Pulping, alkaline soda process

Alternatively cellulose is produced from wood via wood pulp. A number of processes are used in which the overall effect is the removal of the bulk of the non-cellulosic matter. The most widely used are the sulphite process, which uses a solution of calcium bisulphite and sulphur dioxide, the soda process using sodium hydroxide and the sulphate process using a solution of sodium hydroxide and sodium sulphide. (The term sulphate process is used since sodium sulphate is the source of the sulphide.) For chemical purposes the sulphite process is most commonly used. As normally prepared these pulps contain about 88-90% alpha-cellulose but this may be increased by alkaline purification and bleaching. 

The kraft process evolved from the soda process. The soda process uses an alkaline liquor of only sodium hydroxide (NaOH). The kraft process has virtually replaced the soda process due to the economic benefits of chemical recovery and improved reaction rates (the soda process has a lower yield of pulp per pound of wood furnish than the kraft process). 

Schoettler has studied the alkali resistance of the xylan in pulps made from aspen wood. Verifying and extending previous work by March, he" observed a higher content of xylan resistant to 17.5% sodium hydroxide in aspen alkaline pulps (sulfate or soda process) than in aspen, chlorite holo-cellulose. Further chlorite treatment of an alkaline pulp did not reduce its xylan or resistant-xylan content. Alkaline (sulfate) cooking of the holo-cellulose did not increase the resistant-xylan content. These results are summarized in Table X. 

Kraft process - There are two types of alkaline pulping processes. The soda process is based on sodium hydroxide and is the older of the two. It is little used today. The sulfate or kraft process is based on sodium hydroxide and sodium sulfate. It is the largest single pulping method employed for fully cooked chemical pulps. 

S oda—Anthraquinone. A few mills worldwide use soda pulping of hardwoods. In such cases, the addition of anthraquinone is immediately justifiable in terms of increased yield and upgraded pulp quaHty. The conversion of existing kraft mills is not as simple because AQ contributes no alkalinity to the process as sulfide does, and most kraft causticizing systems would have to be expanded by about 33%. This conversion is probably not justifiable in terms of the yield gain. The greatest benefit from AQ is for new mills in which expenditures for air pollution abatement devices can be reduced. 

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. 

Miksche, G. E. Lignin reactions in alkaline pulping processes (rate processes in soda pulping). In Chemistry of Delignification with Oxygen, Ozone, and Peroxides Nakano, J. Singh, R. P.,Eds. Uni Publishers Co Tokyo, Japan, 1980 pp. 107-120. 

FIG U RE 10.18 Oxidation products formed on soda/AQ treatment of a (3-1 lignin model compound. (Hise, R.G., Seyler, D.K., Chen, C.-L., and Gratzl, J.S., Oxidative-Hydrolytic Processes in Alkaline Pulping. Proc Int Symp Wood Pulp Chem, Paris, France, 1, 391-398, 1987.)... 

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