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