Effects of acidic treatments in delignification of kraft pulps

This study explores the role of acid pre-treatments in the delignification of commercial Kraft pulp from both softwood and hardwood. Both the unbleached and the O2-delignified Kraft pulps were examined to reveal the influence of O2 -delignification on the reactivity of residual pulp lignin. Under typical acidolysis conditions conducted in a dioxane-water (82:18) solution at 100°C for1 h, the influence of HCl concentration (0.1--1M) in delignification displayed a similar pattern for a series of Kraft pulp meal samples. The apparent kappa number reduction increased significantly with an initial increase of acid concentration up to about 0.2 M and then leveled off. The maximum kappa number reduction for softwood Kraft was 65% for unbleached and was higher (80%) for the O2-delignified samples. For hardwood pulps, a similar kappa number reduction (~ 70%) was observed for the unbleached and the O2 -delignified samples.;Further acid treatments were then conducted in dilute HCl solution with regular Kraft pulps. Several chemical reactions were observed. The bulk of hexeneuronic acid (HexA) groups (60-70%) in the unbleached and O2-delignified softwood Kraft as well as in the unbleached hardwood Kraft pulp were quite reactive and removed within the 30 min of acid treatments. In contrast, the HexA groups present in the O2-delignified hardwood Kraft were more resistant to acid treatments. Acid pre-treatments also resulted in a significant formation of the phenolic hydroxyl groups (PhOH). This finding implies that the aryl ether cleavage is a significant reaction contributing to the acid-catalyzed delignification of kraft pulps.;The acid treated pulps were shown being very responsive to a subsequent alkaline extraction with dilute alkali (0.1N NaOH 100°C).;Attempts to improve the efficiency of acid treatments were also evaluated with the use of dilute HNO3 solution and also with the addition of nitrite. These modifications were shown to have a positive effect in the overall delignification process.