Chlorine dioxide bleaching lignin reactions

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. 

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. 

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. 

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. 

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... 

Commercially, most cellulose is extracted from wood by one of two methods, the kraft (sulfate) process or the steam explosion process. The product of these reactions is wood pulp, which consists primarily of cellulose. In the kraft process, wood chips are treated with a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at temperatures of about 175°C (35o°F) for two to six hours. This process usually results in a yield of about 40 to 45 percent wood pulp. The pulp is then treated with a bleaching agent, such as calcium or sodium hypochlorite (Ca(0Cl)2 or NaCIO) or chlorine dioxide (C102) to remove the color of lignin and other impurities. 

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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