Chlorine bleaching lignin reactions

In comparison to chlorine (see the next section. Chlorine Bleaching Chemistry), chlorine dioxide has a much higher selectivity in its reactions with lignin, and in analogy to model experiments, oxidation is the major mode of reaction. This has been demonstrated by bleaching experiments of kraft pulp in which the prebleaching was... 

PI Gnnnarsson, SCH Ljnnggren. The kinetics of lignin reactions dnring chlorine dioxide bleaching. Part 1. Infinence of pH and temperature on the reaction of l-(3,4-dunethoxy-phenyl)ethanol with chlorine dioxide in aqneons solution. Acta Chem Scand 50 422-431, 1996. 

Further Preparative Reactions. When pulps are to be used in the production of materials that do not retain the original fiber stmcture, such as rayon or ceUulose acetate film, the lignin, hemiceUulose, and other components must be reduced to the lowest possible concentrations. A surfactant (ionic or nonionic) is often added during a hot, weakly alkaline extraction step after chlorination. Another approach, sometimes used in addition to the surfactant step, is to treat the pulp with 6—10% NaOH after most of the oxidative bleaching is finished. This treatment removes most of the hemiceUulose. In most purification plants the final stage includes use of sulfuric acid chelators are optional. 

Sodium chlorite has also been used for treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans. Chlorine dioxide from chlorite is also useful for microbial and slime control in paper mills and alkaline paper machine systems (164,165). The use of sodium chlorite in textile bleaching and stripping is well known. Cotton is not degraded by sodium chlorite because the oxidation reactions are specific for the hemiceUulose and lignin components of the fibers. 

The spent bleach liquors are complicated mixtures of reaction products, only some of which have been identified. A simple classification according to the type of compounds present has been made in Table 8-4. As can be seen, lignin degradation products dominate in the spent liquors from chlorination and alkali extraction. Because of extensive demethylation of lignin, considerable amounts of methanol are present in the chlorination liquors. The amount of low molecular chlorinated aromatics is very low, but since the most toxic compounds are included in this fraction, clarification of its complicated composition has been the subject of intensive research. 

Panchapakesan B (1990) Photo-induced reactions of lignin and lignin fragments Photooxidation of chlorinated organics in bleach plant effluents and brightness stability of high yield pulps. Ph.D. Thesis, North Carolina State University, Raleigh, North Carolina, USA... 

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 ... 

In other work related to bleaching chemistry, the reactions of chlorine dioxide with monomeric [44] and dimeric [45] lignin model compounds have been studied computationally. These studies closely parallel experimental work in which oxidation mechanisms were proposed [46-51]. In accord with the experimental work, which reports higher reactivity of phenolic compounds, the heats of reaction for these compounds are lower than those for etherified models. The experimentally based mechanisms were generally found to be energetically feasible, but in some cases the electronic results were not consistent with the proposed mechanisms. 

G GeUerstedL E-L Lindfors, M Pettersson, D Robert. Reactions of Lignin in Chlorine Dioxide Bleaching of Kraft Pulps. Res Chem Intermed 21 441 56, 1995. 

An aqueous solution of chlorine constituted the major bleaching agent for cel-lulosic fibers for almost 200 years. The reactions between lignin structures and... 

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