On the role of carbohydrates in oxygen delignification

The decrease in efficiency of the oxygen delignification of chemical pulps at low kappa numbers has been investigated. In order to study the role of the carbohydrates, five different fully bleached pulps were oxygen delignified and carefully analysed. The results show that no permanganate-consuming structures were formed in the carbohydrates during oxygen delignification, but that small amounts of lignin are slowly degraded. Unexpectedly, the ISO brightness was significantly increased, while the yellowing tendency was reduced, in some cases dramatically, making a final oxygen delignification treatment after a bleaching sequence an interesting option for market pulp producers with yellowing problems.