Effects of impregnation time on hardwood kraft pulp characteristics and papermaking potential – a mill study

Abstract

We conducted a series of trials in a commercial-scale pulp mill producing fully bleached birch hardwood kraft pulp to study the effects of impregnation time. Retention time in an impregnation vessel was altered between 5 min and 60 min. Cooking kappa number target and brightness targets were held constant and the rest of the fiber line was operated according to normal practices. After analyzing collected pulp samples to monitor changes in characteristics and papermaking properties, we found that impregnation time has a significant effect on the consumption of effective alkali in the impregnation stage, but the total alkali consumption was not affected when a cooking kappa number was held constant. Reduced impregnation time required higher cooking temperatures to reach the kappa number target. Shorter impregnation times clearly increased the amount of rejects. Viscosity, carbohydrate composition, and fiber saturation point were not significantly influenced by the impregnation time, but some fiber damage occurred with longer impregnation times. The tensile strength and tensile were higher with the shorter impregnation time, whereas tear strength at a given tensile index did not correlate with impregnation time. An obvious reason for the affected papermaking properties was not found.