Investigation of the chemistry of oxygen delignification of low kappa softwood kraft pulp using an organic/inorganic chemical selectivity system

Oxygen delignification (OD) of low kappa softwood kraft pulp was examined in two steps without inter-stage washing as part of an overall program to evaluate the efficiency of a selectivity enhancement system consisting of phenol and magnesium sulfate. Black liquor carryover in the reaction system did not substantially affect delignification and the selectivity of these OD reactions. The residual lignins from both the original pulp and oxygen-delignified pulp with and without the phenol/MgSO4 selectivity enhancement system were prepared and characterized using NMR spectroscopy. The effluent lignins after oxygen delignification were also prepared and characterized. The lignin characterizations provided the basis for the rationalization of the selectivity observed. A significant finding of this study was that the phenol/MgSO4 system in the oxygen delignification reaction appeared to hinder phenolic guaiacyl unit condensation. It also appeared to enrich the levels of p-hydroxyphenyls in the residual lignin.