EXTENDED DELIGNIFICATION WITH A PROLONGED MILD COUNTERCURRENT COOKING STAGE

Enhanced pulping selectivity was attained in kraft extended delignification of softwood (Pinus taeda) by utilizing a prolonged, mild counter-current cooking stage. A softwood kraft pulp c. 20 Kappa number was produced to give pulp viscosity and strength properties at least as good as those of a conventional kraft pulp, Kappa number c.30. Decreased lignin condensation and/or repolymerization, decreased cellulose chain cleavage, and enhanced diffusion of lignin degraded products toward the end of the cook are believed to be responsible for the improved pulping selectivity. Compared to conventional kraft pulping, brownstock and final bleached viscosities were increased by over 50% for softwood and over 30% for hardwood during mill trials. A higher final brightness with much less variation was also observed.