Carry-Over

The dissolved solids entering the oxygen stage originate from two different

sources: the black liquor from cooking; and the filtrate from the oxygen stage.

Experiments conducted by various groups have shown reproducibly that filtrates

from the oxygen delignification stage have no significant effect on the performance

of oxygen delignification. On the other hand, the spent liquor from the

cooking stage causes a clear reduction in delignification efficiency, as reported for

a hardwood kraft pulp [107]. At the same level of COD, the carry-over from the

cooking stage has a more detrimental effect on delignification as compared to the

filtrate of the oxygen stage (Fig. 7.48).

716 7Pulp Bleaching

0 10 20 30 40 50 60

Filtrate from O-stage Spent liquor from cook

Degree of Delignification [%]

carry-over [kg COD/odt]

Fig. 7.48 Influence of amount and type of carry-over on the

degree of delignification in a oxygen-alkali treatment of a

hardwood kraft pulp, kappa number 16.7 (according to [107]).

Conditions of oxygen delignification: 10% consistency, 15 kg

NaOH on pulp, 15 kg O2 on pulp, 30 min, 100 °C.

Therefore, efficient upstream washing is essential to ensure a good performance

of oxygen delignification. The brownstock washing losses are typically in

the range of 10–30 kg COD odt–1 of unbleached pulp, assuming a washing efficiency

of 98–99%. Black liquor solids entering the oxygen stage may also adversely

affect delignification selectivity. The effect of commercial lignin produced from

kraft black liquor (Indulin A from Westvaco) added to the bleach liquor on the

selectivity of oxygen delignification of a softwood kraft pulp was studied by using

different caustic concentrations, while maintaining time and temperature constant

[20]. The data in Fig. 7.49 show that the addition of dissolved lignin causes a

significant decrease in selectivity during bleaching in 0.5 M NaOH.

The results obtained are due to a markedly decreased delignification rate and a

disproportional increase in the rate of depolymerization of carbohydrates. The

drop in the delignification rate may be explained in part by the decreased hydroxide

ion concentration caused by the acid groups formed by the oxidation of the

dissolved lignin. Additionally, an insufficient supply of oxygen may contribute to

the limited delignification rate. At the same time, the depolymerization reactions

of the carbohydrates are accelerated in the presence of dissolved lignin. It is

assumed that the combined presence of dissolved lignin and a rather high hydroxide

ion concentration (0.5 M) promotes the formation of free radicals, which in

turn induces significant chain scissions. Analogous experiments with a low alkali

concentration (0.1M), however, reveal an improved selectivity in the presence of

7.3 Oxygen Delignification 717

8 12 16 20

0.1 M NaOH 0.1 M NaOH + 10 g/l Indulin

0.5 M NaOH 0.5 M NaOH + 10 g/l Indulin

Viscosity [ml/g]

Kappa number

Fig. 7.49 Effect of the addition of commercial

lignin compound, Indulin A from Westvaco,

on the selectivity of oxygen delignification of a

Scots pine kraft pulp, kappa 32, viscosity

1220 mL g–1. Indulin A is added at a concentration

of 10 g L–1 in the bleach liquor; experiments

were run at 97 °C, 0–14 h, 0.2% consistency,

0.7 MPa pressure [20].

dissolved Indulin A (see Fig. 7.49). At this low alkali charge, the dissolved lignin

consumes a great part of the hydroxide ions present. Hence, the pH falls from

12.5 to 8.8 within 1 h, and this explains the very low rate of delignification and

preservation of the carbohydrates due to a lack of free radical formation.

The effect of carry-over on the performance of oxygen delignification can be

understood as a competitive consumption of alkali and oxygen between the residual

lignin in the pulp and the dissolved material in the entrained liquor. Oxygen

delignification appears not to be impaired as long as sufficient caustic and oxygen

are available. The initial rapid phase of delignification is not affected by the presence

of carry-over from the cook, clearly because the hydroxide ion concentration

and oxygen supply are not limiting factors. However, in the continuation of oxygen

delignification, the extent of delignification is clearly impaired by the presence

of dissolved lignin. During this phase caustic and oxygen are consumed by

the dissolved organic matter rather than by the residual lignin. The sole contribution

of the dissolved organic matter on the performance of oxygen delignification

can be studied by neutralizing the carry-over to pH 7. Corresponding experiments

using a eucalyptus kraft pulp were performed by Iijima and Taneda [107]. The

results depicted in Fig. 7.50 show that alkali is preferably consumed by dissolved

black liquor, and this results in a rapid drop of pH.

It can be seen from Fig. 7.50 that delignification discontinues as soon as the pH

falls below 9.5, and at which most phenols are no longer ionized. Thus, it can be

concluded that the presence of carry-over from the cooking stage will increase the

718 7Pulp Bleaching

0 10 20 30 40 50 60

Degree of Delignification:

no carry-over

carry-over, pH 13

carry-over, pH 7

Degree of Delignification [%]

Reaction time [min]

pH

Course of pH:

no carry-over

carry-over, pH 13

carry-over, pH 7

Fig. 7.50 Effect of carry-over from cooking

stage on the performance of oxygen delignification

of a hardwood kraft pulp, kappa

number 16.7, and on the pH profile during the

reaction (according to [107]). Conditions of

oxygen delignification: 10% consistency, 20 kg

NaOH on pulp, 30 kg O2 on pulp, 60 min,

100 °C.

overall oxygen and alkali requirements due to their preferred consumption by

the dissolved organic and inorganic compounds. Moreover, under the conditions

of industrial oxygen delignification the black liquor solids adversely affect

selectivity.

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