A model for the role of carbon on carbochlorination of TiO2

The carbochlorination of TiO2 is characterized by two successive reaction s tages, named the first and the second stage. In both stages, reaction rate increases as carbon mass fraction increases. This kinetic effect of carbon was interpreted on the basis of a model developed to describe the rate of e ach reaction stage as a function of carbon mass fraction in TiO2-C powder m ixture. The model considers, for both reaction stages, two global steps: [I ] the formation of gaseous intermediates at the carbon surface and [II] the reaction of these intermediates with TiO2 to form the reaction products. T he mathematical treatment from these steps involves the adaptation of the g eneral model applied to catalytic gas-solid reactions. The first reaction s tage occurs, for high carbon contents, under strong external chlorine trans port control and, for low enough carbon content, under internal mixed contr ol. The second reaction stage occurs out of external gaseous influence and a net chemical control operates, both at low temperatures and for the lowes t carbon content. The intrinsic activation energies for the first and secon d reaction stages are 40 and 117 kJ-mol(-1), respectively.