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The sulfite process is characterized by its high flexibility. In principle, the entire pH range can be used for pulping by changing the dosage and composition of chemicals. In comparison, kraft pulping can be carried out only with highly alkaline cooking liquor. Thus, the use of sulfite pulping permits the production of many different types and qualities of pulps for a broad range of applications.
The sulfite process can be subdivided into the following types of pulping:
a) acid bislufite pulping
b) bisulfite pulping
c) neural sulfite pulping
d) alkaline sulfite pulping
e) multistage sulfite pulping, and
f) anthraquinone-catalyzed sulfite pulping
All2 these processes are based on the use of aqueous sulfur dioxide and a so-called base, i.e., calcium, magnesium, sodium, or ammonium. In the pulping system, pH-dependent equilibrium reactions occur that depend on the ratio of SO2 to base. If sodium hydroxide is added to the aqueous sulfur dioxide, first bisulfite and then sulfite are formed:
H2O + SO2 "_ SO2 "ª H2O
SO2 "ª H2O + NaOH "_ NaHSO3 + H2O
NaHSO3 + NaOH "_ Na2SO3 + H2O
2.1. Reaction in Sulfite Pulping
In sulfite pulbing , the reactivity of lignin is essentially determined by the pH of the pulping liquor. Fundamental differences exist in the mechanism, rate, and extent of delignification under acidic, neutral, and alkaline conditions.
2.2. The Various Sulfite Processes
Acid Bisulfite Process
In the acid bisulfite process, pulping is performed with a large excess of free SO2 at pH 1.2-1.8. Al of the above-mentioned bases can be employed. No basic differences in their mode of action exist, but the pulping conditions considered to be optimal vary slightly. The course and the result of pulping are influenced by the base selected. For instance, magnesium bisulfite pulping results in the best yields and the highest pulpbrightness. Use of the monovalent bases Na+ and NH+4 results in stronger pulps and a lower shive content. If ammonium is used as base, the pulping process is very fast, but the pulps are dark in color and cannot be uses in the unbleached state. Pulps of the lowest strength are obtained with calcium as base, and this pulping process must be controlled more carefully because calcium precipitates readily.
The total SO2 used in acid bisulfite pulping ist in the range 18 ¡V 25 %, depending on the typoe of wood used, degree of cooking desired etc., and the SO2 to base ratio is adjusted to 4.5-6.0. During the cooking phase, the liquor to wood ration is usually 3-4:1. The cips are first impregnated with a higher liquor ratio to achieve a homogeneous distribution of the pulping liquor. A part of the pulping liquor (side relief) and the SO2gas (top relief) ist hen removed after impregnation and returned to the accumulator to prepare fresh cooking liquor. Pulping temperatures of 125-140¢XC are used for the production of paper pulps. Unlike bisulfite pulping, the acids bisulfite process is also well suited to the production of dissolving pulps because the hemicelluloses can be dissolved to a lare extent in the highly acidic pulping process. However, higher maximum tmperatures of 145-150¢XC are employed for this process. The cooking time is usually 2-5h at this pulping temperature, but con be extended to more than 7h at lower pulping temperatures. A long heating time is also necessary because the SO2 penetrates into the wood much faster than the base due to the high vapor presure of the excess free SO2. Rapid heating can result in lignin condensations due to the high acidity within the chip centers. Since digester relief must also be effected slowly so that the excess SO2 can be reused in the subsequent digestion, the result is a longer cover-to-cover cycle of 8-12 h.
From is first industrial application in 1874 until the end of World War II, the calcium bisulfite process was the principal process for wood pulping used worldwide because of the low consts and good availability of limestone and sulfur dioxide as basic materials for the production of the cooking acid. Today, calcium bisulfite pulping, ammonium bisulfite pulping, and other sulfite pulping, and other sulfite pulping processes based on ammonium are of very little importance.
Bisulfite Pulping
The pulping liquor used in the bisulfite process consists of approximately equal parts of free and combined SO2, i.e., it contains no excess free SO2. The pH value is between 3.5 and 5.5, depending on the temperature and the chemical concentration. The use of calcium as base in this pH range ist not possible. Magnesium is usually employed as base (magnefite process) because the recovery of chemicals is easy and inexpensive. Ammonium bisulfite and sodium bisulfite pulping habe also found industrial applications in individual cases.
Bisulfite pulping is normally carried out with 4-5% total SO2 in the cooking liquor and a liquor to wood ratio of 4:1, i.e., the total SO2used, based on wood, is 16-20%. The pulping temperature is ca. 155¢XC for hardwoods and 165¢XC for softwoods. The higher temperatures required, compared withe the acid bisulfite process, compensate for the lower rate of delignification causes by the lower acidity of the pulping liquor. However, carbohydrates are also dissolved to a smaller extent during bisulfite pulping. Hence, these pulps have a high hemicellulose content, and yields can reach 60 %. In comparison with acid bisulfite pulping, shorter cooking times of 1-3 h are required because of the higher cooking temperatures.
On the whole, the cover-to-cover cycle is much shorter (approx. 6-8h), which increases the production capacity.
A serious disadvantage of the bisulfite process is that the extent of delignification attainable is lower than that obtained under highly acidic pulping conditions, since otherwise the losses of yield and strength that occur are excessively high. The higher residual lignin content of the pulp requires the use of more bleaching chemicals and hence produces a greater effulent charge.
Neutral Sulfite Process
The NSSC (neutral sulfite semichemical) process is not suited for making chemical pulp because the degree of delignification attainable is too low. This process uses sodium as base and is widely applied in the production of semichemical pulps, especially in North America. These pulps are employed mainly for the middle layers of corrugated cardboard and in newsprint. Neutral sulfite pulp mills are sometimes uses in combination with kraft mills, which allows the combined recovery of chemicals (cross recovery). The NSSC waste liquor can be used as make-up chemical to compensate for losses of chemicals in krft process.
The neutral sulfite process is a two-stage process with a chemical delignification step followed by mechanical defibration. In the chemical treatment, neutral to slightly alkaline sulfite solutions are used to dissolve hemicelluloses and lignins and, thus, weaken the bonds between the fibers. In this way, the mechanical effort required to expose fiber bundles and individual fibers is reduced considerably.
In the neutral sulfite pulping of hardwood, between 8 an d18 % of sodium sulfite or ammonium sulfite ist used, bases an o.d. wood. To maintain the pH in the range 7-9, the cooking liquor must be buffered by the addition of 2.5-5% of alkali (NaHCO3, Na2CO3 or NaOH) because acids are released during digestion, particularly acetic acid by the cleavage of acetyl groups form hemicelluloses. The digestion time is 30-180 min at the maximum temperature (ca. 170¢X C).
Alkaline Sulfite Pulping
Alkaline sulfite pulping ist carried out with liqors containing sodium sulfite and sodium hydroxide (sodium carbonate) at initial pH values of 9-13. When, as in NSSC pulping, a low chemical chare, low alkalinity, and low temperature are applied, semichemical pulps can be prepared, especially from annual plants. The boundaries between the two processes are fluid. If the pulping liquor consists of approximately equal amounts of sodim sulfite and NaOH, and if strongly alkaline conditions (pH>13) are chosen, chemical pulps can be produced as well. This pulping exhibits characteristics typical of both the sulfite process (no odors due to formation of reduced sulfur compounds; good bleachability pf pulps) and the kraft process (no raw material restrictions, high pulp strengths, low yields, and low brightness). However, the higher chemicals requirement, longer pulping times, higher residual lignin content, and the more complicated chemicals recovers, compared to the kraft process, prevented establishment of this process in the industrial practice.
2.3. Multistage Sulfite Processes
The various modifications that can be achieved in the sulfite pulping system by adjustment of the pH can be advantageously combined by changing the pH during pulping. In this way, it is possible to produce special pulps that are ideally suited to certain fields of application. Sodium, as a soluble base, is particularly suitable for multistage digestions of this tpye. Magnesium can be used as bas within limits. In the two-stage systems, pulpings is either carried out first at a higher pH and the pH is subsequently decreased or vice versa. The Stora process has achieved commercial importance. In this process, a sodium bisulfite-sulfite solution at pH 6-8 is used in the impregnation and first cooking stages. Subsequently, excess pulpint liquor is removed and the pH is adusted to ca. 1.5 by introducing SO2. This is followed by normal acid bisulfite pulping as the main delignification step. The reactive centers of lignin are sulfonated practically completely in the first stage, and thus condensation reactions with phenolic extractives under strongly acidic conditions are blockes, so that even pine heartwood, for instance, can be digested satisfactorily by this method. Furthermore, glucomannan stabilization takes place in the bisulfite-sulfite step such that the acetyl groups are cleaved first, allowing the glucomannans to attach themselves firmly to the cellulose fibrils by increased formation of hydrogen bonds. The restistance to acid hydrolysis is thereby increased.
A corresponding two-stage process based on magnesium is calles to HO process (high opacity). Impegnation with a bisulfite solution (pH 3,8-4) ist carried out in the first stage. The pH is then adjusted to 6,5-7 by the injecition of magnesium oxide milk and cooking is completed at a high pulping tmeperature (170¢XC)- Since disssolved oranic acids apparently stabilize the pulping liquor, no precipitation occurs in spite of the high pH.
2.4. Antraquinone-Catalyzed Sulfite Pulping
An exceptional feature of sulfite pulping system ist that the catalytic acitvity of anthraquinone is also exerted at slightly alkaline to neutral pH values, whereas effecitve acceleration of delignification in soda puloing requires a pH above 13. This explains how sulfite pulping in a moderately alkaline pH range, which would otherwise permit only inadequate delignification, can be used for the production of chemical pulps if small amounts of AQ (0.1-0,2%) are added.
The pulping liquor used in this process consists predominantly of sodium carbonate is added as additional alkali.
This pulping procedure is faster and more selective than alkaline sulfite pulping with approximately equal sodium sulfite and sodium hydroxide charges and without AQ. Better delignification is achieved in spite of the lower chemicals demand and shortened cooking times.
AQ-catalyzed sulfite upling combines the advantages of the sulfite and kraft processes. First, there is no raw material restriction and high pulp strengths are achieved. Second, high yields are obtained, the pulps are easily bleachable and the emission of malodorous sulfur compounds is avoided. The main disadvantage of this process is the relatively high residual lignin content in the pulps, which have a high chemicals demand for bleaching and require the use of chlorine-containing bleaching agents.
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