Chemistry of (Acid) Sulfite Cooking

Antje Potthast

The composition of the spent sulfite liquor depends to a large extent on the cooking

conditions chosen and the chemical composition of cooking chemicals – that

is, mainly the ratio of free and combined SO2 (for details, see Section 4.3.2). The

degree of delignification is directly related to the concentration of the product

[H+]·[HSO3

– ], while the concentration of [H+]directly affects the rate of cellulose

hydrolysis.

Depending on the progress of the sulfite cook, the composition of the cooking

liquor changes mainly due to consumption of bound SO2 (HSO3

–) and changes in

acidity [1].

SO2

H O 2 H2SO3

H

+

HSO+ 3 +

_

Scheme 4.31 Equilibrium of bound and free sulfur dioxide.

The composition of the cooking liquor in terms of free SO2 and combined SO2

(hydrogen sulfite) must be balanced in a way that assures sufficient delignification

while keeping the condensation reactions to a limit. Kaufmann [2](F ig. 4.155)

illustrated the borderline ratio between total SO2 and combined SO2, which will

4.3 Sulfite Chemical Pulping 405

either result in cooks with acceptable outcome or, if outbalanced, in so-called

“black cooks”, where condensation processes preponderated. Crossing the border

towards lower amounts of combined SO2 and lower total SO2 will yield pulp with

highly condensed lignin fractions impracticable to bleach. Keeping the appropriate

ratio is indispensable to minimize condensation effects and to allow sufficient

delignification.

bisulfite solution

area of black

cooks

area of

acceptable

cooks

Total SO

(%)

Combined SO

(%)

Black cooks

Normal or brownish cooks

0.00 0.25 0.50 0.75 1.00 1.25

Fig. 4.155 Kaufmann diagram, indicating areas of black

cooks in relation to the cooking liquor composition (adopted

from [1]).

Cooking close to conditions of black cooks results in:

_ Increasing dehydration (more free SO2) due to increasing temperature.

_ Decreasing concentration of hydrogen sulfite due to consumption

by lignin:

– Formation of strong acid anions (lignosulfonate anions), which in

turn reduce the available bound SO2

– Less available hydrogen sulfite prevents sulfonation of lignin

(delignification), but increases condensation reactions

– Buffer capacity decreases towards the end of the cook

_ Formation of new H+ ions from sulfur dioxide and water according

to Scheme 4.31, hence increasing in [H+].

406 4 Chemical Pulping Processes

The general reactions in a sulfite cook can be divided into sulfonation, hydrolysis,

condensation, and redox processes. Sulfonation reactions mainly occur with lignin

and to a minor extent also with carbohydrates and low molecular-weight degradation

products. Condensation is mainly observed between lignin units and lignin

intermediates and extractives, and to some extent also with degradation products

of carbohydrates. Carbohydrates (especially hemicelluloses) are affected by

hydrolysis, but lignin moieties are also partly fragmented by this reaction type.

Hydrolysis is especially important to cleave lignin–carbohydrate linkages. Redoxprocesses

are taking place with inorganic compounds, most often with participation

of the degraded carbohydrates and extractives.

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