Tungsten (W) - Properties, Applications

Tungsten, also known as Wolfram, lapis ponderosus or Heavy Stone, has highest melting point of all elements except carbon - sources in scientific literature vary between 3387 °C and 3422 °C.

It also has also excellent high temperature mechanical properties and the lowest expansion coefficient of all metals.

A temperature of about 5700 °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/cm³, 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.

Tungsten has the chemical symbol W and is element 74 of the periodic table.

Properties

Table 1. Properties of Tungsten

Pure tungsten is a shiny white metal and, in its purest form, is quite pliant and can easily be processed. Usually, however, it contains small amounts of carbon and oxygen, which give tungsten metal its considerable hardness and brittleness.

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.

History

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.

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.

The second important breakthrough in tungsten applications was made by W. D. Coolidge 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.

Minerals

Tungsten occurs in the natural state only in the form of chemical compounds with other elements. Although more than twenty tungsten bearing minerals are known, only two of them are important for industrial use, namely wolframite and scheelite.

Table 2. Industrially Important Tungsten Bearing Minerals

Pure scheelite has blue-white fluorescence in ultraviolet light, a property which is utilised in prospecting. Wolframite is a general term for iron and manganese tungstates where the iron/manganese ratio can vary. A mineral with more than 80% FeWO₄ is called Ferberite and a mineral with more than 80% MnWO₄ is called Hübnerite.

Deposits

All tungsten deposits are of magmatic or hydrothermal origin. During cooling of the magma, differential crystallization occurs, and scheelite and wolframite are often found in veins where the magma has penetrated cracks in the earth's crust. Most of the tungsten deposits are in younger mountain belts, i.e. the Alps, the Himalayas and the circum-Pacific belt.

The concentration of workable ores is usually between 0.3 and 1.0% WO₃.

Supply

Over the last few years, sources of supply have changed dramatically. In 1986, the USSR was the world’s largest consumer but, by 1992, the reformed CIS was exporting tungsten and by 1996 was the world’s second largest supplier.

The other principal producing countries today are Austria, Bolivia, Peru and Portugal, whilst mines have closed in the last decade in Australia, Brazil, Canada, France, Japan, South Korea, Sweden, Thailand and the USA.

Not only have the sources of supply altered but so have the tungsten compounds traded, as fluctuating price differentials between concentrate and upgraded products and govermental restrictions played their part in the market.

Table 3. Trading of Tungsten Compounds

Intermediate products include tungstates, tungsten oxides and hydroxides, W and WC powders, and ferrotungsten.

Mining & Beneficiation

Tungsten is usually mined underground. Scheelite and/or wolframite is frequently located in rather narrow veins which are slightly inclined and often widen with the depth. Open pit mines exist but are rare.

Most tungsten ores contain less than 1.5% WO₃ and ore dressing plants are always in close proximity to the mine.

The ore is first crushed and milled to liberate the tungsten mineral crystals.

Scheelite ore can be concentrated by gravimetric methods, often combined with froth flotation, whilst wolframite ore can be concentrated by gravity, sometimes in combination with magnetic separation.

Pricing

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.

Processing

Concentrates

Most tungsten concentrates are processed chemically to ammonium paratungstate (APT). Secondary raw materials like (oxidized) scrap and residues are another important feed for chemical tungsten processing.

Wolframite concentrates can also be smelted directly with charcoal or coke in an electric arc furnace to produce ferrotungsten (FeW) which is used as alloying material in steel production.

Pure scheelite concentrate may also be added directly to molten steel.

Recycling

Recycling is an important factor in the world’s tungsten supply. It is estimated that today some 30% is recycled, and the tungsten processing industry is able to treat almost every kind of tungsten-containing scrap and waste to recover tungsten and, if present, other valuable constituents.

Contaminated cemented carbide scrap, turnings, grindings and powder scrap are oxidized and chemically processed to APT in a way similar to that used for the processing of tungsten ores. If present, cobalt, tantalum and niobium are recovered in separate processing lines. Other tungsten containing scrap and residues might require a modified process.

Clean cemented carbide inserts and compacts are converted to powder by the zinc process (treatment with molten zinc which is dissolved in the cobalt phase and is then distilled off, leaving a spongy material which is easily crushed). This powder is added back to the manufacture of ready-to-press powder. By this process, not only tungsten carbide but also cobalt, tantalum carbide and other carbides are recycled.

Recycling of tungsten in high speed steel is high, and a typical melt contains 60 to 70% scrap, including internally generated scrap.

On the other hand, recycling in such applications as lamp filaments, welding electrodes and chemical uses is low.

Although tungsten seems to be relatively harmless to the environment, environmental concerns have led to an increasing recycling rate, especially when the material contains other metals in addition to the tungsten. Recycling is always more environmentally friendly and usually more economic than waste disposal.

Toxicity

Tungsten and its compounds show generally low toxicity compared to most other metals and their compounds.

Intermediates

Ammonium Paratungstate (APT)

APT [(NH₄)₁₀W₁₂O₄₁ ^. 5 H₂O] is the main intermediate and also the main tungsten raw material traded in the market. APT is usually calcined to yellow or blue oxide (WO₃ or W₂₀O₅₈).

Tungsten Metal Powder (W)

Yellow or blue oxide is reduced to tungsten metal powder by hydrogen. The reduction is carried out in pusher furnaces, in which the powder passes through the furnace in boats, or in a rotary furnace, at 700 - 1000 °C.

Tungsten Carbide (WC)

Most of the tungsten metal powder is converted to tungsten carbide (WC) by reaction with pure carbon powder, e.g. carbon black, at 900 - 2200 °C in pusher or batch furnaces, a process called carburisation.

Tungsten carbide is, quantitatively, the most important tungsten compound. Because of its hardness, it is the main constituent in cemented carbide.

Cast Carbide

By melting tungsten metal and tungsten monocarbide (WC) together, a eutectic composition of WC and W₂C is formed. This melt is cast and rapidly quenched to form extremely hard solid particles having a fine crystal structure. A tough, feather-like structure is preferred over the brittle, blocky structure obtained by insufficient quenching. The solids are crushed and classified to various mesh sizes.

Applications

• Hardmetal is the most important usage of tungsten. Its main constituent is tungsten monocarbide (WC), which has a hardness close to diamond.
• Tungsten mill products are tungsten metal products such as lighting filaments, electrical and electronic contacts, wire, rods, etc.
• Other applications include chemical uses, mainly in the form of catalysts.
• Cemented carbide and high speed steel tools
• Television sets,
• Magnetrons for microwave ovens

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