What are rare earth materials?
Rare earth elements (REE) is a term generally used to describe the elements in the lanthanide series—the second last row of the periodic table. However yttrium and scandium, both Group IIIB transition metals, are often included as REE since they are naturally found together with other lanthanides and have similar chemical properties.
Named for the rarity of the minerals they were recovered from, the term rare earth elements is actually a misnomer. REE are relatively abundant in the earth’s crust and have similar crystal abundance to copper (50 ppm) [1].
REE do not occur naturally as metallic elements but rather in different mineral forms including halides, carbonates, oxides, and phosphates. About 200 minerals contain REE but few are commercially significant [1]. The bastnӓsite, monazite and xenotime minerals are the primary sources of REE [1].
Why are rare earth materials important?
REE have been called the “vitamin” of metals, because a small amount can greatly enhance the properties of the metals they are used with (i.e. alloyed together) [2]. Lanthanum (La), neodymium (Nd), and dysprosium (Dy) are three of the most widely used REE. They have played a key role in the development of batteries and magnetic materials for energy applications.
Lanthanium has been used as an alternative to cadmium or lead currently used in rechargeable batteries [1].
Neodymium-iron-boron magnets are used in wind turbines. It’s estimated that 0.6-1.0 tons of neodymium magnets is used per megawatt of electricity generated [1]. The strength of the magnets used determines the amount of electricity generated. Neodymium magnets are preferred since they are more powerful than the alternatives.
Magnets sometimes lose their magnetism as they heat up. But the addition of dysprosium to magnets prevents this from happening, which is why they are frequently used in magnets [1].
The global production of REE is ~133,600 tons annually [3]. In 2010, the world demand for REE was ~136,100 tons [3]. Above-ground inventories made up the difference [3]. It is predicted that by 2015 the global demand for REE may rise to 210,000 tons per year [3].
REE resources are only found in a handful of countries (see Figure 1), totalling ~121 million tons of reserves in 2010 [4]. Thus concern over REE availability is not the abundance of REE resources but whether the supply or REE can expand to meet future demands [5].
Map showing the global distribution of REE resources [3]
Like most metals, the availability of REE is strongly dependent on production costs and environmental concerns/regulations. Production costs for heavy-REE like Dy is higher than light-REE like La or Nd since there’s a lower abundance in most deposits [1]. The relative difficulty in extracting each element also contributes to the production costs. Separating REE from each other is challenging and requires sophisticated methods because of similarities among REE [6].
In the past, plants have been closed due to environmental concerns such as the radioactivity of ores. For example, xenotime in Malaysian deposits typically contain 2% uranium and 0.7% thorium [1].
Obviously, the gap between demand and supply for REE has important economic and political implications as mentioned by Elisa Alonso and co-authors in [5] and in the news recently.
Outlook
While it may seem logical to substitute rare earth elements with another metal, it isn’t easy or feasible. REE are often used for their highly specific properties and substitutes are either unknown or provide inferior performance. Scientists have been researching alternatives for the neodymium-iron-boron magnet for the past 20 years without success [1].
Recycling is a more promising solution. Wind turbines and electric vehicles concentrate the use of REE in single parts [5]. So recovering constituent materials is possible [5 The feasibility of recycling will be largely dependent on the number of different materials used in the original products since separating REE out of these products can be costly and difficult [6].
However, the impact of recycling REE on the growing demand will only be significant in the long term because of the delay between consumption and recycling [5].
So I guess the future of clean energy technology really is a materials problem.
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[1] British Geological Survey Natural Environmental Research Council. (2010). Rare Earth Elements. Retrieved from http://www.bgs.ac.uk/downloads/start.cfm?id=1638
[2] Wu, C., Yu, D., Law, C., & Wang, L. (2004). Properties of lead-free solder alloys with rare earth element additions Materials Science and Engineering: R: Reports, 44 (1), 1-44 DOI: 10.1016/j.mser.2004.01.001
[3] Humphries, M. (2011). Rare Earth Elements: The Global Supply Chain. Retrieved from http://www.fas.org/sgp/crs/natsec/R41347.pdf
[4] Graedel, T. (2011). On the Future Availability of the Energy Metals Annual Review of Materials Research, 41 (1), 323-335 DOI: 10.1146/annurev-matsci-062910-095759
[5] Alonso, E., Sherman, A., Wallington, T., Everson, M., Field, F., Roth, R., & Kirchain, R. (2012). Evaluating Rare Earth Element Availability: A Case with Revolutionary Demand from Clean Technologies Environmental Science & Technology DOI: 10.1021/es203518d
[6] Zawisza, B., Pytlakowska, K., Feist, B., Polowniak, M., Kita, A., & Sitko, R. (2011). Determination of rare earth elements by spectroscopic techniques: a review Journal of Analytical Atomic Spectrometry, 26 (12) DOI: 10.1039/c1ja10140d
Basal Science Clarified 