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.