Alternative Sulfite Pulping Concepts

The difficulty of pulping certain resin-rich softwoods and hardwoods is certainly a

serious handicap for the conventional acid sulfite process, and was one of the reasons

why sulfite pulping technology lost ground to the kraft process. The development

of the kraft recovery process during the 1930s, including a technically more

convenient and efficient form of combustion furnace, particularly by the contributions

of Tomlinson, made the pine kraft pulps highly competitive with the spruce

and hemlock sulfite pulps [1–4]. Since the early days of sulfite pulping technology,

much effort has been undertaken to overcome this deficiency. An initial breakthrough

in the pulping of resinous wood was provided by the development of

Hagglund and co-workers, who developed a two-stage concept, with the first stage

more alkaline than the second [5–7]. It could be shown that condensation reactions

between reactive lignin groups and some phenolic extractives (e.g., pinosylvin

and its monomethylether) are greatly diminished at pH levels greater than 4.

Therefore, an acceptable delignification of the extractive-rich pine heartwood in

the succeeding acid sulfite stage can be obtained. This (bi)sulfite-acid sulfite twostage

concept was first realized in an industrial-scale operation by Stora Kopparberg

and Svenska Cellulosa, known as the Stora and Kramfors processes for pulping

pine and tannin-damaged spruce, respectively [8,9]. To change the pH from

the acid to the slightly alkaline range, the so-called soluble bases (sodium, ammonia,

and magnesium) are required. Sodium and ammonia both allow the possibility of

changing the acidity of the liquor within the entire pH-range. Sodium is, however, to

be given preference as the ammonia liquors are less stable at higher temperatures. It

is also possible to use magnesium as a base up to pH levels of about 6.5 [10]. It has

been shown that the solubility of magnesium monosulfite suspensions in the pH

range above 5 increases significantly in the presence of recycled spent sulfite

liquor or high sodium salt concentrations (e.g., NaCl or Na2SO4) [10].

The great effort into the development of alternative sulfite pulping concepts during

the 1950s and 1960s, in answer to the rapid development of the kraft process,

has led to the introduction of several pilot- and mill-scale applications. Among the

most successful developments are the following (though this list is far from comprehensive):

_ Magnefite process (Bisulfite)

– Cooking base magnesium

– Cooking liquor magnesium hydrogen sulfite, Mg(HSO3)2, pH 4

_ Two-stage neutral Magnefite (Bisulfite-MgO)

– Cooking base magnesium

– First stage magnesium hydrogen sulfite, Mg(HSO3)2, pH 4

– Second stage magnesium monosulfite/magnesium hydroxide, pH 6–6.5

_ Sivola process(es):

– Bisulfite-soda

– Cooking base sodium

– First stage sodium hydrogen sulfite, pH 4–5

4.3 Sulfite Chemical Pulping 465

– Second stage sodium carbonate/sodium monosulfite, pH 8

– Acid bisulfite-soda

– Cooking base sodium

– First stage acid sodium sulfite, pH 1–2

– Second stage sodium carbonate/sodium monosulfite, pH 8

– Bisulfite-acid bisulfite-soda

– Cooking base sodium

– First stage sodium bisulfite, pH 4

– Second stage acid sodium sulfite, pH 1–2

– Third stage sodium carbonate/sodium monosulfite, pH 8–10

_ Stora process (hydrogen sulfite or monosulfite-acid bisulfite)

There are numerable variations to this process, but the general concept can be

described as a sulfite-acid bisulfite cook.

– Cooking base sodium

– First stage sodium hydrogen sulfite, pH 4, or sodium monosulfite,

pH 6–8

– Second stage acid sodium sulfite, pH 1–2.

The Billerud method, not separately introduced here, may best be described as a

modification of the Stora method, the main variation occurring in the second

stage of the cook (pH 3).

_ ASAM (alkaline sulfite anthraquinone and methanol)

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