Softwood kraft pulps with different kappa numbers were oxygen-delignified using different selectivity enhancing agents, namely, MgSO_4, phenol, MgSO_4/Phenol, and 3-Methoxyl hydroxyl quinone to examine delignification selectivity. The results showed that there is a synergistic effect between MgSO_4 and phenol during oxygen delignification. This effect is more obvious for high lignin content pulp than that for low lignin content pulp. However, examination of the carboxyl groups on pulp fibers along with the carbonate content in the effluent liquors demonstrated that the kappa number of softwood kraft pulp can not drop below 7 without incurring severe cellulose degradation during oxygen delignification despite the application of selectivity enhancement agents.
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