Alkali pulping lignin reactions

Cellulose is sometimes used in its original or native form as fibers for textile and paper, but is often modified through dissolving and reprecipitation or through chemical reaction. The xanthate viscose process, which is used for the production of rayon and cellophane, is the most widely used regeneration process. The cellulose obtained by the removal of lignin from wood pulp is converted to alkali cellulose. The addition of carbon disulfide to the latter produces cellulose xanthate. 

Chemical modification reactions continue to play a dominant role in improving the overall utilization of lignocellulosic materials [1,2]. The nature of modification may vary from mild pretreatment of wood with alkali or sulfite as used in the production of mechanical pulp fibers [3] to a variety of etherification, esterification, or copolymerization processes applied in the preparation of wood- [4], cellulose- [5] or lignin- [6] based materials. Since the modification of wood polymers is generally conducted in a heterogeneous system, the apparent reactivity would be influenced by both the chemical and the physical nature of the substrate as well as of the reactant molecules involved. 

Lignin content in wood is reduced from about 35% to 5%, and further processing is necessary to obtain whiter pulp. This consists of a series of bleaching steps using chlorine (C), alkali extraction (E), hypochlorite (H), and chlorine dioxide (D). The common sequence is CEHDED. The chlorination step C can be basically described by the reactions ... 

J Gierer, L-A Smedman. The Reactions of Lignin during Sulfate Pulping. Part XL Reactions of Pinoresinol with Alkali and with White Liquor. Acto Chem Scand 25 1461-1467, 1971. 

DF Wong, G Leary, and G Arct. The role of stilbenes in bleaching and colour stability of mechanical pulps. I. The reaction of lignin model stilbenes with alkali and oxygen. Res. Chem. Intermed. 21 329-342, 1995. 

He W., Tai D., and Lin S., A comparative study of the reactivities of straw and wood alkali lignin in sulfonation reactions, Appita Proceedings 6th International Symposium on Wood and Pulping Chemistry 1, Atlanta, USA, 1991, pp. 509-515. 

In conventional pulp bleaching, chlorine, hypochlorite, and chlorine dioxide are used with or without intermediate alkaline extraction steps. The first step in conventional lignin-dissolving bleaching is the chlorination. Qilorine water is normally used for this chlorination. It converts the residual lignin to products, that are soluble in water and/or alkali. Chlorine and hypochlorite react primarily with the benzene or phenol rings of lignin, in which substitution and oxidation reactions take place. In a separate extraction step, the unwanted soluble substances are removed with water and/or diluted sodium hydroxide solution. 

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