Kinetics of the direct causticizing reaction between sodium carbonate and titanates

Gasification of kraft black liquor increases the amount of Na ₂CO₃ which needs to be converted into NaOH in the lime cycle because one mole of Na₂CO₃ is created for each mole of gaseous sulfur produced during gasification, while sulfur ends up as Na ₂S in conventional recovery. This disadvantage can be eliminated by gasification of kraft black liquor in the presence of titanates, leading to direct causticization of Na₂CO₃. The purpose of the present research is to determine the kinetics of direct causticization of Na₂CO₃ by both TiO₂ and Na ₂O.3TiO₂ at relative low temperatures (below 750°C) in a novel reactor consisting of a stack of Al₂O₃ trays. The reaction rate is determined from the CO₂ content of a carrier gas flowing over a thin layer of a mixture of Na₂CO₃ and TiO₂ or Na₂O.3TiO₂ in each tray, and by continuous (buoyancy-corrected) weight loss measurement. Excellent agreement is obtained between the two methods. The variables studied are the CO₂ concentration (0 to 10%) and temperature (630°C to 740°C). The products were analyzed by XRD measurements. The final yield of NaOH is tested by titration.