The sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites (SO32−), or bisulfites (HSO3−), depending on the pH. The counter ion can be sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+) or ammonium (NH4+).
The first pulp mill using the sulfite process was built in Sweden in 1874 and used magnesium as the counter ion. Calcium became the standard counter ion until the 1950s. Sulfite pulping was the dominant process for making wood pulp intil it was surpassed by the kraft process in the 1940s. Sulfite pulps now account for less than 10% of the total chemical pulp production.
The sulfite process is acidic and one of the drawbacks is that the acidic conditions hydrolyze some of the cellulose, which means that sulfite pulp fibers are not as strong as kraft pulp fibers. The yield of pulp (based on wood used) is higher than for kraft pulping and sulfite pulp is easier to bleach. Apart from printing and specialty papers, a special grade of sulfite pulp, known as "dissolving pulp" is used to make cellulose derivatives. Lignosulfonates are an important byproduct of sulfite bleaching. These materials are used in making concrete, drilling mud, drywall and so on.
The use of wood to make pulp for paper began with the development of mechanical pulping in Germany by F.G. Keller in the 1840s. Chemical processes quickly followed, first with J. Roth's use of sulfurous acid to treat wood, followed by B. Tilghman's US patent on the use of calcium bisulfite, Ca(HSO3)2, to pulp wood in 1867. Almost a decade later the first commercial sulfite pulp mill was built in Sweden. It used magnesium as the counter ion and was based on work by Carl Daniel Ekman. By 1900 sulfite pulping had become the dominant means of producing wood pulp, surpassing mechanical pulping methods. The competing chemical pulping process, the sulfate or kraft process was developed by Carl F. Dahl in 1879 and the first kraft mill started (in Sweden) in 1890. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s  allowed kraft mills to recycle almost all of their pulping chemicals. This, along with the ability of the kraft process to accept a wider variety of types of wood and produce stronger fibers  made the kraft process the dominant pulping process starting in the 1940s. Sulfite pulps now account for less than 10% of the total chemical pulp production and the number of sulfite mills continues to decrease.
Sulfite pulp remains an important commodity, especially for specialty papers and as a source of cellulose for non-paper applications. Sulfite pulp is used to make fine paper, tissue, glassine. and to add strength to newsprint. A special grade of bleached sulfite pulp is known as "dissolving pulp" which is the raw material for a wide variety of cellulose derivatives, for example rayon, cellophane, cellulose acetate and methylcellulose. Rayon is a reconstituted cellulose fiber used to make many fabrics. Cellophane is a clear reconstituted cellulose film used in wrapping and windows in envelopes. Cellulose acetate was used to make flexible films for photographic use, computer tapes and so on and also to make fibers. Methylcellulose and other cellulose ether derivatives are used in a wide range of everyday products from adhesives to baked goods to pharmaceuticals.
Processes involved in sulfite pulping
Pulping liquor preparation
- S + O2 → SO2
- SO2 + H2O ⇌ H2SO3
- 2 SO2 + O2 → 2SO3
- SO3 + H2O ⇌ H2SO4
Sulfuric acid is undesirable since it promotes hydrolysis of cellulose without contributing to delignification.
The cooking liquor is prepared by adding the counter ions as hydroxides or carbonates. The relative amounts of each species present in the liquid depned largely on the relative amounts of sulfurous used. For monovalent (Na+, K+ and NH4+) hydroxides, MOH:
- H2SO3 + MOH → MHSO3 + H2O
- MHSO3 + MOH → M2SO3 + H2O
For divalent (Ca2+, Mg2+) carbonates, MCO3:
- MCO3 + 2H2SO3 → M(HSO3)2 + CO2 + H2O
- M(HSO3)2 + MCO3 → 2 MSO3 + CO2 + H2O
Sulfite pulping is carried out between ph 1.5 and 5, depending on the counterion to sulfite (bisulfite) and the ratio of base to sulfurous acid. The pulp is in contact with the pulping chemicals for 4 to 14 hours and at temperatures ranging from 130 to 160 °C (266 to 320 °F) , again depending on the chemicals used.
Most of the intermediates involved in delignification in sulfite pulping are resonance-stabilized carbocations formed either by protonation of carbon-carbon double bonds or acidic cleavage of ether bonds which connect many of the constituents of lignin. It is the latter reaction which is responsible for most lignin degradation in the sulfite process. The electrophilic carbocations react with bisulfite ions (HSO3-)to give sulfonates.
- R-O-R' + H+ → R+ + R'OH
- R+ + HSO3- → R-SO3H
The spent cooking liquor from sulfite pulping is called brown or red liquor (compared to black liquor in the kraft process). Pulp washers, using countercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicelulose. The extracted brown liquor is concentrated, in multiple effect evaporators. The concentrated brown liquor can be burned in the recovery boiler to generate steam and recover the inorganic chemicals for reuse in the pulping process or it can be neutralized to recover the useful byproducts of pulping.
Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized to nitrogen and nitrogen oxides when burned. The earliest process used calcium, obtained as inexpensive calcium carbonate and there was little incentive to recover the inorganic materials. Sodium-based processes use a recovery system similar to that used in the kraft recovery process, except that there is no "lime cycle".
The recovery process used in magnesium-based sulfite pulping the "Magnefite" process is well developed. The concentrated brown liquor is burned in a recovery boiler, producing magnesium oxide and sulfur dioxide, both of which are recovered from the flue gases. Magnesium oxide is recovered in a wet scrubber to give a slurry of magnesium hydroxide.
- MgO + H2O → Mg(OH)2
This magnesium hydroxide slurry is then used in another scrubber to absorb sulfur dioxide from the flue gases producing a magnesium bisulfite solution that is clarified, filtered and used as the pulping liquor.
- Mg(OH)2 + 2 SO2 → Mg(HSO3)2
Sulfite pulping is generally less destructive than kraft pulping, so there are more usable byproducts. Chief among these are lignosulfonates, which find a wide variety of uses whereas relatively inexpensive agent is needed to make a water dispersion of a water-insoluble material. Lignosulfonates are used in tanning leather, making concrete, drilling mud, drywall and so on.
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