Delignification of wood pulp

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

In pulp and paper processing, antbraquinone accelerates delignification of wood and im-... 

Wood Delignification. The production of wood pulp (qv) for the paper (qv) industry consists of removing lignin (qv) from wood chips, thus freeing the cellulose fibers. An aqueous solution containing 30—70 wt % sulfolane efficiently extracts the lignin from aspen, Western hemlock, and Southern pine wood... 

Understanding the MnP and LiP mechanism is important not only from a fundamental point of view but also because delignification of wood chips leaves behind pulp, the fibrous polysaccharides (hemicellulose and cellulose) that... 

Lignin condensation reactions have been studied for many years, mainly because they possibly interfere with efficient delignification of wood. Numerous model compound studies established the various types of condensation reactions that may occur between lignin fragments (1-5) or between lignin and carbohydrates (1,2,6,7) during alkaline pulping. An important class of condensation reactions involves formaldehyde. 

RH Atalla, lA Weinstock, JS Bond, RS Reiner, DM Sonnen, CJ Houtman, RA Heintz, CG Hill, CL Hill, MW Wemple, YuV Geletii, EMG Barbuzzi. Polyoxometalate-based closed systems for oxidative dehgnification of wood pulp fibers. In DS Argyropoulos, ed. Oxidative Delignification Chemistry Fundamentals and Catalysis. ACS Symposium Series, 785, pp. 313-326, 2001. 

The alkaline delignification of wood in the pulp industry is an example of a particularly complex system involving hundreds of reactions. This process is the key step in the production of cellulose via chemical pulping. The lignin material in the wood (L) is partially decomposed and dissolved in the cooking liquor, whereas cellulose fibers remain in a solid state. A part of hemicelluloses is also dissolved. The process can be roughly described as (without exact stoichiometry) [35]... 

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. 

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.

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