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

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, X g, 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

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

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

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

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

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

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).

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