Conventional. The main production steps involve the use of tungsten metal powder, which is mixed with high-purity carbon black, filled into graphite boats, and pushed through a high-temperature furnace under hydrogen at temperatures above 1300 °C. In contrast to the reduction step for tungsten, temperatures are much higher, and single tube push type furnaces (Fig. 11) are used. The particle size of the tungsten metal powder, used for carburization, determines directly the particle size of the tungsten carbide powder (Fig. 12).

Fig. 11 Single-tube push type tungsten carbide powder production furnace. Courtesy: Wolfram Bergbau- und Huetten-GmbH Nfg KG, Austria
Fig. 12 Correlation between tungsten metal and tungsten carbide particle size for ultrafine and coarse powders. Courtesy: Wolfram Bergbau- und Huetten-GmbH Nfg KG, Austria

After cooling the tungsten carbide powder is deagglomerated by means of ball mills, jet mills, or crushers. The deagglomeration step just separates particles from sticking to each other and is not used to adjust the particle size (Ref 43). The final process steps are homogenization (in double cone blenders) and sieving.

Fine Grained Powder Production. Recently, there has been an increasing demand for tungsten carbide powders finer than 1 /'m. Together with the limitations of the conventional process concerning the lower efficiency of fine grained tungsten metal powder production (Ref 44) there has been a strong motivation to develop alternative production routes.

Tokyo Tungsten has developed the direct carburization of tungsten oxide (WO3) as an alternative to the conventional process (Ref 45, 46, 47). This process is carried out today on an industrial scale. The main production steps are as follows:

• Production of pellets from a mixture of tungsten oxide with carbon black

• Subsequent reaction in rotary furnaces at temperatures up to 1600 °C (Ref 47) to tungsten carbide powder in two steps: first the direct reduction of the WO3 to tungsten by carbon under nitrogen, followed by carburization to WC under hydrogen. Particle sizes of 0.1 to 0.7 /,!m can be produced (Ref 47).

More recently another direct carburization process has been developed by Dow Chemical (Ref 48, 49, 50):

• A mixture of WO3 and carbon black falls through a vertical reactor at temperatures around 2000 °C. Thereby the reaction time is typically a few seconds, leading to an intermediate mixture of W, W2C, WC, and C.

• This product has to be subjected to a second, more conventional, carburization step.

The typical particle size range for these direct carburization processes is between 0.1 and 0.8 /,!m. which is identical with the lowest limits of feasibility of the conventional process (Ref 43).

To produce powders finer than 0.1 /'m (100 nm), so-called nanopowders, two processes were developed. The spray conversion process has been promoted by Nanodyne Inc. (Ref 51, 52). Aqueous solutions of tungsten and cobalt salts are "spray dried," subsequently reduced, and carburized with gas mixtures like H2/CH4 or C02/C0 in a fluidized bed reactor. The product is a relatively large (about 75 .i,!m). hollow WC/Co composite powder particle, which consists of WC crystallites with a grain size of about 30 nm. This product must be milled in order to get a powder usable for being compacted (Ref 52).

The chemical vapor reaction (CVR) process, developed and patented by H.C. Starck (Ref 53) is based on the gas phase reaction of metal halides with different gas mixtures, to produce nanosized powders (5 to 50 nm). Production of tungsten carbide powder is possible in principle, although the process seems to be better suited for production of nitrides or carbides of other group IV and V refractory metals (e.g., TiN).

Coarse-Grained Powder Production. The conventional tungsten carbide powder production route is usually limited by the application of carburization temperatures around 2000 °C. With these temperatures, it is possible to carburize 50 to 100 /'m tungsten metal particles, but the resulting product is poly crystalline (Ref 43).

The growth of tungsten carbide single crystals can be highly boosted by the use of a liquid phase, which is able to dissolve both tungsten and carbon. The Menstruum process (also called macro process) involves the formation of tungsten carbide within the melt of auxiliary metal, such as Fe (Ref 54, 55). Mixtures of tungsten ore concentrates, Fe3O4, Al, CaC2, and/or C react exothermally (after ignition by using a starter) to form WC, Fe, CaO, and Al2O3. Temperatures of about 2500 °C are reached and complete reduction and carburization of about 22 tons of WC is achieved in 60 minutes. To get the tungsten carbide in powder form, the solidified auxiliary metal has to be dissolved, which is typically done with hot hydrochloric acid. Separation of the obtained tungsten carbide powder according to particle size is done by sieving operations. Menstruum tungsten carbide powders have a high extent of well faceted single crystal particles, contain approximately 0.2% Fe, and have typical particle sizes of 40 mesh (about 400 /'m) and below.

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