Bleaching softwood kraft pulp: the role of chlorine dioxide dosage and final pH in the D stages

The effects of chlorine dioxide dosage and final pH in the D stages of the bleaching process were investigated in laboratory experiments using hatch cooked and oxygen-predelignified softwood kraft pulp, The results clearly indicate that the optimum final pH was different in each stage (D-0, D-1 and D-2) and depended on the chlorine dioxide doses, The more chlorine dioxide used, the lower the optimum final pH after each stage. The lowest kappa number, measured after the first alkali stage, in the D-0 stage was achieved at final pH values of 2.0-3.5 at all the chlorine dioxide doses tested, When the final pH was controlled with alkali in the D-1 stage, the optimum pH range of 3.0-4.0 was attained with an average increase in brightness of 1.5-2.0% ISO at all chlorine dioxide doses, Correspondingly, the optimum pH range in the D-2 stage was 4.0-5.0 and the average improvement in brightness about one pet-cent. The use of alkali lowered the viscosity by an average of 50 dm(3)/kg per additional kg of NaOH at all chlorine dioxide doses in the D-2 stage, Final pH control had no direct effect on chlorine dioxide consumption in the D stages.