Optimal delignification degrees of cooking and oxygen alkali stage in production of ECF bleached softwood kraft

The aim of this study is to prepare, in the laboratory, ECF bleached softwood pulps with good yield and good quality by raising the kappa number from cooking and extending delignification with oxygen. Conventional softwood pulps were cooked to kappa numbers of 19.5 - 28.8 - 47.5 and 58.5. The pulps were further oxygen delignified in either one or two stages without intermediate washing. Two-stage oxygen delignification was improved with interstage treatment (peracetic acid or silicoperoxomolybdate) and by intensifying the second oxygen stage with hydrogen peroxide. The pulps were bleached using the sequence DE,,DD. The effect of anthraquinone was also studied at kappa numbers of 26.6 and 51.1. The usual laboratory analyses were supplemented with computer-aided process simulation of the pulp mill (PulpSim) so that the results could be better understood and compared. According to the results, the degree of delignification during oxygen delignification increases as the kappa number from the cook increases, even when alkali dosage per kappa unit is lowered. Activating two-stage oxygen delignification by intermediate treatment with silicoperoxomolybdate or reinforcing the second oxygen stage with hydrogen peroxide gave promising results in terms of kappa reduction. To maximize pulp yield, pulps should be cooked to high kappa and further delignified with oxygen. However, raising the kappa number over 30-35 is limited by an increase in the amount of rejects. The viscosity of bleached softwood pulp is highest at kappa 45-50. Optimum beatability, expressed in terms of beating time to achieve a tensile index of 70 Nm/g, is obtained at kappa 35-40. Pulps delignified in an intensified two-stage oxygen delignification process are more easily beaten than those subjected to ordinary oxygen delignification. Pulp strength is highest at kappa 40-45 when tear indices are compared at a tensile index of 70 Nm/g. The O-OP pulps had the best strength properties. Process simulation provides an idea of how much the capacities of the cooking plant and recovery boiler can be reduced by raising the kappa number om cooking and by extending oxygen delignification. The increase in the efficiency of washing after the oxygen stage needed to achieve the same washing loss (expressed in terms of Na,SO, to the bleach plant) was also calculated. It was found, for ex ample, that on cooking to a kappa number of about 20 followed by one-stage oxygen delignification, less effective washing is necessary (2,85 units smaller value for washing efficiency E-k), the consumption of steam in the evaporation plant is approximately 10% higher and the load to the recovery boiler is 15% higher than on cooking to a kappa number of about 45 followed by two-stage oxygen delignification. Greater effluent treatment capacity is needed when the kappa number of pulp entering the bleach plant is raised.