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INTERNATIONAL TUNGSTEN INDUSTRY ASSOCIATION

Information on tungsten: sources, properties and uses

Tungsten Processing

Concentrates (Primary Tungsten Sources)

Modern processing methods dissolve scheelite and wolframite concentrates by an alkaline pressure digestion, using either a soda or a concentrated NaOH solution. The sodium tungstate solution obtained is purified by precipitation and filtration, before it is converted into an ammonium tungstate solution. This stage is carried out exclusively by solvent extraction or ion exchange resins. Finally, high purity Ammonium-Paratungstate (APT) is obtained by crystallisation, with the formula (NH4)10(H2W12O42) ·4H2O.

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.

Scrap (Secondary Tungsten Sources)

Tungsten scrap due to its high tungsten content in comparison to ore is a very valuable raw material.

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 (average coil weight: <30 mg; absolutely not economic to recycle), 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 (Co, Ta). Recycling is always more environmentally friendly and usually more economic than waste disposal.

© 2011 International Tungsten Industry Association
Copyright images courtesy of: CERATIZIT (Scrap)