Parameters Units Low-alkali High-alkali

Cooking conditions

EA-charge % as NaOH 17.5 22.5

Time at cooking temp. min 300 105

Unbleached pulp characteristics

Screened yield % 43.8 43.8

Kappa number 18 17.2

Brightness %ISO 30.9 36.7

Viscosity mL g–1 973 913

Light absorption coefficient m2 kg–1 16.5 13.1

b-O-4 in residual lignin lmol g–1 71 115

Bleachability

After “Standard"-O

Total peroxide consumption in

Q-OP-Q-PO sequence

kg t–1 25.7 19.2

After “Extended"-OO

Total peroxide consumption in

Q-OP-Q-PO sequence

kg t–1 12.1 11.6

Thus, it can be concluded that extended oxygen delignification significantly

decreases the demand of bleaching chemicals and evens out variations from the

cooking stage. However, to date no information is available about the selectivity of

extended oxygen delignification.

There are a number of technical set-ups where the insights from delignification

reaction kinetics are realized. All of these seek to provide the best conditions in

order to maximize delignification efficiency and selectivity. The first commercial

two-stage oxygen delignification process was developed at Oji Seishi KK Tomakomai

mill in Japan in 1985, simply by adding a second reactor to an existing onestage

oxygen delignification system. A detailed description of the practical experiences

derived from this first commercial installation was provided [121]. In the

same year, the first patent for a two-stage reactor oxygen delignification process

was granted to Kamyr [122].

726 7Pulp Bleaching

The first commercial application of a two-stage oxygen delignification stage as a

pretreatment of a TCF-sequence for the manufacture of a high-purity eucalyptus

prehydrolysis kraft pulp (PHK) came on stream in 1996 at the Bacell S.A. mill

(since 2004, BahiaPulp) in Bahia near Salvador [123]. This concept of two-stage

oxygen delignification was developed in an extensive laboratory program with a

one-stage oxygen treatment as a reference [124]. In accordance with the final pulp

quality requirements, the task of oxygen delignification was to reduce the kappa

number from about 8–10 to below 3 in order to avoid too high ozone charges and

to be able to control cellulose degradation in the subsequent ozone stage. The preliminary

trials using the conventional one-stage oxygen delignification treatment

resulted in a significant drop in viscosity as soon as the lignin removal rate was

extended beyond 60%. Consequently, a two-stage delignification concept was

investigated to achieve a higher degree of delignification without impairing viscosity.

It was shown that if the given amount of caustic is split into the first and second

stage in a ratio of approximately 60/40 to 75/25, then delignification can be

extended in the final part of the second stage (Fig. 7.52).

The advantage of a higher delignification efficiency in a two-stage oxygen

delignification process at a given charge of sodium hydroxide can be attributed for

the most part to a higher pH-level (or residual effective alkali concentration) in

the final part of the treatment (Fig. 7.53). Similar to kraft pulping, the selectivity

of delignification improves due to the more even pH profile.

0 20 40 60 80 100

Kappa number

Reaction time [min]

one-stage oxygen two-stage-oxygen

Fig. 7.52 Course of kappa number of one- and

two-stage oxygen delignification of a eucalyptus-

PHK pulp, kappa 8.6, viscosity 1131 mL g–1

(according to [124]). One-stage: 25 kg

NaOH bdt–1, 110 °C, 10% consistency, 0.7 MPa

oxygen pressure; Two-stage: first stage 15 kg

NaOH bdt–1, 110 °C, 0.7 MPa oxygen pressure,

15 min; second stage 10 kg NaOH bdt–1,

115 °C, 0.4 MPa oxygen pressure; 10% consistency

in both stages.

7.3 Oxygen Delignification 727

0 20 40 60 80 100

pH-value

Reaction time [min]

one-stage oxygen two-stage oxygen

Fig. 7.53 pH-profile during one- and two-stage oxygen

delignification of a eucalyptus-PHK pulp, kappa 8.6, viscosity

1130 mL g–1 (according to [124]). Conditions as shown in

Fig. 7.52.

The optimum overall selectivity and efficiency of two-stage oxygen delignification

can be achieved by limiting the retention time in the first reactor to 10–

20 min, while about 60 min appears to be the optimum retention time in the subsequent

second reactor. Figure 7.54 illustrates the advantage in selectivity of a

two-stage concept at an extent of delignification higher than 55%. A rather moderate

and almost linear decline in viscosity can be observed as long as the kappa

number is above 3.5 in case of a one-stage, and 2.9 in case of a two-stage oxygenalkali

treatment. This corresponds to an improvement in delignification efficiency

from 61% to 68% (unbleached kappa number 9; see Ref. [124]).

In the meantime, the two-stage oxygen delignification process for the production

of high-purity dissolving pulp has been more than nine years in operation at

the Bahia pulp mill (Salvador, Brazil), and operational results have clearly

exceeded expectations based on laboratory experiments. The average performance

of this prebleaching stage is achieving values of about 76% delignification while

maintaining a moderate level of cellulose degradation of about 0.195 mmol AHG–1,

expressed as the number of chain scissions (corresponds to a kappa number of

2.2 and a viscosity of 785 mL g–1 when compared with the pulp used for laboratory

experiments in Fig. 7.54).

Extended oxygen delignification is certainly more important for paper-grade

kraft pulps than for dissolving pulps, mainly because of the difficulty in removing

the residual lignin present in paper-grade pulps (without prehydrolysis). Therefore,

much effort was undertaken to develop appropriate two-stage delignification

728 7Pulp Bleaching

1 2 3 4 5 6

Viscosity [ml/g]

Kappa number

one-stage oxygen two-stage oxygen

Fig. 7.54 Comparative evaluation of one- and two-stage oxygen

delignification using a eucalyptus-PHK pulp, kappa 9 and

viscosity 1200 mL g–1. Conditions as in Fig. 7.52, and according

to Refs. [124,125].

concepts. In 1995, a two-reactor oxygen delignification process was applied for

patent by Sunds Defribrator (today, Metso) [126]. The delignification stage is manufactured

under the trademark OxyTrac. This technology operates as a two-stage

system with a high-shear mixer before each stage. It has been claimed that the

selected conditions in both stages of the OxyTrac system are based on the kinetics

for oxygen delignification [12,127,128]. The rather long retention time of 30–

40 min in the first reactor is not exactly in line with the current knowledge on

reaction kinetics (see Section 7.3.3). However, it cannot be concluded that the

longer residence time in the first reactor than was predicted from kinetic considerations

results in an inferior delignification performance. The first reactor is

operated at a rather low temperature of 80–85 °C, but with high oxygen pressure;

these conditions lead to a higher concentration of dissolved oxygen. Moreover, the

gas volume at a given charge of oxygen is smaller, and this facilitates mixing of

the three-phase system. All of the alkali and most of the oxygen are charged to the

first reactor (Tab. 7.30).

The second stage is designed as an extraction stage using a higher temperature,

a longer residence time, and a lower chemical concentration to extend delignification

without drastically reducing the pulp viscosity and pulp strength.

Figure 7.55 illustrates schematically a typical OxyTrac process flowsheet. Caustic

is added to the dilution screw of the press before the pulp drops down into the

standpipe of the medium-consistency pump. After oxygen is charged in the first

high-shear mixer, the pulp-liquor-gas mixture passes through the first oxygen

reactor. It then flows down to a static steam mixer, where medium-pressure steam

7.3 Oxygen Delignification 729

Tab. 7.30 Recommended operating conditions of the OxyTrac

system for two-stage delignification (according to

Refs. [127,129]).

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