Rare earth elements
Rare earth elements (REEs) are a group of 17 chemically similar elements, specifically atomic numbers 57 to 71, known for their unique properties and extensive industrial applications. These elements, which include lanthanum, cerium, neodymium, and others, are primarily found in minerals such as monazite and bastnäsite. They are more concentrated in continental rocks compared to ocean basins, with significant deposits located in countries like China, the United States, Australia, and Brazil.
REEs are essential in various industries, serving functions in glass polishing, petroleum refining, catalysts, and electronics, including their use in colorants for displays and high-performance magnets. They do not occur as standalone elements but instead are found in mixtures within mineral deposits. Historically, REEs were first separated and identified in the late 18th and 19th centuries, paving the way for their commercial extraction and utilization today.
Despite their name, rare earth elements are relatively abundant in the Earth's crust, though economically viable concentrations are less common. Their extraction involves complex processes, including magnetic separation and chemical leaching, making their supply chain a focus of interest given their critical role in modern technology.
Rare earth elements
Where Found
Mixtures of the rare earth elements are present, but only in small amounts, in most rocks of the Earth. The rare earth elements are more concentrated in rocks of the continents than in those of the ocean basins. The minerals monazite, a phosphate mineral, and bastnäsite, a fluorine-carbonate mineral, form the main ores for the rare earth elements with lower atomic numbers. Xenotime, another phosphate mineral, is mined for its concentration of the rare earth elements with higher atomic numbers. The largest sources of rare earth elements are from bastnäsites mined in China and the United States. Monazite deposits are found in Australia, Brazil, Chile, India, Malaysia, South Africa, Sri Lanka, Thailand, and the United States.
Primary Uses
Mixtures of the rare earth elements are used for breaking down in to form more gasoline, to remove impurities from iron and steel, as polishing materials, for carbon arcs, and in metallurgy. Pure rare earth elements are used as coloring agents.
Technical Definition
The rare earth elements (abbreviated REE), or lanthanide elements, are a group of elements from atomic numbers 57 to 71. Their atomic weights range from 138.91 to 174.99. This large group of elements is grouped together because they have similar chemical properties. The names of the rare earth elements, from low to high atomic numbers, are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. In addition, the elements scandium and yttrium (atomic numbers 21 and 30 respectively) are sometimes included with the rare earth elements because they have chemical properties similar to those of the rare earth elements. The of the pure metals ranges from 5.23 to 9.84 grams per cubic centimeter. Melting points for the metals range from 798° to 1,663° Celsius.
Description, Distribution, and Forms
The rare earth elements, because of their similar chemical properties, do not occur as separate individual elements in minerals. Rather, they are fractionated in similar ways within or on the Earth. The rare earth elements are not normally soluble in water, so they are not transported in solution by natural waters.
The rare earth elements are widely distributed in the rocks of the world. The concentrations of some of the rare earth elements are as high as those of copper or zinc. For example, the dark, fine-grained rocks composing much of the ocean floor (called basalts) contain about 3 to 5 parts per million lanthanum, whereas and rocks on the continents typically contain 20 to 100 parts per million lanthanum. The rare earth elements are lowest in concentration in carbonate rocks such as limestone.
History
A now called gadolinite was discovered by Johan Gadolin, who subsequently separated an “element” called yttria from gadolinite in 1796. Later it was discovered that yttria actually consisted of a concentration of the heavy rare earth elements. Another “element” called ceria was separated in the early nineteenth century; ceria was later discovered to consist of a concentration of the light rare earth elements.
By the mid-nineteenth century, individual rare earth oxides were separated from yttria and ceria by a series of chemical separations and identified after analytical techniques such as the spectrograph were developed.
Obtaining Rare Earth Elements
Monazite and the associated xenotime are mined from beach sands in Brazil, India, Australia, South Carolina, South Africa, and Russia. Monazite is weakly magnetic and may be separated from the non-ore minerals by magnetic separation. Bastnäsite is mined in Africa, China, and the United States. It occurs in large amounts at Mountain Pass in California mixed with the non-ore minerals quartz, barite, and calcite. The is crushed, and bastnäsite is concentrated by flotation. The rare earth elements are further concentrated by heating and with hydrochloric acid. Minor amounts of the rare earth elements are also produced as by-products from other ore processing, such as uranium production.
Uses of Rare Earth Elements
Rare earth elements have a large variety of end uses, in glass-polishing agents, ceramics, catalytic converters, computer monitors (phosphors), lighting, radar, televisions, X-ray films, chemicals, petroleum-refining catalysts, pharmaceuticals, magnets, metallurgy, and laser and scintillator crystals.
Pure europium mixed with yttrium oxides, for example, produces an intense red fluorescence, so the mixture is used in television screens. Pure lanthanum is used to make quality glass for lenses. The rare earth elements are also used for X-ray screens, high-quality magnets, artificial diamonds, and superalloys.
Bibliography
Brouziotis, Antonios Apostolos, et al. "Toxicity of Rare Earth Elements: An Overview on Human Health Impact." Frontiers in Environmental Science, vol. 10, 6 Sept. 2022, doi.org/10.3389/fenvs.2022.948041. Accessed 6 Jan. 2024.
Delfrey, Keith N., ed. Rare Earths: Research and Applications. New York: Nova Science Publishers, 2008.
Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Gschneidner, Karl A., ed. Industrial Applications of Rare Earth Elements: Based on a Symposium Sponsored by the Division of Industrial and Engineering Chemistry at the Second Chemical Congress of the North American Continent (180th ACS National Meeting), Las Vegas, Nevada, August 25-26, 1981. Washington, D.C.: American Chemical Society, 1981.
Kogel, Jessica Elzea, et al., eds. “Rare Earth Elements.” Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.
Krebs, Robert E. “Lanthanide Series (Rare-Earth Elements): Period 6.” In The History and Use of Our Earth’s Chemical Elements: A Reference Guide. 2d ed. Illustrations by Rae Déjur. Westport, Conn.: Greenwood Press, 2006.
Pecharsky, Vitali, K. and Karl A. Gschneider, Jr. "Rare-Earth Element." Britannica, 21 Nov. 2024, www.britannica.com/science/rare-earth-element. Accessed 8 Jan. 2024.