Dhananjay Sant
Rare Earth Elements (REE) are a group of elements so-called as they were isolated as oxides from rare minerals back in the 18th-19th century. Acquiring technology for the extraction or enrichment of these elements economically is an ongoing technological challenge. The unique magnetic and optical properties of REE make them uncommon and lead to diverse applications that touch many aspects of modern life and culture (see Table 1).
Table 1: List of Rare earth elements and their applications. Letters and numbers in the bracket denote the symbol and atomic number for the rare earth elements in the Periodic Table.
| Rare Earth Elements | Applications |
| Scandium (Sc, 21) | Ceramics, lasers. |
| Yttrium (Y, 39) | Additive in alloys; making of microwave filters for radar and a catalyst in ethene polymerisation. Yttrium-aluminium garnet (YAG) is used in lasers that can cut through metals. |
| Lanthanum (La, 57) | Hybrid engines and metal alloy. |
| Cerium (Ce, 58) | Glass and glass polishing, phosphors, ceramics, catalysts, metallurgy, and the medical field. |
| Praseodymium (Pr, 59) | Magnets. |
| Neodymium (Nd, 60) | Lasers, glass colouring and tinting, dielectrics, and neodymium-iron-boron permanent magnets. |
| Promethium (Pm, 61) | Specialized atomic batteries. |
| Samarium (Sm, 62) | Samarium-cobalt permanent magnets. |
| Europium (Eu, 63) | Colour TVs, computer screens and fluorescent lamps, medical, surgical and biochemical fields. |
| Gadolinium (Gd, 64) | Host for x-ray cassettes and scintillated materials for computed tomography. |
| Terbium (Tb, 65) | Fluorescent lamps and as the high intensity green emitter used in projection televisions. |
| Dysprosium (Dy, 66) | High strength permanent magnets, hybrid engines, and ceramic. |
| Holmium (Ho, 67) | Used to create the highest known magnetic fields, lasers for microwave equipment, medical and dental fields. |
| Erbium (Er, 68) | In glass colouring, in lasers for medical and dental use, and in eye ware and decorative glassware, an amplifier for fibre optic data transfer, surgical applications. |
| Thulium (Tm, 69) | Making crystals and lasers medical area, production of portable x-ray sources, dental diagnosis; detecting defects in inaccessible mechanical and electronic components. |
| Ytterbium (Yb, 70) | Fibre optic technologies and in laser. |
| Lutetium (Lu, 71) | Best host for x-ray phosphors and as dopant. |
REE is a group of 17 metallic elements on the periodic table, 15 from lanthanides (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium) along with scandium and yttrium. REEs are further sub-grouped as Light Rare Earth Elements (LREE) from lanthanum to samarium and Heavy Rare Earth Elements (HREE) from europium to lutetium including scandium and yttrium.
The most abundant REEs are cerium, yttrium, lanthanum and neodymium. They have average crustal abundances that are similar to commonly used industrial metals such as chromium, nickel, zinc, molybdenum, tin, tungsten and lead. Thulium and lutetium are the two least abundant rare earth elements, and yet they each have an average crustal abundance that is nearly 200 times greater than the crustal abundance of gold.
Photo: Peggy Greb. courtesy www.commons.wikimedia.org
REEs occur within the atomic structures of most rock-forming minerals, but their concentration is very low. However, extractions of REEs are only economically viable from a set of few minerals referred to as strategic minerals. At least 245 individual REE-bearing minerals are recognized of which a few are listed in Table 2.
Table 2: Brief information on REE-bearing minerals (Name, composition and host rock) (modified from Balaram, 2018; Singh, 2020).
| Allanite / Orthite • (Ce,Y,Ca,Y)2(Al,Fe3+)3(SiO4)3OH • Sorosilicate group • Metamorphosed clay-rich sediments and felsic igneous rocks (granites, syenites, diorites and their pegmatites). Apatite /Fluorapatite • (Ca, REE, Na)5(PO4 )3(F,OH) • Phosphate group • Granite, pegmatites; hornfels – contact metamorphic rocks; igneous diabase; ore veins and nepheline syenite Bastnaesite • (La, Ce, Y) (CO3)F • Carbonate group • Hydrothermal and primary igneous. Granite, alkali syenites, pegmatites, carbonatites, contact metamorphic deposits, rarely in detrital deposits. Eudialyte • Na4(Ca, Ce, La)2(Fe2+,Mn2+,Y) Zr Si8O22(OH, Cl)2 • Complex trigonal cyclosilicate group • Exotic crystalline igneous rock Peralkaline alkali-feldspar granite / nepheline-syenite Fergusonite • (La,Ce,Nd,Y) NbO4 • Oxides group XO4 type • Pegmatite, gneissic rocks and alkaline rocks Gittinsite • CaZrSi2O7 • Sorosilicates group • Alkaline rocks namely syenite Imoriite • Y2(SiO4) (CO3) • Silico-carbonate group • Granitic pegmatite, veins Kainosite • Ca2(Y,Ce,La)2Si4O12(CO3).H2O • Cyclosilicates group • Granitic pegmatite Loparite • (Ce,La,Na,Ca, Sr) (Ti,Nb)O3 • Oxide group • Perovskite clinopyroxenite, Lamprophyre, Kimberlite | Monazite • (Ce, La,Nd, Th)PO4 • Phosphate group • Placer deposits Mosandrite • (Na,Ce,Ca)3Ca3La (Ti,Nb,Zr) (Si2O7)2(O,OH,F)4 • Sorosilicates silicates group • Shows signs of hydrothermal alteration Parisite • Ca(Ce/La/Nd/REE)2(CO3)3F2 • Carbonate group • Carbonate-alkaline complexes Pyrochlore Family • Ceriopyrochlore (Ce,Ca,Y)2(Nb,Ta)2O6(OH,F) • Plumbopyrochlore (Pb,Y,U,Ca)2-xNb2O6(OH) • Uranpyrochlore (U,Ca,Ce)2 (Nb,Ta)2 O6(OH,F) • Yttropyrochlore (Y,Na,Ca,U)1-2(Na,Ta,Ti)2(O,OH)7 • Oxide group • Granophyric granite Rinkite • (Ca,Y,)4Na(Na,Ca)2Ti(Si2O7)2(O,F)2 • Sorosilicates silicates group • Nepheline syenites to syenites and granitic rocks Steenstrupine • Na14Ce6Mn2Fe2(Zr,Th)(Si6O18)2(PO4)7.3H2O • Cyclosilicates group • Alkaline rocks Synchysite • Ca (La,Ce,Nd,Gd,Y) (CO3)2F • Carbonate group • Carbonatite complex Xenotime • (Y, Er)PO4 • Oxide group • Granite, pegmatite Zircon (Zr,Th,U,REE)SiO4 • Nesosilicates group • Granite pegmatites |
REE-bearing minerals occur in mineral groups such as silicates, oxides, and carbonates (fluoro-carbonates, and hydroxyl-carbonate). The host rocks of the majority of these minerals are (a) carbonatites, (b) peralkaline igneous systems, (c) magmatic magnetite-hematite bodies, (d) iron oxide-copper-gold deposits, (e) mafic gneiss (xenotime-monazite) (f) ion-absorption clay deposits, and (g) monazite-xenotime-bearing placer deposits. Important sources of REE are monazite-bearing placer deposits (Singh, 2020). The world’s main source for LREE is rock carbonatites, whereas ion-absorption clay deposits in southern China are the world’s primary source of HREE.
In India, REEs are produced from mineral monazite contained in heavy-mineral sands (placer deposits). REEs such as Ce, La, Nd, and Th are present within the atomic structure of monazite. The major deposits which contain monazite are from Kerala (Chavara barrier beach and Eastern Extension, Kollam district); Tamil Nadu (Manavalakurichi beach sand deposit, Kanyakumari district, Sathankulam Teri sand deposit, Ovari Manapadu Teri Sand deposit, Navaladi-Ovari Teri Sand deposit, Kuduraimoli Teri Sand deposit); Andhra Pradesh (Bhimunipatnam beach sand deposit, Kandivalasa beach sand deposit, Kalingapatanam beach sand deposit, Srikurman beach sand deposit, Bhavanapadu beach sand deposit); Odisha (Gopalpur beach sand deposit, Chhatrapur beach sand deposit, Brahmagiri beach sand deposit) (Indian Minerals Yearbook, 2019).
The Rare Earth Division of Indian Rare Earths Ltd. (IREL) and Kerala Minerals and Metals Ltd. (KMML) are the two government-owned producers of REEs. IREL produces monazite concentrate of 6,000 metric tons per year, whereas KMML produces 240 metric tons per year. India’s reserves of REE are estimated to be 3.1 million metric tons.
The wide-spread application of REEs in different industries as well as agriculture is alarmingly increasing leading to a constant increase in the concentration of these elements in the environment which will not only disturb the aquatic system but also the plant and soil ecosystem leading to a number of health issues (Balaram, 2019). Cerium (Ce), one of the most abundant rare earth elements, has wide applications in industry, agriculture, and medicine. However, there is evidence of long-term exposure to Ce causing health concerns, such as the development of oral cancer (He et al., 2021). Mixing of REEs with soil in and around the mines leads to soil toxicity and thereby increases environmental acidification (Pagano et al., 2015).
Conclusion
Rare earth elements are essential for green energy generation as well as for green technologies like LEDs and Electric Vehicles. However, we need new research and technological advancement for enriching REEs from minerals with medium to lower concentrations. India has upheld the task by narrowing the exploration of individual REEs in various geological settings and aims to become self-reliant in a few, if not all, industrial REEs, that drive the economy. Stricter environmental regulations framed for the mines and managing the surrounding areas are sure to open up new environmentally safe mines.
References
• Balaram, V., Rare earth elements: A review of applications, occurrence, exploration, analysis, recycling, and environmental impact. Geoscience Frontiers v.10, p.1285-1303 (2019).
• Scott M. McLennan and Stuart Ross Taylor., Geology, Geochemistry, and Natural Abundances of the Rare Earth Elements in The Rare Earth Elements: Fundamentals and Applications Edited by David A. Atwood, p. 1-20. (2012).
• Singh Y., Rare Earth Element Resources: Indian Context Society of Earth Scientists Series, pp.395, (2020)
• Indian Minerals Yearbook 2017, (Part-III: Mineral Reviews) 56th Edition, Rare Earths, Government of India Ministry of Mines, Indian Bureau of Mines. p.1-7 (2018)
• He B, Wang J, Lin J, Chen J, Zhuang Z, Hong Y, Yan L, Lin L, Shi B, Qiu Y, Pan L, Zheng X, Liu F and Chen F., Association Between Rare Earth Element Cerium and the Risk of Oral Cancer: A Case-Control Study in Southeast China. Front. Public Health v.9, 647120. (2021)
• Pagano, G., Guida, M., Tommasi, F.,Oral, R.Health effects and toxicity mechanisms of rare earth elements – Knowledge gaps and research prospects Ecotoxicology and Environmental Safety, V. 115, p. 40-48 (2015)
• Indian Minerals Yearbook 2019, (Part-III: Mineral Reviews) 58th Edition, Rare earths (advance release), Government Of India Ministry Of Mines, Indian Bureau Of Mines. p.1-24 (2020).
The author is with the Department of Geology, Faculty of Science at The Maharaja Sayajirao University of Baroda, Vadodara. He can be reached at sant.dhananjay-geology@msubaroda.ac.in.