Promotion of Solid TiO₂ Reduction by Molten Magnesium from the Perspective of Reaction Mechanisms

Abstract

The possibility of direct reduction of TiO₂ by molten magnesium has been investigated using TiO₂ single crystals and powders. First, reduction experiments were carried out using TiO₂ single crystals at 1173 K for up to 8 h, resulting in formation of two layers consisting of oxide phases with different Mg and Ti concentrations between magnesium and TiO₂, in which layers TiO₂ was reduced only to TiO_0.5 as a maximum and there was no metallic titanium obtained. The thicknesses of both the oxide layers increased according to the parabolic rate law, and Mg is reported to have diffusivity greater than the others, suggesting that the reduction rate of TiO₂ is controlled by the diffusion of Mg across the oxide layers. Second, reduction experiments were carried out using TiO₂ powers having 45 and 100 μm diameters at 1173 K for up to 48 h, resulting in reduction of TiO₂ to form metallic titanium, TiO_0.5 and TiO, indicating the effect of change in figuration of TiO₂ from single crystal to powder. However, difference in powder size did not appear in the degree of reduction presumably due to sintering effect during reduction. Finally, reduction experiments were carried out using TiO₂ powers having 45 μm diameters with additions of MgCl₂ at 1173 K for 30 h because MgCl₂ is miscible with MgO. TiO₂ was reduced to form only metallic titanium and TiO_0.5, without TiO remaining as titanium oxide, suggesting the removal effect of MgO by MgCl₂ which may allow continuous reduction on the TiO₂ surface. Consequently, it has been found that additions of MgCl₂ are effective to promote reduction of TiO₂ by molten magnesium.