Alkali pulping

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

Fig. 7-26. Elimination of proton and formaldehyde from the quinone methide intermediate during alkali pulping (Gierer, 1970).

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. 

Niemela K, Sjostrom E (1986) Simultaneous identification of aromatic and aliphatic low molecular weight compounds from alkali pulp liquor by capillary gas-liquid chromatography-mass spectrometry Holzforschung 40 361-368... 

This type of alkali catalyzed degradation is of significance in alkali pulping as combined with the unzippii process it leads to pulp losses and fiber strength decreases. 

---