Selectivity of oxygen delignification for southern softwood kraft pulps with high lignin content

Abstract

The selectivity of kraft pulping versus the oxygen delignification processes over the range of kappa nos. 25–90 was compared. Kraft pulping was found to be more selective than oxygen delignification for removing lignin from southern softwood kraft pulps. The greater selectivity is thought to be related to hydroxyl radicals that form in the oxygen delignification process that are not present in the kraft process. The hydroxyl radicals attack the carbohydrates and randomly cleave the polymeric chains, causing a significant decrease in the degree of carbohydrate polymerization and thus a loss of viscosity. Kraft pulping generates hydrosulfide ions that are highly selective and attack the lignin. Carbohydrate degradation occurs mainly from peeling reactions, which do not appreciably reduce the degree of polymerization of the cellulose and thus there is less viscosity loss. At low lignin content (i.e., low kappa number), the remaining lignin is likely bound covalently to the carbohydrate portion in both processes. Therefore, removal of the lignin results in significant degradation of the carbohydrates.

Evaluating the Potential to Modify Pulp and Paper Properties through Oxygen Delignification

“…According to Tables and , oxygen-delignified pulps had 100–200 units lower viscosity, indicating that cleavage of cellulose chains was more pronounced by radical attack in the oxygen stage compared to alkaline hydrolysis in the cooking stage. This is in accordance with previous studies. ,,− …”

“…On the other hand, when oxygen delignification is introduced after kraft cooking to higher kappa numbers, the yield can be decreased compared to the same final kappa number achieved by kraft cooking alone. 23 The comparison with the total yield in the present study is however somewhat flawed as oxygen delignification was performed on screened pulp. Because relatively high kappa numbers of 25 and 30 after oxygen delignification were aimed for, the kappa numbers of the cooked pulp were high, ranging between 40 and 57, and thus contained a significant amount of rejects.…”

“…Large yield gains by oxygen delignification have been claimed in the literature. ,,,, However, the yield in the cited studies was compared between extended cooking and oxygen delignification, that is, at such low kappa numbers that the kraft cooking has entered the unselective residual phase delignification. On the other hand, when oxygen delignification is introduced after kraft cooking to higher kappa numbers, the yield can be decreased compared to the same final kappa number achieved by kraft cooking alone …”

Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on chemical and physical properties

“…3. These groups are mainly due to the creation of muconic acid structures in lignin (Dang et al 2006;Snowman et al 1999;Yang et al 2003), but also carbohydrates are oxidized (Tao et al 2011;Zhang et al 2006;Zhao et al 2016). The amount of fiber charges depended on the initial amount of charges as well as on the conditions in the oxygen stage.…”

Limitations on the protective action of MgSO4 for cellulose during kraft pulp oxygen delignification

“…Oxygen delignification is typically conducted at moderate temperatures under an alkaline environment, where the oxygen reacts through complex radical chain reactions with both the lignin and carbohydrates. However, the delignification/cellulose degradation selectivity deteriorates in an oxygen delignification process at a higher delignification rate, or when the initial pulp exhibits a high kappa number (Tao et al 2011;Jafari et al 2014a). The poor selectivity is primarily due to the formation of hydroxyl radicals during oxygen delignification, and these can easily cleave the glycosidic bonds in cellulose via direct attack on the anomeric carbon (Guay et al 2001;Guay et al 2002).…”