BLEACHING SOFTWOOD KRAFT PULP - THE ROLE OF CHLORINE DIOXIDE DOSAGE AND FINAL PH IN THE D-STAGE

The effects of chlorine dioxide dosage and final pH in the D stages of the bleaching process were investigated in laboratory experiments usi ng hatch cooked and oxygen-predelignified softwood kraft pulp, The res ults 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, T he more chlorine dioxide used, the lower the optimum final pH after ea ch stage. The lowest kappa number, measured after the first alkali sta ge, 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 w ith alkali in the D-1 stage, the optimum pH range of 3.0-4.0 was attai ned with an average increase in brightness of 1.5-2.0% ISO at all chlo rine dioxide doses, Correspondingly, the optimum pH range in the D-2 s tage was 4.0-5.0 and the average improvement in brightness about one p et-cent. The use of alkali lowered the viscosity by an average of 50 d m(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.