Effects of Dissolved Solids (Lignin) and Ionic Strength

Delignification selectivity is negatively affected by the organic substances dissolved

during the cook at given liquor-to-wood ratios (3:1 to 5:1). The reason for

the impaired final pulp quality can be attributed to the reduced delignification

rate at a late stage of the cook due to the presence of the dissolved organic matter

[18](see Section 4.2.5.2.2, Reaction kinetics). The removal of dissolved wood components,

especially xylan, during the final cooking stages is however disadvantageous

to total yield as the extent of xylan adsorption on the pulp fibers diminishes.

The xylan content in a pine kraft pulp would be 4–6% without adsorption compared

to 8–10% after a conventional batch cooking process, and hence the total

yield would be reduced by 2% from 47 to 45% [37].

The level of dissolved lignin concentration in the final cooking stage is also a

major determinant of delignification selectivity in batch cooking. In order to avoid

extra dilution and to preserve material balance, a lower lignin concentration in the

final cooking liquor means shifting to a higher concentration in the initial stages

of the cook. A linear relationship between the gain in viscosity at a given kappa

number and the dissolved lignin concentration after displacement with fresh

cooking liquor has been obtained [38]. A reduction in the dissolved lignin concentration

from 62 g L–1 to approximately 40 g L–1 corresponds to an overall increase

in viscosity of 100 mL g–1 [38].

A detailed study on the effects of dissolved lignin has been conducted by Sjoblom

et al. [39]. The results of Pinus silvestris kraft cooks in a continuous liquor

flow digester demonstrate that the presence of dissolved lignin during the later

stages of delignification (bulk and final) impairs the selectivity expressed as viscosity–

kappa number relationship. The effect increases with prolonged delignification.

Interestingly, the presence of lignin during the initial phase and the transi-

244 4 Chemical Pulping Processes

tion phase to bulk delignification results in an increase in pulp viscosity. The addition

of untreated black liquor from a previous cook decreases selectivity more as

compared to the addition of dialyzed black liquor or precipitated kraft lignin

(Indulin AT from Westvaco) at a given lignin concentration (Fig. 4.48). Conventional

batch cooking and continuous liquor flow cooking with the addition of

untreated black liquor in the final part of the cook show comparable selectivity in

the kappa number range 20–32. Figure 4.48 shows that, in comparison to these

cooks, continuous liquor flow cooking without the addition of lignin to the cooking

liquor (CLF reference) produces pulps with 200–250 mL g–1 higher viscosity in

the given kappa number range. The better selectivity may be explained partly by

the low concentrations of dissolved lignin and sodium ions which increases the

delignification rate, and partly by the continuous supply of hydrogen sulfide ions.

In conventional cooking there seems to be a lack of hydrogen sulfide ions at the

beginning of the bulk delignification phase, and this might increase the proportion

of enolic ether structures in the lignin [40].

15 20 25 30 35

CLF reference Batch

CLF untreated black liquor CLF dialysed black liquor

Viscosity [ml/g]

Kappa number

Fig. 4.48 Selectivity plots of laboratory Pinus sylvestris kraft

cooks comparing the concepts of continuous liquor flow

(CLF) and conventional batch (Batch) technology, as well as

the addition of untreated and dialyzed black liquor during the

final cooking stage according to Sjoblom et al. [39].

CLF reference: [OH– ]= 0.38 mol L–1; [HS– ]= 0.26 mol L–1; Batch: 20% EA on

wood, 40% sulfidity, liquor-to-wood ratio 4:1; cooking temperature 170 °C for both

concepts; untreated or unchanged black liquor and dialyzed black liquor in a concentration

of 50 g L–1 lignin, each of which is added at the end of bulk delignification

until the end of the cook.

4.2 Kraft Pulping Processes 245

Moreover, the presence of dissolved lignin in alkaline solution leads to an

increase in the alkalinity when the temperature is raised [41]. The amount of hydroxide

liberated from lignin when increasing the temperature from 25 to 170 °C

equals approximately the amount being consumed by dissolution of precipitated

lignin at 25 °C. The effect of increased temperature on the acid/base reactions is a

displacement towards the acid forms according to the following equation:

B– + H2O _HB + OH–

The release of hydroxide ions at high temperatures is most pronounced in the

pH range 10–12, where the phenolate and carbonate ions react to form phenols

and hydrogen carbonate, respectively [42].

The determination of alkali concentration at 170 °C is measured indirectly by

the extent of cellulose degradation caused by alkaline hydrolysis using high-purity

bleached cotton linters (stabilized with NaBH4 treatment against alkaline peeling

degradation). It has been shown that in a lignin-free cooking liquor (white liquor),

the alkali concentration at 25 °C must be increased by 31% in order to obtain equal

alkalinities at 170 °C with that of a lignin solution of 44 g L–1. Figure 4.49 illustrates

how much the EA concentration must be increased in a lignin-free solution

at 25 °C to reach the alkalinity at 170 °C of a lignin solution of a given concentration.

The relationship is valid at an effective alkali concentration of 0.6 mol L–1 in

the lignin solution at 25 °C.

The selectivity is also impaired by an increasing ionic strength (e.g., sodium

ions) during the final part of the cook. The effect of dissolved lignin on selectivity

is, however, greater than that of sodium ions limited to the concentration levels in

0 20 40 60 80 100

0,0

0,1

0,2

[OH-]-[OH-]

L

, mol/l

Lignin concentration, g/l

Fig. 4.49 Difference in alkali concentration at

25 °C between lignin-free solution and lignincontaining

solutions, [OH– ]-[OH– ]L, as a function

of lignin concentration (according to [41]).

Both solutions show the same alkalinities at

170 °C. The relationship is valid at an alkali concentration

of 0.6 mol L–1 at 25 °C.

246 4 Chemical Pulping Processes

normal cooks. The presence of untreated black liquor during the final part of the

cook causes a lower yield of approximately 0.5% at a given kappa number. The

yield loss originates from the prolonged cooking necessary to reach a given kappa

number.

The sole effect of liquor displacement was studied for kraft cooking of radiata

pine [43]. Liquor displacement means the replacement of black liquor by fresh

white liquor in the late stage of the cook. The results from laboratory cooking

experiments clearly show that if the displacement is conducted earlier in the cook

(1200 H-factor), then the effective alkali split ratio has no influence on pulping

selectivity. However, if displacement is delayed until 1600 H-factor, pulping selectivity

increases for both effective alkali split ratios investigated (see Fig. 4.50).

In another study, a three-stage process with both high initial sulfide concentration

due to a low liquor-to-wood ratio and the use of a sulfide-rich white liquor

(vapor phase cook until H-factor 300) and low final lignin concentration suggests

a substantial selectivity advantage compared to a modified reference cook

[26,44,45]. A dissolved lignin concentration in the final cooking phase as low as

20 g L–1 (compared to 40 g L–1 for the modified reference and 70 g L–1 for the conventional

reference cooks) is achieved through drainage of the free liquor at H-factor

1200. The bisection of the dissolved lignin concentration in the final cooking

phase results in an increase of approximately 90 units in pulp viscosity at a given

kappa number, which again is about 60 and 160 units higher as compared to the

modified and conventional reference cooks, respectively [44].

10 20 30 40

0.83 EA split at 1200 HF 0.83 EA split at 1600 HF

0.70 EA split at 1200 HF 0.70 EA split at 1600 HF Conventional cooking

Viscosity [ml/g]

Kappa number

Fig. 4.50 Selectivity plot as viscosity–kappa

number relationship for radiata pine kraft

pulps (according to [43]). Influence of liquor

displacement at different effective alkali split

ratios at different H-factor levels in comparison

to conventional kraft batch cooking. Constant

conditions: Total EA-charge 15.6% on o.d.

wood; 26.5% sulfidity; max. cooking temperature

170 °C.

4.2 Kraft Pulping Processes 247

The application of liquor exchange at a predetermined H-factor to reduce the

content of dissolved lignin and sodium ions in laboratory kraft pulping of hardwood

(e.g., different Eucalyptus species) was also very successful in improving the

relationship between pulp strength and kappa number [23]. If liquor exchange is

combined with alkali profiling, the benefits gained are substantially additive.

The rate of delignification is determined by the initial fraction of EA alkali

reflecting the higher alkali concentration at the start of the bulk phase. High pulping

rates can be maintained at low EA split ratios if displacement is shifted to earlier

cooking stage [43].

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