Stora Processes (Hydrogen Sulfite or Monosulfite-Acid Sulfite)

The bisulfite-acid sulfite process was primarily developed for the partial use of

extractive-rich pine heartwood and tannin-damaged spruce. Yield and strength

properties are reported to be slightly better compared to acid sulfite pulps, and

even better than bisulfite pulps in case of breaking length (e.g., magnefite pulp)

(see Tab. 4.65). The two-stage neutral sulfite-acid sulfite pulping process was developed

by Stora Kopparberg for utilization of pine raw material, and was first

applied in their mill at Skutskar [8,25–29]. The key principle of the Stora process

involves a two-stage cooking process, comprising lignin sulfonation under slightly

acidic to neutral pH conditions in the first stage, and lowering the pH by adding

SO2 (liquid or hydrated) for completion of the pulping operations in a second

stage. The cooking liquor for the first stage consists of about 50% of liquor drawn

from a previous cook and of fresh liquor prepared in the chemical converting

plant. The digester volume during the first-stage is entirely filled with cooking

liquor, corresponding to a liquor-to-wood ratio for softwood of about 4.5:1, to

ensure proper impregnation. Depending on the applied wood furnish, the reaction

time during the first stage varies from 2–3 h (spruce) to 4–6 h (pine). After

the first stage, cooking liquor is drawn to adjust a liquor-to-wood ratio of about

3.0–3.5:1. The second stage starts with the injection of SO2 and, depending on the

temperature (135–145 °C), proceeds for about 2–4 h. The two cooking stages can

also be characterized as sulfonation (stage 1) and delignification (stage 2) stages.

A degree of sulfonation of about 0.3 S/OCH3 has been identified [25].

As illustrated in Fig. 4.187, only a relatively small amount of lignin is removed

during the first stage, whereas in the subsequent second stage a rapid lignin dissolution

occurs.

The reaction conditions during the first cooking stage efficiently prevent condensation

reactions between reactive lignin groups and the phenolic extractives

originating from pine, larch, Douglas fir and other extractive-rich wood raw material.

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

in the subsequent acid sulfite cook can be obtained. Further investigations have

shown that the conditions of the first stage also affects the carbohydrate yield. As

shown in Fig. 4.188, the yield increase is a clear function of the pH and temperature

in the first cooking stage. At higher temperature the inflexion point is shifted

towards lower pH value.

Interestingly, the increase in yield proceeds parallel to a decrease in the acetyl content

of the wood after the pretreatment (Fig. 4.188). After deacetylation, the molecules

will be packed so closely together that acid hydrolysis and diffusion are prevented

in the final acid sulfite cook.

The research group of Billerud found that the yield advantage obtained with the

neutral sulfite-acid sulfite pulping concept is mainly due to a increased amount of

glucomannan in the pulp [31–33]. The increased stability is thought to result from

an increased degree of lateral order caused by deacetylation of the glucomannan,

followed by adsorption of the linear backbone onto the cellulose surface. Surprisingly,

the degree of polymerization of the retained glucomannan is about 30, independent

of the cooking procedure. Both glucomannan and cellulose are located

472 4 Chemical Pulping Processes

2 4 6 8

Stage I Stage II

pH first stage: 7.0 4.0

Degree of delignification [%]

Time [h]

Fig. 4.187 Delignification of pine wood in sodium-based,

two-stage sulfite pulping at two different pH levels in the first

stage (according to [30]).

predominantly in the S2 cell wall layer. Thus, most of the glucomannans have

only a very short path of diffusion in order to be adsorbed onto the cellulose molecules.

Different results have been published regarding the stabilizing effect of hardwood

pulping. Sanyer and Wenneras were unable to identify any significant yield

increase for all different concepts of multi-stage sulfite cooking using oak, sweetgum,

and birch [8,18,19]. The small content of glucomannan in the hardwood species

was argued to be the reason for the nondetectable stabilization effect of the

two-stage pulping. Janson and Sjostrom, however, reported that two-stage cooking

of birch results in a 2–3% higher pulp yield as compared to conventional acid-sulfite

cooking [34]. The contribution of xylan is very much dependent upon the conditions

during the first stage. An increase in temperature and duration of that

stage, and a decrease in temperature in the subsequent stage, appears to favor

xylan retention in the final pulp [25]. In combination with a starting pH of 8–9

and a temperature of 150 °C for 1–2 h, a yield gain of up to 4–5% on o.d. wood

was also reported for aspen and birch pulps when applying the two-stage Stora

concept [25].

The properties of paper are influenced by the higher amounts of short-chained

hemicellulose in such a way that the fibers are more easily beaten and hydrated.

The fibers swell and become flexible, which promotes fiber–fiber bonding. A higher

hemicellulose content means a higher beating response, a higher tensile

strength and a lower tear factor and opacity at the same freeness. The two-stage

4.3 Sulfite Chemical Pulping 473

4 6 8

125.C, 90 min 160.C, 60 min

Pulp yield [% od wood]

pH-value

0.2

0.4

0.6

0.8

Acetyl content [% od wood]

acetyl content

Fig. 4.188 Pulp yield as a function of pH in the first stage of a

sodium-based, two-stage sulfite pulping of spruce (according

to [25]). Acetyl content of sprucewood residues after treatment

at 125 °C for 90 min as a function of pH of the cooking

liquor (according to [33]).

pulps attain breaking lengths which are considerably above those of acid sulfite

pulps, and are also better than the bisulfite pulps. The tear strength for the twostage

pulps is however lower than for bisulfite pulps; this can be explained by the

high hemicellulose content and also in part by the smaller number of fibers per

gram of pulp. Surprisingly, the two-stage pulp has a higher bulk than the conventional

sulfite at a certain tensile strength.

Although the use of pine heartwood creates no uncontrollable condensation

reactions during two-stage sulfite pulping, the high resin content in the

unbleached pulp would cause a serious pitch problem during further processing

of the pulp (bleaching, papermaking). Fortunately, pitch in pine pulp can be easily

extracted by using an alkaline treatment, and this is facilitated by the wide window

pits in pine and the resin-rich ray parenchymal cells.

The neutral sulfite or bisulfite-acid sulfite concept has also been applied to magnesium

base cooking, due to the finding that the precipitation of magnesium

monosulfite/magnesium hydroxide can be avoided up to pH levels of about 6.5,

as previously mentioned. The process, which was developed and realized by

Weyerhaeuser, comprises a first-stage cooking at 150 °C, using a cooking liquor

with a pH ranging from 5.2 to 6.0. No particular advantage was realized when

cooking at pH values above 6.0, provided that the temperature was about 150 °C

in the first stage [14]. At the end of the first stage, liquor is withdrawn from the

digester to produce a liquor-to-wood ratio of 2:1. Liquid SO2 is then injected into

474 4 Chemical Pulping Processes

the digester to adjust the acidic conditions. The second stage is carried out at 130–

140 °C, adjusting the time to produce a pulp of appropriate properties. The “FB"-

denoted pulp is characterized by its high hemicellulose content and resultant rapid-

hydrating properties. The low opacity and low power requirement for refining

makes the pulp attractive for transparent grades of paper.

The sodium-based, two-stage sulfite process is also used in the production of

highly reactive spruce/pine dissolving pulp at Domsjo [24,35]. The first stage is

operated at a pH of 4.5 (hydrogen sulfite), a temperature above 150 °C, and a high

liquor-to-wood ratio to ensure optimum impregnation and sulfonation. At the end

of the second stage, liquor is withdrawn for reuse in a subsequent cook. The acidic

conditions in the second stage are adjusted by adding SO2 water. Cooking is maintained

until the target viscosity level between 400 mL g–1 and 800 mL g–1 is

attained. The low content of residual lignin (kappa number 3–8) makes it possible

to produce a high-brightness pulp (>92% ISO) using two-stage totally chlorine

free (TCF) bleaching according to an E-P-sequence with fully closed water cycle.

Before the hot caustic extraction (E), the pulp is subjected to a special depitching

treatment. The alkalized pulp is treated mechanically in screw presses, followed

by Frotapulper screws. The combined chemical and physical treatments effectively

disperse the resin, which then is carefully washed out [36].

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