Production Methods of Coarse-grain Tungsten Carbide


Refining tungsten is to use tungsten powder and carbon black as raw materials, first mix them at a specific ratio to get a mixture, then put the mixture into a graphite boat and place that into a charcoal tube furnace or a sense of high-frequency electric furnace. After the carbonization under a certain temperature, tungsten carbide powder can be obtained by ball milling and screening.

The formula of coarse-grain tungsten carbide is WC. It has some special properties and usages different from medium and fine grained WC powder, especially that the high-temperature WC has fewer structural defects, higher microhardness, slighter micro-strain, etc. Thus it is widely used in land mining, oil drilling, lathing and so on. Its hardness is only next to diamond with extremely high value.


Main production methods of coarse-grain WC:

A. High-temperature carbonization of tungsten powder.

Longtime high-temperature carbonization can minimize the lattice imperfection and micro-strain of WC and improve its plasticity. This way so far known is the main approach of tungsten carbide production in China. Carbonization temperature should not exceed 1800-1900 degrees centigrade. When the temperature exceeds 1800 degrees centigrade, WC grains tend to integrate on the boundary and grow, resulting in uneven distribution of WC grain. Some studies indicate that reducing the particle size of the raw material tungsten, increasing the carbonization temperature, and shorten carbonization time can improve the quality of WC obtained.

B. Medium temperature reduction and high temperature carbonization of tungsten oxide doped with lithium salt

Main principles of this method are as follows: add additives to accelerate the volatilization and deposition rate during the reduction process of WO3, resulting in tungsten powder’s particle size growth at lower temperatures. Usually lithium salt is used as additive for tungsten powder growth. This method is mainly applied in making mining alloys and cold micro-alloys.

C. Adding cobalt or nickel during high-temperature carbonization

Adding a small amount of cobalt, nickel or their oxides during blending tungsten powder with carbon can change the mechanism of carbonation and improve its carbonation speed. The amount of cobalt affects the grain size of coarse-grain WC produced in this way greatly, larger amount of cobalt results in more coarse WC.

D. Adding salt

Add salt in ammonium para-tungstate, then coarse tungsten powder with particles larger than 10μm can be obtained after reduction at higher temperatures, finally coarse WC powder can be obtained through high-temperature carbonization.

E. Rapid calcination and reduction of ammonium para-tungstate

The essence of this method is the rapid heating calcination of ammonium para-tungstate in the oxidizing atmosphere at 850-1000 degrees centigrade. Then go through a reduction process in hydrogen furnaces after rapidly heated to a temperature of 1100-1300 degrees centigrade. With this method, tungsten powder with particle size of 25-36μm can be prepared.

F. Hydrogen reduction of halide boiling layer

Use H2 to restore tungsten chlorides or fluorides in boiling layer. First, send H2 and the original tungsten powder into the bottom of the reactor for the preparation of boiling layer, and the halide vapor will run into the reactor through the upper portion of the reactor, and restored into tungsten powder by H2 at the optimum temperature and deposited on the original tungsten powder, gradually coarsening the original powder, then tungsten powder can be collected from the reactor regularly. The particle size of tungsten powder prepared through this method is larger than 40μm.

G. Coarse-grain thermite process

Through high endothermic reaction, WC can be produced directly from tungsten concentrate. WC grains produced via this method are coarse, large, single-phase, and high-purity.

H. Carbonization of tungsten concentrates with fused salt

First, use Na2SiO3-NaCl fused salt to decompose tungsten concentrates at a high temperature of 1050-1100 degrees centigrade, and separate the generated Na2WO4-NaCl fused salt from those silicates containing Fe, Mn, Ca, then injected into the molten phase with methane to produce coarse-grain WC. The advantage of this method is its low cost, while the disadvantage is its high impurity (Mo, Cr, Fe, Ni, Si) content and long time chemical treatment it takes.