On the mechanism of carbothermal reduction processes of TiO2 and ZrO2

In this paper an experimentally determined reaction mechanism for the carbothermal reduction processes of TiO2 and ZrO2 in an inert atmosphere, both of which consist of three reaction steps and are characterized by the appearance of oxide intermediates of low volatility, is proposed and discussed. The mass transfer of the whole reaction is realized by the CO/CO2 mass transport mechanism. An immediate regeneration of CO2 to CO by reaction with solid carbon is necessary. The overall reaction rate therefore depends on the structure of the carbon (graphite, carbon black, active carbon) used in the mixture with the oxide. In carbothermal reduction processes of TiO2 and ZrO2 the oxide particles are the precursors for the hard phase particles formed. However, in the first reaction step CO acts as a reducing agent only and TinO2n-1 and ZrO2-x are formed as intermediates. In reaction step two oxycarbides are formed by further reduction accompanied by the incorporation of carbon. The reaction rates of oxygen loss and carbon incorporation are not equal. In reaction step three TiC and ZrC are formed from the corresponding oxycarbides. The CO/CO2 mass transport mechanism was confirmed experimentally by mass spectrometry, adsorption measurements and field emission scanning electron microscopy of the starting and reacted oxide/carbon powder mixtures. The reaction mechanism proposed in this paper for TiO2 and ZrO2 is in agreement with the reaction model of Weimer et al., for SiO2 which forms a volatile intermediate (SiO). In this case the carbon particle is the precursor of the hard phase particle formed.