Compound Acid Conjugated Base pKa

Sodium sulfide HS–

H2S

S2–

HS–

17.10

7.05

Sodium carbonate HCO3

–

CO2·H2O

CO3

2–

HCO3

–

10.33

6.35

4 Chemical Pulping Processes

0 2 4 6 8 10 12 14

10-7

1x10-5

10-3

10-1

CO

2-

CO

2-

HCO

-

H

CO

3 H

CO

H

CO

H

S

HCO

-

HCO

-

HS-

H

S

H+ OH-

S2-

HS-

H

S

Total concentration [mol/l]

pH value

Fig. 4.2 Bjerrum diagram of the electrolyte system prevailing

at the beginning of a conventional kraft cook, assuming a

hydroxide concentration of 1.15 mol L–1, a hydrogen sulfide

ion concentration of 0.20 mol L–1, and a carbonate concentration

of 0.11 mol L–1.

The initial concentrations of the active cooking chemicals in a conventional

kraft cook, assuming an EA charge of 18.4% NaOH on wood and a liquor-to-wood

ratio of 4:1, are shown by the Bjerrum diagram over the whole pH range (Fig. 4.2).

The shaded area in Fig. 4.2 represents the pH range typical for a kraft cook. The

results depicted in the figure confirm that, during the whole kraft cook, the equilibria

strongly favor the presence of HS– ions and carbonate ions. The sulfide ion

and the hydrogen carbonate ion concentrations are below 0.1 mmol L–1 and thus

of no importance in the electrolyte equilibria of the cooking liquor.

A titration curve of the white liquor at room temperature illustrates the presence

of three equivalence points characterizing the acid–base equilibria shown in

Eqs. (14), (16) and (17). The equilibrium conditions for the titration of salts of

weak acids and strong bases with a strong acid can be expressed according to

Eq. (18):

__ AH _ H _ _ A _ _ OH _ _18_

where [AH]corresponds to the concentration of the conjugated acid of the titrated

weak base (e.g., the carbonate ions). Assuming the total concentration of the sum

of the conjugated base [A– ]and the acid [AH]to be C (in mol L–1), and kB the equilibrium

constant of the conjugated base, the concentration of the acid [AH]is calculated

according to Eq. (19):

4.2 Kraft Pulping Processes

_ AH _

kB _ C

_ kB _ OH _ _ _19_

The concentration of A– can be expressed as the difference between the molar

amount of the titrator acid, C*, and the molar concentration of the strong bases

available in the white liquor sample, equivalent to the EA concentration. The titration

curve can be calculated on the basis of Eq. (18) and by considering the acid–

base equilibria present in the white liquor [Eqs. (14), (16) and (17)]. The pKb values

are used from Tab. 4.5:

kB_CO2_ 3 _ CCO2_ 3

_kB_CO2_ 3 _OH_ _

kB_HCO__3 _ CHCO_3

_HB_HCO_3 _OH_ _

kB_HS__ _ CHS_

_kB_HS_ _OH_ _

10_14

_OH_ _ _C* _ _EA _

_ _OH_

_20_

The course of pH (or the [OH– ]) can be calculated as a function of the molar addition

of titrator acid by solving an equation of the fifth order. A synthetic white

liquor comprising an effective alkali concentration of 110 g L–1, [OH]= 2.75 mol L–1,

a sulfidity of 30.4%, [HS– ]= 0.485 mol L–1, and a sodium carbonate concentration

of 20 g L–1 (as NaOH), [CO3

2–]= 0.25 mol L–1, was titrated with hydrochloric acid of

0.5 mol L–1. A comparison between the experimental and theoretical neutralization

curves shows quite acceptable correspondence (Fig. 4.3). In the high-pH region,

some deviation occurs mainly due to the disregarded high ionic strength

which affects the activity factor of the hydroxide ions.

2,0 2,5 3,0 3,5 4,0

EP3

EP2

EP1

experimental calculated

pH-value

[H+], mol/l

Fig. 4.3 Neutralization curve of white liquor

at room temperature ([OH]= 2.75 mol L–1,

[HS– ]= 0.485 mol L–1, [CO3

2–]= 0.25 mol L–1)

using 0.5 mol L–1 hydrochloric acid as titrator

acid at room temperature. Experimental

curve compared to theoretical curve using

Eq. (20) (Mathematica 4.1.). Equivalence

points: EP1 at pH 11.7, EP2 at pH 8.7, EP3 at

pH 4.1 [7].

4 Chemical Pulping Processes

During the course of cooking, the white liquor gradually enriches with an extremely

complex mixture of degraded lignin and carbohydrate substances, and

finally converts to black liquor. The lignin part of the wood is degraded into low

molecular-weight compounds which contain hydrophilic groups such as phenolate,

catecholate and partly also carboxylate groups [12]. The main part of the carboxylic

acids originate from carbohydrate degradation products, for example,

hydroxyacids such as gluco-, xyloisosaccharinic acid, lactic acid and gluconic acid

[13]. In addition, rather large amounts of formic acid, glycolic acid and acetic acid

are formed through fragmentation reactions. In addition to the various organic

acids, which are ionized in the cooking liquor, a significant amount of inorganic

substances are mostly present as dissolved species, including Na+, K+, CO32–,

SO4

2–, SO3

2–, S2O3

2–, SnS2–, HS– ions and nonprocess-elements (NPEs), comprising,

for example, Mg, Al, Si, Mn, Cl, P, and transition metal ions. The alkalinity of the

black liquor depends on the acid–base properties of all the dissolved and ionized

compounds. The residual effective alkali concentration is a key parameter for the

processability and the control of the kraft cooking process. Thus, knowledge of

the hydroxide ion concentration in black liquor is of great importance, although

most methods provide only approximate values [14]. Following the black liquor

model from Ulmgren et al., the acid–base equilibria of four organic and three

inorganic substances have been considered (Tab. 4.6).

Tab. 4.6 Black liquor composition following the suggestion of

Ulmgren et al. [14]and additional measurements [6].

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