New Insights into Lignin Reactivation Towards Chlorine Dioxide Bleaching After Caustic Extraction, 2002 International Pulp Bleaching Conference Proceedings

Brian N. Brogdon, Douglas G. Mancosky, Lucian A. Lucia

Chlorine dioxide delignification (D0) affords similar amounts of quinones and muconic structures onto the oxidized lignin (1.7 - 2.0 mmol/g lignin each). These groups act as "blocking groups," resisting further attack by ClO2. Alkali extraction (E), in addition to removing solubilized lignin and saponifing muconic acid methyl esters, reactivates the residual D0 lignin to further attack by ClO2 by reducing quinones to polyphenols. This action of alkali on D0 oxidized lignin is similar to that of hydrosulfite reduction (Y). Chlorine dioxide oxidation of D0E and D0Y treated pulps yielded equal amounts of quinones and muconic structures onto the oxidized lignin (1.7 - 2.0 mmol/g lignin each), like that of the D0 stage. This implies that polyphenols react with ClO2 in a comparable way as unbleached phenolic groups. Clearly, the formation of quinones during ClO2 oxidation of phenols and polyphenols represents a blocking group, which can be easily reactivated by alkali treatment. It is envisioned that each subsequent alkali treatment and re-oxidation by ClO2 partitions quinones formed in a previous ClO2 oxidation into hydroxylated quinone and muconic acids groups, thereby assisting lignin solubilization and removal from the fiber.