Acid sulfite pulping

Pulp Manufacture. Sodium sulfite is utilized in neutral semichemical pulping, acid sulfite pulping, high yield sulfite cooling, and some kraft pulping processes (339). Many pulp mills prepare their own sulfite and recycle as much as possible, but use of merchant sodium sulfite by pulp mills is substantial. Much of the by-product sodium sulfite from resorcinol manufacture goes into pulp appHcations as well as a substantial fraction of the lower assay manufactured sodium sulfite. 

The use of the lignin fraction is much more cumbersome currently the best-known chemical of a real commercial importance is vanillin, which is obtained by oxidation of the black liquor. Another example is a product called spray-dried lignosulfonate (as sodium salt) obtained from the older, acidic sulfite pulping process. It is sold as a commercial product primarily as a concrete additive for enhanced strength. Since the cement industry is one of the big contributors of carbon dioxide emissions (due to the production of calcium oxide from calcium carbonate), the use of this renewable, wood-derived product not only is fossil-carbon neutral in itself but also reduces carbon dioxide emission due to the diminished need for cement in large infrastructures made of concrete. 

Acid soak cleaners, for electroplating, 9 783 Acid soap, 22 728 Acid soluble dyes, 7 373t Acid solutions hydrazine, 13 587 in leaching chemistry, 16 152 Acid sulfite process, 23 540-541 Acid sulfite pulping, 21 22-23, 26 Acid treatment, in petroleum refining,... 

Although acidic pulping methods have largely been displaced over the past 50 years by neutral and alkaline processes, there is still a significant amount carried out. Acid sulfite pulping uses combinations of sulfur dioxide and water at high temperatures and pressures. An appropriate base is used to control the pH and, although usually acidic, it is possible to perform these reactions at neutral or even alkaline pH. The most active nucleophile present is the bisulfite ion,... 

After felling of a tree the content of extractives decreases, and their composition changes because of various reactions. For acid sulfite pulping, the changes are beneficial, and wood is usually stored for several months to minimize pitch problems and to lower the resin content of the pulp. In the case of kraft pulping, storage of wood has detrimental consequences, since yields of both turpentine and tall oil are reduced. 

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.

Acid Sulfite (and Bisulfite) Pulping During acid sulfite pulping the a-hydroxyl and the a-ether groups are cleaved readily under simultaneous formation of benzylium ions (Fig. 7-6). This reaction takes place regardless... 

Fig. 7-6. Behavior of /3-aryl ether and open a-ether structures during acid sulfite pulping (Gellerstedt and Gierer, 1971). R = H, alkyl, or aryl group. The first reaction step involves cleavage of the a-ether bond with formation of a resonance-stabilized carbonium ion which is then sulfonated. Note that both the phenolic and nonphenolic structures are sulfonated, while the /3-aryl ether bonds are stable.

Fig. 7-8. Reactions of stilbene structures during acid sulfite pulping (see Gellerstedt, 1976).

Fig. 7-9. Examples of lignin condensation products formed during acid sulfite pulping (Gierer, 1970). Condensation results from the reaction of a carbonium ion with the weakly nucleophilic sites in the benzene nucleus.

During acid sulfite pulping, lignin may also condense with reactive phenolic extractives. Pinosylvin and its monomethyl ether, present in pine heartwood, are examples of phenolic extractives of this type. Dual condensation of pinosylvin with lignin generates harmful cross-links. Consequently, pine heartwood cannot be delignified by the conventional acid sulfite method. 

Acid Sulfite Pulping Since glycosidic bonds are easily cleaved by acids, depolymerization of polysaccharides during acid sulfite pulping cannot be... 

Fig. 7-18. Conversion of a-pinene to p-cymene and taxifolin to quercetin during acid sulfite pulping.

TABLE 7-5. Typical Composition of the Sulfite Spent Liquor Resulting from the Acid Sulfite Pulping of Norway Spruce... 

TABLE 10-1. Products Obtained on Acid Sulfite Pulping"... 

During acid sulfite pulping a-pinene and some monocyclic terpenes are partially converted to p-cymene (Fig. 7-18). Crude cymene can be separated from the digester gas relief condensates and purified by distillation. With spruce and fir the distilled product is 99% p-cymene and only minor quantities of other products, such as borneol and sesquiterpenes, are present. Pine wood contains 3-carene, which affords appreciable quantities of m-cymene besides p-cymene. The crude cymene can be used within the mill as a resin-cleaning solvent. The distilled p-cymene finds use in the paint and varnish industry. 

Treatment of lignin with acid as is the situation, for example, in acidolysis (Chap. 6.1) gives rise to stilbene and phenylcoumarone structures (Adler et al. 1966). Stilbenes similar to those identified in acid sulfite pulping liquors have been detected in the steam hydrolyzate of aspen wood (Bardet et al. 1985), and a coniferyl aldehyde structure was formed when a yS-aryl ether-type lignin model compound was heated with slightly acidified water (Kratzl et al. 1959). 

Conductometric titrimetry can be applied to a wide range of pulp types with a wide range of sulfonate contents. The method has been applied to softwood chips treated with sulfite, softwood sulfite mechanical pulps, high-yield bisulfite pulps, low-yield acid sulfite pulp (Katz et al. 1984, Gummerus 1985, Heitner and Hattula 1988) and sulfite treated aspen wood (Beatson et al. 1985) with sulfonate contents ranging from 20 to 300 mmol kg-1. In addition, the method has been adapted to measure sulfonate content of sulfonated kraft lignin (Oster et al. 1988). 

Digestion liquor for acid sulfite pulping consists of a solution of sulfur dioxide in water containing calcium ion. Sulfur dioxide is obtained by... 

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