Sulfite process

Source: Wikipedia, the free encyclopedia.

The sulfite process produces

lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process
which produces stronger fibers and is less environmentally costly.

idealized scheme for lignin depolymerization by the Sulfite process.

History

The use of wood to make pulp for paper began with the development of

mechanical pulping in the 1840s by Charles Fenerty in Nova Scotia[1] and by F. G. Keller[2] in Germany. Chemical processes quickly followed, first with Julius Roth's use of sulfurous acid to treat wood in 1857, followed by Benjamin Chew Tilghman's US patent on the use of calcium bisulfite, Ca(HSO3)2, to pulp wood in 1867.[3] Almost a decade later in 1874 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

chemical pulp production[3] and the number of sulfite mills continues to decrease.[5][6][7]

Magnesium was the standard

counter ion
until calcium replaced it in the 1950s.

Pulping liquor preparation

The pulping liquor for most sulfite mills is generated by treating various bases (alkali metal or alkaline earth hydroxides) with sulfur dioxide:

SO2 + MOH → MHSO3
MHSO3 + MOH → M2SO3 + H2O

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.

Calcium-based

The earliest process used calcium, obtained as inexpensive calcium 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

Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized to

nitrogen oxides
when burned.

Magnesium-based

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, 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

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
Sodium-based

Sodium-based processes use a recovery system similar to that used in the kraft recovery process, except that there is no "lime cycle".

Processes involved in sulfite pulping

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 are

electrophilic
carbocations react with bisulfite ions (HSO3−)to give sulfonates.

R-O-R' + H+ → R+ + R'OH
R+ + HSO3− → R-SO3H

The sulfite process does not degrade

byproducts
.

Chemical recovery

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 to

second 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% fermentable hexose sugars in the spent liquor.[10]

The sulfite process can use calcium, ammonium, magnesium or sodium as a base.

Applications

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
.

Commodity

Sulfite pulp remains an important

fine paper, tissue, glassine,[11] and to add strength to newsprint
.

Dissolving pulp

Main article: Dissolving pulp

A special grade of bleached sulfite pulp is known as dissolving pulp

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

baked goods to pharmaceuticals.[13]

Byproducts

Sulfite pulping is generally less destructive than kraft pulping, so there are more usable byproducts.

Lignosulfonates

Chief among sulfite process byproducts are

drilling mud, drywall and so on.[14]

Oxidation of lignosulfonates was used to produce vanillin (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]

Other byproducts

Acid hydrolysis of hemicelluloses during sulfite pulping produces

monosaccharides, predominantly mannose for softwoods and xylose for hardwoods,[2] which can be fermented to produce ethanol
.

See also

References

  1. ISBN 978-0-9783318-1-8
    pp.25–30
  2. ^
    ISBN 0-12-647480-X
    .
  3. ^
    ISBN 0-12-097360-X
    .
  4. ^ "History of Paper". Archived from the original on 2006-12-08. Retrieved 2007-10-08.
  5. ^ "Swedish, German mills phase out sulfite". Pulp and Paper. January 1997. Retrieved 2007-10-08.
  6. ^ "Wisconsin sulfite mill shuts down 2005". Retrieved 2007-10-07.
  7. ^ Friederich, Steven (September 25, 2006). "Living on borrowed time its whole life (Weyerhauser sulfite mill)". The Daily World. Retrieved 2007-10-08. [dead link]
  8. ^ "Magnefite process". Archived from the original on 2007-12-17. Retrieved 2007-10-11.
  9. ^ Chemrec web site
  10. ISBN 951-9479-25-2
    .
  11. ^ "Grades and uses of paper". Archived from the original on 2012-09-19. Retrieved 2007-10-12.
  12. ^ "Dissolving pulp by the sulfite process". Retrieved 2007-10-12.
  13. ^ "Applications for Methocel cellulose ethers from Dow Chemical". Archived from the original on 2008-12-24. Retrieved 2007-10-12.
  14. ^ "Uses of lignosulfonates". Archived from the original on 2007-10-09. Retrieved 2007-10-07.
  15. doi:10.1021/ed074p1055
    . Retrieved 2006-09-09.
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