Tungsten

TUNGSTEN, W (Latin: Wolframium; * EN: tungsten; DE: Wolfram; FR: tungstene; ES: tungsteno; RU: вольфрам) is the chemical element of the group VI of the periodic system of Mendeleev, the atomic number is 74, the atomic mass is 183.85. The natural tungsten comprises the mixture of the five stable isotopes, namely, 180W (0.135%), 182W (26.41%), 183W (14.4%), 184W (30.64%), 186W (28.41%). Tungsten has been discovered and isolated in the form of the tungsten anhydride during the 1781 by the Swedish chemist Carl Wilhelm Scheele. During the 1783, the Spanish chemists brothers José and Fausto Elhuyar have isolated the WO3 from the wolframite, and, after having reduced this oxide using the carbon, they have obtained for the first time the pure metal, which has been named by them as tungsten.

Tungsten is the heavy metal, pale gray, difficult to melt. It crystallizes into the volume-centered cubic lattice with the period of a=0.31647 nanometres (3.1647 Ångström). The density is 19300 kilograms per cubic metre, the temperature of the melting point is 3410±20 degrees Celsius (the most difficult to melt element after carbon), the temperature of the boiling point is 5930 degrees Celsius. The thermal conductivity [Watt per (metre • Kelvin)] is 129.89 (at 20 degrees Celsius); 108.94 (at 1300 degrees Celsius). The electrical resistivity (Ohm • metre) is 5.5 • 10^-4 (at 20 degrees Celsius); 90.4 • 10^-4 (at 2700 degrees Celsius). The temperature coefficient of the electrical resistance (at 0-170 degrees Celsius) is 5.1 • 10^-3 per Kelvin. The important properties of tungsten are: the high electron emission during the heating of the metal (milliamperes per square metre) is 1.5 • 10^-6 (at 830 degrees Celsius); 2.3 (at 1630 degrees Celsius); 10^4 (at 1730 degrees Celsius); 298 • 10^4 (at 2230 degrees Celsius), and 1690 • 10^4 (at 2427 degrees Celsius); the high power of the energy, which is radiated by the surface of the metal at the high temperatures (Watt per square metre): 0.9 • 10^4 (at 800 degrees Celsius); 18.0 • 10^4 (at 1600 degrees Celsius); 64.0 • 10^4 (at 2200 degrees Celsius); 153.0 • 10^4 (at 2700 degrees Celsius); 245 • 10^4 (at 3030 degrees Celsius).

Within the compounds, the oxidation state of tungsten may be from +2 to +6. In case of the higher oxidation states, it possesses the acidic properties, while in case of the lower oxidation states, it possesses the basic properties. The compounds of the lower oxidation states of tungsten are relatively unstable. There are most characteristic and stable the compounds of tungsten with the oxidation state of +6. Tungsten has the high propensity for the forming of the complexes. The metallic tungsten under the usual conditions is chemically very resistant. It starts to interact with oxygen at the temperature of above 400 degrees Celsius; it resists the action of the water, but at the temperature of the red heat, it is easily oxidized using the water vapour. Under the cold conditions, tungsten is practically not subjected to the action of the HCl, H2SO4, HNO3, and HF at any concentration, but it is easily dissolved within the mixture of the HNO3 and HF. In the absence of oxygen, tungsten does not dissolve within the alkalis and ammonia. The most important of the compounds of tungsten are: the trioxide WO3, the tungsten acid H2WO4 and its salts, namely, the tungstates.

Tungsten is rare within the nature; the content within the Earth's crust is 1•10^-4% (in terms of the mass). Tungsten is not encountered in the free state. Tungsten forms its own minerals (the tungstates with Ca, Fe, Mn, sometimes with Pb, Zn, rarely the oxides WO3, H2WO4, yet more rarely the sulfides WS2) or, in the form of the isomorphous admixture, it belongs to other minerals, prevalently to the minerals of Mo, Ti, and also to the certain silicates (micas, feldspars). The most important minerals of tungsten are the wolframite and scheelite, which may be formed and accumulated to the level of the industrial concentration during the skarn, greisen, and hydrothermal processes. Within the natural mineral parageneses, tungsten is often associated with Si, Mo, Sn, Be, Ta, F, and is less often associated with Au, Sb, Hg. The forms of the migration of tungsten within the high-temperature ore-bearing solutions are represented mostly by the hydroxo and hydroxo-fluoride complexes. About the major genetic types of the deposits of tungsten, and the schemes of the beneficiation, see within the "Tungsten ores" article. The obtainment of the pure metal from the tungsten concentrates is conducted in three stages: the chemical isolation of the pure tungsten acid or its salts; the reduction of the WO3 to the metallic powder; the transformation of the powder into the metal.

The major field for the usage of tungsten is the production of the steels (approximately 85% of the extraction). The pure tungsten is used for the production of the incandescent filaments for the electric lamps, of the spiral for the heaters within the electric furnaces, of the electrodes, of various parts for the high vacuum and X-ray devices, and during the welding with the usage of the atomic hydrogen.