Viscosity

[mPa.s]

COD

[kg t–1]

Yield

[%]

DEopDD 5.0 85 28.4 37.5 96.0 0.3

DEopDP 3.8 85 27.138.2 96.0 0.25

DEophotDPhot 2.5 98 18.7 44.0 95.15 0.25

Magnesium sulfate (Epsom salt) is an additive applied in peroxide bleaching to

prevent cellulose depolymerization. The benefit of adding magnesium ions

becomes apparent in peroxide bleaching at very high temperatures and with a

high peroxide input. Thus, magnesium salts are added in TCF bleaching but not

normally in an Eop stage or a final P stage. The positive effect of magnesium sulfate

addition becomes apparent in bleaching sequences which used acidic conditions

for transition metals removal. A Q stage under mildly acidic conditions

retains the magnesium and calcium traces in the pulp, thus the impact of additional

magnesium is less pronounced. The same is valid in regions with a high

water hardness and not very narrow water loops. The mechanism of the protective

effect is not clearly understood. Magnesium will be precipitated rapidly as

Mg(OH)2 under the conditions of a peroxide stage. Theories discuss the absorption

of other metal ions during precipitation or an action as scavenger of superoxide

anion radicals [54]. Experiments with peroxide bleaching of deinked pulp in

the disperger (i.e., at high temperature) did indicate any positive effect of another

precipitation. Bicarbonate ions present in the water loop after CO2 neutralization

would decompose peroxide under the conditions of disperger bleaching. The formation

of insoluble magnesium carbonate avoided peroxide losses and improved

the bleaching result [55]. Since carbonate ions are present in caustic soda and are

generated during bleaching, their precipitation may be part of the positive effect

of magnesium salt addition.

7.6 Hydrogen Peroxide Bleaching 865

7.6.5.4 Pressurized Peroxide Bleaching

In TCF bleaching the brightness target of a final peroxide stage might require the

consumption of very large amounts of peroxide, and to achieve sufficient consumption

of H2O2 high temperature will be required. The application of pressure

is also recommended [50,51], with pressure applied ranging from 0.1 MPa to a

maximum of 0.5 MPa. These stages are frequently labeled as P(O) stage because

pressure is applied by oxygen gas addition. The positive effect was described as an

acceleration of an otherwise very slow brightening of the pulp. Later, pressurized

peroxide stages were also recommended for ECF sequences [56]. Several mills

have installed such equipment, although many typically operate without applying

pressure because bleaching is not improved by pressure or oxygen addition

[57,58]. This is in line with current knowledge of the reaction mechanism of alkaline

peroxide bleaching. The peroxide reactions are neither accelerated nor

improved by a moderate pressure increase, however, pressurized equipment is

more expensive than nonpressurized counterparts.

7.6.6

Technology of H2O2 Bleaching

Andreas W. Krotscheck

7.6.6.1 Atmospheric Peroxide Bleaching

The process flowsheet of a typical atmospheric peroxide bleaching system is

shown schematically in Fig. 7.122. Caustic soda is added to the MC pulp coming

from the previous bleaching stage, for example, to the repulper of a drum washer

or to the dilution conveyor after a wash press. The alkaline pulp falls into a standpipe

and is mixed with peroxide as it enters the MC pump.

The pump provides good mixing of the peroxide into the pulp suspension, and

a dedicated mixer is often not required. The pulp proceeds to an atmospheric

upflow reactor where the bleaching reaction takes place. Depending on the feed

requirements of the subsequent washing equipment, the pulp slurry is discharged

from the reactor either at low or medium consistency.

MC PUMP REACTOR WASHING

Pulp from

preceding

stage

Pulp to

next stage

NaOH

H2O2

Fig. 7.122 Process flowsheet of a typical atmospheric peroxide bleaching system.

866 7Pulp Bleaching

Washing after a peroxide stage is usually carried out with single-stage washing

equipment, for example, with a wash press, a single-stage Drum Displacer™, an

atmospheric diffuser, or a vacuum drum washer.

The material of construction for wetted parts in a peroxide stage is typically a

higher grade of austenitic stainless steel.

Further information regarding atmospheric peroxide bleaching equipment,

including medium-consistency pumps and atmospheric upflow reactors, is provided

in Section 7.2. Pulp washing is detailed in Chapter 5.

7.6.6.2 Pressurized Peroxide Bleaching

The equipment used for pressurized peroxide bleaching is very similar to oxygen

delignification equipment. The process flowsheet of a typical pressurized peroxide

bleaching system is shown schematically in Fig. 7.123. Caustic and peroxide are

added to the medium-consistency pulp coming fromthe previous bleaching stage, as

in atmospheric peroxide bleaching. TheMCpump forwards the pulp suspension to a

high-shear mixer which is charged with oxygen and steam, after which the threephase

mixture proceeds to a pressurized upflow reactor where the bleaching reaction

takes place.

As in oxygen delignification, it is essential that the high-shear mixer creates

stable micro-bubbles which ensure a homogeneous bleaching result, without

channeling in the reactor. After its discharge from the reactor top, the pulp suspension

is separated from the gas phase in a blowtank. The offgas from the blowtank

can normally be blown to the atmosphere. Depending on the feed requirements

of the washing equipment, the pulp slurry is discharged from the blowtank

either at low or medium consistency.

Washing after a peroxide stage is usually carried out with single-stage washing

equipment, for example, with a wash press, a single-stage Drum Displacer™, an

atmospheric diffuser, or a vacuum drum washer. Further information on pressurized

peroxide bleaching equipment, including medium-consistency pumps and

mixers, pressurized reactors and blowtanks is provided in Section 7.2. Details of

pulp washing are provided in Chapter 5

MC PUMP

HIGH-SHEAR

MIXER

REACTOR BLOWTANK WASHING

O2 Steam

Pulp from Offgas

preceding

stage

Pulp to

next stage

NaOH

H2O2

Fig. 7.123 Process flowsheet of a typical pressurized peroxide bleaching system.

7.6 Hydrogen Peroxide Bleaching 867

7.6.7

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

2

4

6

8

10

(κ-factor)

D

0

(0.20)-E D

0

(0.20)-Ep D

0

(0.25)-E D

0

(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

70

75

80

85

90

Kappa number

brightness

Brightness [% ISO]

H

2

O

2

-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

82

84

86

88

Temperature [°C]

85 95

Brightness [% ISO]

H

2

O

2

-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.

Eo Eop Eop

70

75

80

85

brightness (%ISO)

Eo Eop Eop

86

88

90

92

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

86

87

88

89

90

91 H

2

O

2

-charge in Ep: 2.5 kg/t

savings in ClO

2

Sequence: DED DEpD

Brightness [% ISO]

Active chlorine charge(D

1

+D

2

) [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

87

88

89

90

91 H

2

O

2

-charge in P: 0,25 %

Bleaching stages: D

1

D

1

D

2

D

1

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.

7.6.7.1 Stabilization of Brightness with H2O2

Brightness stability is affected by a number of parameters, and is typically analyzed

in tests using accelerated aging. The pulp samples are exposed to elevated

temperature under either dry or humid conditions. Brightness losses during dry

heating are normally less pronounced compared with humid reversion tests. A

standard procedure is to heat handsheets over boiling water for 1h. This test

method is described as E.4P method by Paptac [63]. The changes in light absorption

and scattering are measured as post color number [64]; the smaller the number,

the less reversion has taken place. Humid brightness reversion is thought to

correlate more with natural aging occurring in pulp bales [65].

7.6 Hydrogen Peroxide Bleaching 873

The intensity or aggressiveness of the bleaching process certainly has an

impact. Compounds and conditions that affect brightness stability include transition

metals, remaining lignin, hemicelluloses, and the pulping process used [66].

Past experience with hypochlorite pointed to rather negative effects of low pH,

high temperature and high charges of this chemical on brightness stability. Losses

could be attributed to oxidation of the cellulose chain, and were often very significant.

50 years ago kraft pulp was typically not bleached above about 80% ISO

brightness using a typical CEHH sequence. The situation has now changed with

the increasing use of chlorine dioxide, initially in a final stage (CEHD) and later

with -D1E2D2 or -D1EpD2 final bleaching.

Mill experience teaches that the higher the brightness, the greater the stability,

though this applies to the same sequence and moderate changes in bleaching conditions.

The effectiveness of lignin removal or impurities removal is important for

the stability of brightness, and therefore differences between TCF and ECF bleaching

can be expected. Indeed, in TCF bleaching of birch kraft pulp HexA were identified

as a source of high reversion [67]. Likewise, poor brightness stability was

found for ECF “light” bleached softwood pulp [68]. However, in “normal” ECF

bleaching HexA were found not to be the source for reversion [69], as it was

removed completely in the process. These variances explain the importance of

bleaching conditions. The TCF sequence used to bleach birch pulp with poor

brightness stability [67] was conducted exclusively with alkaline bleaching steps.

In order to attribute correctly the reversion to certain sources, it is important to

understand how complete or ineffective potential sources for the development of

colored compounds are destroyed.

A comparison of different ECF sequences, all using sufficient chemical for lignin

oxidation and HexA hydrolysis or destruction [69], showed that cellulose depolymerization

(apparent as a lower viscosity) has no direct influence on brightness

stability. Neither hot acid hydrolysis nor ozone nor aggressive conditions and a

very high temperature in the final peroxide stage had any significant impact on

reversion. The positive impact of using more bleaching chemical in a D0-Eop-D1

sequence is illustrated graphically in Fig. 7.130. A moderate input of chlorine

dioxide (in this example, 1%) provided a reasonable brightness close to 89% ISO,

but the brightness was not stable. With a loss of about 10 points of brightness in

humid reversion, the instability was pronounced. The use of additional chemical

improves bleached brightness, but not to any great degree. A doubling of the

active chlorine input (from 1% to 2%) added only one brightness point, yet reversion

losses decreased from 10 points to only 7 points. The more intense degradation

of lignin or other “impurities” was seen to improve brightness stability.

The advantage of an additional treatment stage to improve brightness and

brightness stability is illustrated in Fig. 7.131. A small amount of active chlorine

(0.5%) applied in the second D stage lifts brightness to 90% ISO, and reduces

losses in humid reversion to about 5 points of brightness. Even more pronounced

is the improvement of a stoichiometric substitution of chlorine dioxide by H2O2

(0.5% active Cl with 0.25% H2O2). Losses in reversion decrease at best to only 3.5

points.

874 7Pulp Bleaching

1 2

act Cl in D1

78

80

82

84

86

88

90

brightness (%ISO) T260

UM200

bleached

1.5

Fig. 7.130 Analysis of brightness reversion of eucalyptus kraft

pulp after three bleaching stages with D0-Eop-D1. Active chlorine

to D0 with kappa factor 0.23at 50 °C, 1 h. D1 at 70 °C, 2 h.

Brightness analyzed after 2 h at 100 °C, 100% humidity and

4 h at 105 °C (UM 200 test).

D1: 1% act. Cl 1.5% 2%

D2 P D2 P D2 P

78

80

82

84

86

88

90

brightness (%ISO) T260 UM200 bleached

Fig. 7.131 Analysis of brightness reversion of eucalyptus kraft

pulp after four bleaching stages with D0-Eop-D1-D2, respectively

D0-Eop-D1-P.

This advantage of an application of H2O2 can be attributed to the destruction of

carbonyl groups and quinoid structures remaining in pulp after the D stage.

Using UV Raman spectroscopy, Jaaskelainen [70] detected these intermediates (or

end products) of chlorine dioxide bleaching [9] in kraft pulp bleached with a final

D stage. In peroxide-bleached pulp such structures were absent, the reason being

the rapid reaction of alkaline peroxide with quinones.

7.6 Hydrogen Peroxide Bleaching 875

The importance of the remaining quinoid structures, respectively their elimination

by alkaline H2O2, explains the rather good brightness stability of TCF or ECF

“light” bleached pulp. Despite their higher lignin residual, these pulps normally

show very moderate reversion losses in accelerated aging. The same applies to

bleached mechanical pulp, which is rather stable during heat-induced aging. Its

sensitivity against light-induced yellowing has phenols as the main source, and

follows a different reaction pathway [71].

The conclusion drawn regarding the source for reversion of the TCF-bleached

birch pulp mentioned above [67] can now be seen from a different angle. This

pulp had been subjected to a very intense peroxide treatment, but not to any acid

stage capable of removing HexA. Therefore, the source for the remaining reversion

of this pulp was HexA. It is not appropriate to generalize this specific finding,

however. In ECF bleaching with sufficient hydrolysis or oxidation of HexA,

other compounds are responsible for brightness losses in aging. The best brightness

stability results from the most effective removal of all impurities. This

includes sufficient bleaching chemical with different reactivities towards the

impurities and definitively sufficient washing. The combination of a very high

temperature in the D0 and the last D stage (90 °C) with a final peroxide stage provides

access to the brightness range above 93% ISO, and simultaneously to

extreme brightness stability. The increase in brightness, and the development of

brightness stability in humid reversion, is shown graphically in Fig. 7.132. The

peroxide-bleached pulp loses less than one point of brightness during the aggressive

aging treatment.

D1 D2 P

88

89

90

91

92

93

brightness (%ISO)

0

0.1

0.2

0.3

0.4

0.5

post color #

brightness

post color #

Fig. 7.132 Development of brightness and brightness

stability of eucalyptus kraft pulp bleached with the stages

hotD0-Eop-D1-hotD2-P, reversion analyzed after 2 h at 100 °C,

100% humidity.

876 7Pulp Bleaching

7.6.7.2 Catalyzed Peroxide Bleaching

Under acidic conditions, H2O2 reacts very slowly with pulp, the reaction being

accelerated by the presence of molybdenum and tungsten salts [72]. Such an

acidic treatment can be used to activate an alkaline peroxide bleaching stage. During

the period of intense searching for alternatives to chlorine bleaching – that is,

before settling on ECF bleaching – molybdate-catalyzed peroxide was intensely

investigated [73]. Obviously just a niche application remained [74]. In order to

achieve a sufficient turnover of peroxide with a molybdate input at 0.04%, the

temperature must be elevated above 80 °C, while the required retention time is

about 2 h.

A catalyst for alkaline peroxide bleaching was described by Patt [75]. The manganese

complex accelerates and intensifies bleaching in both ECF and TCF applications.

Unfortunately, synthesis of the manganese compound [76] (Fig. 7.133) is

complicated, and this results in high – and for an industrial application – prohibitive

costs. On a laboratory scale, the addition of 10–50 ppm of the complex

resulted in a higher brightness at a lower demand for H2O2. To date, no other

compounds with similar properties and lower production costs have been

described in the literature.

N

Mn

N

N N

Mn

N

N

O

O

H3C

CH3

CH3

H3C

2“

O O

H3C

2 [ClO4]”

Fig. 7.133 Model of the manganese complex effective in

activating peroxide bleaching of kraft and sulfite pulp.

7.6.7.3 Application in TCF Sulfite Pulp Bleaching

The brightening of sulfite pulp is rather easy with H2O2. The reason for this is a

low level of residual lignin, and very little lignin condensation occurring during

the pulping process. Today, therefore, most sulfite pulp is bleached under TCF

conditions simply with H2O2. The acidic pulping process can leave a high amount

of hemicelluloses in the pulp. This permits pulping with a high yield, though the

hemicelluloses are soluble under alkaline conditions. Because a higher input of

caustic soda is required to activate large amounts of H2O2, the intensity of bleaching

affects both yield and effluent load. An example of the impact of temperature

and caustic soda on chemical oxygen demand (COD) and yield is provided in

Fig. 7.134. The graph illustrates the high likelihood of a significant yield loss

when caustic soda is applied without care. The combination of a very high charge

of caustic soda and very high temperature is detrimental to pulp yield and effluent

load. Not surprisingly, this correlation is linear [77] within the typical range of

caustic soda addition.

7.6 Hydrogen Peroxide Bleaching 877

30 40 50 60 70 80

20

40

60

80

1

2

1

2

3

3

Temperature [°C]: 70 98

Yield Loss [kg/t]

COD [kg/t]

Fig. 7.134 Impact of alkali amount on chemical oxygen

demand (COD) and yield at two different temperature levels

and NaOH charges (1%, 2%, and 3%), 10% consistency, 1 h.

EopP(hc) Op(MgO)P(hc) EopP(mc) P(O)P(O)

90

92

94

96

98

yield (%)

Fig. 7.135 Yield in bleaching of spruce sulfite pulp to identical

brightness (88% ISO) with four different two-stage processes.

mc: medium consistency; hc: high consistency.

Bleaching to very high brightness typically begins with an MC treatment, with

the application of a moderate amount of caustic soda and H2O2, sometimes in

addition to a small amount of oxygen. Such a first step reduces the lignin level

and prepares a final bleaching step. High-consistency bleaching requires less

caustic soda because the higher concentration results in a higher pH with less

878 7Pulp Bleaching

chemical. This becomes apparent in a comparison of different bleaching technologies.

In Fig. 7.135, the yields are compared following two-stage processes. Medium-

consistency delignification followed by HC peroxide bleaching (Eop-Phc)

results in the highest yield. The aggressive alkalinity of a high-temperature pressurized

peroxide process (PO) [78] causes a significant drop in yield. Consequently,

the recommendation of such process conditions [78,79] leads in the

wrong direction. The corresponding brightness increases are shown in Fig. 7.136,

which also contains details of the applied process variables. The combination of

MC delignification and HC bleaching clearly provides the best response. Alternatively,

a combination of oxygen delignification, chelation and peroxide bleaching

(O-Q-P) is applied.

The use of magnesium oxide in an oxygen/peroxide delignification allows recycling

of the effluent into the recovery system in magnesium sulfite pulping

[80,81]. This decreases the effluent load from final bleaching, although the effectiveness

of brightening is lower – typically only the low 80s are accessible in PMgO

bleaching. A sequence applied in practice uses oxygen and peroxide together with

MgO and countercurrent washing from the acid stage after MgO treatment. This

acid stage is necessary to remove residual MgO completely. An OPMgO-A-P

sequence can reach brightness of 86–88% ISO. The effluent load is significantly

1 2 3 4

78

81

84

87

90

first P(O) stage

Eop brightness

Sequences: P(O)P(O) Eop(MC) Eop(HC)

Brightness [% ISO]

Total H

2

O

2

-charge [%]

Fig. 7.136 Brightness increase of spruce sulfite

pulp (kappa 17.1) in P stage. Pre-bleaching

with Eop, 1.5% NaOH, 0.75% H2O2, 1.5 h,

0.3MPa O2, 10% consistency. Second bleaching

stage: Pmc: H2O2 and NaOH variable, 3h,

80 °C, 10% consistency. Phc: H2O2 and NaOH

variable, 0.5% sodium silicate, 4 h, 75 °C, 25%

consistency. P(O) bleaching at 10% consistency.

1st stage with 2% H2O2, 1.8% NaOH,

0.3MPa O2, 95 °C, 1.5 h; 2nd stage with 1% or

2% H2O2, 1.6% NaOH, 95 °C, 1.5 h.

7.6 Hydrogen Peroxide Bleaching 879

decreased through the recycling procedure. Because the make-up of MgO required

in pulping can be added to the bleaching stage, this process will not lead to

additional costs – on the contrary, the costs for caustic soda are lowered.

7.6.7.4 Activators for H2O2 Bleaching

Activation steps with ozone, peracetic acid, or catalyzed acidic H2O2 have been

described in order to improve the performance of the final peroxide stage [82].

Cyanamide can be used to improve the performance of an MC peroxide bleaching

stage [83,84], but the more effective reaction of H2O2 at HC conditions [85] has

outphased this application. Dissolving pulp is bleached with an ozone step to

remove traces of lignin [86]. Peracetic acid has been used on a large scale to boost

brightness above the 90% ISO range. This process is used to bleach magnefite

pulp with the sequence O-Q-Paa/P. The activation uses on-site-mixed peracetic

acid (the mixture contains equilibrium peracetic acid); thus, peracetic acid, acetic

acid, H2O2 and water are present. Under the slightly acidic conditions of the Paa

stage, H2O2 is not consumed. The excess of H2O2 is activated for bleaching simply

by adding caustic soda. The activation with small amounts of Paa (0.15–0.3%)

allows two additional brightness points, and permits brightening to a level above

91% ISO [87].

7.7

Peracetic Acid in Pulp Bleaching

Bleaching of pulp with peracids is limited on an industrial scale to the application

of peracetic acid, CH3COOOH. In the past, other per-compounds were also promoted

for bleaching, among these being Caro’s acid, H2SO5, or mixtures of Caro’s

acid and peracetic acid. The interest in their application stems from the search for

alternatives to chlorine bleaching. The application of peracids was tested in ECF

and TCF bleaching sequences [1]. With the increasing knowledge about ECF

bleaching and its good environmental performance, peracids became less interesting.

Today, peracetic acid occupies a niche in TCF bleaching. The application of

peracetic acid in TCF bleaching was introduced because of the need to modify residual

lignin to allow its destruction in hydrogen peroxide bleaching, and to

improve the economics of TCF sequences. Typically, a peracid treatment is applied

under weak acidic conditions. This results in an improved delignification and a higher

brightness in the following alkaline peroxide stage.

Peracetic acid has a sharp pungent odor. It has a boiling point of 103 °C and a

vapor pressure of 3325 Pa at 25 °C. It is a weaker acid than acetic acid, and is produced

by mixing (glacial) acetic acid with hydrogen peroxide. The addition of a

strong acid (e.g., sulfuric acid) accelerates the formation of the equilibrium between

acetic acid, hydrogen peroxide, water and peracetic acid:

2CH3COOH _ H2O2 _ H2O _ CH3COOOH _ CH3COOH _ 2H2O

880 7Pulp Bleaching

The equation shows that the equilibrium can be shifted to the right by applying

high concentrations of hydrogen peroxide. However, there will always be an excess

of hydrogen peroxide and unreacted acetic acid in the mixture. This increases the

cost for the application of equilibrium peracetic acid in bleaching, because the

reaction conditions for peracid bleaching will not allow the reaction of hydrogen

peroxide. Another important factor is product safety. Storage and handling of

higher concentrations of peracetic acid with a high H2O2 content are restricted

due to its potential hazards. This prevents application in mill practice.

The peracetic acid equilibrium is shifted to the right by distillation under vacuum.

The resulting peroxide conversion is greater than 90%. The distillation products

are the most volatile compounds, water and peracetic acid (boiling point

103 °C). This distillate must be cooled to prevent re-formation of the equilibrium.

The ideal storage temperature for the mixture is below 0 °C; therefore, storage

tanks require both insulation and refrigeration. Cooled distilled peracetic acid is

commercially available with a content of 35–40% peracetic acid in water. Because

of the absence of a strong acid, re-formation of the equilibrium is very slow. An

accidentally higher storage temperature (e.g., ambient temperature) would not

constitute a safety hazard, but the resulting “new” equilibrium would produce a

lower concentration of peracetic acid.

Another economical alternative for peracetic acid application is on-site mixing

of peracetic acid with hydrogen peroxide. At a temperature slightly above ambient,

and with acid activation, the equilibrium is established within a few hours. Mixtures

with a content >8% peracetic acid and <40% H2O2 are commercially produced

on-site. In order not to waste the content of hydrogen peroxide in this equilibrium,

the Paa treatment must be followed by the peroxide stage, without intermediate

washing. Following an addition of caustic soda, the unused hydrogen peroxide

content in the pulp reacts in the subsequent P step.

The reactions of peracid with lignin follow mainly an electrophilic pathway.

With regard to reactivity, peracetic acid (CH3COOOH) has an advantage over

Caro’s acid (H2SO5). Peracetic acid has a pKa value of 8.2, and is only partly dissociated

at neutral or moderately acidic pH. Peracetic acid reacts via hydroxylation

(OH+), splitting into a cation and an anion, acetate (CH3COO–). In contrast, Caro’s

acid has two pKa values of 1and 9.3. Thus, it is completely dissociated

(SO5

2– + 2H+ ). An electrophilic reaction is only possible via the mono anion

(HSO5

– OH+ + SO42–), which is present only at very low concentration. This

explains the slow reaction of Caro’s acid with lignin. A comparison of both compounds

at identical active oxygen content favors Paa. The final kappa number is

lower, and the final brightness higher after the Paa-P treatment.

The demand for peracetic acid is moderate. Figure 7.137 illustrates an example

of a TCF bleaching application. An input of 0.1–0.5% peracetic acid is sufficient

for the activation. Paa is applied at moderately acidic pH and at a temperature of

about 80 °C. Because the peracid reaction is slow, a retention time of 1h is not

sufficient to consume a charge of more than 0.5% at 80 °C. On the other hand,

because of the high temperature, peracetic acid is hydrolyzed into acetic acid and

7.7 Peracetic Acid in Pulp Bleaching 881

0 0.1 0.3 0.5

peracetic acid (%)

85

86

87

88

89

90

brightness (%ISO)

P Paa-P

Fig. 7.137 TCF bleaching of softwood kraft pulp with and

without peracetic acid activation of a final P stage. Pulp prebleached

with OO-Q-OP; Paa stage at 85 °C, 1 h, 10% consistency;

final P stage with 2% H2O2, 1.4% NaOH, at 95 °C, 3h,

10% consistency.

hydrogen peroxide. After about 1h the remaining peroxo compound will be predominantly

hydrogen peroxide.

Sequences with peracetic acid activation use the Paa step ahead of the final P

stage. Thus, a TCF sequence could be OO-Q-OP-Paa/P or OO-Q-OP-Paa-P. Peracetic

acid was also recommended as a final treatment step to boost the brightness

of TCF pulp [2]. However, because of the type of reactions occurring with the

remaining lignin, brightness stability is affected negatively. There is an improvement

in brightness, but not in its stability. In addition, the high temperature required

makes application in a high-density storage tower difficult.

Figure 7.138 illustrates the impact of a Paa post-treatment of a TCF pulp with

0.5% Paa at 80 °C. The improvement in brightness by more than 2 points is significant.

Brightness stability, however, decreases. This suggests the formation of

potential chromophores with Paa oxidation. These chromophore precursors are

removed by treating pulp with alkaline peroxide. The change in pH changes the

oxidant form from an electrophile into a nucleophile and thus removes quinones,

it is likely that Paa generates phenolic intermediates. Peracid treatment is therefore

more advantageous if followed with a peroxide stage.

The moderate speed of reaction of peracids with lignin is accelerated in the

presence of chloride ions. Because of their high oxidation potential, peracids will

oxidize chloride to chlorine. This acceleration increases the effect [3], it is, however,

not environmentally sound because halogenated compounds are generated.

Higher levels of AOX in the effluent and OX in pulp result from a reaction of peracid

in the presence of chloride [4]. This contradicts the purpose of TCF bleaching.

Peracetic acid should not be applied when there is a high level of chloride in the

water loop.

882 7Pulp Bleaching

P P-Paa P-Paa/P

85

86

87

88

89

90

brightness (%ISO)

0.2

0.3

0.4

0.5

0.6

0.7

post color #

brightness post color #

Fig. 7.138 Impact of a post-treatment with peracetic acid on

brightness and brightness stability. Paa stage with 0.5% at

80 °C, 1 h 10% consistency, Paa/P at 80 °C with 0.5% Paa and

alkali treatment after 0.5 h with 0.5% NaOH and continuation

for 1 h. Stability analyzed after hot humid reversion (E.4P).

7.8

Hot Acid Hydrolysis

Kappa number – that is, permanganate demand – is generally assumed to represent

the lignin content in pulp with sufficient accuracy. Because permanganate is

a strong oxidizing compound, which reacts not only with the aromatic lignin but

also with other double bonds, this assumption applies only with limitations. Relatively

recently, a large number of “other” double bonds were identified in kraft

pulp. The first indication into the source of these double bonds was the identification

of furan-2-carboxylic acid as main product of its hot acid hydrolysis by

Marechal [1]. The origin was soon identified as hexenuronic acid, HexA,

(Fig. 7.139), a compound generated during alkaline pulping by methanol elimination

from 4-O-methylglucuronic acid on the xylan [2]. Therefore, permanganate

consumption of a pulp describes dominantly the sum of its reaction with lignin

and hexenuronic acid [3].

The ratio of substitution of the xylan backbone with 4-O-methyglucuronic acid

in most hardwoods is about 10:1 (Xyl:Me-GluU). It is higher in most softwoods

(5:1or 4:1), however, the amount of xylan being typically significantly lower in

softwood (spruce, Picea abies ~9%) compared with hardwood (birch, Betula verrucosa

~33%) [4]. Therefore, hexenuronic acids are present in softwood kraft pulp at

a low level, and their share in the kappa number is only about one unit. In

unbleached hardwood kraft pulp, about one-third of the permanganate consumption

in kappa analysis is caused by hexenuronic acid. In oxygen-delignified hard-

7.8 Hot Acid Hydrolysis 883

O

OH

O

O

O

-OOC

HO OH O

Fig. 7.139 Model of hexenuronic acid (HexA) on a xylan side chain.

wood kraft pulp, the level of “other compounds” is even higher, and can reach

almost 50%. (This explains why the degree of “delignification” achieved in an oxygen

stage of hardwood pulp seems more moderate compared with softwood pulp.

As removed “kappa number” it might reach just 30%, but as removed real “lignin”

it is typically better than 50%.)

The reason why hexenuronic acid remained undetected for decades is that the

normally applied analytical method for lignin preparation uses acidic conditions,

during which acid hydrolysis of the hexenuronic acid occurs.

The double bond of hexenuronic acid reacts with electrophilic bleaching agents,

such as chlorine dioxide and ozone. The question of how, logically, to remove hexenuronic

acid can be answered with the background of its analytical evasiveness.

At elevated temperature and low pH, hexenuronic acid is rapidly hydrolyzed. The

first report of the effect of hot acid treatment of hardwood pulp by Marechal in

1993 described the conditions required, namely a temperature above 90 °C, a pH

below 3.5, and extended time. The impact of time on the hot acid hydrolysis of an

oxygen-delignified eucalyptus kraft pulp is shown in Fig. 7.140. The time required

at 90 °C and pH 3 for complete hydrolysis of hexenuronic acid is rather long. The

sharp increase of the effluent load with prolonged time indicates an increasing

cellulose degradation. This becomes apparent from the parallel lowering of the

pulp’s viscosity. In mill applications, therefore, a “hot acid stage” will be conducted

for not more than about 2 h, in order to retain control of the pulp yield and

viscosity losses. From a practical standpoint, this also keeps the size of the towers

within reasonable limits. The process could be accelerated by using a higher temperature

or more acid, but typically the temperature is kept below 95 °C in order

to balance steam demand and to avoid pressurized conditions. A lower pH is similarly

unattractive because it would only accelerate cellulose depolymerization.

Hydrochloric acid may be a cost-attractive alternative to sulfuric acid, especially in

mills with an on-site electrolysis for chlorate. However, the potential problem of

using hydrochloric acid is corrosion of the equipment.

The high temperature and low pH used during the hydrolysis have an additional

effect, in that they allow a significant reduction in the manganese content

of the pulp [5,6]. It is speculated that manganese and iron might form complexes

with hexenuronic acid. In bleaching, hexenuronic acid destruction and metals

removal are equally important. A TCF bleaching sequence with only alkaline oxygen

884 7Pulp Bleaching

0 1 2 3 4

3

5

7

9

11

COD [kg/t]

kappa number

Kappa number

Time at 90 °C [h]

5

10

15

20

25

COD

Fig. 7.140 Impact of time in hot acid hydrolysis of eucalyptus

kraft pulp. Pulp delignified with oxygen to kappa 11, temperature

at 90 °C, pH 3(H 2SO4).

and peroxide stages (e.g., OQPP) will maintain all hexenuronic acid sites in the

bleached pulp, the consequence being a poor brightness stability [7]. The inclusion

of an acid hydrolysis stage into a TCF sequence improves the bleaching process

because it eliminates transition metal. At the same time it increases brightness

stability, due mainly to a reduction in the level of hexenuronic acids.

Despite the very substantial drop in kappa number through the hydrolysis of

hexenuronic acids, the mill application of a hot A stage is rare [8]. The reason for

this is the need for an additional treatment stage that consists not only of a tower

but also a washer. This additional equipment is expensive, and normally the

potential savings of chlorine dioxide in an ECF sequence will not pay back the

investment. Therefore, high-density storage towers are converted to a hot A stage.

The lack of a washer between storage and D0 stage can result in the connection of

a hot A pre-treatment and chlorine dioxide addition. The result is a hotA/D0 stage

combination, and this has been implemented in some mills. The alternative is to

use a reverse approach, with a combination of the D0 stage and a subsequent

hydrolysis, or hot D0 with extended retention time (see Section 7.4.5.3, modified

chlorine dioxide delignification).

7.9

Alternative Bleaching Methods

During the past few decades, there has been a constant search for environmentally

benign alternatives to pulp bleaching. The search continues today, and will

do so in the future. Besides the activation of peroxide stages, one such alternative

is offered through biotechnological means. Although hemicellulose-degrading

7.9 Alternative Bleaching Methods 885

enzymes (xylanases) were the first enzymes to be introduced on a large scale for

pulp bleaching [1], they function more as a bleaching aid than as a direct bleaching

agent. This is because they increase the efficiency of subsequent bleaching

steps by loosening the structure of reprecipitated xylans on the unbleached pulp

fibers, thereby saving on the amounts of bleaching chemicals required.

A direct approach might be to use lignin-degrading fungi (Basidiomycetes or

white rot fungi) or their enzyme systems (e.g., peroxidases, laccases), all of which

have long been recognized. These systems are able to selectively degrade lignin

not only in wood, but also in pulp. However, the time required for this process to

proceed to the desired extent is far too long for a modern pulp mill bleaching system.

This problem of extended reaction times was partly tackled by the application

of a so-called mediator. Discovered accidentally during the early 1990s by R. Bourbonnais

of Paprican during experiments with lignin model systems, the laccasemediator-

system (LMS) was found to consist of an enzyme (laccase) and a mediator

(ABTS) [2]. The mediator applied in this first LMS – a laccase substrate used

for an activity assay – was impracticable for large-scale applications, however. An

LMS suitable for pulp mill use was later patented by Call [3,4] which employed

different mediators (e.g., 1-Hydroxy-benzotriazole, HBT) [5], and initial large-scale

trials conducted with this material has shown promise.

The underlying working principle of the LMS can be summarized as follows.

The enzyme laccase, as a macromolecule, is unable to penetrate the pulp fiber,

despite such penetration being a prerequisite for lignin-degrading action. Moreover,

due to its oxidation potential, laccase on its own is only capable of oxidizing

phenolic lignin moieties, which react predominantly by dehydrogenative polymerization

rather than by lignin degradation. However, both of these difficulties were

overcome with the use of a low molecular-weight redox mediator. In this way, the

substrate range is extended to nonphenolic lignin units, as could be shown by

model compound studies, and the mediator can penetrate much more deeply into

the fibers. In the LMS redox cycle, the enzyme oxidizes the mediator to a more

reactive species, mainly of radical type, and these react in turn with the lignin

macromolecule, either via an electron transfer process or by hydrogen atom

abstraction, depending on the mediator used [6]. The reduced mediator is re-oxidized

by the enzyme, which utilizes dioxygen as a co-substrate and which, in turn,

is reduced to water.

Large-scale applications of the LMS remain inoperative, however, and some

major restraints for eventual mill usage have been identified:

_ The mediator should be a low-cost chemical which should exhibit

a minimum of side reactions. These undesired processes can

cause a reduction in enzyme activity or the production of harmful

degradation products, which in turn raises the issue of environmental

compatibility.

_ A sufficient gain in kappa number reduction usually requires several

LMS stages with additional extraction stages in between.

_ The increase in brightness is often limited, so that additional

bleaching is required.

886 7Pulp Bleaching

For further information on the LMS system, the reader is referred to some

excellent reviews [7,8].

Further developments include mediated electrochemical delignification systems

[9], and enzyme mimicking (porphyrin derivatives, manganese-based complexes,

metal–Schiff base-complexes; for a more detailed description, the reader is

referred to Ref. [10]). Mimicking the action of lignolytic enzymes is also the underlying

concept of bleaching system which was developed in the mid-1990s [11,12]

and today is receiving increased attention. A special class of metal clusters of the

Keggin-type – the polyoxometalates [13], often referred to as POMs – are utilized

as the catalyst. Polyoxometalates are metal-oxo anionic clusters with chemical

properties that can be largely controlled by transition metal substitution and the

countercation used. This, combined with their ability to donate and accept electrons

and their stability over a wide range of conditions, makes them attractive

targets for use as bleaching catalysis. To activate the cluster for delignification,

one or more structural metal atoms are donated by a first-row transition metal

atom (e.g., vanadium or manganese) [14]. Specific conditions (e.g., pH, type of

metal cluster) allow POMs to be selective towards lignin degradation and to be

thermodynamically stable in water [15]. The high-valent metal cluster anions oxidize

and thus degrade and solubilize the lignin, while themselves being converted

into the lower-valent reduced state. The re-oxidation with dioxygen is a process

that generates radicals, but this would result in undesired cellulose damage due to

highly unselective side reactions. Consequently, the POMs are reactivated in a separate

stage under conditions that effect the oxidation of dissolved lignin and other

dissolved organic matter to carbon dioxide and water. Actual delignification of the

pulp is carried out under anaerobic conditions. POMs must be applied in stoichiometric

quantities. Current types of POM are advanced products of development:

they are more easily synthesized than the original representatives of this compound

class, are not too costly, recyclable, and have the capability of self-buffering.

Recent progress has also shown that molybdovanadophosphate heteropolyanions

can be used under aerobic conditions in a single-stage process with either oxygen

or ozone as the reactivating agent [16].

Further research and development is required eventually to transfer novel

delignification principles to large-scale applications.

7.10

Bleach Plant Liquor Circulation

Andreas W. Krotscheck

7.10.1