Oxygen delignification of conventional and high alkali cooked softwood Kraft pulps, and study of the residual lignin structure

The selection of the optimum residual pulp lignin content in Kraft cooking has been the focus of many investigations in order to improve both the pulp yield and viscosity. Kraft cooking at a high alkali concentration (HA-Kraft) results in a higher yield compared to conventional Kraft pulping. Furthermore, the (carbohydrates yield/delignification) selectivity of oxygen delignification (O-delignification) below the fiber liberation point to a fully bleachable pulp is more selective than that of Kraft cooking. In order to obtain a high yield and preserve pulp quality, pine HA-Kraft and conventionally prepared Kraft (Ref-Kraft) pulps with kappa numbers 60 and 80 were subjected to O-delignification, during which constant but low-caustic and high-dissolved oxygen concentrations were secured by means of a flow-through (FT) reactor. The effects of operational conditions were investigated at high dissolved oxygen concentration. In addition, the chemical structure of the residual lignin of the pulps before and after O-delignification was investigated by P-31 NMR. It showed that the carboxylic acid content of the residual lignin increased by 50-200% during oxygen delignification. P-31 NMR data also indicated that the carboxylic acid content inside the residual lignin of oxygen-delignified pulps from Ref-Kraft samples was higher (20-70%) than that of HA-Kraft pulps. The results also demonstrated that the guaiacyl phenolic group content in the kappa range of 60 is higher compared to kappa 80 regardless of the cooking procedure that was utilized.