Kraft pulp chemical composition

A feature of the kraft process vital to continued success is its integral, well-tested chemical recovery system. The digestion liquor for each batch of chips to be pulped is mainly obtained from the chemicals recovered from the spent liquor of previous digestions, and has approximately the composition given in Table 15.8. For kraft pulping it is usual to specify all of the components of the digestion liquor on a NaiO equivalent basis. This puts all the active constituents on the same sodium ion content basis. Thus, the actual concentration of sodium hydroxide present for a 73 g/F, NaiO equivalent is given by Eq. 15.12. 

Extractives in native feedstocks comprise an extraordinary large number of diverse substances, mainly with low molecular masses and their amount and composition vary widely from species to species. The most important compounds are volatile and nonvolatile terpenes and their oxygen-containing derivatives (terpenoids) as well as various esters of fatty acids. During softwood kraft pulping, the volatile turpentine (mainly mono- and sesquiterpenes) components are chemically stable, but the fatty acid esters are removed and almost completely hydrolyzed in the early stage of the cook. ° In case of hardwood pulping, more neutral (i.e., chemically unsaponifiable or difficult to saponify) extractives are present and their efficient removal is sometimes... 

Wood chips are processed in mild chemical liquor and subjected to mechanical refining using disc refiners. Semichemical pulping liquors have variable composition ranging from sodium hydroxide alone, alkaline sulfite (sodium sulfite - - sodium carbonate), mixtures of sodium hydroxide and sodium carbonate, to Kraft green or white liquors [3]. Sodium sulfite/ sodium carbonate liquor is most commonly used and the pulp product obtained thereafter is referred to as neutral sulfite semichemical (NSSC) pulp. 

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