EET
Rare earth magnet recycling faces uncertain future

Rare earth magnet recycling faces uncertain future

It’s no secret that rare earth metals have become indispensable for the emerging clean-energy economy. For example, the magnets in many state-of-the-art wind turbines and electric vehicles use rare earth praseodymium, neodymium, and dysprosium. Problem is, many of these materials come from parts of the world that have political tensions with the U.S. So the topic of avoiding rare-earth supply shortages is now top-of-mind for many manufacturers.

It is also top-of-mind for the Dept. of Energy. The DOE recently finished a study that examined the topic. And one of the ideas it investigated was boosting supplies by figuring out ways to improve recycling capabilities. Unfortunately, precious little is taking place at the moment with regard to rare earth recycling, though interest is rising. Research is underway about how to recycle both production scrap and the rare earths in discarded electronics and retired wind turbines.

Among the organizations exploring rare earth recycling is the Center for Resource Recovery and Recycling (CR3), a consortium started by the National Science Foundation and largely funded by its industrial members. The Center is researching how to develop “new technologies for maximizing the recovery and recycling of metals used in manufactured products and structures, including separation, processing, and recycling of rare earth materials.” CR3 is headquartered at Worcester Polytechnic Institute. Partner universities include the Colorado School of Mines (CSM) and Katholieke Universiteit Leuven in the Netherlands. More than 20 industrial companies — from mining companies to manufacturers — partner with the center.

Dr. Corby Anderson, Harrison Western Professor at the Kroll Institute for Extractive Metallurgy at CSM, says several recycling ideas are being evaluated, though no clear winner has emerged. “The rare earth business is ultracompetitive with numerous top secret projects going on,” says Anderson. “Some countries don’t have or respect intellectual property rights, so most of the work in this area is being conducted under strict nondisclosure agreements with the parties involved.”

That said, the basics of a few rare-earth magnet recycling methods are widely understood. One method involves recycling rare-earth “magnet sludge” using selective chlorination. The sludge is a byproduct of neodymium magnet production. FeCl2 is added to it during the recovery process.

“Recovery involves placing the magnetic sludge, iron chloride, and activated carbon in graphite crucibles and heating the samples between 1,073 and 1,273 K in an inert argon atmosphere,” notes Anderson.

Condensate products collect at the top of the furnace in vessels simply called “collectors.” Experimental results show that 96% of the Nd and 94% of the dysprosium present in the sludge get extracted into the chloride phase, producing a 99.2% pure mixture of Nd and dysprosium trichlorides.

Another process explores the recycling of Nd magnets using molten magnesium as an extractant. Magnesium has an affinity for Nd and can form a low-viscosity liquid alloy with it. Molten magnesium also doesn’t interact much with iron, has a high vapor pressure above 800° C, and a melting point of 649° C. In this process, Nd magnet scraps pulverized to smaller than 2 mm go in an iron crucible, which is suspended over a tantalum crucible containing Mg. The reaction vessel is sealed by TIG welding and heated in an electric furnace with a temperature range between 1,073 and 1,273 K.

The Mg in the bottom crucible evaporates because of its high vapor pressure and condenses in the top vessel, says Anderson. The Nd present in the scrap is then transferred to the liquid magnesium and drains to the bottom vessel through slots in the crucible. Because Nd has a low vapor pressure, it concentrates in the bottom crucible, while the Mg is continually evaporated and condensed. Results show that metallic Nd of 98% purity can be directly recovered using this process. What’s more, the Mg can be reused for further extraction of Nd from magnetic scrap.

A third process uses sulfuric acid to leach and recover Nd from Nd-Fe-B magnet scrap. Experimental results show almost 100% of Nd gets extracted without heating or agitation, using an acid-to-scrap ratio of two at a molarity of two. After dissolution, the aqueous Nd is precipitated as either an Nd double salt prior to fluorination or directly fluorinated using hydrofluoric acid.

Yet another process involves a combination of oxidative roasting, hydrochloric acid leaching, and solvent extraction to recycle Nd-Fe-B/Sm-Co alloy magnets. In this process, the Nd-Fe-B compound and Sm-Co alloy are first finely ground and then roasted at 700° C for eight hours prior to separation. Researchers have been able to extract 97% of Nd and 94% of Sm during experiments. The solution was then sent to solvent extraction where the Sm and Nd were extracted using specialized extractants. At a pH of two, extraction rates exceeded 95%.

Whether any of these methods will emerge as practical for recycling rare earth magnets is anybody’s guess. However, with an estimated 30% of magnetic material lost during the machining process involved in magnet production, there’s plenty of opportunity to increase the recovery rate. Economic realities such as raw material prices — along with technology demands and government mandates — will ultimately determine the feasibility of large-scale recycling facilities.

Rare earths by end use

Where do rare earth metals find use?
• Chemical catalysts — 22%
• Metallurgical applications and alloys — 21%
• Petroleum refining catalysts — 14%
• Automotive catalytic converters — 13%
• Glass polishing and ceramics — 9%
• Phosphors for computer monitors, lighting, televisions — 8%
• Permanent magnets — 7%
• Electronics — 3%
• Other — 3%
Source: 2011 U.S. Geological Survey

Endangered species list

According to the DOE’s 2011 Critical Materials Strategy report, shortages of five rare earth metals including dysprosium, terbium, europium, neodymium, and yttrium may affect the deployment of clean energy technology in the years ahead. So DOE is creating its first critical materials research plan (including recycling efforts). To read the full report, visit energy.gov.

Rare earths 101

Wish you knew more about rare earth metals but were afraid to ask? Read on. The 17 rare earth metals, or lanthanides, include yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, scandium, and lutetium. These elements are difficult to separate because of their similar chemical makeup. Monazite and bastnasite are the two main mineral sources of rare earth metals, with bastnasite mining in California now resurging as a major U.S. resource base, via Molycorp Inc.’s Mountain Pass mine.

Mining and processing are anything but simple: Ore is crushed, ground, classified, and concentrated by flotation. It then undergoes an acid (HCl) digestion to produce several rare earth chlorides; this slurry is then filtered to produce rare earth hydroxide cakes, which are then chlorinated to convert the hydroxides to chlorides. Final filtration and evaporation yields the solid rare earth chloride products. These chlorides then undergo further processing to produce individual rare earth metal compounds ready for use.

TAGS: Content
Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish