Managing and remaking metals
Magnetic scrap recovery
Researchers at the DOE's Ames Laboratory have refined a process that makes it commercially viable to recover valuable rare-earth elements from stockpiled magnetic scrap. High-energy neodymium-iron-boron (Nd-Fe-B) magnets are used extensively in automotive, consumer electronics, and biomedical applications, with sales exceeding $2 billion per year. The performance characteristics of Nd-Fe-B magnets allow manufacturers to make electric motors smaller, more powerful, and more efficient, so that, for example, the motors driving automotive power windows can be lighter in weight and less draining on batteries.
The Nd-Fe-B material is quite brittle, so manufacturers wind up with a substantial amount of waste beyond the amount normally generated during machining and handling. Though this magnet scrap contains only about 29% neodymium by weight, the rare-earth element is valued at $30 per kilogram (or just slightly less per ounce than the price of silver), so it's worth recovering. The problem has been how to recover it.
"The magnet material oxidizes when heated to its melting point," said Scott Chumbley, Ames Laboratory metallurgist and lead researcher on the project. "It is not a simple matter to recycle. But it's too valuable to throw away, so there are literally warehouses full of 210-L (55-gal) drums of the stuff waiting to be recycled."
Until now, the best separation method available was to dissolve the Nd-Fe-B scrap in acid, then perform a series of chemical extraction and reduction steps. However, the complexity and expense of such a method was impractical for large-scale, commercial recycling. Building on research pioneered and patented by Ames Laboratory researchers in the mid-1990s, Chumbley focused on using molten magnesium to extract the neodymium from the magnet scrap because neodymium is soluble in liquid magnesium. The magnesium casting industry routinely adds neodymium and other rare-earth elements to make alloys that are corrosion-resistant and offer improved weldability.
The recovery process is relatively simple. After receiving a solvent bath to remove machining lubricant residue, crushed pieces of Nd-Fe-B magnet scrap are immersed in liquid magnesium at 800°C (1472°F). The liquid magnesium leaches the neodymium from the scrap particles, and the Mg-Nd solution is poured off, leaving the Fe-B particles behind.
The resulting magnesium alloy is enriched in neodymium, making it suitable for use as feed material for the magnesium casting industry at a substantially lower cost. Currently, a typical magnesium alloy casting contains only 2% neodymium by weight, yet the neodymium accounts for 40% of the raw materials cost. "It would give them a product that is exactly what they're already used to using," said Chumbley. "They wouldn't have to retool or change any of their processes."
In addition, the leftover Fe-B scrap could be recycled as well, particularly for low-grade iron castings where composition is not critical.
