Alkaline Sulfite Pulping

Alkaline sulfite (AS) cooking refers to a cooking process using a cooking liquor

made up of mainly NaOH and Na2SO3 with a cold cooking pH of 10.0–13.5 [37].

This process was developed as an alternative to kraft pulping during the late

1960s, and the first patents on alkaline sulfite cooking were issued in 1970–1971

[38,39], at a time when many old sulfite mills had been closed in the United

States, Canada, and Scandinavia. The driving force of this new pulping process

was to reduce investment costs, improve bleachability and beatability and, above

all, to reduce the odor problem, while maintaining strength properties at the level

of a kraft pulp. Other alternatives which arose at that time were the “nonsulfur”

cooking processes, the most important methods being two-stage soda-oxygen and

organosolv pulping procedures, both of which have been extensively reviewed in a

recent publication [40]. Soon after the first laboratory results had been obtained,

the AS process was tested in pilot plant and mill trials [37,41]. The results were

quite convincing, and the AS process was shown capable of producing pulp with

paper properties equal to those of the kraft process, but without odor emissions.

However, many disadvantages were associated with the AS process, and these

eventually prevented industrial acceptance. First, the rate of delignification was

very low, despite a high chemical charge. Second, the yield with softwoods was

lower than that of kraft in the kappa number range typical of the production of

fully bleached grades. Third, there were deep concerns about the cost and reliability

of existing sodium sulfite recovery systems. In fact, the extent of delignification

through AS cooking with a high Na2SO3-to-NaOH ratio is rather low because,

under these conditions, lignin degradation is limited to the cleavage of b-O-4 link-

4.3 Sulfite Chemical Pulping 475

ages of phenylpropane units carrying free phenolic hydroxyl groups. Increasing

the proportion of sodium hydroxide in the cooking liquor improves delignification

(nonphenolic linkages are also cleaved via an oxiran intermediate; see Section

4.2.4.1, Chemistry of alkaline delignification), but at the same time the carbohydrate

fraction is heavily degraded due to alkaline primary and secondary peeling

reactions which reduce pulp yield and quality. The use of anthraquinone (AQ) in

alkaline pulping processes initiated a renewed interest in AS cooking. In Japan,

Nomura has applied for a patent for neutral sulfite and sodium carbonate cooking

in the presence of AQ [42]. In many laboratory, pilot plant and even mill trials, the

positive effect of AQ on the efficiency and selectivity of delignification was confirmed

[43]. Compared to the kraft process, AS/AQ pulping under mildly alkaline

conditions behaves very selectively, providing both a higher yield and a higher viscosity

at a given lignin content or kappa number. However, the pronounced

advantage of AS/AQ over kraft pulping becomes diminished as delignification is

extended below a kappa number of about 40–50 [44]. Further insight into the

effects of liquor composition on the rate of the dissolution of various wood components

is provided by the results obtained by McDonough et al. [44]; these are summarized

in Tab. 4.67.

Cooking with a pure sodium sulfite solution (in the presence of AQ) behaves

very selectively towards the carbohydrates. Cellulose is completely preserved, and

only 30% of the glucomannan and about 25% of arabinoxylan are removed, while

about 63% of the lignin is degraded. Replacing 40% of the sodium sulfite with

sodium carbonate caused a further reduction of the lignin content by 45% and a

significant decrease in the glucomannan content, while the arabinoxylan fraction

was only marginally affected. Replacing sodium sulfite with sodium hydroxide

Tab. 4.67 Effect of liquor composition on yields of the major wood

components using Southern pine as wood source (according to [44]).

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