Application in Chemical Pulp Bleaching

Hans-Ullrich Suss

Hydrogen peroxide is applied in ECF and TCF bleaching sequences. Currently,

ECF bleaching is by far the most dominant bleaching technology; indeed, in 2004

over 90% of all wood pulp was bleached with chlorine dioxide as the main bleaching

agent. (In Asia, a relatively large amount of one-year-plant pulps is still

bleached using chlorine and hypochlorite; thus, in relation to all pulp production,

ECF bleaching might represent only 70%.) TCF bleaching has become a niche

specialty, notably in Sweden and in Central European sulfite mills. Its world share

in bleached pulp production is estimated at about 5%. In ECF bleaching, H2O2 is

used in the extraction stages following chlorine dioxide treatment. After the acidic

D stage, a high level of oxidized lignin remains in the pulp, due to its limited solubility

at acid pH. Consequently, the acidic and alkaline stages are applied alternately.

The effect of an extraction is a further decrease in lignin content, because

the formation of sodium salts of the carboxylic acids within the oxidized lignin

residual results in a better solubility. The demand for caustic soda depends on the

carry-over of acid from the D stage and the carboxylic acids content. Increasing

the amount of caustic soda above a certain level has only a limited effect

(Fig. 7.124). With effective washing the demand for caustic soda can decrease significantly.

The temperature in an E stage is between 75 °C and 90 °C, while the pH

is typically about 11 at the start of treatment and about 10 at the end.

1,0 1,2 1,4 1,6 1,8 2,0

(κ-factor)

D

(0.20)-E D

(0.20)-Ep D

(0.25)-E D

(0.25)-Ep

Kappa number

NaOH charge [%]

Fig. 7.124 Impact of increasing amounts of

caustic soda in the extraction stage following a

D0 stage; softwood kraft pulp, kappa 24.6.

Kappa factor is the multiplier for the

kappa number value to calculate the input of

active chlorine to the D0 stage. Conditions: D0

stage 50 °C, 1 h; E(p) stage 0.5% H2O2, 75 °C,

1.5 h, both at 10% consistency.

868 7Pulp Bleaching

The graph in Fig. 7.124 shows the potential for reducing the amount of residual

lignin by an addition of oxidants. The oxidation of quinoid structures improves

the solubility of lignin. In the first E stage, typically oxygen and H2O2 are applied.

Oxygen gas is mixed with the pulp in high-shear mixers, which allow a very thorough

distribution of fine gas bubbles within the fibers. The oxygen level is typically

at 0.3–0.4%. While small amounts of oxygen are consumed rapidly, too-high an

input can result in the re-formation of large oxygen bubbles that may channel

through the tower and negatively affect pulp flow. For a moderate input of oxygen,

the counter-pressure of a tower or pre-tube of 15–20 m height is sufficient. A

potential solution to the problem of higher oxygen charges is to use a pressurized

tower. However, such as investment is questionable because the number of oxidizable

sites in the remaining lignin is normally small. Therefore, a high input of

oxygen does not result in any significant benefits. The exemption are pulps with

unusually high initial kappa numbers (>20). The application of H2O2 does not

require pressure, and in most mills oxygen and H2O2 are applied simultaneously

in the first E stage. The impact of an increasing amount of H2O2 is shown graphically

in Fig. 7.125. Because of the limited availability of easily oxidizable sites, levels

of H2O2 above about 0.5% must be activated by a higher temperature. Brightness

increase is accompanied by a further drop in residual lignin levels, this being

the result of additional oxidation reactions improving lignin extraction. Peroxide

addition can be used to balance the demand for caustic soda during the E stage

(see Fig. 7.124). Increasing the addition of caustic soda has a limited impact on

0,00 0,25 0,50 0,75 1,00

Kappa number

brightness

Brightness [% ISO]

H

O

-charge [%]

3,0

3,5

4,0

4,5

5,0

kappa number

Fig. 7.125 Impact of the addition of H2O2 to an E stage in

bleaching eucalyptus kraft pulp, oxygen-delignified pulp,

D0 stage at 50 °C with kappa factor 0.2. Ep stage at 85 °C for

amounts of 0.25% to 0.5% H2O2, larger amounts applied at

95 °C, constant 1.4% NaOH, 1.5 h.

7.6 Hydrogen Peroxide Bleaching 869

lignin removal. Rather than apply excess caustic soda, the use of moderate

amounts of H2O2 allows the brightness to be increased and the kappa number to

be decreased, simultaneously.

In hardwood pulp bleaching, the impact of peroxide application on Kappa number

is less pronounced. Because neither H2O2 nor oxygen can degrade HexA, the

amount of HexA remaining in the pulp after the D0 stage will remain unaffected

by their addition. Both chemicals will only further oxidize the lignin residual.

Therefore, the additional decrease in kappa number is small compared with softwood

pulp. The impact of H2O2 addition to an E stage following a D0 stage at 50 °C

is shown graphically in Fig. 7.125. Despite moderate changes in kappa number,

the impact on brightness is significant. A temperature increase is required to trigger

the consumption of larger amounts of H2O2. However, despite the higher temperature,

above an input of about 0.4% H2O2 a peroxide residual will remain. The

impact on lignin removal decreases further if large amounts of chlorine dioxide

are applied in D0, or the temperature is raised. The use of a very high temperature

(>90 °C) during the first chlorine dioxide stage allows simultaneous delignification

and hydrolysis, respectively destruction of HexA. In comparison to standard D0

stage conditions (50–70 °C), this significantly reduces the amount of double bonds

measured after the Eop stage. Values between kappa 2 and 3 are achieved with

extraction only. Consequently, the impact of an oxidative support of the extraction

stage with O2 and H2O2 on the remaining double bonds becomes minimal,

though the effect on brightness is still pronounced. An example of the impact of

increasing H2O2 amounts in the E stage following a hot D0 stage with kappa factor

0.2 is shown in Fig. 7.126. It is necessary to raise the temperature to enforce

0,25 0,50 0,75 1,00

Temperature [°C]

85 95

Brightness [% ISO]

H

O

-charge [%]

Fig. 7.126 Impact on brightness of an intensified delignification

by a hotD0 stage on peroxide effectiveness in the subsequent

extraction stage. E stage at 10% consistency, with 1.4%

NaOH.

870 7Pulp Bleaching

the consumption of a higher input of peroxide. Without peroxide addition, the E

stage brightness is only at 73% ISO.

The need to add oxidants to the extraction stage might be questioned. Bleaching

with the stages DEDED is possible in theory, but this would result in a rather high

demand for chlorine dioxide with consequences for cost and effluent load (AOX).

In order to optimize effects it is important to use the potential of other chemicals

to degrade lignin and chromophores. The use of oxygen and H2O2 in the E stages

promotes the E stage from simply an extraction to a brightening and delignification

stage. The improvement in pulp brightness by up to 10 points, compared to

an Eo stage under identical conditions, is shown in Fig. 7.127. This advantage in

brightness is still apparent after subsequent D1 and D2 stages. The right-hand portion

of the graph shows, for the same input of chlorine dioxide to D1 and D2, an

advantage of about one brightness point. This represents an economical and ecological

advantage which is also beneficial with regard to the operational stability of

the bleaching process. The production of off-grade pulp becomes less likely if the

final brightness gains are smaller, because no large variations in chemical addition

are required to compensate brightness.

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