Properties & Intermediates

Tungsten and Wolfram are the names given to element 74 of Mendeleev’s Periodic Table of the Chemical Elements.

Its chemical symbol is W.

In this section:

Properties of the metal

Tungsten has the highest melting point of all metals (3,422 ± 15°C). At this temperature most of the other engineering metals (Fe, Al, Cu, Ti) have already vaporised. Its boiling point of about 5,700°C corresponds to the surface of the sun. With its density of 19.25g/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, and its coefficient of thermal expansion is the lowest of all metals.

Tungsten features the lowest vapour pressure of all metals, very high moduli of compression and elasticity, very high thermal creep resistance and high thermal and electrical conductivity. Tungsten is the most important metal for thermo-emission applications, not only because of its high electron emissivity (which is caused by additions of foreign elements) but also because of its high thermal and chemical stability.

Tungsten is a shiny white metal and, in its purest form, is quite pliant and can easily be processed. Usually, however, it contains small concentrations of carbon and oxygen, which give tungsten metal its considerable hardness and brittleness. For decades, scientists have worked to overcome the brittleness problem.

Most of these unusual properties are due to the half-filled 5d electron shells (d5s1) with a very high binding energy of the tungsten in the bcc tungsten crystal arising from the strong, unsaturated covalent bonds. Based on these properties, tungsten, tungsten alloys and some tungsten compounds cannot be substituted in many important applications in different fields of modern technology.

Property

Value

Atomic number

74

Average relative atomic mass

183.85 ± 0.03

Electron configuration

[Xe]4f145d46s2

Highest melting point of all metals

3,422 ± 15°C

Vapour pressure (2,000°C)

8.15 x 10-8 Pa

Boiling point

5,700 ± 200°C

Crystal structure

Body-centred cubic A2

Lattice parameter

a=3.16524 Å (298K)

Atomic radius (metallic)

137 pm (coordination number 8)

Density

19.25g/cm3 (298K)

Electrical resistivity

5.28µΩ.cm (298K)

Specific heat capacity

135J.kg-1.K-1 (298K)

Enthalpy of fusion

46kJ.mol-1

Thermal conductivity

1.75.W.cm-1.K-1 (298K)

Coefficient of thermal expansion

4.32-4.68 x 10-6.K-1 (298K)

Modulus of elasticity

390-410 GPa (298K)

Hardness

300-650 HV30

Intermediates

In the industrial production process from tungsten raw materials, such as concentrates (primary raw material) and scrap (secondary raw material), to tungsten finished products, such as hardmetals, steels and superalloys, tungsten metal products and chemicals, important intermediates (intermediate products) are produced.

These intermediate products are typical for the tungsten industry, have their specific characteristics and are traded on a global basis.

In the tungsten production process from raw materials to finished products, important tungsten intermediate products are produced.

Tungsten production

The role of tungsten intermediates in the tungsten production process.

Ammonium paratungstate (above) and SEM (scanning electron microscope) image of ammonium paratungstate (below). Copyright images courtesy of HC Starck Tungsten.

Ammonium Paratungstate (APT)

Ammonium Paratungstate  [(NH4)10[H2W12O42]·4H2O] is the most important precursor for the majority of tungsten products. Exceptions are only products of melting metallurgy and some special products with much lower tonnage.

All other intermediates such as tungsten trioxide, tungsten blue oxide, tungstic acid and ammonium metatungstate can be derived from ammonium paratungstate, either by thermal decomposition or chemical conversion.

Ammonium paratungstate is a white crystallised powder having average crystal size between 30 and 100 µm with high purity (trace elements are usually only present in the low ppm – µg/g – range).

Tungsten Oxides

Tungsten trioxide, the yellow coloured WO3, and tungsten blue oxide, a slightly oxygen deficient tungsten oxide with the formula WOn (with n: 2.90 to 2.99), are the main substances used on an industrial scale.

They are produced by thermal decomposition of ammonium paratungstate. In the case of oxidising conditions, tungsten trioxide, WO3, is formed and under slightly reductive conditions the blue tungsten oxide forms. The latter can be described by the formula: (NH4)x·(H2O)y·WOn, with x=0-0.03, y=0-0.06 and n=2.90-2.99.

They have the same high purity as the ammonium paratungstate and microscopically look pseudomorphous to the ammonium paratungstate particles too, but consist of very small tungsten oxide grains, which stick together keeping the original outer shape of the ammonium paratungstate crystals.

The main use for tungsten oxide is to produce tungsten metal powder by hydrogen reduction; they are also used as catalysts or pigments.

SEM image of tungsten trioxide. Copyright image courtesy of HC Starck Tungsten GmbH.

Yellow tungsten trioxide (above) and tungsten blue oxide (below). Copyright images courtesy of HC Starck Tungsten GmbH.

Tungstic acid. Copyright image courtesy of HC Starck Tungsten.

Tungstic Acid

When ammonium paratungstate is decomposed with mineral acid (eg HCl), tungstic acid (H2WO4 or H2O·WO3) is formed.

This makes use of the high chemical purity of ammonium paratungstate and results in high purity tungstic acid.

Tungstic acid is yellow in colour and has a very high active surface and is only used in small quantities for special purposes such as productions of ultrafine tungsten, tungsten carbide powders and tungsten chemicals.

Ammonium Metatungstate (AMT)

Partial thermal decomposition of ammonium paratungstate (or partial replacement of ammonium ions by hydrogen ions using selective ion exchange and subsequent evaporation) leads to ammonium metatungstate, (NH4)6[H6W12O40].3H2O.

Ammonium metatungstate is used to produce tungsten chemicals and catalysts, because of its excellent solubility in water (2,200 g WO3/l at 80°C).

Ammonium metatungstate is a white crystallised powder. It is also used for the preparation of heteropoly acids, which consist of inorganic oxyacids of phosphorous or silicon and that of tungsten. Such compounds are attractive catalysts for many kinds of organic reactions.

Ammonium metatungstate (above) and SEM image of ammonium metatungstate (below). Copyright images courtesy of Xiamen Tungsten.

Tungsten metal powder (above). Copyright image courtesy of GTP. SEM image of typical medium grained tungsten metal powder (below). Copyright image courtesy of Wolfram Berg und Huetten AG.

Tungsten Metal Powder

Yellow or blue tungsten oxide is reduced to tungsten metal powder, W, by hydrogen. The reduction is carried out either in pusher furnaces, in which the powder passes through the furnace in boats, or in rotary furnaces, at 700-1,000oC.

Technical tungsten metal powder qualities are available in average particle sizes from 0.1 to 100µm. The reduction process offers the possibility to produce tungsten powder of any desired average particle size within the above limits only by changes in reduction conditions.

The whole palette of particle sizes is used for further production to tungsten carbide powder, WC, which finds applications in cemented carbide (hardmetal) production. The main portion of tungsten powder is directed to that manufacture.

Starting tungsten powder for tungsten metal products and tungsten alloys covers particle sizes between 2 and 6µm. Extremely coarse powder gained by screening to separate any finer particles has excellent flow characteristics and is used in plasma spraying.

The purity of the tungsten powder is of particular importance in all applications and is mainly influenced by the purity of the original ammonium paratungstate with typical upper limits of trace element concentrations in the low ppm (µg/g) range.

The crucial physical properties are average particle size, particle size distribution, apparent, tap and compact or green density, specific surface area, degree of agglomeration and morphology.

Tungsten Carbide Powder

Tungsten metal powder, W, is mixed with carbon black, C, and treated at temperatures between 1,300 up to 2,200oC in hydrogen atmosphere to produce tungsten carbide powder, WC. Due to its high hardness and comparatively high toughness, it is the main constituent in cemented carbide (hardmetal), a tungsten carbide cobalt alloy. This application accounts for two thirds of global tungsten consumption.

The two main parameters to adjust the properties hardness and toughness in hardmetal are the tungsten carbide grain size and the content of cobalt. The grain size of the tungsten carbide is controlled by the grain size of the tungsten metal powder used for its production and the carburisation temperature (the higher, the coarser).

Stoichiometric tungsten carbide has a carbon content of 6.13wt% C. The main properties of commercial tungsten carbide powders are defined by its grain size, the grain size distribution, the apparent and tap density, the carbon content (which has to be adapted to the further production process to different hardmetal grades, and controlled within strict limits). Of particular importance is the homogeneity of the tungsten carbide powder and, for example, the complete absence of coarser grains in fine grained powders. And, on the other hand, the consistently coarse crystallite size, if coarse grained hardmetal should be produced.

Tungsten carbide powder (above) made from tungsten metal powder. Copyright image courtesy of GTP.  SEM image of medium grained tungsten carbide powder (below). Copyright image courtesy of Wolfram Berg und Huetten AG.

SEM image of tungsten carbide ultrafine grain size. Copyright image courtesy of Wolfram Berg und Huetten AG.

SEM image of tungsten carbide extra coarse grain size. Copyright image courtesy of Wolfram Berg und Huetten AG.

SEM image of fused tungsten carbide. Copyright image courtesy of Xiamen Golden Egret Special Alloy.

Fused Tungsten 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.

The product is called fused tungsten carbide and is used for surface hardening applications.

Ferro Tungsten

Ferro tungsten, FeW, is a master alloy for the production of tungsten-containing steels.

The raw materials for ferro-tungsten production are rich ore or concentrates of wolframite or scheelite. Also, artificial scheelite or soft scrap can be used. The tungsten trioxide in these compounds can be reduced either carbothermically in electric arc furnaces or metallothermically by silicon and/or aluminum. Also, a mixed carbothermic-silicothermic production is in use.

Commercial ferro-tungsten contains between 75 and 85% W. It has a steel grey appearance and a fine-grained structure consisting of FeW and Fe2W. It is supplied in 50–100 mm lumps.

Ferro tungsten. Copyright image courtesy of Tungsten Metals Group Ltd.

Other Intermediates

Other intermediate tungsten compounds and their applications are detailed in the table.

The very soft tungsten disulfide (WS2) has a layer structure similar to graphite and is used as lubricant and for catalytic applications. Copyright SEM image courtesy of Tribotecc.

Compound names

Applications

Tungsten Silicide

Microelectronics

Calcium Tungstate

Phosphors

Heteropolytungstates

  • Lackers and toners
  • Catalysts
  • Passivation of steel

Na-12 Tungstophosphate

  • Manufacture of organic pigments
  • Carroting (surface treatment of furs)
  • Anti-static agent for treatment of acryl-based fibres
  • Leather tanning
  • Water proofing
  • Additive to chrome-plating bath, in cement and adhesives to impart water resistivity

Tungstate Disulfide

Lubricant

Tungstate Diselenide

Lubricant

Tungstate Hexafluoride

For metallisation in the semiconductor industry

Tungstate Hexachloride

Metallisation

Tungstate Hexacarbonyl

Catalyst and organometallic compounds production