Combined parameters Unit Value

Sulfidity (S) % 30.8

Causticity (C) % 82.0

Degree of reduction (DR) % on total S

% on SO4

81.5

89.9

The density of the white liquor is an important property for mass balance considerations

of white liquor management, and correlates with the effective alkali

concentration and sulfidity according to the following relationships [Eq. (11)]:

_ WL _ 0_692 _ EA 0_092 _ S 0_017 _11_

_ WL _ 1_051 _ _ OH _ 0_078 _ _ HS _ 0_014

Using Eq. (11), the density of the white liquor characterized in Tabs. 4.2–4.4 corresponds

to a value of 1.126 g mL–1.

The following equilibria occur in the aqueous solution of white liquor, comprising

the active compounds sodium hydroxide, sodium sulfide, and sodium.

Sodium hydroxide is a strong base and is therefore completely dissociated according

to Eq. (12):

NaOH H 2 O __ Naaq OH _ aq _12_

An aqueous sodium sulfide solution dissociates into sulfide, S2–, hydrogen sulfide,

HS–, and dissolved hydrogen sulfide, H2Saq according to the equilibrium conditions.

Their relative proportions are determined by the OH– ion concentration and

the equilibrium constants, which are particularly dependent on temperature and

ionic strength. Since the pKa of the protolysis of hydrogen sulfide has been agreed

as ca. 17.1, the equilibrium in Eq. (13) strongly favors the presence of HS– ions;

this concludes that, under conditions prevailing in the white liquor, sulfide ions

can be considered to be absent [8,9]:

S 2_ aq H 2 O _ HS _ aq OH _ aq _13_

where the pKa,HS

– and pKb,S

2– equal 17.1 and –3.1, respectively.

Most older publications deal with a pKa value of HS– ions lower than the pKw

value, which in turn provides a wrong picture of the electrolyte equilibria, especially

at initial cooking conditions. Stephens selected a value of pKa = 13.78 based

on calorimetric and thermodynamic consideration, whereas Giggenbach derived a

4.2 Kraft Pulping Processes

value of pKa = 17.1 from an ultra-violet study that involved reassigned absorption

bands [8]. This value was confirmed by Meyer based on Raman spectra for in-situ

measurement of the H–S stretch in high-pH solutions [9]. It has been agreed that

pKa values below 15, which were published previously, were due to oxygen contamination.

Aqueous HS– ions dissociate according to Eq. (14):

HS _ aq H 2 O _ H 2 Saq OH _ aq _14_

where the pKa,H2S and pKb,HS

– equal 7.05 and 6.95, respectively.

The formation of H2S becomes significant when the pH approaches values of

about 8. The dissolved H2S is involved in an equilibrium with hydrogen sulfide in

the gas phase, which only has to be considered in the sodium sulfite recovery systems

where H2S is expelled from the green liquor by carbonation processes (e.g.,

Stora and Sivola processes) [10,11]:

H 2 Saq _ H 2 Sg _15_

where

Kg _ _ H 2 Sg

_ H 2 Saq

Sodium carbonate is also involved in the acid–base equilibria of white liquor

according to Eqs. (16) and (17):

CO 2_ 3_ aq H 2 O _ HCO _3_ aq OH _ aq _16_

HCO _3_ aq H 2 O _ CO 2_ aq H 2 O OH _ aq _17_

where the pKa,HCO3

– and pKb,CO3

2– are 10.33 and 3.67 and the pKa, CO2 and pKb,HCO3

– are

6.35 and 7.65, respectively. The acid dissociation constants of the involved equilibria

in white liquor are summarized in Tab. 4.5.

Tab. 4.5 Acid dissociation constants of important acid–base

equilibria in the white liquor.