Improving Extended Delignification Technology for Kraft Pulping. Part I: A New Two-Stage Batch Cooking Process, 1997 Pulping Conference Proceedings

A new modification of kraft cooking has been developed to provide extended delignification and improved selectivity in a simpler way than has been possible previously. The cooking is performed at conventional high temperature, and in two stages. The first-stage liquor is a mixture of the spent liquor from the second stage of previous cooks and about half of the fresh chemicals normally needed for a conventional cook. The second-stage liquor is made by adding fresh water or washer filtrate to the remaining portion of the fresh cooking chemicals. Pilot-scale cooking results show that this process can reduce kappa number by 25-30% with conventional cooking time and chemical charge. When compared at the same kappa number, the two-stage pulp is about 20-30% higher in viscosity than conventional pulp. Above 20 kappa number, the unbleached yield of the two-stage pulp is lower than that of conventional pulp, but is higher if cooking is extended below 20 kappa number. The physical strength of the two-stage pulp at 16 kappa number is as good as that of conventional kraft pulp at 27 kappa number. We attribute the advantages obtained in this process to the modified cooking chemical concentration and the lignin concentration profiles during the cooking.