Separation anxiety: Magnetic metal recycling

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  • Published: Jun 15, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Separation anxiety: Magnetic metal recycling

Down to earth gadgets

Crystal structures of neodymium complexes. Developed for separating magnetic metals. Credit: Angewandte Chemie Sschechtler et al

Rare earth magnets are widely used in a range of gadgets, instrumentation and even toys but recycling the mixtures of metals from which the magnets are formed is difficult. Now, research published in the journal Angewandte Chemie offers a novel approach to the isolation of neodymium and dysprosium from dismantled magnets. The technique could help ward off a future shortage of these metals.

Rare-earth metals are critical components of many electronic devices and permanent magnets. For instance, the strongest permanent magnets known comprise a neodymium-iron-boron alloy, which may also contain other rare-earth metals such as praseodymium, terbium, and in particular dysprosium. These "neo" magnets are used in loudspeakers and headphones, motors for rechargeable power tool, the read and write head in many computer hard drives and in the electrical dynamo components of wind turbines. The US DoE classifies dysprosium and neodymium as “critical materials”.

Complex simplicity

The rarity and utility of the rare-earth metals make some of them expensive, so recycling of consumer products could be critical to maintaining a supply for industry. This option could become even more important as convenient sources become less accessible to manufacturers due to geopolitical issues. Unfortunately, current recycling techniques cannot easily separate magnetic metals from the complex composite materials and components. Any method currently used is energy- and time-intensive and requires large quantities of volatile organic solvent (VOC). The small amounts of recycled metal available through these routes do not offset the environmental nor the economic cost of the recycling process.

Now, chemist Eric Schelter of the University of Pennsylvania, whose work focuses on synthetic lanthanide and actinide chemistry and addressing the problems faced in their separations may have a solution. He and his colleagues, Justin Bogart, Connor Lippincott and Patrick Carroll, have developed a relatively simple way to tease apart neodymium and dysprosium salts from mixtures of the two. They suggest that this could be an important step towards recycling these valuable materials.

The team has synthesised a nitroxide ligand that can be added to a mixture of the simple salts of the metals in question. This chelating ligand grasps on to the requisite metal ion in a coordination complex like a three-armed claw used to lift cars in the breaker's yard. The key to separation lies in the fact that once chelated, a bound metal ion with a relatively large radius, like neodymium self-associates to form a dimer of the complex. By contrast, the smaller metal ion, dysprosium, is less inclined to form the dimer and remains as individual monomer complexes.

Separation yield

As a result, the neodymium complex is fifty times more soluble in the VOC benzene than the dysprosium complex. This means that only a relatively small amount of VOC is needed to leach the neodymium complex from a 1:1 mixture of both metals to a high purity level. The solid that is left behind in the process contains approximately 95 percent of the dysprosium complex. Another round of purification with benzene can affect an even greater separation percentage. The researchers suggest that it should be possible to apply this simple, cost-effective method to other rare-earth elements.

"We are currently assessing the selectivity of our system across the series of rare earth metals to address other possible recycling opportunities, for example in compact fluorescent light bulbs," Schelter told SpectroscopyNOW. "In future ligand designs we will also work to increase the selectivity to improve the separations factor. Finally, we are working toward making the system practical and easy to use with the goal of a simple, accessible separations recycling technology."

Related Links

Angew Chem Int Edn 2015, online: "An Operationally Simple Method for Separating the Rare-Earth Elements Neodymium and Dysprosium"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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