Sulfite pulping delignification

The soda, kraft, and sulfite pulping processes are used to prepare full chemical pulps. The soda process, which uses sodium hydroxide as the cooking chemical for delignification purposes, has hugely been superseded by the kraft process, which is characterized by its use of sodium hydroxide and sodium sulfide as active delignification agenls in the chip-cooking phase of the process. 

There are several modifications of the sulfite method which are designated according to the pH of the cooking liquor (Table 7-1). For the production of chemical pulps, delignification is allowed to proceed until most of the lignin in the middle lamella is removed after which the fibers can be readily separated from each other. Semichemical pulps are often produced by the neutral sodium sulfite method followed by mechanical fiberization of the partially delignified wood. 

Fig. 7-2. Delignification of the secondary wall (S) and compound middle lamella (ML) during kraft and acid sulfite pulping (Wood and Goring, 1973). Note that the S wall is delignified faster than the ML layer at the earlier stages of the cook.

Fig. 7-4. Selectivity of lignin dissolution during (1) kraft, (2) acid sulfite, and (3) two-stage sulfite pulping of softwood (see Sjostrom, 1964). Note that the delignification proceeds very unselectively at the beginning and at the end of the cook.

J. Luo and P.K. Christensen, Oxygen Delignification with Magnesium as Base A Good Solution for Sulfite Pulp. TAPPI J. 75(6), 183—187, June (1992). 

Some hydroxystilbenes - e.g. pinosylvin - interfere with the delignification process during acid-sulfite pulping (36). The pinosylvin competes with the nucleophilic hydrogensulfite anion and inhibits the formation of soluble lignosulfonates. Instead, black-colored insoluble products are formed. The polyhydroxy stilbenes also contribute to the color of wood since they are easily oxidized to dark-colored products. 

The kinetics of the delignification are of importance especially when considering the control of the pulping process. Since kraft pulping follows simpler kinetics than the sulfite processes, more applications have been adopted for this case. Because of the heterogenity of the system, however, pulping reactions are complicated and can therefore not be treated in the same fashion as homogeneous reactions in solution. 

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