New Method for Reduction of Ultra Coarse Tungsten Powder by Ammonium Paratungstate
Ultra coarse tungsten powder is an important raw material for the preparation of ultra coarse tungsten carbide.
Compared with ultrafine tungsten powder, it has a series of advantages, such as less high-temperature defects, higher microhardness and smaller micro strain.
The common process of reduction of ultra coarse tungsten powder by ammonium paratungstate is usually the reduction of ammonium paratungstate into blue tungsten or yellow tungsten, and then by hydrogen reduction furnace. The tungsten powder prepared by this method is regular polycrystalline, and the particle size is mostly controlled between 2 microns and 5 microns, and the powder size is moderate.
In order to prepare ultra coarse tungsten powder, Li or Na was added to tungsten oxide, as far as the average degree of tungsten powder is concerned, the process of adding Li to ammonium paratungstate is the most effective, but there is a large amount of residue in the powder; At the same time, studies have shown that adding a small amount of Na element in ammonium paratungstate, can not only improve the Fisher particle size of tungsten powder, and can improve the uniformity of doping elements Po tungsten powder can greatly improve the tungsten powder to make the powder of FSSS, but the uniformity is poor.
Some scholars have suggested that Na and Li co permeation of ammonium paratungstate can improve the particle size of tungsten powder and improve uniformity. This is because the particle size of tungsten powder mainly depends on the particle size of ammonium paratungstate, the addition of crystal seeds before the crystallization of ammonium paratungstate solution can inhibit the formation of new nucleation. If the amount of seed "appropriate" grain ammonium paratungstate products obtained mainly by seed growth, product size without seed should be rough. The results show that the average particle size of the ammonium paratungstate powder increases from 4 m to 15 m after adding NaCL, and the average particle size of the secondary ammonium paratungstate powder after adding Li2CO3 grows from 4 um to 23 um, NaCL and Li2CO3 promote the growth of ammonium paratungstate powder particle size significantly, the combination of the two can grow up to 60 um.
In addition, the ammonium paratungstate powder added with the reagent is reduced to yellow tungsten or blue tungsten, and finally the coarse tungsten powder is formed under hydrogen reduction, in which the temperature control of hydrogen reduction also plays an important role.
The use of low hydrogen flow rate, large loading volume, slow push speed, 1000 °C high temperature reduction conditions can further improve the alkali metal salt on the role of particle growth, the particle size of the tungsten powder was doubled to a maximum particle size of 150 um and an average particle size of 68 um.
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