Eo Eop Eop

brightness (%ISO)

Eo Eop Eop

brightness (%ISO)

D1 D2

H2O2: 0.4% 0.6% 0.4% 0.6%

Fig. 7.127 Impact of the addition of H2O2 to the Eop stage on

brightness development in final bleaching.

One positive side effect of adding H2O2 to the first E stage is a significant decrease

in effluent color. Typically, an E-stage effluent is medium to dark brown in color, but

becomes light brown on addition of H2O2 [59]. Therefore, some mills apply H2O2 not

only for its bleaching effect but also to control effluent color. Another positive effect is

the higher intensity of shives bleaching [60,61]. Even when shives are not fully

bleached, they become lighter in color, which reduces their visibility.

Hydrogen peroxide is also applied advantageously in the second E stage of the

longer sequences used in softwood pulp bleaching. In a D0EopD1EpD2 sequence,

7.6 Hydrogen Peroxide Bleaching 871

H2O2 reduces the demand for chlorine dioxide in final bleaching. The substitution

of chlorine dioxide by H2O2 in the sequence follows stoichiometric rules: 1kg t–1

H2O2 replaces 2 kg t–1 active chlorine [62]. This is shown graphically in Fig. 7.128,

where the application of H2O2 results in a higher brightness with lower input of

chlorine dioxide. The resultant flat curve crosses the 90% ISO line at lower input

of ClO2. It becomes easier to achieve a standard deviation of brightness of, for

example } 0.5 points around the 90% ISO value.

10 15 20

91 H

O

-charge in Ep: 2.5 kg/t

savings in ClO

Sequence: DED DEpD

Brightness [% ISO]

Active chlorine charge(D

+D

) [kg/odt]

Fig. 7.128 Substitution of chlorine dioxide by H2O2 in final

bleaching of softwood kraft pulp with the stages D1E(p)D2.

Amount of ClO2 in D1 variable, amount in D2 constant at 5 kg t–1

active chlorine; all stages at 70 °C, 2 h, 10% consistency.

Most modern mills operate an oxygen stage, and therefore have a low level of

lignin entering the bleach plant. This permits shorter bleaching sequences, such

as a four-stage sequence with a D0-Eop-D1-D2 configuration. The two D stages can

be separated by a washing step, or follow each other directly. Another alternative

is a short neutralization with caustic soda after D1, which is followed by mixing

the chlorine dioxide for D2. The target of these modifications is lower investment

costs. The five-stage version D0-Eop-D1ED2 or D0-Eop-D1EpD2 is certainly more

effective, though the differences are not pronounced enough in terms of investment

costs. The demand for chlorine dioxide in a shorter sequence can be rather

high, especially when the target is very high brightness. Several problems of these

shorter sequences are shown in Fig. 7.129. First, it becomes clear that the demand

for chlorine dioxide in a three-stage sequence would become extreme if the brightness

target were to be at 90% ISO. Whilst a level around 89% ISO is within reach,

the flatness of the curve indicates that, to reach a much higher level, would be

872 7Pulp Bleaching

very difficult. After washing the pulp, the addition of further chlorine dioxide (D2)

becomes effective once more. Nevertheless, about 1.5% of active chlorine is required

to achieve (safely) more than 90% ISO. The same brightness is achieved

with much less chlorine dioxide when the second D stage is replaced by a P stage.

The substitution is more than stoichiometric, as one part of peroxide replaces

about four parts of active chlorine. This is the result of a more effective oxidation

by two differently acting chemicals. An additional positive side effect of the P

stage is an improved brightness stability (see the next section).

0,0 0,5 1,0 1,5 2,0

91 H

O

-charge in P: 0,25 %

Bleaching stages: D

D

D

D

P

Brightness [% ISO]

Total active chlorine charge [%]

Fig. 7.129 Substitution of the final D stage with a final P

stage in a D0EopD1D2 sequence. Oxygen-delignified eucalyptus

kraft pulp. D0 with factor 0.2 at 50 °C, Eop kappa 3.0,

brightness 81.5% ISO. D stages at 75 °C, 2 h; P with 0.25%

H2O2, 0.3% NaOH, 85 °C.

Дата добавления: 2015-10-21