Rare Earth Elements and Their Uses

Rare earth elements (REE) consist of 15 elements known as the Lanthanide series. They appear in the Periodic Table from numbers 57 to 71. There are two additional elements, Scandium and Yttrium, numbers 21 and 39 respectively, which have a similar physiochemistry as the lanthanides and are usually included as a part of this group when REEs are discussed.

REE can be found all over the world, however the term ‘rare’ comes from the fact they usually occur in small quantities in the earth’s crust. Deposits do, however, exist in sufficient quantities to be commercially developed around the world. They usually occur in four geological deposits: alkaline igneous rocks, carbonatites, placer deposits (in rocks formed in sedimentary processes) with monazite-xenotime mineralisation and ion-adsorption clay deposits caused by weathering of rocks over local periods forming clays.

REE are divided into light (LREE) and heavy (HREE) based on their atomic weights. Scandium falls outside this classification even though it occurs in the same ore bodies as REE.

LREE include Lanthanum (57), Cerium (58), Praseodymium (59), Neodymium (60), Promethium (61) and Samarium (62).

HREE include Europium (63), Gadolinium (64), Terbium (65), Dysprosium (66), Holmium (67), Erbium (68), Thulium (69), Ytterbium (70) and Lutetium (71). Yttrium (39) has a lower number on the periodic table, however it shares the same properties as the HREE and is classified with them as it occurs in the same ore bodies.

While REE are silvery-white as an ore, when refined into oxide (REO) they take on a range of pastel colours from aqua, blue, lavender, lime-green, white, yellow and pink. This is because ‘… each lanthanide ion, with its unique electronic configuration, will have different excited states and different excitation energies. Therefore, their absorption spectra differ and they display different colours.’¹ Some REE have a biological role and all of them have low or no toxicity. Only Scandium has some level of toxicity.

There are hundreds of applications for REE and they play a huge role in the technologies used today. The information below outlines some of these uses. Due to the increasing demand for REE, companies are increasingly searching for new deposits. As deposits of less common REE are discovered, opportunities for the development of new technologies will evolve.

For more information on individual REE, the Royal Society of Chemistry website (www.rsc.org) is a useful guide for information on each element, its properties and uses.

Light REE

Lanthanum (La, 57) uses:

• a lanthanum nickel alloy to store hydrogen gas for hydrogen-powered vehicles
• in the anode of nickel metal hydride batteries used in hybrid cars
• an important component of mischmetal alloy (about 20%) ², used in ‘flints’ for cigarette lighters
• REE compounds containing La are used extensively in carbon lighting applications, such as studio lighting and cinema projection. They increase the brightness and give an emission spectrum similar to sunlight
• La (III) oxide is used in making special optical glasses to improve the properties and alkali resistance of the glass
• La salts are used in catalysts for petroleum refining
• the ion La3+ is used as a biological tracer for Ca2+
• radioactive La has been tested for use in treating cancer
• electronic devices • rocket propellants
• pyrophoric alloys
• camera and telescope lenses
• water treatment products
• phosphors in x-ray screens

Cerium (Ce, 58) uses:

• as the major component of mischmetal alloy
• the best-known use for this alloy is in ‘flints’ for cigarette lighters.
• This is because Ce will make sparks when struck (the only other element that does this is iron)
• Ce (Ill) oxide has uses as a catalyst
• in the inside walls of self-cleaning ovens to prevent the build-up of cooking residues
• in catalytic converters
• Ce (III) oxide nanoparticles potentially as an additive for diesel fuel to help it burn more completely and reduce exhaust emissions
• Ce sulfide as a pigment that is a rich red colour

Praseodymium (Pr, 59) uses:

• in a variety of alloys
• aircraft engines (it forms a high-strength alloy with magnesium)
• Mischmetal is an alloy containing about 5% Pr
• to make flints for cigarette lighters
• permanent magnets
• carbon arc electrodes for studio lighting and projection (along with other lanthanide elements)
• Pr salts are used to colour glasses, enamel and glazes with an intense and unusually clean yellow
• Pr oxide as a component of didymium glass (along with Nd) used in goggles for welders and glassmakers, because it filters out the yellow light and infrared (heat) radiation

Neodymium (Nd, 60) uses:

• in very strong permanent magnets, when combined with iron and boron to make an alloy, especially for electric vehicles and wind turbines
• Nd is a component, along with praseodymium, of didymium glass (see Pr above
• to colour glass delicate shades of violet, wine-red and grey.
• Nd is used in the glass for tanning booths, since it transmits the tanning UV rays but not the heating infrared rays.
• Nd glass is used to make lasers. These are used as laser pointers, as well as in eye surgery, cosmetic surgery and for the treatment of skin cancers.
• Nd oxide and nitrate are used as catalysts in polymerisation reactions.

Promethium (Pm, 61) uses:

• a little Pr is used in specialised atomic batteries. These are roughly the size of a drawing pin and are used for:
  • pacemakers
  • guided missiles
  • radios
• The radioactive decay of promethium is used to make a phosphor give off light and this light is converted into electricity by a solar cell
• Pr can also be used as a source of x-rays and radioactivity in measuring instruments

Samarium (Sa, 62) uses:

• mainly used in preparing samarium-cobalt alloy magnets for:
  • electric guitars
  • small motors
  • headphones
• remain magnetic at high temperatures, so are used in microwave applications
• to dope calcium chloride crystals for use in optical lasers
• in infrared absorbing glass for carbon arc-lamp electrodes
• as a neutron absorber in nuclear reactors
• Sa oxide in glass and ceramic
• in carbon arc lighting for studio lighting and projection, along with other REE
• a catalyst for dehydrogenating and dehydrating ethanol
• to treat cancer due to its radioactivity
• as an absorber in nuclear reactors

Heavy REE

Europium (Eu, 63) uses:

• in the printing of euro banknotes because it glows red under UV light, and forgeries can be detected by the lack of this red glow
• low-energy light bulbs contain a little Eu to give a more natural light, by balancing the blue (cold) light with a little red (warm) light
• excellent at absorbing neutrons, making it valuable in control rods for nuclear reactors
• Eu-doped plastic has been used as a laser material
• used in making thin super-conducting alloys

Gadolinium (Gd, 64) uses:

• as useful properties in alloys. As little as 1% Gd can improve the workability of iron and chromium alloys and their resistance to high temperatures and oxidation
• in alloys for making:
  • magnets
  • electronic components
  • data storage disks
• Gd compounds are useful in magnetic resonance imaging (MRI), particularly in diagnosing cancerous tumours
• Gd is excellent at absorbing neutrons, so is used in the core of nuclear reactors

Terbium (Tb, 65) uses:

• to dope calcium fluoride, calcium tungstate and strontium molybdate, all used in solid-state devices
• in low-energy lightbulbs and mercury lamps
• to improve the safety of medical x-rays by allowing the same quality image to be produced with a much shorter exposure time
• terbium salts are used in laser devices
• an alloy of terbium, dysprosium and iron lengthens and shortens in a magnetic field. This effect forms the basis of loudspeakers that sit on a flat surface, such as a window pane, which then acts as the speaker

Dysprosium (Dy, 66) uses:

• Dy’s main use is in alloys for Nd-based magnets. This is because it is resistant to demagnetisation at high temperatures. This property is important for magnets used in motors or generators. These magnets are used in wind turbines and electric vehicles, with demand for Dy growing rapidly
• Dysprosium iodide is used in halide discharge lamps. The salt enables the lamps to give out a very intense white light
• Dysprosium oxide-nickel cermet (a composite material of ceramic and metal) is used in nuclear reactor control rods. It readily absorbs neutrons, and does not swell or contract when bombarded with neutrons for long periods

Holmium (Ho, 67) uses:

• absorb neutrons, so it is used in nuclear reactors to keep a chain reaction under control
• used as alloys in some magnets

Erbium (Er, 68) uses:

• when alloyed with metals such as vanadium, Er lowers their hardness and improves their workability
• Er oxide is occasionally used in infrared absorbing glass, for example safety glasses for welders and metal workers
• added to glass to give it pink tinge
• to give colour to some sunglasses and imitation gems
• broadband signals, carried by fibre optic cables, are amplified by including erbium in the glass fibre

Thulium (Tm, 69) uses:

• when irradiated in a nuclear reactor, thulium produces an isotope that emits x-rays. A ‘button’ of this isotope is used to make a lightweight, portable x-ray machine for medical use.
• in lasers with surgical applications.

Ytterbium (Yb, 70) uses:

• in memory devices and tuneable lasers
• as an industrial catalyst
• increasingly to replace other catalysts considered to be too toxic and polluting

Lutetium (Lu, 71) uses:

• as a catalyst for cracking hydrocarbons in oil refineries

Yttrium (Y, 39) uses:

• in the making of microwave filters for radar
• Yttrium-aluminium garnet (YAG) is used in lasers that can cut through metals
• in white LED lights
• Yttrium oxide is added to glass used to make camera lenses to make them heat and shock resistant
• to make superconductors
• as an additive in alloys, such as aluminium and magnesium alloys to increases their strength
• the radioactive isotope yttrium-90 has medical uses. It can be used to treat some cancers, such as liver cancer
• has been used as a catalyst in ethene polymerisation

Other

Scandium (Sc, 21) uses:

• it has great potential because it has almost as low a density as aluminium and a much higher melting point. An aluminium-scandium alloy has been used in Russian MIG fighter planes, high-end bicycle frames and baseball bats
• Scandium iodide is added to mercury vapour lamps to produce a highly efficient light source resembling sunlight, to help television cameras reproduce colour well when filming indoors or at night-time
• the radioactive isotope scandium-46 is used as a tracer in oil refining to monitor the movement of various fractions. It can also be used in underground pipes to detect leaks