Effect of Pulp Consistency

The basic development of ozone bleaching of chemical pulp was exclusively carried

out in the low (LC) and high (HC) consistency ranges. Medium-consistency

ozone bleaching has long been considered impossible because of the lack of

appropriate mixing technology and the need for mixing large volumes of gas with

the pulp suspension. Even with the development of high-shear mixers, MC ozone

bleaching technology has remained initially unsuccessful. The first trials on a

pilot scale were carried out in 1986 at Baienfurt in Germany, but the results were

poor, due mainly to the fact that ozone treatment of the medium-consistency pulp

was performed at almost atmospheric pressure conditions. The breakthrough of

medium-consistency ozone technology occurred only when the ozone-containing

gas was compressed to below a gas void fraction, V g, of about 0.35 to ensure homogeneous

mixing of the gas with the pulp suspension [37–39]. The successful laboratory

and pilot plant trials finally led to the first commercialization of ozone

bleaching using medium-consistency technology at the viscose pulp mill at Lenzing

AG, Lenzing Austria in 1992 [40,41]. There, an ozone stage replaced the hypochlorite

stage in an EOP-H-P sequence, thus converting to EOP-Z-P.

In LC bleaching, vigorous stirring of the very dilute pulp suspension is a prerequisite

to dissolving ozone in water and transferring it to the reaction site by convection

through the bulk water, and by diffusion across the immobile water layer

surrounding the fiber. Under these conditions, a rather homogeneous reaction between

ozone and fibers can be expected. However, during the past 14 years of

commercialization of ozone bleaching technology, it transpired that LC ozone

bleaching was not economically applicable on an industrial scale because of the

very high energy demand for mixing, high investment costs, and low bleaching/

delignification efficiency due to competitive reactions with dissolved oxidizable

matter. The influence of mixing energy as a function of ozone consumption rate

and specific kappa number reduction of a beech acid sulfite dissolving pulp (BAS)

is shown in Fig. 7.89.

806 7Pulp Bleaching

0 20 40 60 80

0.2

0.3

0.4

0.5

0.6

ΔKappa / kg O

-charge

Δ Kappa / kg O

-charge

Ozone consumption yield [%]

Mixing energy [kWh/odt]

Ozone yield

Fig. 7.89 Low-consistency (LC) ozone bleaching

of a beech acid sulfite dissolving wood

pulp (B-AS, kappa after EO-pretreatment 1.8)

carried out in a 500 kg day–1 pilot plant at

Waagner Biro, Austria. Conditions of LC ozone

bleaching: pulp consistency 2%, 50 °C, average

retention time 17 min, pH 2.5, ozone charge

2–2.4 kg odt–1. Influence of mixing energy on

the performance of LC ozone bleaching

(according to Refs. [28,42]).

The results of these pilot plant trials clearly indicate that a high mixing energy

must be employed to ensure a reasonably good performance of ozone delignification.

A mixing energy of about 50–60 kWh odt–1 in LC ozone technology equals

the delignification efficiency of medium-consistency ozonation applying only

11 kWh odt–1 (MC-pump: 5 kWh odt–1, MC-mixer: 6 kWh odt–1). The corresponding

delignification performances of both ozone bleaching technologies are illustrated

graphically in Figs. 7.85 and 7.88.

More detailed results of the pilot plant trials on LC ozone bleaching are

reviewed elsewhere [28,42].

In high-consistency HC ozone bleaching, the fibers are surrounded by a very

thin layer of water, while most of the residual water is embedded in the cell wall

pores [assuming that a fiber saturation point of 1.5 mL g–1 corresponds to a consistency

of (1–1.5/2.5) = 40%]. Local variations in dry solids content may thus create

fibers containing layers of immobile water of different thickness. The reactivity of

ozone with the pulp fibers depends very much on the thickness of the immobile

water layer. A laboratory study using a sulfite pulp revealed a maximum ozone

consumption at consistencies of 30–50% comprising a consistency range from

below 10% to above 90% [5]. Interestingly, the extent of ozone consumption significantly

decreases at consistencies above 50%. This may be explained by a

reduced accessibility of ozone to the reaction sites.

7.5 Ozone Delignification 807

Regions of a thick immobile water layer around the fibers resist the reaction

with ozone, whereas those of a thin water layer receive too high an ozone dose.

Moreover, the pulp fibers must be separated from each other by fluffing as a prerequisite

to enable ozone to come into contact with the water layers surrounding

the single fibers.

Lindholm investigated the influence of consistency on ozone bleaching using

an unbleached pine kraft pulp after acidifying to pH 2.8 [43]. Two different modes

of gas phase ozone bleaching at 35% consistency were compared with LC ozone

bleaching at 1% consistency (denoted low consistency). In one mode of HC ozone

bleaching, extreme heterogeneity was simulated by separating the pulp samples

into four portions which then were ozonated in series. The properties of the pulps

in the four bottles were examined separately, and the average properties were calculated

thereof (samples denoted heterogeneous gas phase). As a reference, HC

ozone bleaching was performed using a standard procedure (samples denoted gas

phase). This standard procedure included a thorough fluffing of the pulp prior to

ozonation. The ozone dosage was increased step-by-step to 5.8% on o.d. pulp in

the case of LC, and to 7.5% on o.d. pulp in the case of HC ozone bleaching (both

modes). The heterogeneity of ozone bleaching is clearly reflected by the relationship

between intrinsic pulp viscosity (average) and the zero-span tensile index.

Figure 7.90 illustrates that this relationship depends heavily on the mode of operation.

200 400 600 800 1000 1200

0.6

0.7

0.8

0.9

1.0

1.1

heterogeneous gas phase gas phase low consistency

Normalized zero-span tensile index

related to unbleached value

Viscosity [ml/g]

Fig. 7.90 Relationship between normalized

zero-span tensile index (related to the value of

the starting pine kraft pulp = 148 Nm g–1) and

pulp viscosity of ozonated pine kraft pulp

comprising various conditions of ozone

bleaching (according to [43]): low consistency

at 1% consistency versus gas phase at 40%

consistency and heterogeneous gas phase.

808 7Pulp Bleaching

In heterogeneous gas phase treatment, the zero-span tensile index begins to

decrease at an average level of about 950 mL g–1, probably due to severe carbohydrate

degradation in the fibers closest to the ozone inlet. By contrast, in normal

gas phase ozonation, much lower pulp viscosity levels can be tolerated without

impairing strength properties. The loss of zero-span tensile strength at rather low

ozone charge may be explained as a result of the greater heterogeneity in the reactions

between ozone and pulp during gas phase ozonation. This may create zones

with very low lignin contents, allowing intensified attack of ozone on carbohydrates.

The highest delignification selectivity was obtained in LC ozone bleaching.

There, strength properties were preserved even at pulp viscosity levels as low as

520 mL g–1 [43]. Lindholm also found that in LC ozone bleaching of a pine kraft

pulp, the zero-span tensile index remained fairly constant up to an ozone consumption

of about 5% on o.d. pulp, while in HC ozonation the strength properties

decreased at ozone consumption levels beyond 2% on o.d. pulp [44]. This behavior

may be explained by a rather homogeneous reaction between ozone and

the single pulp fibers. A similar interpretation of the results has been given when

comparing the degradation processes caused by ozonation and acid hydrolysis

[45]. The molar mass distribution revealed the formation of two distinct cellulose

distributions during ozonation of an unbleached birch kraft pulp. However, HC

ozone bleaching shows a higher efficiency of lignin removal at a given ozone

charge. As a combined effect, equal selectivity was obtained for HC and LC

bleaching, provided that the kappa number after ozonation was maintained above

17 (equals kappa number reduction smaller than 50% with a starting kappa number

of 34). When bleaching to lower kappa numbers, the selectivity of HC ozone

bleaching was inferior to that of a LC operation [44]. The higher selectivity of LC

as compared to HC ozone bleaching was attributed, at least in part, to dissolved

lignin fragments, which act as carbohydrate protectors [46].

The performance of medium-consistency ozone bleaching is described as resembling

that of LC ozone bleaching rather than that of HC ozone bleaching,

especially with respect to bleaching and delignification selectivity. Laxen et al. concluded

that in the range of 1to 10% consistency, the selectivity of ozone delignification

does not depend on consistency [37]. Similar conclusions were made by

others [39], though no differences with regard to delignification selectivity between

laboratory HC and medium-consistency ozone bleaching have been observed

for oxygen-delignified pine ASAM paper pulp and beech ASAM dissolving

pulp [47]. This can be explained by both efficient fluffing prior to ozonation and to

the presence of a highly accessible and easily oxidizable kappa number.

Commercial ozone bleaching installations worldwide are operated at medium

(about 10%) and high (about 35%) pulp consistencies, with the majority using

medium consistency (Tab. 7.39).

7.5 Ozone Delignification 809

810 7Pulp Bleaching

Table 7.39 Ozone bleaching installations in 2004 [48, 49]

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