Sulfite pulping selectivity

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. 

Fig. 11 Cellulose (beech sulfite pulp) dissolved in NMMO (Lyocell dope). Left DSco and differential MWD of the starting pulp, molecular weights of DP = 50 and DP = 200 are indicated by vertical dashed lines. Right Time course of the overall carbonyl content for three selected molecular weight ranges. Reprinted with permission from Biomacromolecules (2002) 4 743. Copyright (2002) American Chemical Society...

Fig. 20 Group selective labeling agents for chemical microscopy by TOF-SIMS. Analysis of a CCOA-labeled (right) and an FDAM-labeled beech sulfite pulp...

Fig. 7-4. Selectivity of lignin dissolution during (1) kraft, (2) acid sulfite, and (3) two-stage sulfite pulping of softwood (see Sjostrom, 1964). Note that the delignification proceeds very unselectively at the beginning and at the end of the cook.

The furnish of a paper machine varies widely, depending on the grade of paper being made. Newsprint usually consists of about 80 percent groundwood and 20 percent chemical fiber (sulfite or semibleached kraft). Bag papers and linerboard are usually 100 percent unbleached softwood kraft. Printing papers are made from bleached pulps and contain both hardwoods and softwoods. By selecting the proper pulps and refining conditions, a wide variety of paper qualities can be achieved. 

Anyone who has examined the chapters leading up to this one, even in the most cursory fashion, must be well aware of the role of extractives in utilizing wood. Intensity and uniformity of color affect the selection and value of fine veneers for furniture manufacture. Absence of extractives permits the invasion of microorganisms that promote sap-stain or actual wood destruction. On a positive note, absence of extractives permits pulpability by the sulfite process. Extractives harm bleached wood pulp absorptivity when used in sanitary applications such as disposable diapers. Traces of extractives in pulpmill effluents threaten the life of important marine organisms. Extractives impede pressurized wood treatment. The list could go on and on. One thing that is particularly evident about all the industrially important aspects of these effects of extractives is that they are accepted by the user simply as a fact of nature and nothing much is done about them other than the simplest sorts of physical or chemical treatment. 

Both processes have advantages and disadvantages in raw material selection, in pulp quality, in yields, in possibility to use the by-products and in environmental acceptance. The Kraft process is still the favorite. In 1995107 x 10 mt of pulp were made by this process and only 8.2 x 10 mt by the sulfite process, which tends to shrink further with time. 

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