Lanthanum

Lanthanum () is a chemical element with the symbol La and atomic number 57. Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and other rare earth elements. Lanthanum is a malleable, ductile, and soft metal that oxidizes rapidly when exposed to air. It is produced from the minerals monazite and bastnäsite using a complex multistage extraction process. Lanthanum compounds have numerous applications as catalysts, additives in glass, carbon lighting for studio lighting and projection, ignition elements in lighters and torches, electron cathodes, scintillators, and others. Lanthanum carbonate (La₂(CO₃)₃) was approved as a medication against renal failure.

Properties

Physical properties

Lanthanum is a soft, malleable, silvery white metal which has hexagonal crystal structure at room temperature. At 310 °C, lanthanum changes to a face-centered cubic structure, and at 865 °C into a body-centered cubic structure.) and is therefore used as in elemental form only for research purposes. For example, single La atoms have been isolated by implanting them into fullerene molecules. If carbon nanotubes are filled with those lanthanum-encapsulated fullerenes and annealed, metallic nanochains of lanthanum are produced inside carbon nanotubes.

Chemical properties

Lanthanum exhibits two oxidation states, +3 and +2, the former being much more stable. For example, LaH₃ is more stable than LaH₂.

:2 La(s) + 3 H₂SO₄ (aq) → 2 La³⁺(aq) + 3 SO (aq) + 3 H₂ (g)

Lanthanum combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming binary compounds.

Monazite (Ce, La, Th, Nd, Y)PO₄, and bastnäsite (Ce, La, Y)CO₃F, are the principal ores in which lanthanum occurs, in percentages of up to 25 to 38 percent of the total lanthanide content. In general, there is more lanthanum in bastnäsite than in monazite. Until 1949, bastnäsite was a rare and obscure mineral, not even remotely contemplated as a potential commercial source for lanthanides. In that year, the large deposit at the Mountain Pass rare earth mine in California was discovered. This discovery alerted geologists to the existence of a new class of rare earth deposit, the rare-earth bearing carbonatite, other examples of which soon surfaced, particularly in Africa and China.

Production

Lanthanum is most commonly obtained from monazite and bastnäsite. The mineral mixtures are crushed and ground. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3-4. Thorium precipitates out of solution as hydroxide and is removed. After that, the solution is treated with ammonium oxalate to convert rare earths to their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO₃. Lanthanum is separated as a double salt with ammonium nitrate by crystallization. This salt is relatively less soluble than other rare earth double salts and therefore stays in the residue.

Modern uses of lanthanum include: One material used for anodic material of nickel-metal hydride batteries is La(Ni_3.6Mn_0.4Al_0.3Co_0.7. Due to high cost to extract the other lanthanides a mischmetal with more than 50% of lanthanum is used instead of pure lanthanum. The compound is an intermetallic component of the AB₅ type. As most hybrid cars use nickel-metal hydride batteries, massive quantities of lanthanum are required for the production of hybrid automobiles. A typical hybrid automobile battery for a Toyota Prius requires 10 to 15 kg (22-33 lb) of lanthanum. As engineers push the technology to increase fuel mileage, twice that amount of lanthanum could be required per vehicle. Hydrogen sponge alloys can contain lanthanum. These alloys are capable of storing up to 400 times their own volume of hydrogen gas in a reversible adsorption process. Heat energy is released every time they do so; therefore these alloys have possibilities in energy conservation systems. Mischmetal, a pyrophoric alloy used in lighter flints, contains 25% to 45% lanthanum. Lanthanum oxide and the boride are used in electronic vacuum tubes as hot cathode materials with strong emissivity of electrons. Crystals of LaB₆ are used in high brightness, extended life, thermionic electron emission sources for electron microscopes and Hall effect thrusters. Lanthanum fluoride (LaF₃) is an essential component of a heavy fluoride glass named ZBLAN. This glass has superior transmittance in the infrared range and is therefore used for fiber-optical communication systems. Cerium doped lanthanum bromide and lanthanum chloride are the recent inorganic scintillators which have a combination of high light yield, best energy resolution and fast response. Their high yield converts into superior energy resolution; moreover, the light output is very stable and quite high over a very wide range of temperatures, making it particularly attractive for high temperature applications. These scintillators are already widely used commercially in detectors of neutrons or gamma rays. Carbon lighting applications, especially by the motion picture industry for studio lighting and projection. These applications consume about 25% of the rare-earth compounds produced. Small amounts of lanthanum added to steel improves its malleability, resistance to impact and ductility. Whereas addition of lanthanum to molybdenum decreases its hardness and sensitivity to temperature variations. Lanthanum oxide additive to tungsten is used in gas tungsten arc welding electrodes, as a substitute for radioactive thorium. Various compounds of lanthanum and other rare-earth elements (oxides, chlorides, etc.) are components of various catalysis, such as petroleum cracking catalysts. Lanthanum-barium radiometric dating is used to estimate age of rocks and ores, though the technique has limited popularity. Lanthanum carbonate was approved as a medication (Fosrenol, Shire Pharmaceuticals) to absorb excess phosphate in cases of end-stage renal failure. Lanthanum fluoride is used in phosphor lamp coatings. Mixed with europium fluoride, it is also applied in the crystal membrane of fluoride ion-selective electrodes.

Biological role

Lanthanum has no known biological role. The element is not absorbed orally, and when injected its elimination is very slow. Lanthanum carbonate was approved as a medication named Fosrenol to absorb excess phosphate in cases of end-stage renal failure.

Precautions

Lanthanum has a low to moderate level of toxicity and should be handled with care. In animals, the injection of lanthanum solutions produces hyperglycaemia, low blood pressure, degeneration of the spleen and hepatic alterations.

See also

Lanthanum compounds

References

Books

The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium, by R.J. Callow, Pergamon Press 1967

Extractive Metallurgy of Rare Earths, by C.K. Gupta and N. Krishnamurthy, CRC Press 2005

Nouveau Traite de Chimie Minerale, Vol. VII. Scandium, Yttrium, Elements des Terres Rares, Actinium, P. Pascal, Editor, Masson & Cie 1959

Chemistry of the Lanthanons, by R.C. Vickery, Butterworths 1953

External links

WebElements.com – Lanthanum

Category:Chemical elements Category:Lanthanides Category:Lanthanum Category:Reducing agents