As NaOH as compound

NaOH 90.0 90.0

Na2S 40.0 39.0

Na2CO3 19.8 26.2

Na2SO4 4.5 8.0

Na2S2O3 2.0 4.0

Na2SO3 0.6 0.9

Other compounds 2.5

Total alkali (TA) 156.9 170.6

Total Sulfur (TS) 47.1 19.7

Effective alkali (EA) 110.0

Active alkali (AA) 130.0

4 Chemical Pulping Processes

In kraft cooking technology, the concentration of active chemicals are expressed

according to the following terminology:

_ Total alkali (TA) comprises the sum of all sodium salts, the

amount of total titratable alkali (TTA), the sum of sodium hydroxide,

sodium sulfide and sodium carbonate. Most important for

the characterization of the cooking liquor are the expressions

effective alkali (EA), active alkali (AA) and sulfidity (S), which are

defined according to Eqs. (3–5).

_ The EA-concentration (eq. 3) is equivalent with the concentration

of hydroxide ions because the sulfide ions are completely dissociated

to hydrogen sulfide ions and hydroxide ions under the conditions

occurring throughout kraft pulping (see Fig. 4.2).

EA = NaOH + 0.5 Na 2 S (3)

_ The AA-concentration (eq. 4) includes the whole sodium sulfide

concentration and can be used to express the sulfidity of the aqueous

solution according to Eq. (5):

AA =NaOH + Na 2 S (4)

S _

Na 2 S

AA _ 100_ _5_

For simplicity, all the chemicals are calculated as sodium equivalents and

expressed as weight of NaOH or Na2O (see Tab. 4.2).

_ The inter-relationship between EA- and AA-concentrations can be

displayed by Eq. (6):

EA = AA _1 _ 0_5 _ S _

AA=

EA

_1 _ 0_5 _ S _ _6_

where S is expressed as a fraction.

In pulping chemistry, molar units are preferred over weight units. As mentioned

previously, the OH– ion concentration is equivalent to the effective alkali

concentration. The HS– ion concentration, however, corresponds to only half of

the sodium sulfide concentration. The OH– ion and the sulfide ion concentrations

in molar units of the white liquor introduced in Tab. 4.2 are depicted in Tab. 4.3.

Based on molar units, the sulfidity of the white liquor can be calculated by

using Eq. (7):

S _

2 _ _ HS _

_ HS _ OH _ _ 100_ _7_

4.2 Kraft Pulping Processes

Tab. 4.3 Molar concentrations of hydroxide ions, hydrogen

sulfide ions and carbonate ions in the white liquor presented in

Tab. 4.2.

Species mol/l

OH– 2.75

HS– 0.50

CO3

2– 0.25

Reversed, the molar concentration of H2S may be derived from sulfidity (fraction)

and [OH– ]according to Eq. (8):

_ HS _ _

S _ _ OH _

_2 _ S _ _8_

Residual amounts of sodium carbonate in the white liquor are determined by the

extent of the causticizing reaction, which is an equilibrium reaction. The efficiency

of this reaction is characterized by the causticity (C) or causticizing efficiency,

and thus the concentration of sodium carbonate according to Eq. (9):

C _

NaOH

NaOH Na 2 CO 2 _ 100_ _9_

expressed as sodium hydroxide equivalents.

The causticity is a function of reaction time and the total salt concentration

(e.g., TTA). In dilute aqueous solutions and with longer retention times, the equilibrium

is shifted to the sodium hydroxide side, which in fact corresponds to a

higher causticizing efficiency. Table 4.2 shows also oxidized sulfur compounds,

such as sodium sulfate, sodium sulfite or sodium thiosulfate, which can be considered

inert with respect to the pulping reactions. From a process point of view,

the amount of oxidized sulfur compounds must be kept at the minimum achievable

level. The ratio between the reduced sulfur compounds (sodium sulfide) and

the total sulfur is designated as degree of reduction (DR). Actually, DR can be

related to both the total sulfur or the sulfate sulfur only, respectively [Eq. (10)]:

DRtot _

Na 2 S

Na 2 S Na 2 SO 4 Na 2 SO 3 Na 2 S 2 O 3 _ 100_ _10_

DRSO 2_ 4 _

Na 2 S

Na 2 S Na 2 SO 4 _ 100_ _10_

Table 4.4 lists the characteristics of the white liquor given in Tab. 4.2 by using the

expressions of Eqs. (5), (9) and (10).

4 Chemical Pulping Processes

Tab. 4.4 Sulfidity and efficiency of transformation to active

cooking chemicals based on the white liquor composition

given in Tab. 4.2.

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