Effect of Sodium Ion Concentration (Ionic Strength) and of Dissolved Lignin

The sodium concentration of the cooking liquor (an approximate measure of the

ionic strength) exerts an influence on the amount of residual phase lignin in a

similar manner as a decrease in [OH– ]. For a sodium concentration of up to

1.9 mol L–1 the effect is seen to be small, but in the range typical for industrial

kraft pulping (2–3 mol L–1) the amount of residual phase lignin is significantly

increased. It has been reported that in kraft pulping of both birch and spruce

wood, an increase in sodium ionic strength from 1.3 to 2.6 mol L–1 causes an 80%

increase in the amount of residual phase lignin [38], while the rate of delignification

remains unchanged. It has been speculated that this effect might be due to a

decreased solubility of the lignin fragments, but this is unlikely because a reduced

solubility would also result in a decreased rate of delignification – which definitely

is not the case. This strengthens the hypothesis that the main influence on the

amount of residual phase lignin is via the activity of the reactants. The desirable

decrease in sodium ion concentration of an industrial cooking liquor below a critical

level of 1.8 mol L–1 can only be accomplished by increasing the dilution factor,

but this would require additional evaporation capacity.

The rate constant kj in Eq. (74) is influenced by the concentration of dissolved

lignin in various ways. The presence of dissolved lignin in the early stages of kraft

cooking (e.g., the beginning of the bulk phase) causes an increase in the bulk

delignification rate, which in turn results in a higher viscosity at a given kappa

number. Moreover, the addition of dissolved lignin during the bulk delignification

results in a yield increase of about 0.5% on wood (valid for the use of Pinus silvestris

L. as a wood source) [59]. The mechanism of this increased selectivity is not

known, but it has been speculated that the added lignin may increase the absorption

of sulfur by the wood during the precooking stage [59]. Another reason for

this positive effect might be the generation of polysulfide ions, which may accelerate

the cleavage of ether linkages in the lignin via the oxidation of enone and quinone

methide-type intermediates [60]. However, adding dissolved lignin at a late

stage of the cook (e.g., kappa number <50) reduces the delignification rate during

the final phase. Consequently, the presence of dissolved lignin during extended

delignification inevitably leads to a decrease in selectivity. The ratio of the reaction

rates of bulk (kb) and final delignification (kf) is reduced by 17–50%. The reasons

for this negative effect of dissolved lignin on delignification rate during the final

phase are not physical effects, such as the sorption of lignin by fibers or a reduction

in the rate of diffusion of released lignin fragments. Condensation reactions

between the dissolved lignin and the fiber lignin or carbohydrates are thought to

be responsible for the disadvantageous effect of dissolved lignin on delignification

selectivity [59]. The addition of black liquor from previous cooks already in the

early pulping stages (as is the case in modern displacement cooking procedures)

leads to an increase in the concentration of dissolved lignin throughout the whole

cooking process. The presence of dissolved lignin early in the cook almost compensates

for the negative effect in the later part of the cook.

4.2 Kraft Pulping Processes 207

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