Extended oxygen delignification of high kappa softwood pulp in a flow-through reactor

Oxygen delignification (O-delignification) is an important tool for improving yield, reducing lignin content of pulp and lowering the bleach chemical consumption in the pulp and paper industries. This process is designed after cooking to maximize the delignification rate in comparison to the final phase of Kraft cooking without sacrificing pulp quality. In order to improve the yield of O-delignification, combinations of different approaches were tested in this study. The first was to introduce a high kappa number Kraft pulp into the O-delignification stage to take advantage of the higher yield-delignification selectivity of O-delignification compared to that of the final stage of Kraft cooking. Another was to keep the alkali concentration at a relatively low but constant level throughout the O-delignification stage to minimize the cellulose degradation during delignification. This was achieved by using a flow-through reactor (FT-R) configuration. A high degree of delignification (> 76%) was achieved at low alkali but high concentration of dissolved oxygen. In addition, reaction kinetic models for delignification and cellulose degradation were proposed for FT-R oxygen delignification of high kappa number pulps to allow optimization of the delignification and selectivity of the oxygen stage to reach a target kappa number. Finally, to avoid excessive cellulose degradation at higher degree of delignification over 76%, the addition of different viscosity-preserving chemicals was examined while the temperature was raised to a higher level than that during conventional oxygen delignification (110-115 deg C). At these high temperature conditions, the condensed and nonphenolic lignin units which are not degraded at the conventional conditions can now be dissolved. An extraordinary high degree of delignification (> 83%) was obtained in the flow-through setup. Furthermore, the results were also compared to batch or multiple batch O-delignification of conventional softwood Kraft pulp (24 kappa) and high kappa softwood pulps (65 kappa) respectively.