Oxygen-alkali pulping

Figure 7-38 illustrates the recovery and conversion of the kraft cooking chemicals. In the case of sulfur-free pulping (soda process, soda-oxygen process, and anthraquinone-alkali pulping) only sodium carbonate is recovered and chemical losses are compensated by adding sodium carbonate. The hydroxide-carbonate and lime cycles are the same as for the kraft process. 

The residual lignin in a semi-bleached pulp such as oxygen-bleached pulps or chlorinated and alkali extracted pulps can be isolated using the same procedure as described previously. However, during the successive enzymatic hydrolyses, some residual lignin in the semi-bleached pulp becomes soluble in the acetate buffer solution. The water-soluble residual lignin can be isolated by combining... 

K Ala-KaUa, J Li, O Sevastyanova, G Gellerstedt. Apparent and actual delignifica-tion response in industrial oxygen-alkali delignification of birch kraft pulp. Tappi J 2(10) 23-27, 2003. 

The fact that most pulp and paper applications use dilute solutions of hydrogen peroxide in alkali has kindled interest in the electrosynthesis of alkaline peroxide solutions by cathodic reduction of oxygen. 

Thus, similarities in color formation exist between the treatment of hardwood meal with alkali and the treatment of syringyl alcohol with alkali. In either instance, when oxygen is absent or suppressed, color formation is minimized. It may be concluded then, that oxygen is necessary for color formation when wood is pulped under alkaline conditions, and that units of the lignin component can be responsible for the color. 

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