Thesulfite process produceswood pulp that is almost purecellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, includingsodium (Na+),calcium (Ca2+),potassium (K+),magnesium (Mg2+), andammonium (NH4+). The lignin is converted tolignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with theKraft process which produces stronger fibers and is less environmentally costly.
The use of wood to make pulp for paper began with the development ofmechanical pulping in the 1840s byCharles Fenerty inNova Scotia[1] and byF. G. Keller[2] inGermany. Chemical processes quickly followed, first withJulius Roth's use ofsulfurous acid to treat wood in 1857, followed byBenjamin Chew Tilghman'sUS patent on the use ofcalcium bisulfite, Ca(HSO3)2, to pulp wood in 1867.[3] Almost a decade later in 1874 the first commercial sulfite pulp mill was built inSweden. It used magnesium as the counter ion and was based on work byCarl 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 orkraft process was developed byCarl F. Dahl in 1879 and the firstkraft mill started (in Sweden) in 1890.[3] The first sulphite mill in the United States was the Richmond Paper Company inRumford, Rhode Island in the mid-1880s. The invention of therecovery boiler byG. H. Tomlinson in the early 1930s[2] 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[4] made the kraft process the dominant pulping process starting in the 1940s.[3] Sulfite pulps now account for less than 10% of the totalchemical pulp production[3] and the number of sulfite mills continues to decrease.[5][6][7]
Magnesium was the standardcounter ion until calcium replaced it in the 1950s.
The pulping liquor for most sulfite mills is generated by treating various bases (alkali metal or alkaline earth hydroxides) with sulfur dioxide:
Similar reactions are effected with divalent cations (Mg2+, Ca2+) and using carbonates in place of hydroxide.
The ratio of sulfite to bisulfite depends on pH; above pH=7, sulfite predominates.
The earliest process used calcium, obtained as inexpensivecalcium carbonate, and there was little incentive to recover the inorganic materials. At least in Sweden the brown liquor from this process was previously frequently used for producing ethanol, while with other brown liquors the fermentable hexose sugars are left to contribute to the energy needed in the recovery process. Calcium sulfite, which is poorly soluble, converts to calcium bisulfite only at low pH. Therefore calcium-based sulfite processes require acidic conditions.
Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized tonitrogen andnitrogen oxides when burned.
The recovery process used in magnesium-based sulfite pulping the "Magnefite" process is well developed.[8] The concentrated brown liquor is burned in a recovery boiler, producingmagnesium oxide andsulfur dioxide, both of which are recovered from the flue gases. Magnesium oxide is recovered in a wetscrubber to give aslurry ofmagnesium hydroxide.
This magnesium hydroxide slurry is then used in another scrubber to absorbsulfur dioxide from theflue gases producing a magnesium bisulfite solution that is clarified, filtered and used as the pulping liquor.
Sodium-based processes use a recovery system similar to that used in thekraft recovery process, except that there is no "lime cycle".
The process is conducted in large pressure vessels called digesters. Sulfite pulping is carried out between pH 1.5 and 5. 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 areresonance-stabilizedcarbocations 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.[2] Theelectrophilic carbocations react with bisulfite ions (HSO3−)to give sulfonates.
The sulfite process does not degradelignin to the same extent that thekraft process does and thelignosulfonates from the sulfite process are usefulbyproducts.
The spent cooking liquor from sulfite pulping is usually called brown liquor, but the terms red liquor, thick liquor and sulfite liquor are also used (compared toblack liquor in thekraft process). Pulp washers, usingcountercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicellulose. The extracted brown liquor is concentrated, inmultiple effect evaporators. The concentrated brown liquor can be burned in therecovery 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. Recent developments inChemrec'sblack liquor gasification process, adapting the technology to use in the sulfite pulping process, could makesecond generation biofuels production an alternative to the conventional recovery boiler technology.[9] Around 1906 Gösta Ekström a Swedish engineer patented a process of ethanol generation from the residual 2-2.5% fermentablehexose sugars in the spent liquor.[10]
The sulfite process can usecalcium,ammonium,magnesium orsodium as a base.
The sulfite process isacidic and one of the drawbacks is that the acidic conditionshydrolyze some of the cellulose, which means that sulfite pulp fibers are not as strong as kraft pulp fibers. Theyield of pulp (based on wood used) is higher than for kraft pulping and sulfite pulp is easier tobleach.
Sulfite pulp remains an importantcommodity, especially for specialty papers and as a source of cellulose for non-paper applications. It is used to makefine paper,tissue,glassine,[11] and to add strength tonewsprint.
A special grade of bleached sulfite pulp is known asdissolving pulp[12] which is the raw material for a wide variety of cellulose derivatives, for examplerayon,cellophane,cellulose acetate andmethylcellulose.
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 fromadhesives tobaked goods topharmaceuticals.[13]
Sulfite pulping is generally less destructive than kraft pulping, so there are more usable byproducts.
Chief among sulfite process byproducts arelignosulfonates, which find a wide variety of uses where a relatively inexpensive agent is needed to make a water dispersion of a water-insoluble material. Lignosulfonates are used intanning leather, makingconcrete,drilling mud,drywall and so on.[14]
Oxidation of lignosulfonates was used to producevanillin (artificial vanilla), and this process is still used by one supplier (Borregaard, Norway) while all North American production by this route ceased in the 1990s.[15]
Acid hydrolysis of hemicelluloses during sulfite pulping producesmonosaccharides, predominantlymannose for softwoods andxylose for hardwoods,[2] which can be fermented to produceethanol.
