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Friday, 11 February 2011 21:56

Tungsten

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Gunnar Nordberg

Occurrence and Uses

Tungsten (W) never occurs free in nature and is found only in a few minerals as tungstate of calcium, iron or manganese. Of the known tungsten-bearing minerals, scheelite (CaWO4), wolframite ((Fe,Mn)WO4), hubnerite (MnWO) and ferberite (FeWO4) are commercially important. Total world reserves of tungsten trioxide (WO3 ) are estimated to be about 175,000,000 t. These tungsten minerals are mostly mined from underground workings, but open-cut operations and more primitive methods are also applied. The tungsten content of the ore mined is usually 0.5 to 2.0%. The more common impurities are gangue minerals such as quartz and calcite, and metallic minerals of copper, bismuth, tin and molybdenum.

Tungsten is a component in hard metals. It is used to increase the hardness, toughness, elasticity and tensile strength of steel. It is used in the production of tungsten steels for automobiles and high-speed cutting tools. Tungsten is also used in lamps, vacuum tubes, electric contacts, x-ray tubes and fluorescent light tubes. It serves as a flame retardant in the textile industry.

Tungsten carbide (WC) has replaced diamond in large drawing dies and rock drills because of its extreme hardness. Tungsten compounds are also used in lasers, dyes, inks and ceramic frits. Some tungsten alloys are used in the nuclear and space industries for nozzles of rocket motors and for protecting shields for spacecraft.

Hazards

Little is known of the toxicity of tungsten. The LD50 of sodium tungstate for 66-day-old rats was between 223 and 255 mg/kg and showed significant postprandial and age effect. Of three tungsten compounds, sodium tungstate is most toxic, tungstic oxide is intermediate, and ammonium paratungstate is least toxic. The feeding of 2.5 and 10% of diet as tungsten metal over a period of 70 days has been shown to be without marked effect upon the growth of male rats, as measured in terms of gain in weight, though it caused a 15% reduction in weight gain for female rats from that of control.

Industrial exposure is related chiefly to substances associated with the production and uses of tungsten, its alloys and compounds, rather than tungsten itself. In the mining and milling processes, the main hazards seem to be exposure to quartz-containing dust, noise, hydrogen sulphide, sulphur dioxide and chemicals such as sodium cyanide and sodium hydroxide. The exposure may be associated with other metals in the ore, such as nickel.

Hard metal is the mixture of tungsten carbide and cobalt, to which small amounts of other metals may be added. In the tool-cutting industry workers may be exposed to dust of tungsten carbide, cobalt fumes and dust, and carbides of nickel, titanium and tantalum. Following occupational exposure to tungsten carbide dust by inhalation, cases of pneumoconiosis or pulmonary fibrosis have been reported, but it is generally agreed that this “hard-metal disease” is more likely to be caused by the cobalt with which tungsten carbide is fused. Where machining and grinding of tungsten carbide tools is performed, the hard-metal workers may be at risk for the development of interstitial obstructive lung disease, a serious hazard associated with elevated air concentrations of cobalt. The effects of hard metals on the lungs are discussed elsewhere in this Encyclopaedia.

Tungsten carbonyl is a moderate fire hazard when exposed to flame. When heated to decomposition, it emits carbon monoxide. The incidence of accidents and diseases in tungsten mines and mills is not well documented. However, from the scarce data available it can be said that it is less than that of coal mines.

 

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