Rare earth elements consist of a group of 15 metals. In most cases and usage patterns in the modern economy, these 15 elements are oxides. The names of the elements are Cerium, Dysprosium, Erbium, Europium, Gadolinium, Holmium, Lanthanum, Lutetium, Neodymium, Praseodymium, Samarium, Terbium, Thulium, Ytterbium, and Yttrium.
The bulk of the world’s supply of rare earth elements comes from the mineral bastnasite. Bastnasite is a mixed lanthanide fluoro-carbonate mineral (Ln F CO3) that is found in rocks called carbonatites.
Carbonatites are igneous carbonate rocks. Specifically, this means that the rock masses contain more than 50% carbonate minerals, and cooled from a melt. Despite extensive research, no one is entirely certain about the origins of carbonatites. The general viewpoint is that carbonatites are carbonate rocks that were buried deep enough to melt via metamorphic processes or in the presence of igneous intrusions. Some geochemists have speculated that carbonatites can form when mantle rocks melt in the presence of carbon dioxide. There is almost always significant calcite in carbonatites.
Most carbonatites are intrusive igneous rocks. Structurally, they occur as volcanic plugs, dikes and cone sheets. Carbonatites often occur as smaller components of large igneous intrusions of silicate rocks, such as nepheline syenite. In these cases the general term is to refer to a “carbonatite complex.”
According to the geological literature, there are about 330 known occurrences of carbonatites worldwide, but almost all are small and noncommercial. There are only a few carbonatite deposits of commercial significance in the world.
Currently there are two deposits that are up and running. One is at Mountain Pass, California and operated by Molycorp, a subsidiary of Chevron (formerly owned by Unocal). The other major deposit is at Baiyun Ebo in Inner Mongolia, China. Mount Weld, Australia is also a large commercial body in development stages.
Major Uses of Rare Earth Elements
Lanthanum comes from the mineral bastnasite, and is extracted via a method called “solvent extraction.” Lanthanum is a strategically important rare earth element due to its activity in catalysts that are critical in petroleum refining. By one estimate, lanthanum “cracking-agents” increase refinery yield by as much as 10%, while reducing overall refinery power consumption.
Cerium is the most abundant of the rare earth elements. Cerium is critical in the manufacture of environmental protection and pollution-control systems, from automobiles to oil refineries. Cerium oxides, and other cerium compounds, go into catalytic converters and larger-scale equipment to reduce the sulfur oxide emissions. Cerium is a diesel fuel additive for micro-filtration of pollutants, and promotes more complete fuel combustion for more energy efficiency.
Neodymium is a critical component of strong permanent magnets. Cell phones, portable CD players, computers and most modern sound systems would not exist in their current form without using neodymium magnets. Neodymium-Iron- Boron (NdFeB) permanent magnets are essential for miniaturizing a variety of technologies. These magnets maximize the power/cost ratio, and are used in a large variety of motors and mechanical systems.
Europium offers exceptional properties of photon emission. When it absorbs electrons or UV radiation, the europium atom changes energy levels to create a visible, luminescent emission. This emission creates the perfect red phosphors used in color televisions and computer screens around the world. Europium is also used in fluorescent lighting, which cuts energy use by 75% compared to incandescent lighting. In the medical field, europium is used to tag complex biochemical agents which helps to trace these materials during tissue research.
Praseodymium comprises just 4% of the lanthanide content of bastnasite, but is used as a common coloring pigment. Along with neodymium, praseodymium is used to filter certain wavelengths of light. So praseodymium finds specific uses in photographic filters, airport signal lenses, welder’s glasses, as well as broad uses in ceramic tile and glass (usually yellow). When used in an alloy, praseodymium is a component of permanent magnet systems designed for small motors. Praseodymium also has applications in internal combustion engines, as a catalyst for pollution control.
Yttrium is rare in bastnasite, so is usually recovered from even more obscure minerals and ores. Still, almost every vehicle on the road contains yttriumbased materials that improve the fuel efficiency of the engine. Another important use of yttrium is in microwave communication devices. Yttrium- Iron-Garnets (YIG) are used as resonators in frequency meters, magnetic field measurement devices, tunable transistors and Gunn oscillators. Yttrium goes into laser crystals specific to spectral characteristics for high-performance communication systems.
Other Rare Earth Elements
Most of the remaining lanthanides fall into the group known as the “heavies” and include: Samarium, Gadolinium, Dysprosium, Terbium, Holmium, Erbium, Thulium, Ytterbium, and Lutetium.
Samarium has properties of spectral absorption that make it useful in filter glasses that surround neodymium laser rods.
Gadolinium offers unique magnetic behavior. Thus this element is at the heart of magneto-optic recording technology, and other technology used in handling computer data.
Dysprosium is a widely used rare earth element that helps to make electronic components smaller and faster.
Terbium is used in energy efficient fluorescent lamps. There are various terbium metal alloys that provide metallic films for magnetooptic data recording.
Holmium is exceedingly rare and expensive. Hence it has few commercial uses.
Erbium has remarkable optical properties that make it essential for use in long-range fiber optic data transmission.
Thulium is the rarest of the rare earth elements. Its chemistry is similar to that of Yttrium. Due to its unique photographic properties, Thulium is used in sensitive X-ray phosphors to reduce X-ray exposure.
Ytterbium resembles Yttrium in broad chemical behavior. When subject to high stresses, the electrical resistance of the metal increases by an order of magnitude. So ytterbium is used in stress gauges to monitor ground deformations caused, for example, by earthquakes or underground explosions.
Lutetium, the last member of the Lanthanide series is, along with thulium, the least abundant. It is recovered, by ion-exchange routines, in small quantities from yttrium-concentrates and is available as a high-purity oxide. Cerium-doped lutetium oxyorthosilicate (LSO) is currently used in detectors in positron emission tomography (PET).
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