Modified Kraft Cooking

From an environmental standpoint, it would be highly desirable to lower the residual

lignin content (kappa number) as much as possible before entering the

bleach plant. In commercial practice, most softwood kraft pulps are, however,

delignified only to a kappa number in the range from 20 to 35, depending on the

technology applied. The reason for this constraint can be referred to limitations in

pulp quality and pulp yield. Pulp with lower strength properties will not be

accepted by customers.

The strength properties of an unbleached kraft softwood pulps reach an optimum

in the kappa number range from 22 to 35. A mill study including both continuous

and batch digesters revealed that conventional pulping in kraft softwood

mills can be extended to kappa numbers close to 25 without deteriorating

unbleached pulp strength (Fig. 4.41).

20 30 40 50

at breaking length

Mill I - continuous digester: tear index at 12 km tensile strength

Mill II - batch process: tear index at 11 km tensile strength

Tear Index [mNm2/g]

Kappa number

Fig. 4.41 Tear index at given tensile strength as a function

kappa number. Results from different kraft mills. Mill I operates

a continuous digester using a spruce/pine mixture; Mill

II operates batch digesters using softwood furnishes (according

to [12]).

The optimum target kappa number, however, is determined not only by pulp

strength properties but also by yield and other parameters. Reinforcing delignification

from kappa number 32 to 25 reduces the yield of screened pulp from

47.2% to 45.7% in case of conventional cooking [12].

In a given process, prolonged cooking results in a gradual degradation of the

carbohydrate chains, observed as a drop in viscosity and in a decrease in yield.

The pulp viscosity of a softwood kraft pulp can be correlated to pulp strength,

expressed by the product of specific tearing strength and tensile strength [13]. The

4.2 Kraft Pulping Processes 235

500 700 900 1100

Pulp strength

min. viscosity:

850 ml/g

(Tear index * tensile index)

Intrinsic Viscosity [ml/g]

Fig. 4.42 Strength, estimated by the product of tensile index

and tear index, of a softwood kraft pulp related to its intrinsic

viscosity (according to [13]).

relationship between viscosity and pulp strength can be approximated by the type

of saturation curve shown in Fig. 4.42.

Teder and Warnquist have chosen a value of 850 mL g–1 as the lowest acceptable

viscosity after bleaching for a softwood kraft pulp [13]. This relationship is valid

for conventionally and ECF bleached pulps, including an oxygen stage. As seen

from Fig. 4.42, pulp strength is seriously deteriorated when the viscosity falls

below 850 mL g–1. Taking a viscosity drop in the course of ECF-bleaching of

approximately 150 viscosity units into account, the viscosity should be about

1000 mL g–1 after cooking. In the case of TCF-bleaching, the overall viscosity loss

during bleaching accounts for more than 300 units, which in turn requires an

unbleached viscosity of more than 1150 mL g–1 at a given kappa number.

The selectivity of conventional kraft cooking improves by increasing the sulfidity

of the white liquor. Raising the sulfidity from 25% to 35% and further to 45%

increases the viscosity by 110 and 125 mL g–1 at a given kappa number of 30,

respectively [14]. Considering the pros and cons of high sulfidity, in general the

disadvantages prevail slightly. The potential drawbacks of higher sulfidity (>35%)

can be summarized as more costs for malodorous gas collection, incineration and

recovery, the tendency to more corrosion in the recovery furnace, the more

reduced sulfur to oxidize in the white liquor, and a higher amount of inert sulfur

and sodium compounds. However, in case of high wood costs and high wastewater

treatment costs, raising the sulfidity might be a favorable measure.

The need to reduce environmental pollution by simultaneously keeping the

pulp quality at the desired level (see Fig. 4.42) was the basis of seeking possibili-

236 4 Chemical Pulping Processes

ties to modify the kraft cook so that selectivity would be improved. These modifications

should it make possible either to enter the bleach plant with a lower kappa

number, or – in order to gain also the yield advantage – to sufficiently increase the

viscosity at a given kappa number (in the range of 25–30) so that a subsequent

TCF- or ECF-bleaching treatment would be applicable. The principles of modified

cooks, with the focus on increasing the ratio of delignification to carbohydrate

degradation rates, rL/rC, are primarily based on the results of pulping kinetics

investigations (see Section 4.2.5, Kraft Pulping Kinetics). The principles of modified

cooking are summarized in the next section.

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