History
Lanthanum was isolated from cerium nitrate in 1839 by Mosander. The element in its oxide form was called “lanthana” meaning “hidden”. Although the electron configuration of the element shows a vacant 4f orbital and, therefore, does not belong to the “true rare-earth elements,” the metal exhibits striking similarities to other rare-earths. In nature, lanthanum never occurs in free state and is always found associated with other rare-earth metals. The principal minerals are monazite and bastnasite. Its concentration in the earth’s crust is estimated to be 30 mg/kg. Lanthanum as a pure metal has limited applications. However, in the form of alloys, the metal has several metallurgical applications. When alloyed with iron, chromium, nickel, and molybdenum, it improves resistance of these metals to oxidation. It also improves the impact strength, fluidity, ductility and other mechanical properties of the alloys. The pure metal is used only for research.
Production Methods
Extraction of lanthanum from monazite is discussed below first, followed by that from bastnasite. The mineral mixtures are crushed and ground.After its separation the mineral is treated with hot concentrated sulfuric acid, which converts thorium, lanthanum, and rare-earth metals present into their sulfates. Alternatively, thorium may be precipitated as pyrophosphate by adding sodium pyrophosphate to the acid solution. The solution after removal of thorium is treated with ammonium oxalate. This converts all lanthanide elements in the ore into their insoluble oxalate salts. The oxide mixture is, therefore, treated with dilute nitric acid to dissolve lanthanum oxide and other rare-earth oxides to separate them from cerium. Lanthanum is separated from this cerium-free rareearth mixture as a double salt with ammonium nitrate by crystallization. The lanthanum-ammonium double salt is relatively less soluble than other rareearth double salts and stays in the most insoluble fraction.
Chemical Properties
solid
Physical properties
Lanthanum is a soft silvery-white metal that, when cut with a knife, forms an oxide withthe air (tarnishes) on the exposed area. It is the most reactive of the elements in the series. Itreacts slightly with cold water but rapidly with hot water, producing hydrogen gas (H
2) andlanthanum oxide (La
2O
3). It directly interacts with several other elements, including nitrogen,boron, the halogens, carbon, sulfur, and phosphorus.
Its melting point is 918°C, its boiling point is 3,464°C, and its density is 6.15 g/cm
3.
Isotopes
There are 49 isotopes of lanthanum. One, La-139, is stable and makes up99.910% of the known amount found on Earth. Another isotope has such a long halflifethat is considered stable: with a half-life of 1.05×10+11 years, La-138 makes up just0.090% of the known abundance on Earth. All the other isotopes are radioactive andhave half-lives ranging from 150 nanoseconds to several thousand years.
Origin of Name
From the Greek word lanthanein, meaning “to be hidden.”
Occurrence
The main ore in which lanthanum is found is monazite sands, and it is also found in themineral bastnasite. Monazite sands contain all of the rare-earth elements as well as some elementsthat are not rare-earths. Its ores are found in South Africa, Australia, Brazil, and Indiaand in California, Florida, and the Carolinas in the United States.
The prices of lanthanide elements are somewhat reasonable and are less than gold per kilogram.(Gold is about $1,800 per kg.) Cesium (Ce), which is relatively common, is often alloyedwith La, Nd, and Pr and iron to form misch metal. This alloy has several uses based on its uniqueability to spark when scratched. The most common use is as flints for cigarette lighters.
Lanthanum is the fourth most abundant of the rare-earths found on the Earth. Its abundanceis 18 ppm of the Earth’s crust, making it the 29th most abundant element on Earth. Itsabundance is about equal to the abundance of zinc, lead, and nickel, so it is not really rare.Because the chemical and physical properties of the elements of the lanthanide series are sosimilar, they are quite difficult to separate. Therefore, some of them are often used together asan alloy or in compounds.
Characteristics
Lanthanide, as a pure metal, is difficult to separate from its ores, and it is often mixed withother elements of the series. It is mostly obtained through an ion-exchange process from the sandsof the mineral monazite, which can contain as much as 25% lanthanum as well as the oxides ofseveral other elements of the series. The metal is malleable and ductile and can be formed intomany shapes. Lanthanum is considered the most basic (alkaline) of the rare-earth elements.
History
Mosander in 1839 extracted a new earth lanthana, from impure cerium nitrate, and recognized the new element. Lanthanum is found in rare-earth minerals such as cerite, monazite, allanite, and bastnasite. Monazite and bastnasite are principal ores in which lanthanum occursin percentages up to 25 and 38%, respectively. Misch metal, used in making lighter flints, contains about 25% lanthanum. Lanthanum was isolated in relatively pure form in 1923. Ion-exchange and solvent extraction techniques have led to much easier isolation of the so-called “rare-earth” elements. The availability of lanthanum and other rare earths has improved greatly in recent years. The metal can be produced by reducing the anhydrous fluoride with calcium. Lanthanum is silvery white, malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. It oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, and hot water attacks it much more rapidly. The metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur, and with halogens. At 310°C, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865°C it again transforms into a body-centered cubic structure. Natural lanthanum is a mixture of two isotopes, one of which is stable and one of which is radioactive with a very long halflife. Thirty other radioactive isotopes are recognized. Rareearth compounds containing lanthanum are extensively used in carbon lighting applications, especially by the motion picture industry for studio lighting and projection. This application consumes about 25% of the rare-earth compounds produced. La2O3 improves the alkali resistance of glass, and is used in making special optical glasses. Small amounts of lanthanum, as an additive, can be used to produce nodular cast iron. There is current interest in hydrogen sponge alloys containing lanthanum. These alloys take up to 400 times their own volume of hydrogen gas, and the process is reversible. Heat energy is released every time they do so; therefore these alloys have possibilities in energy conservation systems. Lanthanum and its compounds have a low to moderate acute toxicity rating; therefore, care should be taken in handling them. The metal costs about $2/g (99.9%).
Uses
Lanthanum is the first element in the rare earth or Lanthanide series. It is the model for all the other trivalent rare earths. After Cerium, it is the second most abundant of the rare earths.
Lanthanum-rich Lanthanide compounds have been used extensively for cracking reactions in FCC catalysts, especially to manufacture high-octane gasoline from heavy crude oil.
Lanthanum-Rich Rare Earth metals play the important roles in hydrogen storage batteries.
It is utilized in green phosphors based on the phosphate (La0.4Ce0.45Tb0.15)PO4;in laser crystals based on the Yttrium-Lanthanum-Fluoride (YLF) composition.
Lanthanum Metal is the very important raw materials in producing Hydrogen Storage Alloys for NiMH batteries, and is also used to produce other pure Rare Earth metals and specialty alloys. Small amounts of Lanthanum added to Steel improves its malleability, resistance to impact, and ductility; Small amounts of Lanthanum are present in many pool products to remove the Phosphates that feed algae. Lanthanum Metal can be further processed to various shapes of ingots, pieces, wires, foils, slabs, rods, discs and powder.
Uses
Carl Auer Baron van Welsbach (1858–1929) of Austria developed misch metal as a methodof igniting a gas flame. In 1903 he patented an alloy of 70% Ce and 30% Fe that gave offsparks when scratched by steel. Baron van Welsbach is also the inventor of the gas mantle.Today, China manufactures most of the misch metal used in the world. The alloys that Chinauses consist of Ce, La, and Nd. They use whatever mixture of these elements are found intheir ores, and thus there is no need to refine them. Lanthanum is used to make electrodes forhigh-intensity, carbon-arc lights that are used in motion picture studios and searchlights. Italso used in the refining of high-grade europium metal and the creation of glass with a highrefractive index as well as for quality lenses in cameras and scientific instruments. It is also usedin the manufacture of strong permanent magnets.
Lanthanum is used for electronic instruments, as a rocket fuel, as a reducing agent, andin automobile catalytic converters.
Definition
lanthanum: Symbol La. A silverymetallic element belonging to group3 (formerly IIIA) of the periodic tableand often considered to be one of thelanthanoids; a.n. 57; r.a.m. 138.91;r.d. 6.146 (20°C); m.p. 921°C; b.p.3457°C. Its principal ore is bastnasite,from which it is separated by an ionexchangeprocess. There are two naturalisotopes, lanthanum–139 (stable)and lanthanum–138 (half-life10
10–10
15 years). The metal, being pyrophoric,is used in alloys for lighterflints and the oxide is used in someoptical glasses. The largest use of lanthanum,however, is as a catalyst incracking crude oil. Its chemistry resemblesthat of the lanthanoids. Theelement was discovered by CarlMosander (1797–1858) in 1839.
Definition
A soft ductile
malleable silvery metallic element that is
the first member of the lanthanoid series. It
is found associated with other lanthanoids
in many minerals, including monazite and
bastnaesite. Lanthanum is used in several
alloys (especially for lighter flints), as a catalyst,
and in making optical glass.
Symbol: La; m.p. 921°C; b.p. 3457°C;
r.d. 6.145 (25°C); p.n. 57; r.a.m.
138.9055.
Production Methods
Lanthanum is most commonly obtained from the two naturally occurring rate-earth minerals, monazite and bastnasite. Monazite is a rare earth-thorium phosphate that typically contains lanthanum between 15 to 25%. Bastnasite is a rare earth-fluocarbonate-type mineral in which lanthanum content may vary, usually between 8 to 38%. The recovery of the metal from either of its ores involves three major steps: (i) extraction of all rare-earths combined together from the non-rare-earth components of the mineral, (ii) separation or isolation of lanthanum from other lanthanide elements present in the mineral extract, and (iii) preparation of high-purity grade lanthanum from its isolated product.
Industrial production processes also may vary depending on the nature and availability of mineral, manufacturing cost, demand for other byproducts, purity level of the metal desired, and its end use. Recovery processes are quite similar to other rare-earth metals and chemistry of the processes does not differ noticeably from one metal to another. Extraction of lanthanum from monazite is discussed below first, followed by that from bastnasite.
The mineral mixtures are crushed and ground. Monazite, because of its magnetic properties, may be separated from other minerals that may be either magnetic or nonmagnetic by repeated electromagnetic separation using electromagnets of varying intensities. After its separation the mineral is treated with hot concentrated sulfuric acid, which converts thorium, lanthanum, and rare-earth metals present into their sulfates. These sulfates are soluble in water. The soluble products are leached into water. The insoluble residues and impurities are filtered out. Then the acidic filtrate is partially neutralized with caustic soda to pH 3 to 4. Thorium precipitates out of solution as hydroxide. Alternatively, thorium may be precipitated as pyrophosphate by adding sodium pyrophosphate to the acid solution. The solution after removal of thorium is treated with ammonium oxalate. This converts all lanthanide elements in the ore into their insoluble oxalate salts. The oxalates decompose into oxides when calcined in air. Cerium is the major component of this mixture of rare-earth oxides. Also, it is the only lanthanide element oxidized to its tetravalent state, Ce4+. The tetravalent cerium(IV) oxide is insoluble in dilute nitric acid. The oxide mixture is, therefore, treated with dilute nitric acid to dissolve lanthanum oxide and other rare-earth oxides to separate them from cerium. Lanthanum is separated from this cerium-free rareearth mixture as a double salt with ammonium nitrate by crystallization. The lanthanum-ammonium double salt is relatively less soluble than other rareearth double salts and stays in the most insoluble fraction.
The most efficient method of separating lanthanum from rare-earth salts solution involves ion exchange. In this process, lanthanum and other tripositive rare-earth metal ions are sorbed onto suitable cation-exchange resin beds by exchange with hydrogen, ammonium, or cupric ions incorporated into the resins. The rare-earth ions are then selectively removed in successive steps by eluting with solutions of suitable complexing agents. Several complexing agents have been used successfully. These include ammonium citrate (a buffered solution, pH 2-4); buffered nitrilotriacetate; buffered ethylenediamine tetraacetic acid (EDTA); and ammonium or metal salts of EDTA. Buffered solution of ammonium-EDTA (pH ~8.4) has been found to be highly effective in such ion-exchange separation, yielding high purity products. EDTA complexes of lanthanide elements have high formation constants in a significantly wide range, between 10
4 and 10
9. Lanthanum also may be separated from aqueous solutions of rare-earth nitrates by liquid-liquid extraction using a suitable water-immiscible organic liquid such as tributyl phosphate or another complexing agent dissolved in it. The method, however, is not as efficient as the ion-exchange technique discussed above.
Lanthanum in purified metallic state may be obtained from its purified oxide or other salts. One such process involves heating the oxide with ammonium chloride or ammonium fluoride and hydrofluoric acid at 300° to 400°C in a tantalum or tungsten crucible. This is followed by reduction with alkali or alkaline earth metals at 1,000°C under argon or in vacuum. A typical reaction is:
La
2O
3 + 6NH
4Cl → 2LaCl
3 + 6NH
3 + 3H
2O LaCl
3 + 3Li → La + 3LiCl
Also high purity lanthanum may be produced by electrolysis of a molten mixture of anhydrous lanthanum chloride and sodium chloride or potassium chloride at elevated temperatures.
When lanthanum is produced from the mineral bastnasite, all processes except ore extraction discussed above are the same. The mineral is crushed and concentrated by flotation process. This is followed by treatment with dilute HCl, which converts lanthanum and the rare-earths contained in the mineral into their chlorides. Calcination in air results in rare-earth oxides.
LaCl
3 + 3Li → La + 3LiCl
.
Hazard
In powder form, lanthanum will ignite spontaneously. If ingested, it can cause liver damageand prevent blood from clotting. Many of its compounds are toxic.
Flammability and Explosibility
Flammable
Industrial uses
A chemical element, lanthanum, symbol La, thesecond most abundant element in the rare earthgroup, is a metal. The naturally occurring elementis made up of the isotopes and is one ofthe radioactive products of the fission of uranium,thorium, or plutonium. Lanthanum is themost basic of the rare earths and can be separatedrapidly from other members of the rareearth series by fractional crystallization. Considerablequantities of it are separated commercially,because it is an important ingredient inglass manufacture. Lanthanum imparts a highrefractive index to the glass and is used in themanufacture of expensive lenses. The metal isreadily attacked in air and is rapidly convertedto a white powder.
Lanthanum becomes a superconductorbelow about –267°C in both the hexagonal andface-centered crystal forms.
Purification Methods
It is a shiny metal that slowly tarnishes in air due to oxidation. It slowly decomposes by H2O in the cold and more rapidly on heating to form the hydroxide. The metal is cleaned by scraping off the tarnished areas until the shiny metal is revealed and stored under oil or paraffin. It burns in air at 450o. It exists in three forms: -form, -form and -form with transition temperatures of 310o and 864o, respectively. [Spedding et al. Ind Eng Chem 44 553 1952.]
