Don't stop using rare earths – that is the message from me as a geologist. Although the headlines recently have been about supply restrictions, volatile prices, and a rush to find substitutes; on behalf of the geological community, I can reassure materials scientists that there are plenty of rare earths and a wide variety of potential new deposits. It is just a matter of patience (please) whilst the challenges of developing new deposits are overcome.
Rare earth elements (scandium, yttrium and the 15 elements of the lanthanide series) are used in a wide range of environmental technologies. Examples include low energy light bulbs, catalytic converters and magnets in large wind turbines [1]. Rare earths also turn up inside every computer, mobile phone, and TV. The applications of rare earths rely on distinct properties that result from their complex electronic structures and so substitution is difficult. Despite the fact that rare earths have so many applications, most of them require only small amounts of material and the global market, of about 130 000 tonnes per year, is small compared with more common metals, such as copper or zinc.
Rare earths and several other so-called ‘technology metals’ are required in such small quantities that they come from only a few mines, and one or two countries in the world. If supply were to be disrupted, even at just one mine, world supply could be seriously affected. The European Union published a report in June 2010 correlating uses and the difficulty of substitution with supply risk, and this led to the definition of fourteen materials as ‘critical’ for Europe [2]. Rare earths (counting as one of the fourteen) are high on the critical list. Other critical materials include platinum group elements, tungsten, niobium and tantalum. To compound the problem, recycling rates of many of these metals are low.
So, coming back to my headline, there are plenty of potential economic deposits of rare earths, in alkaline igneous rocks and carbonatites, hydrothermal veins, sedimentary placers, as by-products of fertilizer production, or reworking of mineral wastes, and, looking further to the future, from ocean floor deposits. So why then are there problems with supply? Well, the answer is that although there are many potential deposits to consider (822 according to one report [3]) it is not easy to bring them to production. Whilst China was supplying cheap and plentiful rare earths, there was little incentive for companies to explore or develop new deposits, or even to keep running existing mines, and we have reached a stage where China supplies 97 % of the world's requirement. Following the restriction of exports from China, and the rise in prices, there are now hundreds of rare earth exploration projects, and two mines, at Mountain Pass, USA and Mt Weld, Western Australia, on their way to full production. Mountain Pass was for many years the World's main supplier of rare earths; Mt Weld is a new mine.
Exploration and development teams looking for new deposits have a number of challenges to overcome. Challenges in addition to the usual technical, financial, environmental, and social constraints of setting up a new mine! The first is that although the rare earths form a series, most deposits contain large amounts of the light rare earths, lanthanum and cerium, which are as abundant in the crust as copper [4]. Finding geological environments that produce high concentrations of the less abundant, higher atomic number ‘heavy rare earths’ is much more difficult. Another geological problem is that individual rare earths do not form minerals (let alone ore deposits) of their own. For example, dysprosium may be particularly valuable at the moment but there are no known dysprosium minerals. Instead, dysprosium sits alongside yttrium, which is far more abundant, in other minerals such as xenotime (YPO4), or is adsorbed in small amounts onto clays with other similar heavy rare earths. A third challenge is that the radioactive elements thorium and uranium often substitute into rare earth minerals. The handling of radioactive ores or the generation of radioactive waste is not desirable. Some deposits of the rare earth mineral, monazite, are not mined for this reason. Predicting which geological environments will concentrate rare earths but select against thorium and uranium is an important role for geologists. Moving on to mineralogy, there is the challenge that some deposits contain minerals that have not been commercially exploited before. I now have an excuse to list some exotic sounding mineral names, here goes: ancylite, eudialyte, and steenstrupine, all require new metallurgical processes for beneficiation and extraction. The subsequent separation of the rare earths from each other is also a difficult process; some methods contain 1000 steps! Innovation here is also crucial to widening the number of potential suppliers.
Few economic geologists have studied such ‘strange’ elements as the rare earths. However, we now find materials scientists are using a wider range of these kinds of elements in new technologies. It is timely to pay more geological attention to a wider range of elements. The Natural Environment Research Council has published a Theme Action Plan that includes security of supply of minerals and will be encouraging more research. Elements such as the rare earths are, however, unlikely to secure the interest of the multinational mining companies. The markets are still too small. Support is coming from higher up in the supply chain, with more vertical integration from mines to manufacturers, encouraging links from geologists through to materials scientists.
Further Reading
[1] Rare Earth Elements Mineral Profile (2011), British Geological Survey, Keyworth, Nottingham, UK,www.mineralsUK.com, 54 pp.
[2] Critical raw materials for the EU. (2010) The ad-hoc Working Group sub-group of the Raw Materials Supply Group, European Commission, June 2010, 85 pp. http://ec.europa.eu/enterprise/policies/raw-materials/files/docs/report-b_en.pdf
[3] Orris, G.J. and Grauch, R.I. (2002) Rare Earth Element Mines, Deposits, and Occurrences USGS Open-File Report 02-189.
[4] Geological Society (2011) Rare Earth Elements Briefing Note 13 pp www.geolsoc.org.uk/ree