Ammonium Paratungstate (NH₄)[H₂W₁₂O₄₂].4H₂O is the most important precursor for the majority of tungsten products. Exceptions are only products of melting metallurgy and Menstrum WC produced directly from ore concentrates.

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

APT is a white crystallized powder having average crystal size between 30 and 100 µm. Especially crucial for the quality is the purity. Typical levels of today’s commercial APT are (upper limits in µg/g):  Al 1-7, As 5-10, Bi 0.5-1, Ca 1-10, Co 1-10, Cr 1-10, Cu 1-3, Fe 3-10, K 2-10, Mg 1-7, Mn 1-10, Mo 5-30, Na 5-10, Ni 1-7, P 5-7, Pb 1-5, S 5-7, Si 1-10, Sn 1-10, Ti 3-10, and U 3-10. APT is packaged in polyethylene-lined drums or woven bags up to 1 ton.




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Ammonium paratungstate is a relatively insoluble salt which dissolves only to the extent of 4.5 parts in 100 parts of water at 22℃. and has limited solubility in hot water. On the other hand, ammonium metatungstate dissolves to the extent of 120 parts per 100 parts of water at 15℃. and is very soluble in hot water.

It is frequently desirable to obtain a relatively high concentration of tungsten in solution, Le. 30% W0 or more at ambient temperature. This is done most readily by dissolving ammonium metatungstate. On the other hand, ammonium paratungstate is a more common product which can be obtained by several methods during the recovery of tungsten from its ores. Heretofore, the market price of ammonium metatungstate has been much higher, compared to ammonium paratungstate, because of the difficulty entailed in manufacturing the former.




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At the present time, crystalline AMT is being produced from APT. Although this process overcomes some of the disadvantages of the-prior art, in practice it is somewhat difficult to control for maximum conversion. As a consequence, on crystallization of AMT from solution, the residual APT must be removed in order to produce completely water-soluble AMT products. Another disadvantage of this process is its intermittent batch nature, requiring baking, digesting, filtering and concentrating of solutions before crystallization. On a production scale, such stepwise operations tend to be time consuming and costly.

As an improvement over the above process, a method for producing AMT directly from ammonium tungstate (AT) solution is came up with. AT solution is contacted with a liquid extraction system consisting of Di-Z-ethyhexyl phosphoric acid (D-2-EHPA)-tri-n-butyl phosphate (TBP)- kerosene, which extracts ammonium ion and rapidly lowers the pH to the AMT-forming region. The solution is then digested to allow formation of AMT.

While this method substantially avoids the formation of APT and is amenable to continuous processing, it has not found widespread use due to a tendency toward instability of the organic system, and resultant formation of insoluble phospho-tungstates in amounts up to percent or more of the total solid product, depending upon the conditions of formation.


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There are many benefits belonging to tungsten copper electrodes.

High Wear Resistance
Due to the high melting point of tungsten (3 420 °C), WCu electrodes have a considerably longer service life than copper or graphite electrodes, in particular in the case of small electrodes which have undergone complex machining. Because the production costs of complex electrodes often far exceed the pure material costs.

Extremely Sharp Contours
The low wear levels of tungsten copper electrodes make it possible to focus the arc precisely and achieve greater geometrical accuracy in the workpiece.

High Surface Quality
The use of tungsten copper electrodes result in outstanding surface quality and, in particular, eliminates the need for circular electrode movements.

Good Machinability
The considerably greater modulus of elasticity and increased hardness offered by tungsten copper electrodes lead to exceptionally good machinability.

Improved Environmental and Working Conditions
The machining of graphite requires the use of special extraction systems to remove dust, prevent graphite-smeared floors and keep the air clean. In contrast, the machining of tungsten copper electrodes does not require any additional equipment.

 

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