Experiences with extended delignification of hardwood and softwood kraft pulp in a continuous digester

A multistage continuous kraft pulping process that provides a more selective delignification than conventional kraft cooking is in commercial operation at a mill in Finland. This improved selectivity is used to lower the pulp's kappa number, which has contributed to reductions in the consumption of bleaching chemicals, to reductions in the effluent load of the bleach plant, and to an increase in the bleach plant's production capacity. When compared at the same kappa number, the modified pulp has a high viscosity, better strength, and consumes less bleaching chemicals than a conventional kraft pulp. Operating experiences, consumption data, and pulp-quality data for hardwood and softwood kraft pulps are reported. 6 references.