REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 7

Carbothermal reduction of tungsten oxides with carbon monoxide, or gas mixtures of CO/CO₂, CO/H₂, CH₄/H₂, C₂H₄/H₂, and C₂H₄/H₂, as well as by reaction between metal oxide vapor and solid carbon have recently attracted attention for produci.ng high surface area tungsten carbides (up to 100m²/g), for use as catalyst, and for nanophase WC/Co composite powders.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 6

Direct carburization is advantageous mainly in the production of submicron tungsten carbide powders. The idea behind the process is to circumvent the expensive hydrogen reduction of fine tungsten powder by excluding water vapor during the reduction step, which is responsible for grain growth reactions and low capacity. Roughly speaking, the grain size of the final carbides is related to the size of the internmediate oxides.

Only recently was a new carbothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process). Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor WC_1-x) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then undergoes a second heat treatment to convert the precursor into substantially pure WC.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 5

Under industrial conditions, the process can be performed in two steps, one following the other. The first step-reduction-affords the complete exclusion of hydrogen and is performed in nitrogen atmosphere. In the presence 0f hydrogen, water vapor would form which generates tungsten crystal growth and consumption of carbon (C+H₂O→CO+H₂). This carbon consumption is not only restricted to the carbon black or graphite used as reagent (disturbance of the carbon balance), but also to the furnace carbon tube (shortening its lifetime). The second step-carburization-needs a hydrogen atmosphere, which supports the carbon transport via methane to the tungsten particle surface.

Accordingly, the direct carburization process starts with a pelletized mixture of WO₃ and graphite passing step-by-step two rotary furnaces, the first operating at 950-1250℃ under nitrogen (reduction) and the second at 1400℃ under hydrogen (carburization).

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 4

Carbothermic reduction of WO₃ 0r ore concentrates is of technical importance in melting metallurgy-preparation of ferrotungsten, melting base, and cast carbide.

The reduction of WO3 by carbon in the solid state, however, has gained technical interest and importance m the case when the desired final product is tungsten carbide. This process is named “Direct Carburization” and the basic reaction is described by the following equation:

   WO₃+4C→WC+3CO

In contrast to the conventional procedure for WC production-W03 reduction by hydrogen followed by W+ C mixing and carburization-this method allows the formation of WC in only one step, while, carbon acts as carburizing and as reducing agent.

The above equation is only of theoretical value because, besides the main reaction, the following reactions also occur:

WO₃+5/2C→WC+3/2CO₂

C+CO₂→2CO

Consequently, in this procedure, an uncertainty in the carbon demand is given. This means that, depending on the operating conditions, the carbon content of the final product may vary significantly.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS -3

Although the reducing agent-carbon-is very cheap, and carbon reduction was the early basis of tungsten powder production, so far none of the numerous carbothermic procedures has been established in the production of pure tungsten.

The reasons for that are:

·The carbon balance is difficult to control; furthermore, the metal forms carbides. Although the theoretical carbon consumption to reduce W03 to W is 15.5 wt%, only 12-14 wt% are normally sufficient at 1200℃ t0 1400℃because the reaction does not only proceed via WO₃ + 3C→ W +3CO but also to a certain extent via 2W03+3C W+3.C02. Therefore, in most cases the reduction is either incomplete or the metal powder is contaminated by carbides.

·Carbon is always a source of increased tungsten contamination because it contains impurities like Ca, Si, Fe, S, and P.

·Another disadvantage of the carbothermic reduction is the limited possibility of steering the grain size by varying the reduction conditions as compared to the hydrogen reduction process.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS-1

The reduction kinetics are based on an adsorption-autocatalytic reaction. Reduction enthalpy and temperature are linked by a linear relationship. The calculated activation energy is about 75 kJ•mol^-1. Carbide formation occurs only when all oxygen is removed.

The apparent activation energy at the beg, inning of the reaction is 121kj/m0l WO₃ and increases during further progress t0 205 kJ•mol^-1 WO3, due to the fact that the reaction becomes diffusion-controlled (diffusion of oxygen to the particle surface).

The assumed reaction mechanism is as follows: direct reaction of WO3 and C at contact zones and. For the main portion, sublimation of W03 to the carbon particles, reduction at the surface of the carbon particles, and desorption of CO and CO₂.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS-1

The reduction of tungsten oxides by carbon or carbon-containing compounds can be easily performed. Statements about the starting temperature for the reaction between WO₃ and solid carbon (carbon blacks, graphite) vary in the current literature between 655℃ and 783℃. Differences in WO₃ and C properties (particle size of the powders, preparation history, crystallinity, etc.) as well as in atmospheres may be responsible for that. The temperature range coincides with the beginning of self-conductivity and sublimation of W03: Carbon monoxide starts to react with W03 at 535℃.

The reduction/carburization sequence can be assessed from so-called predominance area diagrams (Kellogg diagram s), which can be derived from free energy of formation data of the compounds occurring in the W-C-O system (WO_3, WO_2.9, WO_2.72, WO₂, W, W₂C, WC, CO, CO₂). It predicts that at 1100℃ the reduction/carburization proceeds via the oxide phases（WO2.9→ WO_2.72→WO₂→W→W₍₂₎C）, with the possibility of a direct carburization of WO_2.72 and WO₂.

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