DESCRIPTION
Wolfram, also called tungsten, is a chemical element with atomic number Z=74 and is found in group VI B of the periodic table of the elements. It is a rare metal in the earth’s crust, 57º in abundance, found as an oxide and as salts in certain minerals. The word wolfram comes from the German wolf and rahm, translated as “Wolf’s slime” in reference to the superstitions of medieval Saxon miners who believed that the devil appeared in the form of a wolf and inhabited the depths of the mines corroding the cassiterite. The word tungsten comes from Swedish; tung translates as “heavy” and sten, “stone”.
It is never found free in nature, but in the form of salts combined with other elements, mainly as scheelite (CaWO4) and wolframite ((Fe,Mn)WO4), which are its most important minerals. On the other hand we also have cuproscheelite (CuWO4), ferberite (FeWO4), Hübnerite (MnWO4) and stolzite (PbWO4). These ores are mined and used to produce about 35,000 tons of tungsten concentrates per year. China produces more than 75% of this total and most of the remaining production comes from: Austria, Bolivia, Portugal and Russia.
To extract the element from its ore, it is smelted with sodium carbonate to obtain sodium wolframate, Na2WO4. The soluble sodium wolframate is then extracted with hot water and treated with hydrochloric acid to obtain wolframic acid, H2WO4. The latter compound, once washed and dried, forms the oxide WO3, which is reduced with hydrogen or carbon in an electric furnace. The fine powder obtained is reheated in molds in a hydrogen atmosphere and pressed into bars which are rolled and hammered at high temperature to make them compact and ductile. It is also possible to obtain it by hydrogen reduction of WF6.
PROPERTIES
It is steel-gray in color, very hard and dense, has the highest melting point of all metals and the highest boiling point of all known elements. From the chemical point of view, tungsten is relatively inert. It is not easily attacked by common acids, alkalis or aqua regia. It reacts with a mixture of nitric and hydrofluoric acids. It is easily attacked by molten oxidizing salts, such as sodium nitrite. Gaseous chlorine, bromine, iodine, carbon dioxide, carbon monoxide and sulfur react with tungsten only at high temperatures. Carbon, boron, silicon and nitrogen also form compounds with it at elevated temperatures; with hydrogen it does not react.
Tungsten is not classified as a hazardous substance by the E.U. nor is it classified as dangerous goods for transport.
APPLICATIONS
- Manufacture of special steels.
- Manufacture of tungsten carbide for the manufacture of machining tools.
- TIG welding electrodes.
- Incandescent filaments and electrical resistors.
- Luminaires.
