12.2.1.1 Origin and Isolation

Kraft pulping is the dominant process for production of pulp for paper [21]. The delignification is carried out in a strong alkaline solution composed mainly of OH — and HS — ions, removing around 90% of the initial lignin [22, 23]. In this process lignin undergoes reactions involving sulfidolytic cleavage of a — and b-aryl ether bonds in both phenolic and non-phenolic lignin units. Reactions of conjugate addition of carbanions to quinone methide intermediates lead to the increase of condensed structures in lignin [22]. Additionally, some other complex reactions between lignin and other wood components can also occur [8, 24].

Unmodified kraft lignin renders insoluble in aqueous solution at pH bellow the pKa values of the phenol groups of lignin, which are in the range 10.0-11.5 [25]. The pH currently used for precipitation and further filtration of lignin is near 7. Sulfuric acid or carbon dioxide have been used for this propose. Recently, the isolation of high purity kraft lignin (both softwood and hardwood lignins) has met a new advancement with Lignoboost process [26-28]. With this process, the washing at controlled conditions of precipitated lignin was improved leading to a final product with rather low content of ashes and carbohydrates, and then opening the perspectives to improve the existent applications or upgrading lignin in new valuable applications [29, 30]. Significant work concerning the recovery [31, 32] and fractionation [33, 34] of lignin from kraft liquors has been published in recent years, many of them via membrane processes.