Oxidation pathway and kinetics of titania slag powders during cooling process in air

The oxidation pathway and kinetics of titania slag powders in air were analyzed using differential scanning calorimetry (DSC) and thermogravimetry (TG). The oxidation pathway of titania slag powder in air was divided into three stages according to their three exothermic peaks and three corresponding mass gain stages indicated by the respective non-isothermal DSC and TG curves. The isothermal oxidation kinetics of high titania slag powders of different sizes were analyzed using the ln-ln analysis method. The results revealed that the entire isothermal oxidation process comprises two stages. The kinetic mechanism of the first stage can be described as

,

$f\left( \alpha  \right) = 1.97\left( {1 - \alpha } \right){\left[ { - \ln \left( {1 - \alpha } \right)} \right]^{\left( {1.97 \;-\; 1} \right)/1.97}}$

, and

$f\left( \alpha  \right) = 1.18\left( {1 - \alpha } \right){\left[ { - \ln \left( {1 - \alpha } \right)} \right]^{\left( {1.18 \;-\; 1} \right)/1.18}}$

. The kinetic mechanism of the second stage for all samples can be described as

$f\left( \alpha  \right) = 1.5{\left( {1 - \alpha } \right)^{2/3}}{[1 - {\left( {1 - \alpha } \right)^{1/3}}]^{ - 1}}$

. The activation energies of titania slag powders with different sizes (d₁ < 0.075 mm, 0.125 mm < d₂ < 0.150 mm, and 0.425 mm < d₃ < 0.600 mm) for different reaction degrees were calculated. For the given experimental conditions, the rate-controlling step in the first oxidation stage of all the samples is a chemical reaction. The rate-controlling steps of the second oxidation stage are a chemical reaction and internal diffusion (for powders d₁ < 0.075 mm) and internal diffusion (for powders 0.125 mm < d₂ < 0.150 mm and 0.425 mm < d₃ < 0.600 mm).