SULFITE PULPING

Around 1857 in Philadelphia, Julius Roth and Benjamin Tilgham found that treatment of wood with a mixture of sulfur dioxide (SO2) in hot water softened the wood considerably. However, the resulting pulp was extremely dark. This was due to the acidic conditions of the reaction, as sulfur dioxide in water forms sulfurous acid (SO2 + H2O <---> H2SO3). The acid degraded and eventually precipitated the lignin onto the cellulosic fibers. They then found that if a base was added (originally limestone, CaCO3), an excellent pulp was obtained without significant darkening. The addition of the base buffered the aqueous solution so as to be resistant to changes in pH. By controlling the pH, a more lignin-selective (sulfonation) reaction occurred which solubilized the lignin without precipitating it. A simple set of two equilibrium reactions governs the sulfite process and is exemplified with sodium hydroxide:

H2SO3 + NaOH <---> NaHSO3 + H2O (Eq. 1)

NaHSO3 + NaOH <---> Na2SO3 + H2O (Eq. 2)

A solution of sulfurous acid without base is acidic (pH<1.7; SO2 + H2O <---> H2SO3). As base is added the pH increases as shown in the titration curve at the right. At specific pH values (1.76 and 7.19), a large amount of base is required for relatively little change in pH. The solution is buffered around these pH values, which are termed pKa values. A pKa represents the pH of half-dissociation for equilibrium reactions such Eqs. 1 and 2 above. At pH 1.76 the concentration of H2SO3 = NaHSO3. Below pH 1.76 sulfurous acid predominates. At pH 7.19 the concentration of NaHSO3 = Na2SO3. Between pH 1.76 and 7.19 the bisulfite anion (HSO3-1) predominates, and at pH > 7.19 the sulfite anion (SO3-2) is found in highest concentrations. With three different pulping species available (H2SO3, NaHSO3, and Na2SO3), different types of sulfite pulping can be performed depending on the pH (Table I).

Table I. Types of Sulphite Pulping
 Nomenclature  Predominant reagent in cooking liquor Approx. initial pH at 25 C
 Acid sulfite H2SO3 + XHSO3  1-2
 Bisulfite XHSO3 2-6
 Neutral sulfite XSO3 + XCO3 6-9
 Alkaline sulfite XSO3 + XOH >10
X denotes the cation used in the process (Ca+2, Mg+2, NH4+, Na+).

The essentials of the sulfite (sulphite) process are as follows. The mill receives elemental sulfur (S) and burns it in an exothermic reaction at about 1100 °C to form SO2 gas. The gas is cooled to about 200 °C and trapped by an aqueous solution of base. The base will include the counterion used in the process (i.e., Ca+2, Mg+2, NH4+, Na+), and the common bases used for sulfite pulping are shown in Table II. For example if Na+ was the cation (or counterion) the aqueous solution would contain Na2CO3.

Table II. Bases Used in Sulfite Pulping.
 Cation Anion Added Base Common Name
Ca+2 (CO3)-2  CaCO3 limestone
Mg+2 OH- Mg(OH)2 magnesia
Na+ (CO3)-2 Na2CO3 soda ash
NH4+ OH- NH4OH anhydrous ammonia

Mixing the the gas with an aqueous solution of base in known concentration generates the raw liquor used for cooking. This liquor is usually fortified with gases from the previous digester run to bring it up to the desired concentration for pulping. The pulping process is typically batchwise with digesters holding somewhere in the vicinity of 30 cords of wood at a time. Staggering the cooking times provides a constant flow of pulp to the paper machines. Recovery of the cooking chemicals was typically not done in the early days of sulfite pulping as the base used was CaCO3 (calcium carbonate or limestone). Now, all sulfite mills recover chemicals due to strict environmental controls as well as the increased cost of using bases other than limestone.

Digester heatup is typically slow and maximum temperatures in the acid sulfite process are usually under 140 °C. This is to avoid undesirable lignin condensation reactions which would afford a dark pulp. Cooking times are generally in the vicinity of 6-8 hours. The length of cooking depends on the desired product, longer times results in lower yield but better overall removal of lignin. Lower yields and increased lignin removal would be desired of a pulp that is to be bleached. If the reaction goes too far, acid hydrolysis of the polysaccharides will occur causing drastic decreases in pulp strength. The relationship between polysaccharide yield and overall pulp yield is shown below. Delignification is usually taken to a known lignin content, measured by the ability of the remaining lignin to be oxidized by permanganate (MnO4-). The value obtained by titration of the pulp vs. permanganate is the Kappa number, and this value is typically directly related to the lignin content (especially at low lignin levels, less than 5% total lignin).

Sulfite pulps are typically a bit more flexible than kraft pulps but do not have the strength properties. Sulfite mills have been slowly disappearing from North America (Table III) due to the better qualities of the kraft pulp. However this trend appears to be on the reverse as lignosulfonates find new uses in things such as plywood adhesives and printing inks.

Table III. US and Canadian Sulfite Mills (1990 vs. 1980)
US US Canada Canada Total Total
Pulping Base 1980 1990 1980 1990 1980 1990
Calcium 7 5 3 0 10 5
Magnesium 9 7 3 3 12 10
Sodium 4 1 25 15 29 16
Ammonia 9 5 4 3 13 8
Totals 29 18 35 21 64 39
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