Chemistry at Work With Tungsten Products

The chemistry of tungsten is complex. The metal has different oxidation states in compounds, the most usual being +6. The yellow oxide WO, is an important example. In alkaline water solutions the hexavalent element is present as tungstate ion 

The industrial chemical treatment of tungsten concentrate uses scheelite, either concentrated scheelite or synthetic scheelite, manufactured from woframite concentrates. In this latter case wolframite is treated with sodium hydroxide or sodium carbonate to form sodium tungstate. Dissolution in water and filtering removes the hydroxides of iron and manganese. Addition of calcium chloride to the filtrate precipitates calcium tungstate - synthetic scheelite. 

Scheelite, natural or synthetic, is attacked by hydrochloric add in an autoclave. An almost insoluble residue of WO,-5 1120, contaminated by silicates from the gangue, is obtained. On treatment with ammonia this oxide-hydrate is dissolved to form a colorless solution and purified by filtration. After evaporation a white substance crystallizes, ammonium paratungstate APT (or ammonium parawolframate APW) with the formula (NH ) j H j Wj O g. APT has been an important substance in the worldwide trade in tungsten products. When this compound is ignited pure WO, is formed. 

Tungsten powder is obtained by reduction of WO, with hydrogen gas at a temperature of800°C. The very hard, wear-resistant, tungsten carbide WC is prepared by reaction between W powder and carbon powder at a temperature above 1600°C. Tungsten carbide WC has a density of 15.72 g/cm and a Vickers hardness of 2350. 

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