Process recovers valuable neodymium from magnetic waste
Ever own an expensive wool suit that went out of style or got a little too small? Even though you can't wear it, the price tag makes it too hard to part with. So, the suit hangs at the back of the closet, gathering dust, on the chance that you'll drop some pounds or the fashion will come back into vogue.
In a way, that's similar to the situation faced by manufacturers of high-energy neodymium-iron-boron magnets. These rare-earth magnets allow the production of smaller, more powerful and more efficient electric motors that are used to make everything from portable CD players to automotive power windows more compact, lighter in weight and easier on batteries.
Despite those good characteristics, the magnet material is brittle and processing it creates a larger than normal amount of machining waste. This leftover scrap can't be reformed into magnets or easily recycled. But because it's roughly 29 percent neodymium (by weight) that's valued at $30 per kilogram — just slightly less per ounce than the price of silver — it's worth too much to dispose of. So it's been stockpiled on the chance that it might someday have a use.
Molten magnesium solution
That day could be here thanks to Scott Chumbley and Alan Russell. The two Ames Laboratory researchers have refined a process that makes it commercially viable to recover the neodymium from tons of stockpiled magnetic scrap.
"The neodymium-iron-boron material decomposes peritectically — it changes composition — when heated to its melting point," says Chumbley, lead researcher on the project. "So it can't just be melted down and reused. But it's too valuable to throw away, so there are literally warehouses full of 55-gallon drums of the stuff waiting to be recycled."
Until now, the best separation method available was to dissolve the neodymium-iron-boron 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 Tim Ellis and Rick Schmidt in the mid-1990s, Chumbley focused on using molten magnesium to extract the neodymium from the magnet scrap. Neodymium is soluble in liquid magnesium. In fact, 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 residue, crushed pieces of magnet scrap are immersed in liquid magnesium that's been heated to 800 degrees Celsius (1,472 degrees Fahrenheit). The magnesium leaches the neodymium from the scrap particles. The liquid magnesium-neodymium (Mg12Nd) solution can then be poured off, leaving the iron-boron particles behind.
What Chumbley and Russell discovered was that there is actually a two-phase zone that forms as the magnet material is submerged in the liquid magnesium. Initially, most of the neodymium is still tied up in an iron compound (Nd12Fe17). Then the neodymium gradually moves into the magnesium at the interface between the iron compound and the liquid magnesium. Over time, the composition of neodymium in the magnesium rises at a fairly constant rate and is spread evenly throughout the liquid, while the width of the transitional, two-phase region increases.
The resulting alloy is roughly 30 percent neodymium, making it perfect for use as feed material for the magnesium-casting industry. And it should substantially lower the cost. Currently, a typical magnesium alloy casting contains only two percent neodymium by weight, yet the neodymium accounts for 40 percent of the raw materials cost.
"It would give them a product that is exactly what they're already accustomed to using," Chumbley says. "They wouldn't have to retool or change any of their processes."
Tweaking the process
The researchers are looking at the effect that variables, such as the size of the magnet pieces, temperature and duration have on the process. In addition to recovering the neodymium, the leftover iron-boron scrap could be recycled as well, particularly for low-grade iron castings where composition isn't critical.
Russell has fielded a number of inquiries about the process from metal producers and recyclers, but so far "no one seems to be on the verge of licensing the technology," he says. One company, a magnesium-casting firm, has requested samples of the magnet material to take a closer look at the process.
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