Inorganic Side Reactions during Chlorine Dioxide Bleaching of Wood Pulps

The drawback of chlorine dioxide bleaching is the rather low efficiency due to the

formation of chlorate and residual chlorite. The loss in oxidation power leads to a

further increase in bleaching costs. Additionally, chlorate has been shown to exhibit

toxic effects on brown algae, and has therefore been the focus of many studies

to evaluate possibilities to minimizing the formation of chlorate and chlorite.

The pH profile exhibits a significant influence on the performance of chlorine

dioxide bleaching. At high pH, the efficiency of chlorine dioxide bleaching is very

7.4 Chlorine Dioxide Bleaching 737

low. Svenson found that when bleaching to pH 11.2 approximately 70 mol% of

the initial chlorine dioxide charge is converted to chlorite, wasting 56% of the initial

oxidizing power charged to the pulp [14]. Chlorite is not consumed by reactions

with the residual lignin, and accounts for a large portion of lost oxidation

potential. This is also reflected in a higher residual kappa number as compared to

chlorine dioxide bleaching at lower pH levels. Chlorite is formed through a oneelectron

transfer reaction between the phenolic and nonphenolic structures present

in the residual lignin and chlorine dioxide. The low chloride ion concentration

also indicates that less hypochlorous acid forms at high pH (Fig. 7.60).

2 4 6 8 10 12

Chlorite Chlorate Chloride Chlorite+Chlorate

Mole% Initital Chlorine Dioxide

End pH

Fig. 7.60 Effect of final pH in chlorine dioxide bleaching of a

27 kappa number softwood kraft pulp on chlorate, chlorite,

and chloride formation (according to [14]). D0– conditions:

5.4% active chlorine charge or 20.5 kg chlorine dioxide odt–1

pulp equal to a 0.2 kappa factor; 50 °C, 120 min reaction time.

The reactivity of chlorite ions increases as soon as the pH of the bleaching

liquor decreases, because most reactions that consume chlorite require acidic conditions.

The chlorite ions are in equilibrium with chlorous acid, its conjugated

acid:

ClO 2 H _ ClO _2 _ H _ _62_

Figure 7.60 shows that the concentration of the chlorite ions linearly decreases

to a very low level until a pH of 3.4 is achieved and remains constant at lower pH

levels. Chlorous acid readily oxidizes lignin structures, forming hypochlorous acid

according to Eq. (63):

738 7Pulp Bleaching

ClO 2 H _ L _ LO _ HOCl _63_

where LO represents oxidized lignin.

Under acidic conditions – preferably at a pH equal to the pKa of chlorous acid –

chlorite also undergoes a dismutation reaction that generates chlorate and hypochlorous

acid as expressed in Eq. (64):

ClO 2 H _ ClO _2 _ ClO _3 _ HOCl _64_

It has been confirmed that the amount of hypochlorous acid increases when the

reaction pH decreases during chlorine dioxide bleaching with a kraft pulp [15]. As

a result, bleaching efficiency increases significantly due to the regeneration of

chlorine dioxide through chlorite oxidation. The loss of oxidation power due to

chlorite formation can be recovered by oxidizing chlorite with hypochlorous acid,

as illustrated in Eq. (65). This reaction represents the key step for the better performance

of chlorine dioxide bleaching at acidic conditions:

2 ClO _2 _ HOCl _ 2 ClO 2 _ Cl _ _ OH _ _65_

Hypochlorous acid is in equilibrium with chlorine according to Eq. (66). However,

a significant amount of elemental chlorine is present only at pH < 2

(pKa = 1.8).

HOCl _ H _ _ Cl _ _ Cl 2 _ H 2 O _66_

In contrast to chlorite, the oxidation potential of chlorate cannot be reactivated

by adjusting the reaction conditions. Figure 7.60 reveals that chlorate formation

clearly increases while the final pH decreases. Chlorine dioxide decomposes in

alkaline media to form chlorate and chlorite ions according to Eq. (67), with a

reaction mechanism that is still the subject of debate [16]:

2 ClO 2 _ OH _ _ ClO _3 _ HClO 2 _67_

The initial rate of chlorine dioxide decomposition is rather slow, but largely

influenced by the presence of hypochlorite ions. The reaction displayed in Eq. (68)

is discussed as a further contributor to chlorate formation under neutral to alkaline

conditions [17]:

ClO _2 _ HOCl _ H 2 O _ _ _ ClO _3 _ H _ _ Cl _ _ H 2 O _68_

Reactions containing excess hypochlorous acid favor chlorate formation according

to Eq. (68), while reactions with excess chlorite generate chlorine dioxide as

depicted in Eq. (65). In contrast to the results reported for reactions with wood

pulps, chlorate formation increases with rising reaction pH during the reaction of

chlorine dioxide with nonphenolic lignin model compounds [17]. This behavior is

7.4 Chlorine Dioxide Bleaching 739

attributed to the slower reaction kinetics of etherified lignin moieties as compared

to phenolic ones, thereby allowing hypochlorous acid to react with chlorine dioxide

to form chlorate.

Under acidic conditions chlorite oxidation was shown to proceed via a dichlorodioxide

(Cl2O2) intermediate [10]. This intermediate may undergo a number of

possible reactions. Both nucleophiles, chlorite and water, compete for the reaction

intermediate, which results in the formation of either chlorine dioxide [Eq. (65)]

or chlorate [Eq. (68)].

At a pH below 3.4, where only little chlorite is present, chlorate production

clearly dominates. This concludes that chlorate formation during bleaching is

more pronounced when the chlorine dioxide concentration is high relative to

chlorite. However, in the case of a high chlorite to chlorine dioxide ratio, and

when the pH is shifted to higher values, chlorite ions react with the dichlorodioxide

intermediate to form chlorine dioxide, rather than the hydrolysis product

chlorate. Consequently, the level of chlorate formation can be kept at a minimum

when the pH is adjusted from a high to a low level throughout chlorine dioxide

bleaching. This can be achieved by splitting chlorine dioxide bleaching into two

stages, where the first stage is conducted to a final pH around 7, and the second

stage is run to a final pH below 3. In the first stage, a large part of chlorine dioxide

is converted to chlorite, thus preventing the generation of additional chlorate in

the subsequent acidic stage. The pH profiling ensures a sufficiently high chlorite

to chlorine dioxide ratio to effectively suppress chlorate formation. The concentration

of chloride ions increases parallel with the reduction of the chlorite ion concentration

and passes a maximum at a pH level of about 3.4 (see Fig. 7.60). The

decrease in chloride can be explained by a further increase in hypochlorous acid

formation at low pH. This agrees well with the observation that chlorine dioxide

bleaching causes an increase in AOX formation with decreasing pH below 3.4.

On closer examination of Fig. 7.60, it can be seen that the total amount of

wasted oxidation potential (sum of chlorite and chlorate) does not significantly

change below a final pH of 3.4. These results differ slightly from those obtained

by chlorine dioxide bleaching of delignified pulps (e.g., D1, D2) [18–20]. Chlorine

dioxide bleaching (in a D1-stage) of a CE pre-treated softwood kraft pulp requires

the final pH to be between 3 and 4 in order to exhibit an optimum bleaching efficiency.

The low reactivity of the oxidized lignin structures of prebleached kraft

pulps favors the enhanced execution of side reactions at pH levels below 3.5, such

as the disproportionation of chlorous acid to form chlorate. The higher reactivity

of the unbleached kraft lignin towards chlorine dioxide bleaching promotes the

oxidation of the lignin while maintaining a constant chlorate concentration.

It can be concluded, that chlorine dioxide bleaching of both unbleached and

prebleached kraft pulps is most efficient when adjusting the final pH to about 3.5.

In the case of unbleached kraft pulp, the oxidation power of chlorine dioxide (in a

D0-stage) remains constant even when the pH falls below 3. In contrast, chlorine

dioxide bleaching of prebleached kraft pulps in D1– or D2-stages displays a maximum

reaction efficiency only between pH 3 and 4 because the lower reactivity of

740 7Pulp Bleaching

the oxidized lignin structures promotes the inorganic side reactions, predominantly

associated with chlorate formation.

7.4.3

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