The effect of lignin demethylation on oxygen delignification

Treatment of radiata pine kraft and kraft-oxygen pulps with sodium ethanethiolate (NaSEt) in dimethylformamide (DMF) reduced the methoxyl content of the fibre lignin. Demethylation increased as the level of NaSEt applied was raised. Lignin removal in a following oxygen stage increased as the extent of demethylation was raised. For example, after NaSEt pretreatment low consistency oxygen delignification reduced the kappa number of a kraft-oxygen pulp from 17.9 to 3.8, versus 7.9 for the non-pretreated pulp. The methoxyl content of lignin isolated from pretreated kraft or kraft-oxygen pulps was approximately linearly related to the kappa number of the pulp after oxygen delignification. Improved methods for demethylation of the fibre lignin are required if their enhanced reactivity to oxygen is to be exploited, as treatment with NaSEt in DMF led to significant losses in both pulp viscosity and pulp yield.