Chemical Basis for a Selectivity Threshold to the Oxygen Delignification of Kraft Softwood Fiber As Supported by the Use of Chemical Selectivity Agents

Softwood kraft pulps having different lignin contents (κ numbers) were subjected to alkali oxygen under the application of selectivity enhancing agents, namely, MgSO4, phenol, MgSO4/phenol, and 3-methoxyl hydroxyl quinone, to examine the effect on delignification selectivity. The results demonstrated that there is a synergistic effect between MgSO4 and phenol during oxygen delignification. This effect is more obvious for high lignin content pulp than that for low lignin content pulp. Remarkably, examination of the carboxyl groups on pulp fibers and the carbonate content in the effluent liquors demonstrated that the κ number of softwood kraft pulp cannot drop below 7 without incurring cellulose degradation during oxygen delignification despite the application of selectivity enhancement agents.