Viscosity

[mL g–1]

DKappa/CS

0 34.0 1280

19 Z 17.6 920 12.5

19 ZE 12.6 905 15.3

19 ZEP 11.9 885 14.7

19 ZE0 9.5 850 14.2

The use of an E-stage following an ozone stage reduces the ozone charge by 25–

45% when bleaching to a certain kappa number target. Intermediate washing or

neutralization does not affect the extent of lignin removal during subsequent alkaline

extraction. However, neutralization directly after the ozone stage appears to

improve selectivity when followed by alkaline extraction.

7.5 Ozone Delignification 825

In the case of oxygen prebleaching, being the more realistic alternative, the saving

of ozone reaches almost 50% [82]. The viscosity values of the OZE-bleached

pulps correspond to those determined for the OZ-bleached pulps after reduction

with borohydride. Fiber strength (zero-span tensile index) is almost not impaired

by the E-stage (in relation to the Z-treated pulp), at least when using LC bleaching

technology. There are indications that more lignin is removed after LC and MC

ozone bleaching than after HC bleaching [90], but as yet this observation is not

understood.

7.5.6

Technology of Ozone Treatment

Andreas W. Krotscheck

7.5.6.1 Medium-Consistency Ozone Treatment

The process flowsheet of a typical medium-consistency ozone delignification system

is shown schematically in Fig. 7.99. MC pulp coming from the previous

bleaching stage falls into a standpipe after sulfuric acid has been added to adjust

the pH. The pump forwards the pulp suspension to a high-shear mixer which is

charged with compressed ozone/oxygen gas.

MC PUMP

HIGH-SHEAR

MIXER

BLOWTANK WASHING

O3

Pulp from

preceding

stage

H2SO4

Pulp to

next stage

Offgas

Fig. 7.99 Process flowsheet of a typical medium-consistency

(MC) ozone delignification system.

It is of utmost importance that the ozone and pulp are mixed intensively,

because the predominant portion of the delignification occurs inside the mixer.

This is why the medium-consistency ozone system does not have a reactor comparable

to other bleaching applications. Instead, the mixing time is prolonged at

high power dissipation and, on occasion, a second high-shear mixer is installed

for that purpose. Additional time for the reactions to complete after the mixer is

usually provided by the pipe to the blowtank. This pipe may be increased slightly

in diameter to offer about 1min of retention time.

826 7Pulp Bleaching

The pressurized three-phase flow coming from the mixer expands into the blowtank,

where the pulp suspension is separated from the gas phase. The offgas is

cleaned of fibers in a scrubber and proceeds to the ozone destruction unit. The

pulp slurry is discharged from the blowtank either at low or medium consistency,

depending on the feed requirements of the subsequent equipment.

Washing after the ozone stage is often omitted, and the pulp is forwarded to the

subsequent stage at medium consistency. Otherwise, washing can be carried out

with single-stage washing equipment, for example with a single-stage Drum Displacer

™, a wash press, or a vacuum drum washer.

The material for the construction of wetted parts in an ozone stage is typically a

higher grade of austenitic stainless steel.

Further information regarding ozone delignification equipment including medium-

consistency pumps, mixers and blowtanks is provided in Section 7.2. Details

of pulp washing are provided in Chapter 5.

7.5.6.2 High-Consistency Ozone Treatment

High-consistency bleaching requires a press to be utilized before the stage of efficient

pulp dewatering. A plain dewatering press is preferable as it achieves higher

consistencies (up to 40%) than a wash press. It is necessary to adjust the desired

pH in the press feed, because there is no means by which sulfuric acid can be

mixed in between the press and the reactor. The fiber mat leaving the press nip

must be thoroughly disintegrated in order to ensure good accessibility for the

bleaching chemicals to all fiber surfaces.

The Metso ZeTrac ozone delignification system is illustrated in Fig. 7.100. Although

former HC systems required a screw feeder and pulp fluffer, the Metso

ZeTrac does not. Instead, the specially designed shredder screw of the press delivers

well-fluffed pulp which falls into the horizontal reactor and is brought into

contact with the ozone/oxygen gas mixture [100]. Paddles keep the pulp in motion

as it travels concurrently with the gas through the reactor. The reactor operates at

a slight vacuum, thus ensuring that no gas can escape to the ambient air [101].

The delignified pulp is discharged into a dilution screw conveyor, where it is

alkalized. The medium-consistency slurry then falls into a tank before being

pumped to the subsequent stage.

Fig. 7.100 The Metso ZeTrac high-consistency (HC) ozone bleaching system [100].

7.5 Ozone Delignification 827

7.5.6.3 Ozone/Oxygen Gas Management

The oxygen gas needed for the generation of ozone can be supplied from a liquid

oxygen storage tank, or it can be produced on site by oxygen plants using either a

pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) process.

Oxygen is by far the predominant gas in any commercially generated ozone/

oxygen gas mixture. When entering the pulp delignification process, each kilogram

of ozone is accompanied by 6–9 kg of oxygen. This amount of oxygen, together

with oxygen which has been created by the decomposition of ozone, forms

the major part of the offgas leaving the delignification process.

The flow scheme of the common once-through gas system is shown in

Fig. 7.101. Oxygen is fed to the ozone generator and, if used for medium-consistency

delignification, is subsequently compressed. The compression step can be

omitted in the case of high-consistency delignification. After addition to the pulp,

ozone is partly consumed during the delignification reaction and partly decomposed.

Only small concentrations of ozone are left in the offgas, and these are

destroyed in a dedicated destruction unit. Catalytic destruction is the most popular

approach, followed by thermal destruction. The residual gas after destruction is available

for other applications which normally require its re-compression. In the bleaching

plant there are several points where residual oxygen from ozone delignification

can be re-used. These include oxygen delignification, oxygen-reinforced extraction or

peroxide bleaching, and white liquor oxidation. Other opportunities for re-use may

exist in other areas of the mill.

Offgas

separation

Ozone

generation

(Ozone

compression)

Ozone

delignification

Ozone

destruction

Pulp Pulp

Oxygen

compression

Oxygen to oxygen

delignification and

Oxygen other utilisation

(Excess oxygen

to atmosphere)

Fig. 7.101 Once-through gas management.

If the oxygen supplied with the offgas exceeds the mill’s demands, the most appropriate

option is to exhaust some offgas into the atmosphere. Although at first sight

this may not be the most economic solution, the ambivalent operational experiences

fromthe so-called long loop system(Fig. 7.102) seembarely worth the effort. The long

loop contains an additional purification step, where gas components known to interfere

with ozone generation (including moisture) are eliminated as far as possible.

Since the ozone generators are very sensitive to inappropriate gas feed, the perfect

function of gas purification is vital for the availability of ozone from a long loop

system.

828 7Pulp Bleaching

Oxygen to oxygen

delignification and

other utilisation

Offgas

separation

Ozone

generation

(Ozone

compression)

Ozone

delignification

Ozone

destruction

Pulp Pulp

Oxygen

compression

Gas

purification

Oxygen

Fig. 7.102 Long loop gas management.

With modern ozone generators delivering 14% by weight of ozone, the oxygen

supply and demand in the mill is balanced at ozone charges between 4 and

6 kg ton–1 of pulp. Even at lower ozone concentrations in the feed gas, the majority

of ozone delignification applications features specific charges below the balanced

maximum. As a result, once-through gas management has become the system of

choice over the past years.

7.5.7

Application in Chemical Pulp Bleaching

7.5.7.1 Selectivity, Efficiency of Ozone Treatment of Different Pulp Types

7.5.7.1.1 Basic Considerations on the Selectivity of Ozone Bleaching

It is generally agreed that radical formation is a crucial factor that affects the selectivity

and performance of an ozone bleaching stage [75,102,103]. On the basis of

model compounds, the ratio of rate constants for delignification to that of carbohydrate

degradation (kL/kC) is more than 105 in the case of molecular ozone,

whereas for hydroxyl radicals a value of only 5–6 has been determined [72,104].

However, there remains much debate about the pathways of radical formation

during the ozonation of pulp. It is well-documented that radicals are formed in

aqueous medium by self-decomposition of ozone [54,104]. However, the decomposition

of ozone is rather slow in acidic media (see Fig. 7.82). Therefore, additional

reasons have been suggested as being responsible for the unselectivity of ozone

treatment. Radicals are formed in the presence of transition metal ions

[56,103,105] and in a direct reaction between ozone and aromatic lignin structures

[57,58,106]. Recent trials using the TNM (tetranitromethane) method, however,

concluded that the addition of the transition metal ions Fe(II), Cu(II), Co(II) and

Mn(II) to a lignin model compound (e.g., vanillin) do not promote additional radical

formation. It has been argued that the loss in pulp viscosity in the presence of

transition metal ions reported in the literature may be attributed to radical formation

from the reaction with hydrogen peroxide formed during ozonation

7.5 Ozone Delignification 829

[57,103,107]. Instead, radical formation is caused by a direct reaction between lignin

model compounds and ozone [57,58]. In acidic solution, syringyl structures

yield more radicals at a given ozone charge as compared to the corresponding

guaiacyl compounds, mainly due to the lower oxidation potential of the former.

However, no radicals are formed in direct reaction between ozone and carbohydrate

model compounds. Following the finding that syringyl structures yield significantly

more radicals than do corresponding guaiacyl compounds, the viscosity

loss during ozone bleaching should be much more pronounced for hardwood

than for softwood pulps. However, in practice the opposite is true. Ragnar has

shown that the better selectivity of hardwood kraft pulps can be attributed to their

higher amount of hexenuronic acids (HexA), since ozone reactions with this component

do not yield radicals [108]. Magara et al. also reported that the presence of

lignin model compounds with free phenolic hydroxyl groups enhanced cellobiose

degradation during ozonation in water [78]. However, in the presence of nonphenolic

lignin model compounds, cellobiose degradation was retarded. Based on

these model compound studies, it can be concluded that the selectivity of ozone

bleaching gradually improves with decreasing incoming kappa number of a pulp.

In fact, this may explain the superior selectivity of oxygen-bleached pulps over

unbleached kraft pulps, since oxygen reduces both the total lignin content and the

phenolic structures in residual lignin [75,107]. The latter yields more radicals as

compared to nonphenolic structures [57]. Cellulose is also degraded by molecular

ozone according to an insertion mechanism (see Section 7.5.4.3) [53,109]. It is

however doubtful if this type of reaction is responsible for the degradation reaction

of pulps in the presence of residual lignin, because the reaction rate of ozone

towards intact carbohydrate structures is very low (0.21m –1 s–1), while the reaction

rate of hydroxyl radicals towards similar structures is several orders of magnitudes

higher [104].

During the course of final bleaching, when the residual lignin content

diminishes, it seems likely that the direct reaction between ozone and cellulose

gradually becomes the predominant reaction responsible for cellulose degradation

in ozone bleaching. Model compound studies using methyl 4-O-ethyl-b-d-glucopyranoside

were conducted to elucidate the participation of radical species in the

degradation of the polysaccharide during ozone treatment [53]. From the results

obtained it was concluded that both ozone itself and radical species participate in

the glycosidic bond cleavage of carbohydrates during ozonation in aqueous solutions.

The free radical-mediated reaction may lead to both direct cleavage and to

the conversion of hydroxyl to carbonyl groups. The contribution of radical species

was estimated to be about 40–70% during ozonation in distilled water acidified to

pH 2 (the ratio of ionic to radical reactions was calculated by the relative reactivities

at C1towards ozone in anhydrous dichloromethane as a reference for pure

ionic and toward Fenton’s reagent as a reference for radical reactions). Furthermore,

the model compound studies revealed that oxidation of hydroxyl groups at

C2, C3, and C6 positions to carbonyl groups is caused predominantly by radical

species.

830 7Pulp Bleaching

7.5.7.1.2 Efficiency and Selectivity of Ozone Treatment

The use of ozone for the production of paper-grade pulps is limited to low charges

to prevent strength losses. Most of the industrial installations of ozone bleaching

operate on hardwood kraft pulps because of a better selectivity performance compared

to softwood kraft pulps; this is particularly expressed in a better preservation

of strength properties. The higher selectivity of ozone towards hardwood kraft

pulps may be attributed to the presence of a high proportion of HexA [106]. Ozone

is known to be very effective and selective in removing HexA, without simultaneously

impairing pulp properties. Therefore, it can be concluded that the use of

ozone in industrial installations is primarily focused on the removal of HexA.

Ozone is also used for the production of TCF-bleached dissolving pulps. The

ozone treatment is preferably placed between oxygen prebleaching and the final

hydrogen peroxide stage. The tasks of ozone for dissolving pulp production are

both the removal of residual oxidizable impurities (measured as kappa number)

and the controlled adjustment of viscosity. The ozone charge is predominantly

chosen to adjust pulp viscosity, while the final brightness is regulated in the subsequent

hydrogen peroxide stage. Ozone replaces the hypochlorite treatment in a

conventional bleaching sequence for dissolving pulp production. Godsay and

Pearce found a clear relationship between the number of chain scissions and

ozone consumption (in this case even a linear relationship), and this is an important

prerequisite for a controlled viscosity adjustment [99]. During the course of

the development of medium-consistency ozone bleaching, a similar shape was

recognized for the relationship between the number of chain scissions and the

consumption of both ozone and hypochlorite (as active chlorine); this latter point

was verified by Herbst and Krässig [110]. At the start of the reaction, the linear

function has a shallow slope, indicating a minimal effect on carbohydrate degradation.

During the second phase of the reaction, the slope increases and finally

becomes straight, showing that the number of bonds broken is now proportional

to the amount of chemicals consumed. The relationship between the amount of

ozone and hypochlorite consumed and the number of chain scissions in a selection

of experiments using beech sulfite dissolving pulp is depicted in Fig. 7.103.

With respect to chain scissions, the efficiency of 1kg of consumed ozone is

equivalent to that of about 2.8 kg of consumed active chlorine (hypochlorite). If

both oxidants are expressed as oxidation equivalents (OXE), 1.0 OXE of ozone corresponds

to only 0.63 OXE of active chlorine. This means that from the maximum

oxidative power of ozone, representing 6 mol electrons per mol, only 3.8 are transferred,

whereas in the case of hypochlorite all 2 mol electrons per mol are

received.

Furthermore, hypochlorite reacts slightly more selectively with the readily available

residual lignin as compared to ozone, which is characterized by the lower

slope during the first phase. The intercept with the abscissa and the slope of the

curve are characteristic parameters for each pulp. The intercept represents the

amount of ozone or hypochlorite consumed without any significant chain scissions,

while the slope depends on the efficiency of bonds broken. Both parameters

are related to the kappa number, the hemicellulose content, the amount of

7.5 Ozone Delignification 831

0 4 8 12

0

1

2

3

4

Chain scissions

Ozone charge [kg/odt]

Hypochlorite

Chain scissions

Active chlorine consumption [kg/odt]

0 2 4

0

1

2

3

4

Ozone

Fig. 7.103 Carbohydrate degradation, indicated

as number of chain scissions, depending upon

the amount of oxidants consumed (according

to Sixta et al. [41]). Pulp: EO-pretreated beech

acid sulfite dissolving wood pulp (B-AS), kappa

number 2.0, viscosity 560 mL g–1, alpha-cellulose

content 90.2%. medium-consistencyozone

bleaching: 10% consistency, pH 2, 50 °C,

10 s mixing time; hypochlorite treatment: 4%

consistency, 50 °C, initial pH = 9.5, reaction

time 60 min.

reactive groups in the cellulose chain (e.g., carbonyl groups) and the accessibility

to ordered regions under given conditions of ozone bleaching. There is no indication

that the selected wood species exerts any significant influence on the course

of degradation during ozonation, provided that the purity (measured as R18 or

hemicellulose content) and the kappa number of the corresponding pulps are at a

comparable level. The development of chain scissions as a function of ozone

charge for both beech and spruce sulfite dissolving pulps at two different purity

levels, 93% and > 96% R18, respectively, are shown in Fig. 7.104.

The results confirm that a correlation between cellulose degradation and ozone

charge is not discernible for spruce and beech sulfite dissolving pulps at a given

R18 level. The data in Fig. 7.104 also show that the presence of low molecularweight

hemicelluloses protect the pulps against cellulose degradation. Thus, highpurity

dissolving pulps are exposed to more severe carbohydrate degradation at a

given ozone charge.

832 7Pulp Bleaching

0 2 4 6 8 10

0

2

4

6

Beech-sulfite: R18 = 93% R18 = 96%

Spruce-sulfite: R18 = 93% R18 = 96%

Chain scissions

Ozone charge [kg/odt]

Fig. 7.104 Course of chain scissions as a function

of ozone charge for oxygen-delignified

beech and spruce Mg-based sulfite dissolving

pulps of two different purity levels, 93% R18

and 96% R18, respectively (according to [131]).

The remaining properties of the selected

dissolving pulps, such as hemicellulose composition

and kappa number are included in

Tab. 7.42 medium-consistency laboratory

ozone treatment: 50 °C, 10% consistency, 150 g

O3 m–3, 8 bar, 10 s mixing time.

0 2 4 6 8 10 12

0

2

4

6

Euca-PHK, κ = 2.0; Euca-PHK, κ = 4.1

Pine-PHK, κ = 6.9; Pine-PHK, κ = 4.4

Chain scissions

Ozone charge [kg/odt]

Fig. 7.105 Course of chain scissions as a function

of ozone charge for oxygen-delignified

pine and eucalyptus prehydrolysis kraft pulps

at comparable purity level, 97% R18, and different

kappa numbers. Reaction conditions see

Fig. 7.104 and pulp properties see Tab. 7.42.

7.5 Ozone Delignification 833

Table 7.42 Comparative evaluation of the degradation and

delignification behaviour during medium-consistency ozone

bleaching of oxygen delignified pulps of different origin and

composition (according to [131]).

Substrate Initital

kappa

number

R18 value

[%]

Xylan

[%]

Mannan

[%]

Ozone charge

do obtain

Kappa/O3-charge

CS* = 2.0 CS = 3.0 at j after

Z = 1

at j after

Z =0,5

Beech sulfite 1.2–2.0

1.0–1.3

93.3

96.7

3.4

1.9

0.9

0.3

3.1

2.3

4.2

2.9

1.0

0.8

Spruce sulfite 1.4–2.6

0.5–2.0

93.1

96.8

2.0

1.4

2.5

0.7

3.4

2.1

4.7

2.8

1.1

0.7

Beech PHK 4.4

2.3

1.5

1.4

95.5

95.8

96.4

97.3

15.6

5.9

3.1

2.1

0.5

0.4

0.3

0.2

4.6

2.7

2.1

1.1

3.4

2.7

1.8

Pine PHK 6.9

4.4

96.8

96.7

2.2

2.2

2.2

2.2

4.0

3.2

5.9

4.5

0.7

0.8

Eucalypt PHK 2.0

4.0

97.1

97.1

2.6

2.6

0.7

0.7

2.6

3.5

3.5

5.5

1.0

0.9

Pine kraft 3.4

17.5

87.1

86.8

7.1

7.3

6.5

6.8

3.8

9.3

4.8 0.5

CS = chain scissions given as 104

Pt _ 104

PO _ in mmol AGU–1.

The course of cellulose degradation caused by ozonation is also independent on

the wood species for prehydrolysis kraft pulps, as depicted in Fig. 7.105. Despite

major differences in fiber morphology, oxygen-delignified pine and eucalyptus

PHK pulps reveal a similar degradation pattern during ozone treatment in case of

a comparable initial kappa number.

Moreover, the data in Fig. 7.105 demonstrate that the effect of ozone charge on

cellulose degradation decreases with rising kappa number prior to ozone treatment.

Surprisingly, the applied cooking technology for the production of dissolving

pulps appears also not to have any influence on the behavior of cellulose degradation

as a function of ozone charge, provided that both pulps are of comparable

R18 content. Figure 7.106 shows that the response of spruce sulfite and eucalypt

PHK pulps on the number of chain scissions is quite comparable for a broad

range of ozone charges.

As previously indicated, cellulose purity, determined as R18 content or residual

xylan and/or mannan concentrations (see Tab. 7.42), significantly affects the degradation

pattern during MC ozonation. The progressive removal of short-chain

834 7Pulp Bleaching

0 2 4 6

0

2

4

6

Spruce-Sulfite, R18 = 97%, κ = 0.5 -2.0 Euca-PHK, R18 = 97%, κ = 2.0

Chain scissions

Ozone charge [kg/odt]

Fig. 7.106 Comparative evaluation of the

response of oxygen-delignified spruce sulfite

and eucalypt prehydrolysis kraft pulps on chain

scissions as a function of ozone charge at a

comparable purity level, 97% R18 and kappa

numbers (according to [131]). Mediumconsistency

laboratory ozone treatment: 50 °C,

10% consistency, 150 g O3 m–3, 8 bar, 10 s mixing

time.

carbohydrates leads to a growing susceptibility of the remaining high molecularweight

cellulose molecules towards ozone-induced chain scission (Fig. 7.106).

Apparently, the hemicelluloses are preferentially degraded and eventually provide

a sacrificial barrier for cellulose attack by ozone, and as a result, the fall in viscosity

of the remaining polysaccharides is somewhat suppressed.

The high resistance of the beech pulp with the highest hemicellulose content

(P-factor 50) towards chain scissions is partly due to a higher initial kappa number

as compared to the other pulps of the comparison (Fig. 7.107; Tab. 7.42). The

results demonstrate that the presence of both short-chain hemicelluloses and residual

oxidizable impurities (kappa number) protect the high molecular-weight

cellulose against degradation during ozonation. Furthermore, the laboratory

results outlined in Figs. 7.104–7.107 indicate that ozone is suitable for adjusting

viscosity, provided that the kappa number and viscosity of the oxygen-prebleached

pulp are within certain limits. It has been shown previously that, when mediumconsistency

technology is applied, the reaction of ozone with pulp constituents

occurs entirely in the mixer. Unlike laboratory conditions, the residence time in

commercial high-shear mixers is very short, with typical retention times ranging

from less than 1s to 4 s (maximum), compared to 10 s in a typical laboratory application.

The extent of reaction during medium-consistency ozone bleaching is

characterized by the ozone consumption rate inside the high-shear mixer. Parallel

to the increase in ozone charge, the gas void fraction, Xg, increases which in turn

impairs the efficiency of ozone mass transfer. In Fig. 7.108, the relationship between

ozone charge in the range from 1.0 to 5.5 kg odt–1 and the extent of ozone

consumption is compared for laboratory and industrial medium-consistency

7.5 Ozone Delignification 835

0 1 2 3 4 5 6 7

0

2

4

6

Beech-PHK:

P-Factor 50, κ = 4.4; P-Factor 500, κ = 2.3

P-Factor 1000, κ = 1.6; P-Factor 2000, κ = 1.4

Chain scissions

Ozone charge [kg/odt]

Fig. 7.107 Influence of cellulose purity of

beech prehydrolysis kraft pulps on the course

of cellulose degradation during ozonation

(according to [131]). The cellulose purity is

adjusted by prehydroly sis intensity

characterized by the P-factor. Medium-consistency

laboratory ozone treatment: 50 °C, 10%

consistency, 150 g O3 m–3, 8 bar,

10 s mixing time.

bleaching. The rather long residence time of approximately 3.5 s during high-shear

mixing in the commercial system has been obtained by the installation of two mixers

in series. The yield of reacted ozone declines in the industrialMCsystem, from about

75% at an ozone charge of 1.5 kg odt–1 to less than 50% at an ozone charge of

5.5 kg odt–1, while the laboratory mixer keeps an ozone consumption rate beyond

80% throughout the given range of ozone charges.

The lower ozone consumption in the commercial MC ozone installation is

expressed in a reduced extent of reaction between ozone and pulp constituents as

compared to the laboratory system. The data in Fig. 7.109 illustrate that, in an

industrial high-shear mixing system, the number of chain scissions levels off at

ozone charges exceeding 4 kg odt–1. A further improvement of the ozone consumption

yield in an medium-consistency installation can only be obtained by

extending the mixing time, and by reducing the gas void fraction while keeping

the specific energy dissipation, e, at a fairly constant level.

The selectivity of ozone bleaching is an important criterion not only for papergrade

but also for dissolving-grade pulp production, in order to ensure an efficient

delignification and bleaching performance. It has been mentioned previously that

the selectivity of ozone bleaching is also affected by the type and properties of the

pulps. It is well known that ozone bleaching of hardwood kraft pulp is more selective

than for softwood kraft pulp in terms of the kappa number–viscosity relationship

[106]. Moreover, Soteland established that sulfite pulps respond more selectively

to ozone treatment than do kraft pulps [111]. The better response of sulfite

pulps to ozone treatment is attributed to the lower lignin content of the

unbleached pulp [112].

836 7Pulp Bleaching

1 2 3 4 5 6

50

60

70

80

90

Gas void fraction, X

g

[-]

Ozone consumption rate:

industrial scale, τ ~ 3.5 s lab scale, τ = 10 s

Ozone consumption yield [%]

Ozone charge [kg/odt]

0.1

0.2

0.3

0.4

Gas void fraction:

industrial scale

Fig. 7.108 Comparison of industrial and

laboratory medium-consistency ozone bleaching

with respect to the ozone consumption

rate as a function of ozone charge according to

[131]). The development of the gas void fraction

in the commercial system is followed over

the range of ozone charges investigated.

The set-up of the commercial system comprises

the installation of two high-shear mixers

in series. Conditions of the commercial ozone

stage: pH 2.5, ozone concentration prior to

compression: 120–160 g m–3, consistency

8.5%, pressure inside the mixers 7.5 bar, 43°C.

0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

7

industrial application laboratory

Chain scissions

Ozone charge [kg/odt]

Fig. 7.109 Comparison of industrial and laboratory mediumconsistency

ozone bleaching with respect to the ozone consumption

rate as a function of ozone charge (according to

[131]).

7.5 Ozone Delignification 837

The selectivity of delignification and bleaching reactions in general – and that of

ozone bleaching in particular – is defined as the ratio of the rate constant for the

desired delignification or bleaching reactions (removal of chromophores) to that

of the non-desired carbohydrate degradation reaction. A practical way to compare

the selectivity of ozone bleaching of different pulps, and of different levels of initial

viscosity, can be achieved by relating the brightness gain (D brightness) per

number of chain scissions (CS) to the brightness after ozonation. It can be

expected that the bleaching selectivity, expressed as D brightness/CS, decreases

with increasing brightness after ozone treatment. The selectivity behavior of different

types of dissolving pulps and of one softwood paper-grade kraft pulp was

studied in a laboratory medium-consistency system under comparable conditions.

The results outlined in Fig. 7.110 reveal three areas of different selectivity. The

group of highest selectivity comprises the hardwood sulfite dissolving pulps, followed

by the hardwood PHK pulps and the softwood kraft pulps, which cover the

least-selective group of pulps. The superior selectivity of hardwood and sulfite

pulps both with low initial kappa numbers is in accordance with reported values

[108]. Although ozone reacts more readily with lignin structures than with carbohydrates,

the bleaching selectivity decreases with increasing kappa number, due

to a more efficient chromophore reduction at lower residual kappa number. These

results imply that, with respect to pulp viscosity at a given kappa number, it is preferable

to intensify oxygen delignification and to apply less ozone.

50 60 70 80 90

0

10

20

30

40

B-AS: R18 = 93%, κ = 1.3 B-AS: R18 = 96% κ = 1.0 E-PHK, R18 = 97%, κ = 2.0

E-PHK: R18 = 97%, κ = 3.0 P-PHK: R18 = 96%, κ = 3.1 P-PHK, R18 = 96%, κ = 4.4

P-PHK: R18 = 96% , κ = 6.9 P-KA: κ = 3.4

Δ Brightness per

number of chain scissions

Brightness after Z [% ISO]

Fig. 7.110 Bleaching selectivity of a variety of

oxygen-prebleached dissolving pulps and of

one softwood kraft pulp during medium-consistency

ozone treatment in a laboratory highshear

mixer (according to [131]).

The pulps subjected to ozone treatment are

characterized in Tab. 7.42. Constant conditions

of ozone bleaching: pulp consistency 10%,

50 °C, pH 2.0, mixing time 10 s.

838 7Pulp Bleaching

The reason for the higher selectivity of a hardwood over a softwood kraft pulp

has been attributed to the presence of a higher amount of HexA in the former

[106]. However, dissolving pulps derived from both sulfite and PHK technology

contain only minor amounts of HexA, or are even free of HexA at high cellulose

purity levels [113]. Therefore, the presence of HexA alone is not decisive for the

superior selectivity of hardwood pulps. It may be speculated that the residual

kappa number of a dissolving pulp contains no relevant amounts of phenols of

the syringyl- and guaiacyl-type which promote radical formation in different yields

[106]. Nevertheless, ozone bleaching is less selective for both paper-grade and dissolving-

grade pulps rich in kappa number and hemicellulose content. The data in

Fig. 7.110 demonstrate clearly that the pine PHK pulp behaves more selectively

during ozonation as compared to a pine paper-grade pulp of comparable initial

kappa number (kappa number 3.4 and 3.1, respectively).

To better elucidate the influence of wood species on the selectivity of ozonation,

the performance of spruce and beech acid sulfite dissolving pulp (S-AS versus BAS)

of comparable kappa number content (1.4–2.6) and cellulose purity (R18 ~

93%) has been investigated with regard to delignification and bleaching selectivity

(Fig. 7.111).

The data in Fig. 7.111 show clearly that the selectivity of kappa number reduction

is not dependent on the wood species, provided that the compositions of noncellulosic

material in the pulps are at comparable levels. The wood species, how-

0 1 2 3

0

1

2

3

4

Δ Brightness per

number of chain scissions

Brightness after Z [% ISO]

S-AS: R18 = 93%, κ

(E/O)

= 1.4 - 2.6

B-AS: R18 = 93%, κ

(E/O)

= 1.3 - 2.0

Δ Kappa number per

number of chain scissions

Kappa number after Z

60 70 80 90

0

5

10

15

20

25

30

Brightn.

(E/O)

= 67 - 77 % ISO

Brightn.

(E/O)

= 76 - 81 % ISO

Fig. 7.111 Bleaching selectivity of a variety of

oxygen-prebleached dissolving pulps and of

one softwood kraft pulp during medium-consistency

ozone treatment in a laboratory highshear

mixer. The pulps subjected to ozone

treatment are characterized in Tab. 7.42. Constant

conditions of ozone bleaching: pulp consistency

10%, 50 °C, pH 2.0, mixing time 10 s.

7.5 Ozone Delignification 839

ever, may exhibit an influence on the selectivity of brightness gain, as indicated in

Fig. 7.111. Clearly, the beech pulp is slightly more susceptible to an increase in

brightness at a given number of chain scissions as compared to the spruce pulp.

The differences are small, but significant, and may be attributed to the different

reflectance behavior rather than to differences in the light absorption properties

of hard- and softwood fibers.

At first glance, these results appear to contradict those reported by Simoes and

Castro [64], who stated that selectivity was higher for pine than for eucalyptus

pulp when comparing the viscosity versus kappa number profiles. However, when

converting the given changes in viscosity caused by ozonation into the number of

chain scissions, in order to normalize polysaccharide degradation, the delignification

selectivity was exactly the same for both pine and eucalyptus pulps [eucalyptus

pulp: D Kappa/D viscosity = (15.5 – 4.0)/(1270 – 770) = 0.023 converted to

kappa number reduction per chain scissions = 11.5/2.27 × 10–4 = 51 ·j ·AGU·

mmol–4; pine pulp: DKappa/D viscosity = (18.1 – 4.0)/(970 – 615) = 0.040 corresponds

to 50 · j ·AGU· mmol–1) [64]. Thus, it can be summarized that the

delignification selectivity of ozonation is predominantly influenced by the initial

kappa number and the amount of noncellulosic components (e.g., the hemicellulose

content).

7.5.7.2 Effect of Ozonation on the Formation of Carbonyl and Carboxyl Groups

The formation of carbonyl and carboxyl groups has great significance in bleaching

operations. It is well known that the introduction of carbonyl groups along the

polysaccharide chains leads to cleavage of glycosidic bonds when the pulp is subsequently

exposed to alkali. Furthermore, carbonyl groups and uronosidic carboxyl

groups exert a detrimental effect on color stability. The presence of carboxyl

groups also affects the swelling characteristics and water affinity of pulp fibers,

which in turn governs the sheet formation and bonding properties.

Chandra and Gratzl monitored the formation of these groups during ozonation

of alpha-cellulose (produced from a softwood sulfite dissolving pulp further purified

by extraction with 18% sodium hydroxide at 25 °C) [60]. The carboxyl content

increased in a stepwise fashion with the degree of ozonation, while the carbonyl

content underwent steep rises followed by sharp declines throughout ozonation,

resulting in an overall increase at the end of the treatment. The observed pattern

of carbonyl groups as a function of ozone charges was explained by the assumption

that the introduction of carbonyl groups sensitizes this particular part of the

polymer towards further attack by ozone [60,114]. This may lead to excessive degradation

followed by dissolution of the oxidized fragments, and exposes previously

protected domains to continued ozonation. The results of a comprehensive study

on the generation of carbonyl and carboxyl groups along the carbohydrate chain

during the course of ozonation before or after hydrogen peroxide treatment is

included in Section 11.2 (paper grade pulps) and 11.3.2.2 (dissolving grade

pulps)).

840 7Pulp Bleaching

7.5.7.3 Effect of Ozonation on Strength Properties

Unfortunately, ozone delignification is accompanied by a concomitant degradation

of the polysaccharide fraction. As illustrated in Fig. 7.109, cellulose degradation

(characterized in terms of number of chain scissions) is clearly related to

ozone charge. The correlation between strength properties and carbohydrate degradation

(pulp viscosity) of ozone-bleached pulps was found to differ somewhat

from those of pulps subjected to conventional bleaching sequences [111,115].

Ozone-treated pulps are characterized by rapid beating, high-tensile strength but

low tearing resistance. In general, the tearing strength of ozone-bleached softwood

kraft pulps was found to be 10–20% lower as compared to conventionally

bleached pulps of the same provenance [116]. Lindholm has investigated the

impact of various ozone treatments on the tearing strength at a tensile strength of

70 Nm g–1 using a pine kraft pulp [116]. He reported that pulps subjected to Z,

OZ, and OZE treatments had comparable strength properties to those after (CD)E

and (CD)(EO) treatments, provided that the pulp viscosity of the ozone-treated

pulps was greater than 700 mL g–1 (Fig. 7.112). The kappa numbers were about 6

(range: 5–8) for both types of pulp.

400 600 800 1000 1200 1400

10

12

14

16

Pine kraft pulps, after treatments:

unbleached O (CD)E, (CD)(EO) Z, OZ, OZE

Tear Index [mNm2/g] at 70 Nm/g

Viscosity [ml/g]

Fig. 7.112 Tear index at 70 Nm g–1 versus viscosity for differently

treated pine kraft pulps (according to Lindholm [116]).

Figure 7.111 demonstrates a clear relationship between viscosity and tear index

at a given tensile index of the ozone-treated pulps. From this result it can be concluded

that strength properties of ozone-bleached pine kraft pulps are not deteriorated,

provided that pulp viscosity can be maintained above 700 mL g–1. Axegard

et al. reported that the tear strength at a given tensile index of an OAZQPbleached

softwood kraft pulp with a viscosity of 710 mL g–1 was only 5–10% lower

7.5 Ozone Delignification 841

as compared to an OD(EO)DD softwood kraft pulp with a viscosity of 890 mL g–1

[117]. Similar results have been reported by Dillner and Tibbling [118], indicating

that the strength–viscosity relationship presented for conventionally bleached

pulps by Rydholm [119] was not valid for TCF-bleached pulps, including ozone

treatment.

Strength properties of fully bleached hardwood kraft pulps with a sequence

including ozone were comparable to those of a conventionally bleached pulp, although

the viscosity of the ozone-bleached pulp was 20% lower [120]. The preservation

of strength properties despite cellulose degradation through ozone treatment

is also known for hardwood kraft pulps.

Quite recently, the relationships between the molecular weight distributions

(MWDs), intrinsic viscosity and zero-span tensile index of a birch kraft pulp subjected

to HC ozone bleaching were evaluated [121]. The relationship between

rewetted zero-span tensile strength and viscosity is shown graphically in

Fig. 7.113.

400 600 800 1000 1200

0

120

140

160

180

200

Zero-span tensile index [Nm/g]

Viscosity [ml/g]

Fig. 7.113 Zero-span tensile index versus viscosity for ozonetreated

birch kraft pulp (according to [121]). Unbleached

pulp: kappa number 15.5, intrinsic viscosity 1160 mL g–1.

A substantial decrease in fiber strength occurred only when pulp viscosity

decreased below 800 mL g–1. At the highest ozone dosage, the fiber strength was

still 75% of the initial value, corresponding to a viscosity of 510 mL g–1. Apparently,

ozone-treated pulps maintain their initial fiber strength at relatively high

level, despite a substantial reduction in molecular weight. Based on gel permeation

chromatography (GPC) measurements, it was shown that the degradation

pattern through ozonation of kraft pulp was different from that of cotton linters.

In contrast to unbleached birch kraft pulp, ozone-induced cellulose degradation

842 7Pulp Bleaching

did not generate a bimodality of the cotton cellulose peak. The different action of

ozone on MWD was attributed to the presence of lignin in the unbleached birch

kraft pulp, as lignin is known to promote the formation of secondary radicals during

ozone delignification [57]. Due to the enrichment of lignin at the surface of

fibers, it is suggested that cellulose degradation of an unbleached birch kraft pulp

occurs predominantly on the exterior of the fiber, thus generating two distinct cellulose

distributions [21,23].

7.5.7.4 Typical Conditions, Placement of Z in a Bleaching Stage

The placement of an ozone stage within a bleaching sequence must consider both

technological and chemical aspects. The low pH and high sensitivity towards

carry-over from the washing stage of an unbleached kraft pulp suggest that ozone

should not be used in a first delignification stage. Moreover, ozone degrades part

of the phenolic units and makes oxygen less reactive towards lignin in a ZOsequence.

In contrast to the observations of Lachenal et al. [122]. who found that a

single ozone stage (Z) behaves as selectively as an OZ-sequence, Brolin et al. [75],

Ragnar [106], as well as the results shown in Fig. 7.110, show that ozone bleaching

becomes more selective in terms of brightness increase per number of chain scissions

by lowering the incoming kappa number; this means that oxygen delignification

prior to the ozone stage is desirable for reasons of delignification selectivity.

In addition, OZ is favored over Z because of better process economy due to lower

chemical costs (lower ozone consumption and the possibility of recycling oxygen

from the Z-stage) and better possibilities to close the water cycle. The choice between

OZ and Z also depends on the applied ozone bleaching technology. In HC

ozone bleaching, a sufficient quantity of ozone can be reacted in order to achieve

the necessary extent of delignification in a single ozone stage, whereas ozonation

at medium-consistency is limited to a kappa number reduction of maximum 5–7

units (assuming a specific kappa number reduction of about 1unit per kg ozone

charged; see also Tab. 7.36) which in most cases is not enough to complete

delignification.

In a TCF-bleaching sequence consisting of O-, Z-, and P-stages, the use of

hydrogen peroxide (P) is essential to remove the chromophores by oxidizing the

carbonyl groups. As expected, the placement of a P-stage within such a sequence

affects the final bleached pulp properties. OZP- and OPZ-sequences show the

same delignification efficiency, while the latter appears to be more selective as

compared to OZP [122,123]. In a recent study, the effect of placing the Z-stage

prior to (ZP) and after (PZ) standard peroxide bleaching of an (E/O) pretreated

beech dissolving pulp was evaluated by charging different amounts of ozone while

all other reaction conditions were kept constant [123]. GPC measurements

revealed that cellulose degradation was more pronounced for ZP- than for PZtreated

pulps, while the latter had slightly lower brightness values (see Tab. 7.41

and Section 11.3.2.2.2). Figure 7.114 illustrates the course of cellulose degradation

in terms of weight (MW) and number (MN) average molecular weights.

7.5 Ozone Delignification 843

0 2 4 6

30

40

50

200

300

400

PZ-sequence: MW MN

ZP-sequence: MW MN

Molecular weight [kDa]

Ozone charge [kg/odt]

Fig. 7.114 Course of weight (MW) and number (MN) average

molecular weights of beech sulfite dissolving pulps as a function

of ozone charges with Z-stage prior to (ZP) and after Pstage

(PZ), applying identical conditions in each stage [123].

In contrast to the results obtained from Godsay and Pearce [99] and Berggren et

al. [121], the polydispersity index (PDI) – that is, the ratio of the weight average to

the number average molecular weights (MW/MN) – did not increase but rather

was slightly decreased, from about 6.8 in the untreated pulp to 5.5 in the most

severely degraded pulp. This may be attributed to the fact that the beech sulfite

dissolving pulps were subjected to significantly less ozone dosages (2–6 kg odt–1)

than those reported by either Godsay and Pearce (47.7–75.4 kg odt–1) or Berggren

et al. (1–35 kg odt–1).

The placement of Z within the TCF sequence also influences the shape of the

differential MWD. All samples displayed a shift of the MWD towards a lower molecular

weight range as degradation proceeded. The high molecular-weight cellulose

fraction of the pulp subjected to ZP treatment was considerably degraded in

the presence of ozone. From the high molecular-weight peak, with a peak molecular

mass (log Mp) = 5.3, a part of the pulp cellulose fraction was degraded and the

maximum shifted to the second cellulose peak, having a log Mp = 4.7. In the case

of PZ treatment, the shape of the MWD was virtually unaffected by the ozone

charge (Fig. 7.115).

It is well known that ozone treatment of pulp introduces carbonyl groups into the

AHG unit along the polysaccharide chain (see Tab. 7.41an d Section 11.3.2.2.2). In

a subsequent alkaline hydrogen peroxide stage (P), depolymerization of the oxidized

polysaccharide components in the pulp (cellulose and hemicellulose)

is favored due to b-elimination reaction. The high alkali instability of Z-treated

844 7Pulp Bleaching

3 4 5 6 7

6 kg O

3

/odt

4 kg O

3

/odt

2 kg O

3

/odt

ZP-sequence

dW/d(log M)

log Molecular Weight

3 4 5 6 7

6 kg O

3

/odt

4 kg O

3

/odt

2 kg O

3

/odt

PZ-sequence

dW/d(log M)

log Molecular Weight

Fig. 7.115 Differential MWDs of beech sulfite dissolving

pulps prepared by TCF bleaching applying different amounts

of ozone with Z-stage before (upper) (ZP) and after P-stage

(lower) (PZ), applying identical conditions in each stage

[123].

7.5 Ozone Delignification 845

pulps is also the reason why pulp viscosities of PZ-treated pulps are quite comparable

to those of ZP-treated pulps (despite the significantly higher MW and MN

values determined by GPC measurements), provided that the pulps are not subjected

to sodium borohydride reduction prior to viscosity measurements. Although

OPZ bleaching results in superior strength properties, an OZP-sequence

is preferred because of a significantly better brightness stability upon heat or light

exposure. This better brightness stability is achieved by partly oxidizing the carbonyl

groups that are introduced during ozonation. Brightness stability can also

be improved by reducing the carbonyl groups with sodium borohydride after ozonation

[92].

The effect of placing Z-stage on the generation of functional groups as a function

of ozone dosage is discussed in detail in Section 11.3.2.2.2.

Interestingly, in an ECF-sequence comprising O-, Z-, and D-stages, OZD was

found to be more selective than ODZ [124]. This can be explained by the fact that

chlorine dioxide bleaching following a Z-stage shows no adverse effect on cellulose.

The brightness stability after OZD is lower than after OZP bleaching, since a

final chlorine dioxide treatment is less effective in oxidizing or removing carbonyl

group-containing material. Contrary to a treatment in two separate bleaching

stages, a sequential application of chlorine dioxide (D) and ozone (Z) without

intermediate washing was shown to be very selective in delignifying softwood

kraft pulp [125]. This means that the Z- and D-stages are combined into one treatment

(DZ). (DZ) has been found to be more effective for unbleached pulps,

whereas (ZD) seems to be superior for oxygen-delignified kraft pulps [126]. In the

latter sequence, chlorine dioxide partially stabilizes the carbohydrate chain against

alkaline peeling reactions due to oxidation of the carbonyl groups introduced by

ozonation. In the case of an unbleached hardwood kraft pulp, however, chlorine

dioxide reacts with free phenolic groups before the highly reactive ozone is introduced,

the conclusion being that reaction kinetics clearly favors the (DZ) approach

relative to the (ZD) treatment [127]. Furthermore, after chlorine dioxide treatment

the pulp suspension is sufficiently acidic for a subsequent ozone stage. The (DZ)

concept is also advantageous with respect to AOX formation, as ozone has the

ability to destroy some AOX generated during D bleaching. Chlorine dioxide may

act as a radical scavenger, suppressing the extent of radical reactions during the

subsequent ozone treatment. However, in another study it was shown that the

selectivity was not impaired when washing was carried out between D and Z, thus

showing that the presence of residual chlorine dioxide seems not to be essential

for maintaining a high viscosity [128]. The actual reasons for improved selectivity

of a (DZ) treatment remain to be elucidated. In full ECF bleaching sequences, the

replacement of a D0 stage by (DZ) stages was shown to be particularly efficient,

since in the case of a hardwood kraft pulp 1kg charged (consumed) ozone could

replace 1.58 kg charged chlorine dioxide, as shown in Tab. 7.43 [128]. Ozone is

added to the pulp suspension 10 min after the introduction of chlorine dioxide.

846 7Pulp Bleaching

Table 7.43 Comparison of different ECF bleaching sequences of

a hardwood kraft pulp where DO is substituted either by Z or by

(DZ) stages according to [128].

Sequence Stage Chemical Chemical charge Kappa

no.

Bright-ness

[%ISO]