Everything about The Sulfite Process totally explained
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 (SO
32−), or
bisulfites (HSO
3−), depending on the
pH. The counter
ion can be
sodium (Na
+),
calcium (Ca
2+),
potassium (K
+),
magnesium (Mg
2+) or
ammonium (NH
4+).
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.
Lignosulfonates are an important byproduct of sulfite bleaching. These materials are used in making
concrete,
drilling mud,
drywall and so on.
History
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 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.
Current status
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"
Processes involved in sulfite pulping
Pulping liquor preparation
The pulping liquor for most sulfite mills is made by burning
sulfur with the correct amount of
oxygen to give
sulfur dioxide, which is then absorbed into water to give
sulfurous acid.
» S + O
2 → SO
2
SO
2 + H
2O ⇌ H
2SO
3
Care must be taken to avoid the formation of
sulfur trioxide since it gives undesired
sulfuric acid when it's dissolved in water.
» 2 SO
2 + O
2 → 2SO
3
SO
3 + H
2O ⇌ H
2SO
4
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 NH
4+) hydroxides, MOH:
» H
2SO
3 + MOH → MHSO
3 + H
2O
MHSO
3 + MOH → M
2SO
3 + H
2O
For divalent (Ca
2+, Mg
2+) carbonates, MCO
3:
» MCO
3 + 2H
2SO
3 → M(HSO
3)
2 + CO
2 + H
2O
M(HSO
3)
2 + MCO
3 → 2 MSO
3 + CO
2 + H
2O
Pulping
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 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 of
magnesium hydroxide.
» MgO + H
2O → 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's clarified, filtered and used as the pulping liquor.
» Mg(OH)
2 + 2 SO
2 → Mg(HSO
3)
2
Byproducts
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.
Acid hydrolysis of cellulose during sulfite pulping produces
monosaccharides, predominanently
mannose,
which can be fermented to produce
ethanol.
Further Information
Get more info on 'Sulfite Process'.
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