The effect of incoming kappa number on optimal ECF bleaching of softwood kraft pulp - Finding the right balance of chemicals is essential

Single and double O(2)states, with and without H2O2 reinforcement, were carried out on a softwood kraft pulp kappa 26.8 to optimize bulk delignification before ECF bleaching. Based on percentage delignification, retained fibre strength and operating cost, a double O-2 stage without H2O2 was found to be most effective delignification process; ECF bleaching experiments were undertaken in the laboratory to determine the effect of incoming kappa on chemical consumption. In comparing D(0)EopDEpD to D(0)EoDED, 1 kg H2O2 replaced 1.17 kg ClO2 when 3.5 + 0.5 kg H2O2 were added in the Eop and E(2)p stages, while reducing the kappa factor in D-0 from 0.20 to 0.16 for a 26.8-kappa pulp. When the same experiments were carried out on a 10.9-kappa pulp, the optimum replacement ratio was reduced to 0.95 kg ClO2 / kg H2O2. For fully bleached pulps at 90% ISO and 400 CSF made from 26.8 and 10.9-kappa pulps, OOD(0)EopDEpD retains 1-2% less tensile index and 3 to 5.5% less tear strength than OOD(0)EoDED. Cumulative AOX and BOD5, were reduced by 36 to 51% and 24 to 57%, respectively, and O-2 delignification before ECF bleaching deceased TOX by 17 to 33% in the final bleached pulps.