Modified Chlorine Dioxide Bleaching

During the delignification process in alkaline pulping, double bonds are generated

by methanol elimination from 4- O -methylglucuronic acid on the xylan [7].

The first indication of the source of these double bonds was the identification of 2-

furancarboxylic acid as main product of its hot acid hydrolysis by Marechal [8] (see

Section 7.8, Hot acid hydrolysis). Bleaching chemicals with a reactivity towards

double bonds are consumed by HexA. Consequently, the detection of HexA was

directly followed by an evaluation of the different options for their removal and an

analysis of the resulting savings in bleaching chemical. Hot acid hydrolysis with a

retention time of about 2 h at >90 °C and a pH below 3 degrades HexA and lowers

the kappa number. However, the resulting decrease in the demand for chlorine

dioxide is moderate. Potential savings [9] of 1.5% active chlorine in hardwood

pulp bleaching and 0.8% active chlorine in softwood pulp bleaching are too small

to pay for the investment in a huge tower and an additional washing step. Additional

costs result from the demand for up to 0.5 tons of (low-pressure) steam to

heat the pulp to the required hydrolysis temperature.

In ECF bleaching a logical consideration was to combine the first chlorine dioxide

stage (D0) with the acid hydrolysis (Ahot). Both treatments require an acidic

pH. The typical temperature in a chlorine dioxide stage at the beginning of an

ECF sequence is 50–60 °C. The application of chlorine dioxide at even higher temperature

is not a problem, because chlorine dioxide is a rather selective chemical.

For the addition point for chlorine dioxide, two options exist – it could be added

either at the start or at the end of a hydrolysis step. In theory, both options have

advantages and disadvantages. Adding chlorine dioxide at the start of an acid

hydrolysis stage will lead to a consumption of ClO2 not only by lignin but also by

HexA, and therefore savings in the demand for chlorine dioxide will be difficult to

7.4 Chlorine Dioxide Bleaching 761

verify. However, the same is valid for adding chlorine dioxide (without intermediate

washing) after an hydrolysis treatment. The products of the hydrolysis are 2-

furancarboxylic acid and 2-furanaldehyde-5-carboxylic acid [10]. These water-soluble

compounds react very rapidly with ClO2. If ClO2 is added to an unwashed pulp

after a hydrolysis treatment, it will react therefore not only with lignin but also

with the hydrolysis products. Consequently, neither combination will allow full

advantage to be taken of the hydrolysis and chlorine dioxide to be saved.

A comparison of these approaches to combine hot acid hydrolysis with chlorine

dioxide treatment shows visible differences (Figs. 7.65–7.67) [11]. Figure 7.65 illustrates

the resulting kappa numbers analyzed after a subsequent EOP stage. There

is a visible disadvantage of adding ClO2 to the pulp containing hydrolysis products,

as the reaction of ClO2 with 2-furancarboxylic acid or lignin is clearly rapid.

This is certainly also affected by the water solubility of the furan compound and

the need for ClO2 to diffuse into the fiber in order to oxidize the lignin. These

results contradict those of Juutilainen [12], who reported a slow reaction of the

hydrolysis compounds with ClO2, but did not compare both alternatives. This disadvantage

of Ahot/D compared to hotD0 only disappears at very high chlorine dioxide

input. With high availability of chlorine dioxide, the kappa numbers become

identical. Starting with ClO2 is a clear advantage, as it reacts more rapidly with

lignin than with HexA. Thus, after the rapid consumption of ClO2, sufficient

HexA sites are left to be removed by hydrolysis.

0.01 0.1 0.2 0.3

0.1

D

hot

A

hot

D

D(EOP) Kappa number

Kappa factor

Fig. 7.65 Effect of increasing addition of ClO2

on delignification in a hot D0 treatment. Oxygen-

delignified eucalyptus kraft pulp (kappa

10). Hot D0 with ClO2 addition initially, 2 h at

95 °C, pH 3. Ahot/D with 110 min hydrolysis

time at pH <3followed by ClO2 addition and

additional 10 min reaction time, all at 10% consistency.

EOP remained constant with 1.4%

NaOH, 0.4% H2O2, at 85 °C, 0.3MPa O2 pressure

for 0.5 h, 1 h without pressure [11].

762 7Pulp Bleaching

This effect on kappa number is mirrored by the development of brightness. The

more effective reaction with lignin lowers the number of colored sites and increases

brightness, which again is most obvious at low active chlorine input (Fig. 7.66).

0.00 0.1 0.2 0.3

D

hot

A

hot

D

Brightness [% ISO]

Kappa factor

Fig. 7.66 Impact of addition of ClO2 in the hot D0 stage or in an

Ahot/D treatment on EOP brightness. For conditions, see Fig. 7.65 [11].

0.00 0.1 0.2 0.3

0.0

0.6

0.9

1.2

1.5

D

hot

A

hot

D

Post Color Number

Kappa factor

Fig. 7.67 Impact of addition of ClO2 in the hot D0 stage or in

an Ahot/D treatment on EOP brightness stability (post color

number). For conditions, see Fig. 7.65. Analysis of reversion

with humid aging (2 h, 100 °C, 100% humidity) [11].

7.4 Chlorine Dioxide Bleaching 763

The higher brightness of the hot D0-EOP treatment in addition shows a higher

stability. The data in Fig. 7.67 compare the resulting post color numbers. At low

active chlorine input, the improvement is very pronounced. Even following the

addition of a higher input of chlorine dioxide (high kappa factor), the advantage of

keeping the chlorine dioxide treated pulp at very high temperature is still pronounced.

It is a safe assumption that, at this temperature level, all chlorine dioxide

added will be consumed within a few seconds. Therefore, the obvious advantage

of keeping the pulp after the reaction with ClO2 for an extended time at above

90 °C must have its background in additional reactions. In chlorine dioxide

delignification, one may speculate that the degradation processes involve quinones

as intermediates. Quinones are not stable molecules and, provided that the

temperature is sufficiently high, they will react further. For example, hydrochloric

acid or methanol – both compounds are present in a D stage – add to quinones in

a 1,4 addition. The resulting hydroquinones can be reoxidized to another quinone.

The very visible differences in AOX load are in support of this theory of additional

oxidation and degradation reactions.

This hot chlorine dioxide process – which is sometimes denoted as D*-stage

and marketed as the DUALD™ process – enables an overall reduction of the AOX

discharge by approximately 50%, presumably through an accelerated degradation

of chlorinated structures formed in this stage to, for example, harmless chloride

ions [13,14]. Moreover, when bleaching an oxygen-delignified eucalyptus kraft

pulp to full brightness, the demand for chlorine dioxide could be reduced from

33 kg a.Cl odt–1 to 23 kg a.Cl odt–1 when replacing a conventional D0 by a D* stage

in a D(EO)D sequence. The organic chlorine content of the pulp (OX), however, is

not reduced in such trials, because the second D stage – which is conducted at the

conventional level of 75 °C – causes a repeated production of halogenated compounds.

Ragnar and Torngren have shown that the chlorination of the pulp (and

thus OX formation) is related to the presence of elemental chlorine which is

formed in situ during chlorine dioxide bleaching [13]. One way to reduce the OX

level is to perform a subsequent alkaline extraction; this may be rather effective in

terms of brightness increase and OX reduction if applied at the hydrogen peroxide

stage [15].

Alternatively, the addition of sulfamic acid to a final D stage provides an efficient

means of reducing the OX content in the pulp by almost 50%. Sulfamic acid

acts as a chlorine/hypochlorous acid scavenger to form chlorosulfamic acid

according to the following expression:

H 2 NSO _3 _ HOCl _ HClNSO _3 _ H 2 O _81_

The oxidizing power of chlorine dioxide decreases (by about 20%) due to the

capture of elemental chlorine by sulfamic acid. The drawback of a hot chlorine

dioxide treatment (D*) is the lower selectivity as compared to a conventional D

stage. Replacing a conventional D (20 min at 70 C) by a hot D* stage (120 min at

90 °C) using an oxygen-delignified hardwood kraft pulp, kappa number 10.9, viscosity

1021 mL g–1, leads to an increase in the number of chain scissions from

764 7Pulp Bleaching

0.224. 10–4 to 0.527. 10–4 mol AGU–1, while the kappa number reduction over

the D stage increases from 4.7 to 6.3 [14]. Considering the significantly higher

efficiency of the D* stage, the overall extent of cellulose degradation remains quite

moderate in comparison to conventional chlorine dioxide bleaching. The increase

of temperature from 70 °C to 90 °C, and prolongation of the reaction time from

20 min to 120 min during a first chlorine dioxide stage, is significantly more detrimental

to the viscosity of a softwood kraft pulp as compared to that of a hardwood

kraft pulp. Ragnar has shown in an example using an oxygen-delignified softwood

kraft pulp, kappa number 12.9, viscosity 984 mL g–1 that, when changing from conventional

chlorine dioxide conditions to those characteristic for a D* stage, the number

of chain scissions increases from almost zero to 0.83. 10–4 mol AUG–1, while the

kappa number reduction over the D stage increased only by one unit, from about 7.3

to 8.3 [14]. A slight improvement in selectivity can be obtained when the hot chlorine

dioxide stage is supplemented by a prestage without an interstage washing,

carried out at a conventional temperature of about 60 °C with very short retention

time of about 3–6 min, while preserving the beneficial effects of a D* stage [16].

A high temperature in the final chlorine dioxide stage improves brightness stability

[11]. Clearly, intermediates of chlorine dioxide bleaching are degraded and

the precursors of potential chromophores are destroyed. Figure 7.68 illustrates the

impact of temperature on brightness and reversion, where an oxygen-delignified

pulp was pretreated with a hot D0 and an EOP stage. The positive impact of a

higher temperature on brightness stability already becomes visible with only a low

input of active chlorine. At a higher input of chlorine dioxide, the effects are even

more pronounced. Figure 7.68(a) also illustrates the potential of applying only

three bleaching stages to reach full brightness. However, the need for a high input

of chemical in the D1 stage to guarantee >90 %ISO brightness is clear. If the

brightness target is 89–90% ISO, then three bleaching stages are sufficient.

Brightness stability is even more improved when the hot D1 stage is followed by a

final peroxide stage (see Section 7.6, Peroxide bleaching). With the D0-EOP-D1-P

sequence, a brightness of >92% ISO and post color numbers as low as 0.1are

achieved.

Seger et al. found that the efficiency of chlorine dioxide bleaching can be

improved by using a two-step process, denoted as Dh/l, comprising a first stage for

5–15 min at an end pH of 6–7.5, and a long second stage for 150 min at an end

pH of 3.5–4.0 [17]. Results obtained for OD(EOP)D and D(EO)D sequences indicate

that the use of the Dh/l-technology in both the D0 and/or D1 positions

increases final brightness at a given charge of chlorine dioxide, provided that the

pulp entering the D stage has a kappa number below 10. The Dh/l concept is particularly

efficient when applied in bleaching stages following an oxygen prebleaching

stage. The results include higher final brightness and chemical savings of 4 kg

ClO2 odt–1, which equals a ClO2 reduction of more than 20% (Tab. 7.35). The

improved performance of the high/low-pH method appears to be due to a considerable

reduction in chlorate formation (Fig. 7.69). Comparing the chlorate concentration

at a given D1 brightness, an even higher reduction (up to 45% at 78.3%

ISO) can be observed using Dh/l as compared to conventional chlorine dioxide

7.4 Chlorine Dioxide Bleaching 765

70 80 90

active chlorine charge (%)

0.75 1.25 1.75 2.25

Brightness [% ISO]

Temperature in D1 [. C]

70 80 90

0.00

0.4

0.6

0.8

1.0

active chlorine charge (%)

0.75 1.25 1.75 2.25

Post Color Number

Temperature in D1 [. C]

Fig. 7.68 Impact of active chlorine amount and temperature

in a D1 stage on (a) brightness development and (b) brightness

reversion in humid aging. Pre-bleaching with high kappa

factor (0.25) in hot D0 [11].

766 7Pulp Bleaching

4 6 8 10

conventional D D

h/l

Conversion of Cl

to ClO

-, %

ClO

charge in D

-stage, kg/odt

Fig. 7.69 Chlorate formation as a function of

ClO2 charge in D1 stage for a CE-prebleached

softwood kraft pulp of kappa number 4.4

(according to [17]). D1 conventional: 10%

consistency, 180 min, 70 °C, end pH 1.4–4. Dh/l:

first stage: 10% consistency, 5–15 min, 70 °C,

end pH 6–7.5, second stage: 10% consistency,

150–174 min, 70 °C, end pH 3.5–4.0.

bleaching. The lower chlorate formation during high/low-pH chlorine dioxide

bleaching is, however, not reflected in a lower AOX formation. The generation of

organochlorine compounds is seen to be comparable for D stages carried out conventionally

or using the two-step technique [17].

The advantage of the two-step chlorine dioxide bleaching process over the conventional

one-step technology with respect to chlorate formation diminishes with

increasing kappa number prior D1 stage. It may thus be speculated that the

decreasing efficiency of the two-step approach can be attributed to the changing

structure of the residual lignin.

Ljunggren et al. introduced a two-step chlorine dioxide bleaching concept where

the first step operates at low and the second at high pH levels, providing lower

levels of AOX and OX than conventional one-stage chlorine dioxide bleaching

[18]. In the first step, the pH is adjusted to about 2.8 for only 30 s after which, in

the second step, the pH it is raised by injection of alkali to about pH 10 for about

60 min. To avoid complete consumption of chlorine dioxide during only the first

stage, two-thirds of the chlorine dioxide is charged in the first step, and one-third

in the second step. The effect of chlorine dioxide bleaching with a two-step, lowto-

high pH profile was investigated in the first D stage of a D(EOP)DD sequence

using a pre-delignified pine kraft pulp, kappa number 12 and intrinsic viscosity

1020 mL g–1 [18]. Under the most favorable conditions, comprising a pH profile

from 2.8 to 10 at a kappa factor of 0.18, the AOX load in the D1E effluents could

7.4 Chlorine Dioxide Bleaching 767

768 7Pulp Bleaching

Tab. 7.35 Overview of literature data on the efficiency of chlorine dioxide in ECF bleaching sequences of kraft pulps.

Chemical charges [kg odt–1]

Pulp Sequence Unbleached Pulp Kappa Viscosity Kappa factor D0 E, EO(P) D1+ D2 E2 Total

Kappa

No.

HexA

[mmolkg–1]

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