Tungsten

                                      In 1779 Irish chemist Peter Woulfe deduced the existence of a new element – tungsten – from his analysis of the mineral wolframite (an iron manganese tungstate mineral).Tungsten was isolated as tungstic oxide (WO3) in 1781, in Sweden, by Carl W. Scheele from the mineral scheelite (calcium tungstate). However he did not have a suitable furnace to reduce the oxide to the metal.Tungsten was finally isolated by brothers Fausto and Juan Jose de Elhuyar in 1783, in Spain, by reduction of acidified wolframite with charcoal. The element name comes from the Swedish words ‘tung sten’ meaning heavy stone.The chemical symbol, W, comes from the original name of the element, Wolfram.

                Pure tungsten is a steel-gray to tin-white metal. Tungsten has the highest melting point and lowest vapour pressure of all metals, and at temperatures over 1650°C has the highest tensile strength. The metal oxidises in air and must be protected at elevated temperatures. It has excellent corrosion resistance and is attacked only slightly by most mineral acids.

Physical Properties:
  *Melting point: 3695 [or 3422 °C (6192 °F)] K
  *Boiling point: 5828 [or 5555 °C (10031 °F)] K
  *Density of solid: 19250 kg m^3

Charasterstics:
          Tungsten is a very hard, dense, silvery-white, lustrous metal that tarnishes in air, forming a protective oxide coating. In powder form tungsten is gray.The metal has the highest melting point of all metals, and at temperatures over 1650 oC also has the highest tensile strength. Pure tungsten is ductile, and tungsten wires, even of a very small diameter, have a very high tensile strength.Tungsten is highly resistant to corrosion. It forms tungstic acid (H2WO4), or wolframic acid from the hydrated oxide (WO3) and its salts are called tungstates, or wolframates.When present in compounds, tungsten exists mostly in the oxidation state VI.Tungsten is one of the five major refractory metals (metals with very high resistance to heat and wear).

Health Effects of Tungsten:
               Tungsten is a very hard, dense, silvery-white, lustrous metal that tarnishes in air, forming a protective oxide coating. In powder form tungsten is gray.The metal has the highest melting point of all metals, and at temperatures over 1650 oC also has the highest tensile strength. Pure tungsten is ductile, and tungsten wires, even of a very small diameter, have a very high tensile strength.Tungsten is highly resistant to corrosion. It forms tungstic acid (H2WO4), or wolframic acid from the hydrated oxide (WO3) and its salts are called tungstates, or wolframates.When present in compounds, tungsten exists mostly in the oxidation state VI.Tungsten is one of the five major refractory metals (metals with very high resistance to heat and wear).

Tungsten in the Environment:
             Very little tungsten has been detetced in the dew soils that have been analysed for it, although around an ore-processin plant in Russia levels as high as 2000 ppm were found. The concentration of the element in natural waters is very low.There are several minerals of tungsten, the most important are scheelite and wolframite. The main mining area is China, which today accounts for more than two-thirds of the world's supply. Other places with active tungsten mines are Russia, Austria, Bolivia, Peru and Portugal. World production is around 40.000 tonnes per year and reserves are estimated to be around 5 million tonnes. Tungsten is also recycled and it meets 30% of demand.

Applications:
                   Among its compounds can be stressed out the carbide (WC), representing about 40% of the world production of the metal. This compound is extremely hard and resistant, being used in the production of saw blades and other sharp utensils, abrasives, trunnions, ball-point pens, etc. The second largest application is in the production of special hard steels, with extreme resistance to corrosion and improved behavior at high temperatures. There happen to exist about 300 commercial ferrous alloys, with variable amounts of tungsten. It is also an important constituent of several non ferrous alloys, such as those formed with cobalt and chromium, with high hardness and resistance to abrasives, and still those formed with nickel, tantalum or niobium. All these alloys can be used in machinery pieces where high mechanical resistance is required. The most important applications of the pure metallic tungsten are the production of filaments for bulb lamps, electric contacts, arch-welding electrodes, heating elements in high temperature furnaces, valves for reaction propellers used in missiles and airships, etc. Among its compounds are the calcium and magnesium tungstates, used in fluorescent lamps, the disulfide used as lubricant at high temperatures and catalyst in the oil industry, the trioxide and the "tungsten bronzes" (by-products from the reduction of tungstates of alkaline and earth-alkaline metals), in the production of paints, etc.

                                                                          Published by Ravindra.K(Mechanical Engineering)