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Tantalum
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Tantalum (formerly tantalium ) is a chemical element with the symbol Ta and atomic number 73. A rare, hard, blue-grey, lustrous transition metal, tantalum is highly corrosion-resistant and occurs naturally in the mineral tantalite, always together with the chemically similar niobium. It is part of the refractory metals group, which are widely used as minor component in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum, but its main use today is in tantalum electrolytic capacitors.
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Encyclopedia
Tantalum (formerly tantalium ) is a chemical element with the symbol Ta and atomic number 73. A rare, hard, blue-grey, lustrous transition metal, tantalum is highly corrosion-resistant and occurs naturally in the mineral tantalite, always together with the chemically similar niobium. It is part of the refractory metals group, which are widely used as minor component in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute for platinum, but its main use today is in tantalum electrolytic capacitors. The exploitation of coltan, a tantalum ore, in the conflict regions of the Congo raised ethical questions and human rights issues, and endangered wildlife.
Characteristics
Physical
Tantalum is dark, dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide. Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded only by tungsten and rhenium for metals, and carbon.
Chemical
It is able to form oxides with the oxidation states +5 (Ta2O5) and +4 (TaO2), The most stable oxidation state is +5, tantalum pentoxide. Tantalum pentoxide is the starting material for several tantalum compounds. The compounds are created by dissolving the pentoxide in basic hydroxide solutions or by melting it in another metal oxide. Such examples are lithium tantalate (LiTaO3) and lanthanum tantalate (LaTaO4). In the lithium tantalate, the tantalate ion TaO3- is not alone, but part of a perovskite-like structure; while the lantanum niobate contains lone TaO43- ions.
The fluorides of tantalum can be used for its separation from niobium. Tantalum forms halogen compounds in the oxidation states of +5, +4, and +3 of the type , , and , although multi core complexes and substoichiometric compounds are also known. Tantalum pentafluoride (TaF5) is a white solid with a melting point of 97.0 °C and tantalum pentachloride (TaCl5) is a white solid with a melting point of 247.4 °C. Tantalum pentachloride is hydrolyzed by water and reacts with additional tantalum at elevated temperatures by forming the black and highly hygroscopic tantalum tetrachloride (TaCl4). While the trihalogen compounds can be obtained by reduction of the pentahalogenes with hydrogen, the dihalogen compounds do not exist.
Like most of the other refractory metals, the hard forms are stable nitrides and carbides. Tantalum carbide, like the more commonly used tungsten carbide, is a very hard ceramic used in cutting tools. Tantalum (III) nitride is used as a thin film insulator in some microelectronic fabrication processes. Los Alamos National Laboratory scientists have developed a tantalum carbide-graphite composite material that is one of the hardest materials ever synthesized. Korean researchers have developed an amorphous tantalum-tungsten-copper alloy which is more flexible and two to three times stronger than traditional steel alloys. There are two tantalum aluminides, TaAl3 and Ta3Al; they are stable, refractory and reflective, and have been proposed as mirror coatings for use in the IR.
Isotopes
Natural tantalum consists of two isotopes: 180mTa (0.012%) and 181Ta (99.988%). 181Ta is a stable isotope. 180mTa (m denotes a metastable state) is predicted to decay in three ways: isomeric transition to the ground state of 180Ta, beta decay to 180W, or electron capture to 180Hf. However, any radioactivity of this nuclear isomer was never observed. Only a lower limit on its half life of over 1015 years has been set. The ground state of 180Ta has a half life of only 8 hours. 180mTa is the only naturally occurring nuclear isomer (excluding radiogenic and cosmogenic short-living nuclides). It is also the rarest isotope in the Universe, taking into account the elemental abundance of tantalum and isotopic abundance of 180mTa in the natural mixture of isotopes (and again excluding radiogenic and cosmogenic short-living nuclides).
Tantalum has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of 181Ta, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 182Ta with a half-life of 114.43 days and produce approximately 1.12 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several months. Such a weapon is not known to have ever been built, tested, or used.
Occurrence
Tantalum is estimated to make up about 1 ppm or 2 ppmof the Earth's crust by weight. There are many species of tantalum minerals, only some of which are so far being used by industry as raw materials: tantalite, microlite, wodginite, euxenite, polycrase. Tantalite [(Fe,Mn) Ta2O6] is the most important mineral for tantalum extraction. Tantalite has the same mineral structure as columbite [(Fe,Mn) (Ta,Nb)2O6]; when there is more Ta than Nb it is called tantalite and when there is more Nb than Ta is it called columbite (or niobite). The high density of tantalite and other tantalum containing minerals makes the use of gravitational separation the best method. Other minerals include samarskite and fergusonite.
The main production of tantalum occurs in Australia, where the largest producer, Talison Minerals (formerly part of the Sons of Gwalia company), operates the Wodgina mine.The mine produces tantalum oxide from tantalite. While the large scale production of niobium in Brazil and Canada, yields also a comparable small amount of tantalum, other countries like China, Ethiopia and Mozambique mine the minerals with a higher rate of tantalum and produce a significant amount of the world production. Tantalum is also produced in Thailand and Malaysia as a by-product of tin mining and smelting. During gravity separation of the ore from placer deposits not only yield Cassiterite (SnO2) but also small amounts of tantalite are enriched in the final ore concentrated. The tin smelter slag derived from ore of these deposits contains significant amounts of tantalum and is leached from the slag.. Future large sources of supply, in order of magnitude, are being explored in Saudi Arabia, Egypt, Greenland, China, Mozambique, Canada, Australia, the United States, Finland and Brazil..
In central Africa the colloquial term coltan is used to refer to the two minerals equally, an example being the Democratic Republic of the Congo which the United States Geological Survey reports in its 2006 yearbook as having produced a little less than 1% of the world's tantalum for the past four years. Ethical questions have been raised about responsible corporate behaviour, human rights and endangered wildlife, due to the exploitation of resources such as coltan in the conflict regions of the Congo. According to United Nations report smuggling and exportation of coltan helped fuel the war in the Congo, a crisis that has resulted in approximately 5.4 million deaths since 1998 – making it the world’s deadliest documented conflict since World War II.
Production
Several steps are involved in the extraction of tantalum from tantalite. First the mineral is crushed and concentrated by gravity separation. This is generally carried out near the mine site. Further processing by chemical separation is usually done by treating the ores with a mixture of hydrofluoric acid and sulfuric acid at over 90°C. This causes the tantalum and niobium to dissolve as complex fluorides and be separated from the impurities.
- Ta2O5 + 14HF ? 2H2[TaF7] + 5H2O
- Nb2O5 + 10HF ? 2H2[NbOF5] + 3H2O
The first industrial scale separation developed by de Marignac used the difference in solubility between the complex niobium and tantalum fluorides K2[NbOF5].H2O (dipotassium oxypentafluoroniobate monohydrate) and K2[TaF7] (dipotassium heptafluorotantalate) in water. Newer processes use the liquid extraction of the fluorides from aqueous solution by organic solvents such as cyclohexanone. The complex niobium and tantalum fluorides are extracted separately from the organic solvent with water and either precipitated by the addition of potassium fluoride to produce a potassium fluoride complex, or precipitated with ammonia as the pentoxide:
- H2[TaF7] + KF ? K2[TaF7]? + HF
- 2H2[TaF7] + 14NH4OH ? Ta2O5? + 14NH4F + 9H2O
The resulting potassium fluorotantalate salt is generally treated by reduction with molten sodium to produce a coarse tantalum powder.
Applications
Electronics
The major use for tantalum, as the metal powder, is in the production of electronic components, mainly capacitors and some high-power resistors. Tantalum electrolytic capacitors exploit the tendency of tantalum to form a protective oxide surface layer, using tantalum powder, pressed into a pellet shape, as one "plate" of the capacitor, the oxide as the dielectric, and an electrolytic solution or conductive solid as the other "plate". Because the dielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a high capacitance can be achieved in a small volume. Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, pagers, personal computers, and automotive electronics.
Alloys
Tantalum is also used to produce a variety of alloys that have high melting points, are strong and have good ductility. Alloyed with other metals, it is also used in making carbide tools for metalworking equipment and in the production of superalloys for jet engine components, chemical process equipment, nuclear reactors, and missile parts. Because of its ductility, tantalum can be drawn into fine wires or filaments, which are used for evaporating metals such as aluminium. Due to the fact that it resists attack by body fluids and is nonirritating, tantalum is widely used in making surgical instruments and implants. For example, porous tantalum coatings are used in the construction of orthopedic implants due to tantalum's ability to form a direct bond to hard tissue.
Tantalum is inert against most acids except hydrofluoric acid and hot sulfuric acid, also hot alkaline solutions cause tantalum to corrode. This property makes it an ideal metal for chemical reaction vessels and pipes for corrosive liquids. Heat exchanging coils for the steam heating of hydrochloric acid are made from tantalum. Tantalum was extensively used in the production of ultra high frequency electron tubes for radio transmitters. The tantalum is capable to capture oxygen and nitrogen by forming nitrides and oxides and therefore helps to sustain the high vacuum needed for the tubes.
Other uses
The oxide is used to make special high refractive index glass for camera lenses. The high melting point and oxidation resistance lead to the use of the metal in the production of vacuum furnace parts. Due to it's high density, shaped charge and explosively formed penetrator liners have been constructed from tantalum.
Precautions
Compounds containing tantalum are rarely encountered in the laboratory. The metal is highly biocompatible and is used for body implants and coatings, therefore attention may be focused on other elements or the physical nature of the chemical compound. A single study is the only reference in literature ever linking tantalum to local sarcomas. It is possible the result was due to other factors not considered in the study. The study was quoted in IARC Monograph vol. 74 which includes the following "Note to the reader": "Inclusion of an agent in the Monographs does not imply that it is a carcinogen, only that the published data have been examined."
External links
- - Tantalum uses, ore mining and extraction
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