Ding, Y.; Harvey, D.; Wang, N.-W. L.Green Chem., 2020, Advance Article. DOI:https://doi.org/10.1039/D0GC00495B Reproduction by permission of The Royal Society of Chemistry.
Mountain Pass is a tiny, unincorporated scatter of flat-roofed buildings at the eastern edge of San Bernardino County, California – the largest county by area in the United States. The sole reason for any habitation in this desolate, dun-coloured outpost just shy of the Nevada border was the Mountain Pass Mine, the only rare earth metal mine in America.
Here, from the 1950’s to the 2000’s, ores of the mineral family bastnäsite were extracted from a 1.4 billion-year-old Precambrian deposit. The ores – primarily bastnäsite, hydroxyl bastnäsite-(Ce) and hydroxylbastnäsite-(Nd) – underwent comminution, or crushing to a small particle size, then separation by flotation from other accompanying minerals. Hydrochloric acid leaching, and a process of sequential precipitations, separated cerium, europium, gadolinium, samarium, lanthanum, praseodymium, and neodymium, pure or as oxides.
With the rapid expansion of the periodic table over the last 150 years, scientists widened the buffet of elements from which technology may choose. Rare earth elements (REE), defined as the lanthanide series plus yttrium and scandium, augment the workhorse elements of the Industrial Revolution, copper, zinc and lead.
In the late 1990s, the Mountain Pass Mine, once the world’s top producer, struggled to remain a viable mine and was over taken by China as the world’s top producer of rare earth elements. By 2002, it ceased production. Attempts to revive it have lead to changes in ownership, a bankruptcy filing by then-owner Molycorp in 2014, and another closure in 2015. Reopened in 2018, the Mountain Pass Mine was renamed MP Materials. The joint venture between US and Chinese investors is focused on revitalizing the U.S. rare earth elements industry.While the US mine competes with China, whichcontrols 80% of global suppliesof REE, refining of those metalscontinues to be done in China.
But prospecting for precious metals remains part of the romance of the old west. The famous gold rushes of 1848-55 in California, and the late 1890’s in the Canadian Klondike, the writing of Jack London and Robert Service, and the hardy, tough, risk-taking men and women of the frontier lend themselves to an old and incomplete North American narrative – glossing over, of course, the destruction of habitat and the indigenous populations on whom the influx of white prospectors often took a terrible toll.
The human and environmental costs of mining remain, and environmental stewardship depends on the policies of the countries where mines are located. Mining displaces plants and topsoil and often risks contaminating groundwater; tailings ponds left behind must be remediated. The refinement process of the ores generates large amounts of strong acid and organic solvent wastes, some of which is also radioactive. Considering treatment, production of one ton of rare earth oxides generates an average of30 tonsof wastewater released into the environment.
Might we envision recycling metals from end-of-life human-made materials as a more sustainable, alternative “mining” for the Anthropocene?
If we can, researchers such as Professor Nien-Hwa Linda Wang of Purdue University might represent the new frontier’s men and women. In March, Professor Wang, along with her graduate students Yi Ding and David Harvey,reporteda significant advance in REE recovery. In the Royal Society of Chemistry (RSC) journal Green Chemistry, they detail an improved, scalable, commercially viable method for recovering high-purity REE’s from magnets.
How much might be recovered from magnets, you ask?Actually, a huge amount. Neodymium magnets, made of a neodymium-iron-boron alloy which forms aNd2Fe14B crystalline repeating unit, are the most widely used type of rare earth magnet. They account for an astonishing 30% by mass of REE use – the single largest application.
First reported in simultaneous papers by Sumitomo Special Metals and General Motors in the Journal of Applied Physics in 1984, NdFeB magnets represented a significant step forward from the samarium-based magnets then widely in use. A generation of rapid technological progress, particularly in information systems and energy storage, has for now entrenched dependence on them. They remain the strongest type of commercially-available permanent magnet, effecting the requisite magnetic force for applications in hard drives, electric motors, and wind turbine generators.
More esoterically,at the border where neuroprosthetics melds with science fiction, the idea of a “magnetic 6th sense” is occasionallyresurrectedthrough the idea of fingertip implants of powerful magnets. We may yet see NdFeB magnet-equipped cyborgs.
But recent trade instability with China, and the dim prospects of the Mountain Pass Mine bode poorly for price and supply-chain stability for manufacturers of wind turbines or cyborgs.
And recovering REE’s is not easy. Typical methods for recovery from end-of-life magnets or other industrial junk are often similar to those used for extraction from ore. The challenge is, of course, in the separation, first of the REE’s from bulk materials, and second, of REE’s with often similar chemical properties from each other. Comminution, oxidation to the metal oxides, dissolution in strong acids, and solvent extraction are involved. Professor Wang highlights other methods reported in the research literature, including conversion of REEs to soluble chlorides via roasting with ammonium chloride under inert atmospheres, chlorination with chlorine gas, or chlorination with the molten salts of other metal chlorides. As one can imagine, these have their own safety, waste and energy drawbacks.
Wang’s group took a different tack. Ligand-assisted displacement (LAD) chromatography incorporates chelating ligands in the mobile phase (in this case the classic EDTA) to enhance separation of metals. This is an idea dating to the 1950’s, but it hasn’t gained traction industrially – partly, as the researchers note, because there was “no general theory for predicting the formation of a constant pattern in LAD until 2018.” That year, theyreporteda method for predicting the conditions under which so-called “displacement trains” – the areas of pure substance which traverse the displacement chromatographic column and ultimately elute – can be predicted. Building on this achievement, they have now shown on a mixture of Nd, Pr, and Dy how two-zone LAD chromatography radically improves the productivity of neodymium, dysprosium, and praseodymium recycling at high purities. By isolating the metals at 99% purity from the first zone, then loading less pure bands onto a second zone for further separation, they recover 99% of the metals at 99% purity. Chelants such as EDTAand Cu2+ salts are used, and 95% of these can be recovered. Although a full life cycle assessment is not presented, the authors’ thorough economic models demonstrate the potential for this method to be profitable, and scalable.
Prospecting for rare earth metals today may not have the allure of a 19th century gold rush, but instead – to recycle a figure of speech – the new frontiers are electronic waste facilities and laboratories such as Professor Wang’s.