Influence of kraft pulping on the kinetics of oxygen delignification

Conventional kraft pulping was performed on mixed northeastern hardwood chips in a laboratory digester. Four pulps of kappa no. 15-16 were produced with the sulfidity held constant while the effective alkali charge and H-factor were varied. The pulps were used in oxygen delignification experiments performed under industrially relevant conditions. Response variables were: kappa number, viscosity, COD, residual alkali, and final pH. Values were found to depend on the effective alkali concentration used in pulping. The results suggest that more refractory lignin moieties occur during kraft pulping at low alkali levels and high H-factors. An alternative explanation is that pulps with high xylan content are more difficult to delignify in an oxygen stage.