Stage Substrate Unit Values Comment

D0 Consistency

Time

Temperature

Kappa factor

Final pH

%

min

°C

10–14

30–60

40–60

0.20–0.28

2–4

(EOP) Consistency

Time

Temperature

NaOH-charge

H2O2-charge

Pressure

Kappa number

%

min

°C

kg odt–1

kg odt–1

kPa

10–12

60–90

70–90

a

25–50

atmospheric

3–6

2–5

upward flow

downward flow

SW-Kraft

HW-Kraft

a. typically between 15–30, more related to the incoming kappa

number usually a charge equal to the kappa number entering

the D0 stage.

754 7Pulp Bleaching

7.4 Chlorine Dioxide Bleaching 755

The chlorine dioxide charge is calculated in relation to the kappa number of the

pulp entering the D0 stage expressed as kappa factor, kf:

kf _

a _ Cl _ charge__ odt

kappa number _79_

The stoichiometry of the chlorine dioxide delignification reaction in a D0 position

(softwood kraft, initial kappa number 29–32, kappa factor 0.17–0.23, 40–

50 °C) shows a linear relationship, independent of the kappa number entering the

D0 stage according to the following equation [2]:

D_

D ClO 2 _ 0_58 _80_

where ClO2 is expressed in kg chlorine dioxide per o.d. ton of pulp.

Equation (80) allows the calculation of the extent of delignification during the

D0 operation alone. However, this information is not very useful because a great

part of the oxidized lignin compounds remains in the pulp fiber after the D0 stage,

and can only be removed by a subsequent alkaline extraction stage. As expected,

the charge of sodium hydroxide necessary to extract the oxidized, water-soluble

material quantitatively is related to the kappa number of the pulp entering the D0

stage (Tab. 7.32). Today, it is very common to enhance the extraction stage with

oxygen (EO) or hydrogen peroxide (EP), or both (EOP), to compensate for a lower

chlorine dioxide charge in the preceding D0 stage. The data in Fig. 7.61illustrate

0.20 0.24 0.28 0.32

D(EO) kappa number:

SW-Kraft, ê0

= 29.5 HW-Kraft, ê0

= 16.5

D(EOP) kappa number: SW-Kraft HW-Kraft

D(EO)/D(EOP) Kappa number

Kappafactor

Fig. 7.61 Kappa number of D(EO)-treated softwood

and hardwood kraft pulps as a function

of kappa factor (according to [1]). D0 delignified

softwood and hardwood kraft pulps, each

treated with an active chlorine charge

corresponding to a kappa factor 0.22, were

subjected a subsequent extraction stage,

where, in addition to oxygen, 3kg H2O2 per ton

pulp were applied.

that the addition of 3 kg H2O2 odt–1 in the EOP stage decreases the D(EOP) kappa

numbers from 4.4 to 3.4 for the softwood kraft pulp, and from 4.0 to 2.4 for the

hardwood kraft pulp.

The use of 3 kg H2O2 odt–1 in the EOP stage decreases the D(EOP) kappa number

to a level which can be obtained after a D(EO) sequence only by increasing the

kappa factor to 0.27 for the softwood and to 0.25 for the hardwood kraft pulp.

Thus, the charge of 1kg H2O2 per ton pulp replaces active chlorine charges in an

amount of 4.9 kg odt–1 [(0.27 – 0.22). 29.5. 10/3 = 4.9] for the former and

1.7 kg odt–1 [(0.25 – 0.22). 16.5. 10/3] for the latter. However, the absolute

amount of active chlorine charge being replaced by the addition of H2O2 in an

EOP stage depends on many factors, including the amount and reactivity of residual

lignin as well as the applied reactions conditions.

Chlorine dioxide is a very efficient and selective brightening agent. Therefore,

chlorine dioxide stages are most commonly used in the final bleaching (ECF)

sequences. A selection of different ECF-bleaching sequences comprising one or

more chlorine dioxide stages which are applied to obtain full brightness (89+%

ISO) are detailed in Tab. 7.33.

Tab. 7.33 Examples of typical chlorine dioxide bleaching sequences.

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