The effect of pulp screening on oxygen delignification of high lignin content pulps

This study investigates the impact of pulp screening on oxygen delignification of high lignin content kraft pulps from spruce wood. An alternative process is proposed: terminating kraft cooking at higher kappa numbers and applying oxygen delignification directly to unscreened and non-defibrated pulp. The objective is to evaluate whether this non-standard approach can maintain delignification efficiency while improving yield and reducing energy input. The findings demonstrate that screening prior to oxygen delignification is not essential for effective lignin removal or pulp quality. Similar delignification degrees and ISO brightness levels were obtained after oxygen delignification, whether it was performed on screened or unscreened pulps. Notably, the delignification rate in the oxygen stage was the same for the non-standard procedure as for pulp from the standard procedure with the reject fraction removed prior to the oxygen stage. No significant differences were seen in fiber morphology, brightness level, or brightness stability. The amount of total fiber charges in pulps not screened before oxygen delignification was slightly higher than in screened pulps. In conclusion, the study shows that screening before oxygen delignification is not essential for effective and homogeneous lignin removal or pulp quality. Applying oxygen delignification directly to unscreened pulp with high lignin content can enhance yield, when applied on high kappa numbers, and reduce energy demand, since the wood chips do not need to be mechanically refined to continue the process. This change can offer a more sustainable and efficient approach to kraft pulp production.