Extended Delignification Kraft Pulping of Softwoods - Effect of Treatments on Chips and Pulp with Sulfide-Containing Liquors

Abstract

Pretreatment of softwood chips with Na₂ S liquors at moderately-elevated temperatures prior to a kraft stage delignification resulted in higher pulp viscosity at a given kappa number as compared to that of a conventionally-cooked reference kraft pulp. Final pulp yields were equivalent to, or slightly better than for conventional kraft pulps. The high viscosity is due to the lower alkali requirement during the kraft stage as a result of the pretreatment. The decreased alkali requirement is due to the extraction and removal of hemicelluloses during pretreatment which normally produce sugar acids via peeling reactions. The presence of sulfide in the pretreatment promotes lignin degradation and also enhances polysaccharide stability during the subsequent kraft stage. When kraft pulps, pretreated with sulfide-contaim'ng liquors, were further delignified in an oxygen stage, lower kappa numbers at equivalent viscosity were obtained versus conventional kraft-0? treatment. This enhanced polysaccharide stability toward an oxygen stage can also be achieved by treating a conventional kraft pulp with sulfide-containing liquors, including green liquor and black liquor, but a lesser effect is achieved compared with chip pretreatment.