Pulping sulfite

Sulfur dioxide is a two-basic acid and the following equilibria prevail in its aqueous solution  

Since the concentrations of sulfur dioxide in its free (S02) and hydrated (S02 H20 or H.5SO3) forms cannot be determined separately, equations (7-1) and (7-2) are combined to give expression (7-4) in which the total sulfur dioxide concentration is in the denominator. This defines the first equilibrium constant Kt  

The second equilibrium constant derived from equation (7-3) is 

By taking the logarithms of both sides of equations 7-4 and 7-5, the following expressions are obtained  

It should be noted that Kt and K2 are not thermodynamic constants since the activity coefficients have been neglected, and hence they are strictly valid only at a given concentration. 

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Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

Fig. 3. Reactions at the quinonemethide center during pulping (a) sulfite pulping, and (b) kraft pulping.

Fig. 2. Optical miciogiaphs of a softwood sulfite pulp (a) before beating and (b—d) after beating in a valley beater for different periods of time. Beating...

Sulfite pulps have properties that are desirable for tissues and top quaHty, fine papers. Because sulfite pulping is not as versatile as kraft pulping, various options have been developed, and the choice of a specific process is dependent on individual mill situations. The unbleached pulp has high (60+) brightness compared to kraft pulp and is easily bleached. However, it is limited to select wood species. The heartwood of pine, Douglas fir, and cedars are not easily pulped. Additionally, pulps produced from hardwood have limited economic value because of low strength. 

ChemicalRecope . There are advantages and disadvantages to each of the base systems employed in sulfite pulping (see Table 4). Each has its own potential recovery systems except the calcium system, which is obsolete. Calcium-based Hquors can be burned, but scaling problems are severe, and conversion of the calcium sulfate to CaO is not economical. 

In all sulfite pulping systems, any excess SO2 in the digester can be collected easily from the blow gas by adsorption in fresh Hquors. 

Liquid sulfur dioxide discolors iron, copper, and brass at ca 300 ppm moisture and produces light scale at ca 0.1 wt % moisture and serious corrosion at ca 0.2 wt % or higher moisture content. Copper and brass can be used to handle wet sulfur dioxide where some corrosion can be tolerated, or where the moisture level is low. Wooden tanks are widely used for sulfurous acid preparation, handling, and storage. Sulfite pulp digestors are made of steel lined with acid-resistant brick. 

Uses. The dominant use of sulfur dioxide is as a captive intermediate for production of sulfuric acid. There is also substantial captive production in the pulp and paper industry for sulfite pulping, and it is used as an intermediate for on-site production of bleaches, eg, chlorine dioxide or sodium hydrosulfite (see Bleaching agents). There is a substantial merchant market for sulfur dioxide in the paper and pulp industry. Sulfur dioxide is used for the production of chlorine dioxide at the paper (qv) mill site by reduction of sodium chlorate in sulfuric acid solution and also for production of sodium dithionite by the reaction of sodium borohydride with sulfur dioxide (315). This last appHcation was growing rapidly in North America as of the late 1990s. 

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. 

Although the hydrolysis of wood to produce simple sugars has not proved to be economically feasible, by-product sugars from sulfite pulping are used to produce ethanol and to feed yeast (107). Furthermore, a hemiceUulose molasses, obtained as a by-product in hardboard manufacture, can be used in catde feeds instead of blackstrap molasses (108). Furfural can be produced from a variety of wood processing byproducts, such as spent sulfite Hquor, bquors from the prehydrolysis of wood for kraft pulping, hardboard plants, and hardwood wastes (109). 

Sulfite pulp sheet Wood flour Fluffing for acetylation, etc. 10/50 sieve to. 35% < 100 C 1,500 50... 

Sulfit, n. sulfite, -ablauge, /. sulfite waste liquor, -atze, /, sulfite discharge, -aus-lauger, m. Paper) sulfite digester, -cellulose, /, Paper) sulfite pulp, sulfitieren, v.i. sulfite. 

Various permutations of the black liquor recovery boiler design are used to combust soda liquors in the soda pulping process and alkaline earth liquors and ammonium liquor (red liquor) in the sulfite pulping process. 

Lignins are most usually employed as various types of sodium lig-nosulfonate and are products of sulfite pulping. (Kraft lignins from the Kraft pulping process are also available.) They are used as dispersants, binders, sequestrants, and emulsifiers. Sodium lignosulfonate product variables include color, MW, and the degree of carboxylation and sulfonation on the lignin backbone. 

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. 

Sulfite pulps have less color than kraft pulps and can be bleached more easily however, they are not as strong. The efficiency and effectiveness of the sulfite process is also dependent on the type of wood furnish and the absence of bark. For these reasons, the use of sulfite pulping has declined in comparison to kraft pulping over time. 

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