Patent ID: 7919067

Claim:
A method for controlling a causticizing process for producing white liquor from green liquor by feeding green liquor (H 2 O, Na 2 CO 3 , and Na 2 S) and lime (CaO) into a slaker to produce hydrated lime (Ca(OH) 2 ) according to the equation H 2 O+CaO—>Ca(OH) 2 +heat, caustizing the green liquor by reacting the hydrated lime and sodium carbonate (Na 2 CO 3 ) within the green liquor to produce sodium hydroxide (NaOH) and calcium carbonate (CaCO 3 ) according to the equation Ca(OH) 2 +Na 2 CO 3 —>NaOH+CaCO 3 , and separating the calcium carbonate from the sodium hydroxide, the method comprising controlling the causticizing process by: measuring the density of the green liquor being fed to the slaker; controlling the density of the green liquor being fed to the slaker towards a set value with a density controller wherein green liquor density is controlled on the basis of a total titratable alkali by applying the following equation: D=(TTA+os)/kk, where D is the green liquor density; TTA is the total titratable alkali of the green liquor; os is an offset; and kk is a coefficient, wherein the coefficient is a constant angular coefficient having a value selected from between 0.9 and 1.4 when the unit used for expressing the TTA and the density is the same, the offset being determined on the basis of the model; and, specifying the set value by: determining a target value for the total titratable alkali; measuring the total titratable alkali in the green liquor being fed to the slaker; providing a model that relates green liquor density to the measured total titratable alkali in the green liquor; calculating the set value for green liquor density based on the measured total titratable alkali in the green liquor, the target value for the total titratable alkali, and the model; and, updating the model by using the measurement result of the density and the measurement result of the total titratable alkali.