waste intensive. An interesting alternative to mining is to recycle elements that have been processed into materials. But here again, the cost of re-separation and purification is a limitation. Only a tiny fraction of rare-earth-containing products is recycled. Recently, a group of researchers discovered a separation process that could make purifying recycled rare earth elements much less expensive.
A substantial portion of the cost of recycling rare earth elements is tied to their difficult separation. To improve the economic benefits of recycling, simple chemistry is needed that purifies targeted rare earth elements from technologically relevant mixtures. Adding recycled rare earths as a new source to the supply chain is expected to reduce environmental contamination and energy costs associated with their primary mining and separations. Additionally, a new domestic source of rare earths would be a positive contribution to U.S. technology at competitive prices.
Rare earth elements are crucial materials in many consumer products, such as electronics and automobiles. These elements currently have a significant environmental burden. Despite their capability for reuse, the vast majority are discarded into the trash after only one use. Recycling rare-earth-containing products would provide a steady, domestic source of rare earths to manufacturers while also reducing waste. Currently, the main roadblock to recycling rare earth elements is the cost required to purify the mixtures obtained from consumer devices. Recently, a group of researchers from the University of Pennsylvania discovered a separation process that could make purifying recycled rare earth elements much less expensive. By developing a new organic compound (H3TriNOx) for binding rare earth cations, this group formed 15 different rare earth compounds. Solution studies revealed that the “early” rare earth compounds (containing lanthanum, cerium, praseodymium, neodymium, samarium, or europium) preferred to aggregate and form dimeric species. The team observed no such aggregation for “late” rare earth compounds (containing gadolinium, terbium, dysprosium, yttrium, holmium, erbium, thulium, ytterbium, or lutetium). As a result, the solubility difference of these compounds was large enough to enable efficient separation for all early/late rare earth combinations through a single filtration step. Optimization of the separation conditions was used to improve the effectiveness of specific combinations, most notably the neodymium/dysprosium and europium/yttrium pairs. These pairs are widely used in permanent magnets and compact fluorescent light bulbs, respectively. The TriNOx separations system is expected to contribute to the recycling of these and other end-of-life rare-earth-containing products, providing a cheap and green new source for these critical raw materials.
J.A. Bogart, C.A. Lippincott, P.J. Carroll, and E.J. Schelter, “An operationally simple method for separating the rare-earth elements neodymium and dysprosium.” Angewandte Chemie International Edition 54, 8222-8225 (2015). [DOI: 10.1002/anie.201501659]