The history of tungsten goes back to the 17th century. The miners in the Erz Mountains of Saxony noticed that certain ores disturbed the reduction of cassiterite (a tin mineral) and induced slagging. "They tear away the tin and devour it like a wolf devours a sheep", a contemporary wrote in the symbolic language of those times. The miners gave this annoying ore German nicknames like "wolfert" and "wolfrahm" (which means wolf froth).
In 1758, the Swedish chemist and mineralogist, Axel Fredrik Cronstedt, discovered and described an unusually heavy mineral that he called "tung-sten", which is Swedish for heavy stone. He was convinced that this mineral contained a new and, as yet undiscovered, element. Yet, it was not until 1781 that a fellow Swede, Carl Wilhelm Scheele,who worked as a pharmacist and private tutor in Uppsala and Köping, succeeded in isolating the oxide (tungsten trioxide).
Independent of Scheele, two Spanish chemists, the brothers Elhuyar de Suvisa, first reduced the mineral wolframite to tungsten metal in 1783. Jöns Jacob Berzelius (1816) and later also Friedrich Wöhler (1824) described the oxides and bronzes of tungsten and gave the new metal the name "wolfram". While this established itself in Germany and Scandinavia, the Anglo-Saxon countries preferred Cronstedt’s "tungsten".
In 1821, K.C. von Leonhard proposed the name "Scheelite" for the mineral CaWO4. The first attempts to produce tungsten steel were made in 1855, but industrial use was not possible because of the high price of tungsten metal.
The first industrial application of tungsten was the alloying and hardening of steels late in the 19th century. Rapid growth and widespread application followed the invention, and the launch of high speed steels by Bethlehem Steel took place in 1900 at the Paris World Exhibition.
W. D. Coolidge made the second important breakthrough in tungsten applications in 1903. Coolidge succeeded in preparing a ductile tungsten wire by doping tungsten oxide before reduction. The resulting metal powder was pressed, sintered and forged to thin rods. Very thin wire was then drawn from these rods. This was the beginning of tungsten powder metallurgy, which was instrumental in the rapid development of the lamp industry.
The year 1923 is the next important milestone
in the chronology of tungsten. It marks the invention of hardmetal (combining
WC and Cobalt by liquid phase sintering) by K. Schröter and the corresponding
application for a patent, which was granted to Osram Studiengesellschaft in
Berlin and licensed to Krupp in Essen in 1926. Nowadays, hardmetal (cemented
carbide) is the main application for tungsten. [i]
Tungsten
metal has a nickel-white to grayish luster. Among metals it has the highest
melting point, the highest tensile strength at temperatures of more than 1,650º
C (3,002º F), and the lowest coefficient of linear thermal expansion (4.43 
10-6
per ºC at 20º C). Tungsten is ordinarily brittle at room temperature. Pure
tungsten can, however, be made ductile by mechanical working at high
temperatures and can then be drawn into very fine wire. Tungsten was first
commercially employed as a lamp filament material and thereafter used in many
electrical and electronic applications. It is used in the form of tungsten
carbide for very hard and tough dies, tools, gauges, and bits.[ii]
A temperature of about 5,700°C is needed to
bring tungsten to boil - which corresponds approximately to the temperature of
the sun’s surface. With a density of 19.25 g/cu.cm, tungsten is also among the
heaviest metals. Its electrical conductivity at 0°C is about 28% of that of
silver which itself has the highest conductivity of all metals.[iii]
Tungsten features the lowest vapour pressure of all metals, very high moduli of compression and elasticity, very high thermal creep resistance, high thermal and electrical conductivity and, last but not least, a very high coefficient of electron emission. The latter can even be improved by alloying tungsten with certain metal oxides.
Most of these unusual properties are due to
the half-filled 5d electron shell with a very high binding energy of the tungsten
metal lattice. Based on these properties, tungsten, tungsten alloys and some
tungsten compounds cannot be substituted in many important applications in
different fields of modern technology.[iv]
The amount of tungsten in the Earth's crust is estimated to be 1.5 parts per million, or about 1.5 grams per ton of rock. Tungsten is about as abundant as tin or as molybdenum, which it resembles, and half as plentiful as uranium. The two economically important minerals are wolframite and scheelite.[v]
Tungsten ranks 57th in abundance among the elements in the crust of the earth. It is never found free in nature, but occurs in combination with other metals, notably in the minerals scheelite and wolframite [vi]
The first, wolframite [(Fe, Mn) WO4], contains iron and manganese tungstates in all proportions between 20 and 80 percent of each. The second, scheelite (CaWO4), fluoresces a bright bluish color under ultraviolet light.
Tungsten deposits occur in association with metamorphic rocks and granitic igneous rocks. The most important mines are in the Nan Mountains in the Kiangsi, Hunan, and Kwangtung provinces of China, which possesses about 50 percent of the world's reserves. In Russia, mines are located in the northern Caucasus and around Lake Baikal. There are also deposits in Kazakstan. About 90 percent of South Korea's tungsten is at Sang Dong. Canada's Northwest Territories is home to the largest tungsten mine in the Western world, and a mine at Chojlla, Bol., is the largest producer in South America. Deposits in the United States are spread along the Rocky Mountains.[vii]
After finding the tungsten ore in the ground mining is done through Open-pit methods, which are mainly used in Australia and Canada, while underground mining is generally necessary for other mines in the world.
Tungsten ores are beneficiated by crushing followed by gravity concentration. Flotation separation is used for scheelite that has been ground to a fine size to liberate the tungsten; leaching, roasting, and magnetic or high-tension separation when required further supplement this.
Tungsten ores frequently occur in association with sulfides and arsenides, which can be removed by roasting in air for two to four hours at 800º C (1,450º F). In order to produce ammonium paratungstate (APT), an intermediate compound in production of the pure metal, ores may be decomposed by acid leaching or by the autoclave-soda process. In the latter process, the ground ore is maintained for 11/2 to 4 hours in a solution of 10-18 percent sodium carbonate at temperatures of 190º to 230º C (375º to 445º F) and under a pressure of 14.1-24.6 kilograms per square centimeter (200-350 pounds per square inch). Prior to the removal of unreacted gangue by filtration, the acidity is adjusted to pH 9-9.5, and aluminum and manganese sulfates are added at 70º-80º C (160º-175º F) and stirred for one hour. This can eliminate phosphorus and arsenic and reduce silica to a level of 0.03-0.06 percent. Molybdenum is removed by adding sodium sulfide at 80º-85º C (175º-185º F) at a pH of 10, holding for one hour, and then acidifying the solution to pH 2.5-3 and stirring for seven to nine hours to precipitate molybdenum sulfide. A liquid ion-exchange process, using an organic extractant consisting of 7 percent alamine-336, 7 percent decanol, and 86 percent kerosene, can further purify the remaining sodium tungstate solution. During the countercurrent flow of the extractant through the solution, tungstate ions transfer from the aqueous phase to the organic phase. The tungsten is then stripped from the extractant into an ammonia solution containing ammonium tungstate. The resultant APT solution is sent to an evaporator for crystallization.
In the acid-leaching process, scheelite concentrate is decomposed by hydrochloric acid in the presence of sodium nitrate as an oxidizing agent. This charge is agitated by steam spraying and is maintained at 70º C (160º F) for 12 hours. The resultant slurry, containing tungsten in the form of a solid tungstic acid, is diluted and allowed to settle. The tungstic acid is then dissolved in aqueous ammonia at 60º C (140º F) for two hours under stirring. Calcium from the resulting solution is precipitated as calcium oxalate, while phosphorus and arsenic may be removed by the addition of magnesium oxide, which forms insoluble phosphates and arsenates of ammonium and magnesium. Adding a small amount of activated carbon and digesting for one to two hours remove iron, silica, and similar impurities that form colloidal hydroxides. The solution is clarified through pressure filters and evaporated to obtain APT crystals.[viii]
When APT is decomposed to tungsten oxides, it displays different colors according to its composition: the trioxide is yellow, the dioxide is brown, and the intermediate oxide is purple-blue. APT can be decomposed to yellow oxide when heated to above 250º C (480º F) in a furnace under a flow of air. In the industrial production of tungsten, however, APT is usually decomposed to the intermediate oxide in a rotary furnace under a stream of hydrogen, which partially decomposes the ammonia in the crystals into nitrogen and hydrogen while maintaining a reducing atmosphere. The rotary furnace is divided by partitions into three zones maintained, respectively, at 850º, 875º, and 900º C (1,550º, 1,600º, and 1,650º F). The furnace is tilted at a small angle and rotated to provide a continuous flow of powder through the central holes of the partitions.
The blue oxide is then reduced by hydrogen to metallic tungsten powder in stationary furnaces at temperatures ranging from 550º to 850º C (1,025º to 1,550º F). In this process the oxide is loaded into "boats" made of Inconel, a nickel-based alloy noted for its strength at high temperatures. These are stoked into tubes, usually arranged in two rows, and the tubes are heated in three separate zones along their lengths.
APT may also be reduced by carbon, although the powder is usually contaminated with tungsten carbide and some mineral elements contained in the carbon. When APT and carbon are mixed and reacted at 650º-850º C (1,200º-1,550º F), the product is a blue oxide. When heated in the range of 900º-1,050º C (1,650º-1,925º F), the brown oxide is formed. For complete reduction to metal, a temperature higher than 1,050º C is required. The purity of the metal is about 95 percent.[ix]
Tungsten powder is compacted into bars or billets with a mechanical or isostatic press prior to sintering. The "green," or unfired, density of these compacts, obtained from powder particle sizes ranging from 1 to 10 micrometres, is usually 65 to 75 percent of the theoretical. After being presintered at 1,000º-1,200º C (1,800º-2,200º F), tungsten bars of small diameter are sintered in a hydrogen atmosphere, with heat being provided by the direct-resistance method--that is, by an electric current passed through the bar. A spring attachment to the water-cooled clips holding each bar is necessary so that one end is free to move as the bar shrinks during sintering. The current is gradually increased to raise the temperature from room temperature to 2,700º-3,100º C (4,900º-5,600º F). After holding at the final temperature for 30 to 60 minutes, the density reaches 88.5 to 96 percent of the theoretical.
An indirect sintering process is used for large tungsten billets. The heating elements of the furnace are constructed of molybdenum strips and supported by molybdenum or tungsten frames, and they are surrounded by molybdenum heat shields. A slow heating in the early stage of sintering is essential for deoxidizing the material and releasing gases at a controlled rate. At higher temperatures--i.e., from 800º C up to the final sintering temperature of 2,400º C (4,350º F)--the heating rate also should be controlled, since too fast a temperature buildup within the billet would cause thermal stresses and would result in the cracking of the material. A final sintering for 10 hours is required for densification.[x]
The average annual price of tungsten since 1950 has fluctuated between a nadir of US $10 per metric ton unit in 1963 and a peak of US $175 in 1977. During the last five years, trade in concentrates has diminished and the market has relied more and more upon the APT quotation as a price guide since APT is the product traded in the largest quantity. Prices are mainly based on the quotations published twice a week by London’s "Metal Bulletin", although other trade journals also publish quotations or indicative prices.[xi]
The following uses for tungsten are gathered from a number of sources as well as from anecdotal comments. [xii]
- Useful for glass-to-metal seals since the thermal expansion is about the same as borosilicate glass
- Tungsten and its alloys are used extensively for filaments for electric lamps, electron and television tubes, and for metal evaporation work
- Electrical contact points for car distributors
- X-ray targets
- Windings and heating elements for electrical furnaces
- Missile and high-temperature applications
- High-speed tool steels and many other alloys contain tungsten
- The carbide is important to the metal-working, mining, and petroleum industries
- Calcium and magnesium tungstates are widely used in fluorescent lighting
- Tungsten salts are used in the chemical and tanning industries
- Tungsten disulphide is a dry, high-temperature lubricant, stable to 500°C
- Tungsten bronzes and other tungsten compounds are used in paints
- TV tubes (electron tubes)
- X-ray targets
The main chemical application of tungsten is in the form of catalysts.
- DeNOx catalysts for the removal of nitrogen oxides from combustion power plant stack gases by selective catalytic reduction with ammonia; the products are harmless nitrogen and water vapour. Typical DeNOx catalysts are honeycomb-shaped TiO2WO3V2O5 ceramics.
- Catalysts for hydrocracking, hydrodesulphuration and hydrodenitrification of mineral oil products, where tungsten and nickel oxides on ceramic carriers are used. These catalysts help to increase the yield of gasoline and other light hydrocarbons in crude oil processing and to make the products more environmentally friendly by reducing the contents of aromatic hydrocarbons, sulphur and nitrogen compounds.
- Other tungsten containing catalysts for various applications in the chemical industry, for example dehydrogenation, isomerisation, polymerization, reforming, hydration and dehydration, hydroxylation, epoxidation, etc
For the manufacture of chemical products, commercially available tungsten compounds such as sodium tungstate, ammonium tungstates, tungstic oxide or tungstic acid are commonly used as raw materials. The following list gives a few examples.
- Inorganic pigments for
ceramic glazes and enamels:
Tungstic acid or tungsten oxide is used for bright yellow glazes.
Tungsten bronzes, i.e. partly reduced alkali and alkaline earth tungstates, are available in many bright colours. - Barium and zinc
tungstate are examples for bright white pigments.
Coloured organic dyes and pigments based on phosphotungstic acid and phosphotungsto-molybdic acid are made for paints, printing inks, plastic, rubber and other materials. - Tungsten disulphide is a lubricant for temperatures above the application range of molybdenum disulphide. It has also been used to form a self-lubricating surface on razor blades.
- Organic tungsten compounds have been patented as viscosity stabilisers in lubricant oils.
In laboratories, tungsten is used in several applications, for example
- High purity sodium tungstate as a reagent in biochemical analysis
- Sodium metatungstate for the preparation of heavy liquids to be used for the separation of minerals by density in mineralogy or for density gradient centrifugation in biochemical analysis
- High purity tungsten granules as an accelerator in the determination of carbon and sulphur in metals by combustion in an induction furnace.
The versatility of tungsten and its compounds is further highlighted by a few examples:
- Tungsten wire coils, or small boats made of W foil, are used for vacuum metallizing reflector lamps, headlamps reflectors, primarily with aluminium.
- Vapour deposited tungsten oxide layers are functional in self-darkening windowpanes. Tungsten trioxide is yellow-white and fully transparent in the form of a thin layer. Light converts WO3 to dark blue W20O58 in a reversible reaction, so the light transmission through the pane is reduced in bright sunshine, and the light transmission returns with clouds or at sunset.
- The corrosion resistance of optical glasses can be improved by the addition of tungsten oxide to the molten glass mixture.
- Tungsten shot is an environmentally friendly alternative to lead shot for hunting.
- Fishing weights are made of tungsten weighted plastic to replace the conventional lead weights.
Back to Contents Typical Compositions of Selected High Speed Steels (%)
|
Grade |
C |
Cr |
Mo |
W |
V |
Co |
|
T-1 |
0.75 |
- |
- |
18.0 |
1.1 |
- |
|
M-2 |
0.95 |
4.2 |
5.0 |
6.0 |
2.0 |
- |
|
M-7 |
1.00 |
3.8 |
8.7 |
1.6 |
2.0 |
- |
|
M-42 |
1.10 |
3.8 |
9.5 |
1.5 |
1.2 |
8.0 |
Consolidation of Powders to Hardmetal (cemented carbide)
