Recausticizing Kinetics with Mill Liquor and Lime, 1991 Forest Products Symposium Proceedings

The kinetics of the recausticizing reaction were determined using a 3.8 L batch reactor with kraft pulp mill green liquor and lime at different stoichiometric ratios. Several pressurized causticizing experiments were made at elevated temperatures and using different green liquor strengths. A kinetic model was compared with the experimental data using nonlinear least squares and numerical integration of the rate expression. The use of reaction temperatures above 377.6 K (220 degrees Fahrenheit) increases the reaction rate and sodium carbonate conversion. Operation of the recausticizing process with a high TTA (total titratable alkali) liquor reduces the reaction rate and sodium carbonate conversion. The equilibrium conversion of the sodium carbonate in a typical mill green liquor is 89.4% which is a causticity of 90.4%. Lime charges greater than 90% stoichiometric are more than the amount required for equilibrium conversion. A change in CaO:Na2CO3 ratio from 90 to 100% using mill lime has a substantial effect on the reaction rate while increasing the ratio from 100 to 110% is not significant. The use of the kinetic model for estimation of causticity of white liquor produced in a reactor sequence of a slaker and several causticizers is shown for several cases.