Extended Delignification by Kraft Pulping Followed by Oxygen/Alkali Treatment: Technical and Economic Evaluation, 1993 Pulping Conference Proceedings

It is well established that the removal of lignin by an oxygen/alkali treatment is more selective than by conventional pulping. The aim of this study was to evaluate the feasibility of pulping to kappa numbers ranging from 30 to 60 followed by one or two oxygen stages to lower the lignin content of the pulps entering the bleach plant and at the same time to minimize the load to the boiler. The kappa numbers of the pulps after oxygen treatment ranged from 8 to 24. Computer aided process simulation of the mill, using GEMS, were carried out for all the cases. It could be shown that by pulping to a kappa number of 40 followed by two stage oxygen treatment [40(00)] to kappa 13, the road to the recovery boiler is lower than in the base case - pulping to kappa 30. In addition, the operating costs as well as the pollution potential measured by AOX and BOD of the fully bleached pulps by a conventional sequence (DEopD) were lower when compared to the base case. Even lower loading to the boiler could be achieved if the pulping is terminated at kappa number 50 and followed by two stage oxygen delignification [5O(OO)].

A number of kraft pulps delignified with oxygen, (30(O), 40(00), and 50(00)), were bleached by DEopD as well as ZEopZP sequences. Yields and strength properties of these pulps were compared to 30 kappa number pulps bleached with DEopD or OZEopZP sequences.

These studies demonstrated that pulping to higher kappa number followed by extended delignification by oxygen can be applied to lower the kappa number of pulps sent to the bleach plant without increasing the load to the recovery boiler, Thus, this technology may enable recovery-limited kraft mills to implement oxygen delignification and TCF bleaching in a closed effluent cycle without increasing their recovery loading.