A mathematic model was developed for direct causticization of sodium carbonate with:, titanium dioxide in a semi-batch reactor. The model predictions of the titanate yield and pentatitanate yield showed a reasonable agreement with the experimental data from a pilot-scale fluidized bed operating in semi-batch mode. The rate constant of the present mode. was generally consistent with those obtained from the small-scale batch experiments in the literature. A review:, rate constant considering carbonate, pentatitanate was introduced. All the results provided support that the solid-state direct causticization of sodium carbonate with titanium dioxide in a semi-batch fluidized bed follows, the shrinking model.