Delignification, kraft pulping

T Dumonceaux, K Bartholomew, L Valeanu, T Charles, F Archibald. Cellobiose dehydrogenase is essential for wood invasion and nonessential for kraft pulp delignification by Trametes versicolor. EnzMicrob Technol 29(8-9) 478-489, 2001. 

Table II. Reductions of kappa numbers after peroxide delignification of en matically pretreated pine kraft pulps using of different en me preparations...

Tran, A.V. Chambers, R.P. Delignification of an unbleached hardwood Kraft pulp by Phanerochaete chrysosporium. Appl. Microbiol. Technol. 1987, 25, 484-490. 

The actual mechanisms of Kraft delignification are highly complex, revolving around the ionization of acid phenolic units in lignin by OH and nucleophilic displacement of lignin units with HS. The chemistry of delignification is reviewed in detail elsewhere. It is sufficient to note here the conditions specified above and the pulp yields typically 45-55% of the dry weight of wood furnish is produced as Kraft pulp. 

Kirk TK, Yang HH. Partial delignification of unbleached kraft pulp with ligninolytic fungi. Biotechnol Lett 1979 1 347-352. 

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. 

The overall rate of the bulk delignification in kraft pulping, during which the variations in hydroxyl and hydrosulfide ion concentrations are moderate, follows pseudo-first-order kinetics, approximately in conformity with the following equation ... 

Figure 7-32 illustrates the glucomannan and xylan losses during kraft pulping of pine wood. As can be seen, an appreciable portion of the lost xylan is actually not degraded but dissolves in the cooking liquor as a polysaccharide. The amount of dissolved xylan reaches a maximum around the midpoint of the delignification process. 

The discovery that small amounts of anthraquinone (0.05-0.25% on wood) added to alkali liquors can enhance delignification and produce kraft quality pulp has been of great interest. Although the price of anthraquinone (AQ) is high and the chemical is not recovered, only small quantities are required, and the benefits outweigh the costs sufficiently that a number of mills around the world now routinely add AQ to the kraft pulp-wood digester. 

Gaspar, A. R., Evtuguin, D. V, and Neto, C. P., Polyoxometalate-catalyzed oxygen delignification of kraft pulp A pilot-plant experience. Industrial Eng Chem Res 2004,43 (24), 7754-7761. 

From an inspection of the values in Table 7.2.2, it is clear that the phenolic content of lignin decreases in the order kraft lignin kraft pulp > wood, a trend which underscores the significance of aryl ether cleavage in delignification and of the effect of the phenolic hydroxyl groups on lignin solubility. 

Tai D-s, Chen C-L, Gratzl JS (1990) Chemistry of delignification during kraft pulping of bamboos. J Wood Chem Technol 10 75-99... 

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