| Abstract: | The solid state reaction between titanium dioxide and sodium carbonate forming sodium titanates was investigated. Reactions between sodium carbonate and titanium dioxide and/or sodium tri-titanate play a key role in the direct causticization of kraft black liquor. Experiments were carried out in a microdifferential reactor made of quartz glass at varying temperatures up to a maximum of 880 °C. Kinetic data were obtained by measuring the release of carbon dioxide. The physical and chemical properties of the reactants and products were analysed in order to obtain a maximum understanding of the reaction path. Scanning electron microscopy (SEM) and the specific surface area of the reactants and products were applied for morphology determination. X-Ray diffraction (XRD) was employed to characterize the phase composition of the product. The results showed that 100% conversion can be obtained at temperatures above 830 °C. Different kinetic models were taken into consideration, such as the Jander and Valensi-Carter models for diffusion-controlled reaction rates and the phase-boundary model for first-order reaction kinetics. One model was based on the theory that the kinetics can be described by the phase-boundary theory at the beginning of the reaction but, as the reaction proceeds, the rate becomes diffusion controlled. This model gave a good fit to the experimental data collected at 840 °C. However, this model did not work as well at high temperature (880 °C) or at lower temperatures as at 840 °C. This was explained by the fact that the reaction path is different at these temperatures, i.e. other reaction products are involved. |
