SO2 Balance

Conventional titrimetric methods, such as iodometric titration, cannot be applied

to the quantitative determination of concentrations of inorganic sulfur ions present

in the acid sulfite cooking liquor, mainly because the dissolved organic compounds

interfere with correct measurements. A method based on capillary electrophoresis

(CE) has been successfully developed for quantitative determination of

thiosulfate, sulfite, and sulfate ions in acid sulfite liquors [9]. Using this CE method,

it is now possible to balance the whole cook with respect to all sulfur compounds,

including determination of the relieved SO2 gas (top relief, pressure

relief) by conventional iodometric titration. Unfortunately, the CE method cannot

differentiate between free and combined SO2 since the cooking liquor is immediately

absorbed in an alkaline solution to prevent losses of volatile SO2.

Figure 4.159 shows the course of specific amounts of sulfite ions (hydrogen sulfite

and dissolved SO2 hydrate), sulfate ions, and released gaseous SO2 during two

magnesium sulfite cooks with different cooking liquor composition, temperature,

and H-factor profiles.

As expected, the concentration of dissolved sulfur (IV) compounds continuously

decreases with progress of cooking due to consumption reactions (e.g., sulfonation,

redox reactions with reducing end groups, formation of ketosulfonates, etc.).

Simultaneously, a slight increase in sulfate ion concentration can be observed.

As expected, the amount of gaseous SO2 during the pressure release of the cook

correlates with the specific amount and proportion of free SO2 in the cooking

acid. Assuming that the decrease in the specific amount of sulfite-sulfur compounds

can be attributed solely to consuming reactions (see above), the sulfur balance

can be easily completed. Although no information about the stoichiometry

of reactions is available, an evaluation of the sulfur balance data reveals that the

434 4 Chemical Pulping Processes

0 50 100 150 200

[g/kg od wood]

Pressure [bar]

ΣSO

: 140 g/kg od w; TFree SO

: 80 g/kg od w

relief sulfate-SO

sulfite-SO

ΣSO

: 116 g/kg od w; TFree SO

: 50 g/kg od w

relief sulfate-SO

sulfite-SO

Amount SO

H-Factor

Fig. 4.159 Specific amounts of dissolved SO2

(hydrogen sulfite and SO2 hydrate), sulfate and

released gaseous SO2 during two different

beech magnesium acid sulfite cooks: (a) Total

SO2: 140 g kg–1 o.d. wood, free SO2: 80 g kg–1

o.d. wood, maximum cooking temperature:

140 °C, H-factor 148, unbleached viscosity:

590 mL g–1. (b) Total SO2: 116 g kg–1 o.d. wood,

free SO2: 50 g kg–1 o.d. wood, maximum cooking

temperature: 148 °C, H-factor 210,

unbleached viscosity: 590 mL g–1.

sulfur consumption reactions follow a type of saturation function, which indicates

that the consumption rates are highest at the beginning and level off in the later

stages of the cook. The course of the consumption reactions for both cooks is

shown in Fig. 4.160.

Clearly, the extent of sulfur consumption reactions is virtually independent of

the cooking conditions, provided that the target viscosity is achieved. As an example,

the course of one-stage acid sulfite cooking to a target viscosity of 590 mL g–1

comprising two different compositions of cooking acid are compared (Fig. 4.160).

Despite the totally different specific amounts of total SO2 and the proportion of

free and bound SO2, the overall reaction stoichiometry during acid sulfite cooking

is quite comparable for both sulfite cooks. This result is important when designing

the gaseous SO2 recovery, as knowledge of the specific amount of bound SO2

compared to dissolved organic matter, allows easy calculation of the maximum

amount of free SO2 recovery (Fig. 4.161).

In both cooks, approximately 74 g SO2 kg–1 o.d. wood is consumed by reactions

with dissolved organic matter (e.g., to lignosulfonates, etc.). The remaining sulfur

species after the cook include the released gaseous SO2 fraction (top relief and

pressure relief), the dissolved sulfur(IV) compounds as hydrated SO2 or hydrogen

sulfite ions, and a small fraction as oxidized sulfate ions. No thiosulfate ions have

been detected on a level of <0.2 g L–1. The specific amounts and relative proportions

of the sulfur compounds are listed in Tab. 4.57.

4.3 Sulfite Chemical Pulping 435

0 50 100 150 200 250

[g/kg odw]

Temperature [°C]

ΣSO

: 140 g/kg od w, TFree SO

: 80 g/kg od w

ΣSO

: 116 g/kg od w, TFree SO

: 50 g/kg od w

SO

bound to dissolved dry matter

H-Factor

Pressure [bar]

Fig. 4.160 Course of the specific amount of

bound SO2 to dissolved organic matter during

two different beech magnesium acid sulfite

cooks. (a) Total SO2: 140 g kg–1 o.d. wood, free

SO2: 80 g /kg–1 o.d. wood, maximum cooking

temperature: 140 °C; H-factor 148, unbleached

viscosity: 590 mL g–1. (b) Total SO2: 116 g kg–1

o.d. wood, free SO2: 50 g kg–1 o.d. wood, maximum

cooking temperature: 148 °C, H-factor

210, unbleached viscosity: 590 mL g–1.

Before After Before After

ΣSO

: 140 g/kg odw

Free-SO

: 80 g/kg odw

ΣSO

: 116 g/kg odw

Free-SO

: 50 g/kg odw

COOK COOK

Amount SO

[g/kg odw]

Bound-SO

Free-SO

Reacted SO

SO

-relief Sulfate-SO

Sulfite-SO

Fig. 4.161 Gross balance of different sulfur

species prior to and after two different beech

magnesium acid sulfite cooks. (a) Total SO2:

140 g kg–1 o.d. wood, free SO2: 80 g kg–1 o.d.

wood, maximum cooking temperature: 140 °C,

H-factor 148, unbleached viscosity: 590 mL g–1.

(b) Total SO2: 116 g kg–1 o.d. wood, free SO2:

50 g kg–1 o.d. wood, maximum cooking temperature:

148 °C, H-factor 210, unbleached

viscosity: 590 mL g–1.

436 4 Chemical Pulping Processes

Tab. 4.57 Gross balance of different sulfur species as specific

SO2 (g kg–1 o.d. wood) after beech magnesium acid sulfite cooks

of two different acid compositions: A, higher proportion of free

SO2, and B, lower proportion of free SO2 (57% and 43% of total

SO2, respectively).

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