Tungsten Carbide Preparation by Sodium Tungstate
Tungsten carbide (W2C) is composed of a tungsten and carbon black compound, six crystal, metal luster, hardness and diamond are similar, extremely difficult to melt.
Tungsten carbide is a good conductor of electricity and heat, mainly used in the production of cemented carbide materials. The traditional tungsten carbide process, must be go through the production of ammonium paratungstate, tungsten powder preparation and carbonation and many other processes, the production cost is high and the process is lengthy. In order to reduce the production cost of tungsten carbide, the approach is to reduce the process from mineral raw materials to product manufacturing processes.
There are many methods for the preparation of tungsten carbide. For example, high temperature melt extraction, carbonization and reduction by aluminum, tungsten carbide is directly obtained from high-purity tungsten concentrates. These methods are simple, crude and effective, but the temperature requirements are particularly high. They need to be realized by the high temperature calcination equipment above 2000 °C, and some enterprises are difficult to achieve. Therefore, we have to explore another new shortcut for the direct reduction of tungsten carbide from sodium tungstate.
Sodium tungstate is an intermediate transition from tungsten concentrate to APT. The chemical composition is Na2WO4·2H2O crystal or colorless crystalline powder. Some scholars used calcium acid as carbon source to synthesize tungsten carbide with sodium tungstate.
In the experiment, the sodium tungstate powder was first dried at 120 °C in a vacuum drying chamber for 2 hours, the sodium tungstate powder without crystalline water is obtained, with carbide powder grinding in a planetary ball mill for 15 minutes at the same time, and then the columnar mass is obtained by the briquetting mechanism, after weighing, the graphite crucible was placed in a vacuum reduction furnace for reduction and carbonization. The reduced products obtained by reduction remove the calcium oxide by acid washing.
2NaWO4+6CaC2=W2C+6CaO+2Na2O+11C
After reduction of sodium tungstate by calcium carbide, can obtained purer main phases W2C, tungsten and carbon can produce two kinds of tungsten carbide, WC and W2C. WC is a near normal compound. It can hardly dissolve carbon or tungsten. Therefore, in order to get the WC, the W2C should be obtained with calcination twice, the calcination temperature is 1600 °C, the heat preservation time is 2h. In the two times carbonization process, increasing the heating rate and prolonging the moisture retention time are beneficial to the growth of crystal and decreasing the cooling rate, which is beneficial to refine the grain size of tungsten carbide. Finally, the compound is formed by two times carbonization at high temperature to produce more pure tungsten carbide powder.
The process of converting tungsten sodium tungstate to tungsten carbide eliminates the need for ammonium paratungstate, relatively, it saves a lot of process and cost. Compared with the high temperature melt extraction carbonization and aluminum thermal reduction method, it requires a lower temperature of the reaction furnace, in general, is a law-abiding approach. In industrial practice, however, this shortcut has not attracted much interest, the reason is probably: 1. It is not as straightforward as the high temperature heat reduction method; 2. Tungsten produces tungsten carbide although the cost is slightly higher, its ultimate quality is better controlled.
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