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 Krassig [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

Chain scissions

Ozone charge [kg/odt]

Hypochlorite

Chain scissions

Active chlorine consumption [kg/odt]

0 2 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

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

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]).

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