Rare Earth Elements
The lanthanide series of rare earth elements (REEs), whose atomic numbers range from 57 through 71, are listed in Box D-1. Scandium and yttrium are also often included in lists of REEs. All except promethium occur in nature. Lanthanum through samarium, as well as scandium and yttrium, are often termed the light rare earth elements (LREEs) and europium through lutetium are the heavy rare earth elements (HREEs).1
The term “rare earths” is a misnomer since REEs are fairly abundant in Earth’s crust, although when they were originally discovered they were thought to be scarce. “Earth” is an obsolete term for oxide, since they were commonly found as oxides.
The rare earths are no longer mined in the United States. A rare earth fluocarbonate, bastnasite, used to be mined and processed near Mountain Pass, California, by Molycorp. However, concentrates, intermediate compounds, and some oxides were available in 2006, and the United States consumed them internally and even exported them. It is estimated that the rare earths consumed in this country every year have a value of more than $1 billion. In 2005, rare earths were estimated to be used as automotive catalytic converters (32 percent); metallurgical additives and alloys (21 percent); glass polishing and ceramics (14 percent); rare earth phosphors for lighting, televisions, computer monitors, radar, and x-ray intensifying film (10 percent); petroleum refining catalysts (8 percent); permanent
For information on the REEs see http://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/.
magnets (2 percent); and other uses (13 percent) (USGS, 2007). The little recycling that is done is mostly carried out on permanent magnet scrap. In the aggregate between 2002 and 2005 the countries that supplied REEs to the world market, along with the share of the demand they satisfied, are as follows: China (76 percent); France (9 percent); Japan (4 percent); Russia (3 percent); and other (8 percent). In 2006 imports to and exports from the United States increased over 2005, and this trend may be expected to continue in the future.
Rare earth compounds are used in automotive catalytic converters and other applications. For instance, cerium compounds are used mainly for automotive catalytic converters, glass polishing, and glass additives. Yttrium compounds are used in fiber optics, lasers, oxygen sensors, phosphors for fluorescent lighting, color televisions, electronic thermometers, x-ray intensifying screens, pigments, super-conductors, and other items. Mixed rare earth compounds and rare earth metals and their alloys are used in permanent magnets, base-metal alloys, superalloys, pyrophoric alloys, lighter flints, and armaments; the amounts used have increased lately. The use of rare earth chlorides for fluid cracking catalysts in oil refining has declined.
Molycorp has shut down mining at the Mountain Pass location because of wastewater disposal problems but was expected, at the time of writing, to restart processing stockpiled ore late in 2007. Mining may not start for a couple of years (Thorne, 2007, personal communication).
REEs do not need to be as concentrated as most minerals for economic mining. The best source minerals for them are bastnasite and monazite. The largest deposits of bastnasite are those at Baiyun Obo (Inner Mongolia, China) and Mountain Pass, California, with the later already having been mentioned. Monazite deposits occur in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, Thailand, and the United States. Other minerals containing rare earths include apatite, cheralite, eudialyte, secondary monazite, loparite, phosphorites, rare-earth-bearing (ion adsorption) clays, spent uranium solutions, and xenotime. It has been speculated
Actinide Series of Rare Earth Elements
that undiscovered deposits are large compared to expected demand. Exploration for rare earths is continuing in Canada near Yellowknife and in Quebec around Strange Lake (Hedrick, 2007).
Sometimes the elements with atomic numbers 89 through 103 are also considered to be REEs. Listed in Box D-2, they are referred to as actinides. Actinium, thorium, protactinium, and uranium are the only elements of the actinide series that occur in nature. The rest are manmade and obtained by bombarding the naturally occurring actinides with neutrons or heavy particles in cyclotrons; they are known as transuranium elements.
Actinium is found along with uranium, a product of the latter’s decay. It is used in research as a tracer element and as a thermoelectric power source in satellites.
Protactinium is scarce and used only for research. It is highly radioactive and toxic. It occurs in very small amounts in pitchblende and in some ores in the Democratic Republic of the Congo.
Thorium occurs naturally, primarily in monazite but also in other minerals; it is used as a fuel in some nuclear reactors. It is also used to alloy magnesium and coat tungsten and in tungsten-arc welding, heat-resistant ceramics, and sometimes as a shield against radiation. Thorium oxide is used in gas lamp mantles, high-temperature crucibles, lenses, and as a catalyst for oil refining and other chemical reactions. The largest known reserves of thorium are in Australia, India, the United States, Norway, Canada, South Africa, and Brazil.
Uranium occurs naturally, primarily as U238 (over 99 percent) but also as U235 and in very small amounts as U234. Since the half-life of U238 is 4.47 billion years, it is useful for dating materials. The minerals in which it occurs include uraninite (the most common), autunite, uranophane, tobernite, and coffinite. It is also found in some monazite sands. Uranium also occurs in seawater, and in the 1980s the Japanese proved that it could be extracted from the water (JAERI, 1999).
Uranium is fissile; upon bombardment with slow neutrons its isotope U235 becomes the very short-lived U236, which instantaneously divides into two smaller
nuclei, releasing its binding energy along with more neutrons. If these neutrons are absorbed by other U235 nuclei, then a chain reaction occurs. If the excess neutrons are not absorbed, slowing down the reaction, an explosion results. The first nuclear bomb was based on this principle of nuclear fission.
Depleted uranium alloyed with small amounts of other elements is used for high-density penetrators. This use has been criticized because of the residual uranium left in the soil. It serves as counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and in inertial guidance devices. It is also the fuel source for nuclear submarines.
The main civilian use of uranium is in nuclear reactors for power generation. Before its radiation characteristics were understood, uranium was used in yellow glass, pottery dyes, and photographic film.
Canada, Australia, and Kazakhstan are the largest producers of uranium, with the largest reserves being in Australia and Canada. In the United States it occurs in the Colorado Plateau, which spans Colorado, Utah, New Mexico, and Arizona.
Hedrick, J., Personal communication, 2007.
U.S. Geological Survey (USGS). 2007. Mineral Commodity Summaries 2007.