The in-flight reductions of Fe
2O
3, Cr
2O
3, TiO
2 and Al
2O
3 were carried out in the Ar-H
2 and Ar-CH
4 plasma in order to elucidate the reduction ability of hydrogen and carbon in thermal plasma.
The Ar-H
2 plasma reduces Fe
2O
3 and Cr
2O
3 to their metals. TiO
2 is reduced to Ti
2O
3 and Ti
3O
5. Al
2O
3 is not reduced. The products contain non-spherical and spherical particles, which are quenched and cooled from gas and liquid respectively. The reaction mechanisms are explained as follows, from the experimental observations, the phase diagram calculation and the heat conduction calculation. The oxide particles in the plasma are heated quickly, and then they melt and vaporize. Fe
2O
3 and Cr
2O
3 are vaporized as a metallic state in Ar-H
2 plasma. They are quenched and precipitated as non-spherical particles of their metals. TiO
2 and Al
2O
3 are vaporized as their suboxides such as TiO, AlO and Al
2O. They are precipitated as non-spherical particles and oxidized to stable oxides such as Ti
2O
3, Ti
3O
5 and Al
2O
3 during the cooling. The particles which are not vaporized completely are cooled from liquid phase as spherical particles. The phase diagram calculation is used to predict the quenched phase and the quenched temperature. The results of the heat conduction calculation of the particles in the plasma support this vaporizing and quenching mechanism.
The Ar-CH
4 plasma reduces Fe
2O
3 and Cr
2O
3 to their metals. Chromium carbide and titanium carbide are obtained from Cr
2O
3 and TiO
2 by this treatment. The mechanisms are induced as follows. Fe, Cr, Ti and Al are vaporized as metallic sates or the suboxides. When they are cooled, Cr and ti make their carbides and Al precipitates as Al
2O
3. The production of carbides makes it difficult to obtain metals by reduction of oxide with carbon.
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