Introduction. The discovery of chlorine dioxide is generally credited to Sir Humphrey Davy, who

The discovery of chlorine dioxide is generally credited to Sir Humphrey Davy, who

reported the results of the reaction of potassium chlorate with sulfuric acid which

destroyed the color of vegetable dyes [1]. In the 1920s, Schmidt et al. reported that

chlorine dioxide is a very selective bleaching agent which does not react with carbohydrates

[2,3]. However, the use of chlorine dioxide on an industrial scale began

only after World War II, when both suitable manufacturing processes from

sodium chlorate and corrosion-resistant materials became available. In 1946, three

Swedish kraft pulp mills began to apply chlorine dioxide for the production of

highly bleached kraft pulps. Shortly after that, Canadian and US American kraft

mills followed suit by installing chlorine dioxide bleaching stages. It can be stated

that, together with the invention of the Tomlinson furnace, chlorine dioxide

bleaching technology contributed to the breakthrough for the kraft process against

the sulfite process. Since such modification, it became possible to produce fully

bleached pulps (>88% ISO) with high strength properties (no significant decrease

in strength during bleaching operations). In its early application, chlorine dioxide

was solely used at, or near the end of bleaching sequences such as CEHD,

CEHDED, or CEDED refer to (Tab. 7.2). The use of chlorine dioxide in the delignification

stage began only when it became known that a partial replacement of

chlorine by chlorine dioxide improves final pulp quality [19]. In the bleaching of

hardwood sulfite pulps, it was found that the formation of sticky chlorinated res-

734 7Pulp Bleaching

ins could be avoided when chlorine dioxide was used instead of chlorine. In the

case of kraft pulps, 5–10% of chlorine was replaced with the equivalent amount of

chlorine dioxide to further improve the strength properties of the pulps. The

debate about the use of molecular chlorine for pulp bleaching evolved from

reports of the research conducted during the Swedish project “Environment/Cellulose”

[4]. As a result, the study showed that pulp mill effluents entering the Gulf

of Bothnia, Sweden, severely affected the diversity, biomass, and distribution of

invertebrates and plants. In the sediments outside the Swedish coast, chlorinated

compounds related to bleach plant effluents were detected which indicated a high

persistence of such substances. Moreover, the use of molecular chlorine for pulp

bleaching produces polychlorinated dibenzo- p -dioxins (PCDD) and dibenzofurans

(PCDF). The pattern of the tetrachlorinated congeners, as well as their concentration,

depends drastically on the amount of molecular chlorine applied in the first

delignification stage. It has been shown that the concentration of these compounds

decreases as the degree of substitution of chlorine with chlorine dioxide

increases, and cannot usually be detected at 60% chlorine dioxide substitution or

higher [5].

With this knowledge, authorities around the world began to implement new

regulations that limited the amount of chlorinated organic material discharged

from bleaching operations into the environment. Consequently, chlorine dioxide

has become a very popular bleaching agent in place of molecular chlorine. Today,

chlorine dioxide is certainly the most important bleaching chemical, since the use

of elemental chlorine is increasingly abandoned. Chlorine dioxide’s high selectivity

towards the oxidation of chromophoric structures makes it the first choice for

both delignification and pulp brightening, while retaining strength properties.

Furthermore, it meets the current environmental regulations in most countries by

generating approximately five times less chlorinated organic material as compared

to chlorine.

7.4.2

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