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The origin of the sulfite process is attributed to the efforts of Benjamin Chew
Tilghman, an American chemist, who was granted U.S. patent 70,485, dated
November 1867, entitled Treating Vegetable Substances for Making Paper Pulp. The
invention was based on the results of the experiments at the mills of W.W. Harding
and Sons at Manayunk, near Philadelphy, in 1866, and covers the pulping of
wood with aqueous solutions of calcium hydrogen sulfite and sulfur dioxide in
pressurized reactors. However, the first sulfite mill started its production in Europe
at Bergvik,Sweden, in 1874 under the direction of C.D. Ekman usingmagnesium
hydrogen sulfite solution, Mg(HSO3)2, as the cooking agent. The mill was equipped
with small rotating digesters heated indirectly by means of a steam jacket. In 1875,
theGerman chemist A. Mitscherlich was developing a sulfite cooking process using a
horizontal, stationary, cylindrical digester lined with brick and indirectly heated by
means of coils of lead or copper pipe. Cooking was carried out under moderate temperature
and pressure conditions. Consequently, the Mitscherlich process was characterized
by a much higher retention time as compared to the directly heated Ritter–
Kellner process, which was developed at the same time in Austria.
The sulfite process was developed around the acid calcium bisulfite process, as
mentioned in the Tilghman patent. It remained the principal process for wood
pulping because of the low costs and high availability until the beginning of the
1950s, when the need to recover the waste liquor and pulping chemicals slowly
emerged, mainly for reasons of environmental protection. Since calcium sulfite is
soluble only below pH 2.3, it can solely be used in acid bisulfite pulping in the
presence of excess SO2. At cooking temperature, the calcium hydrogen sulfite
decomposes to calcium sulfite and hydrated SO2:
Ca HSO 3 _ _2__
T CaSO 3 SO 2 _ H 2 O _155_
Thus, high charges of free SO2 and low cooking temperatures must be maintained
to prevent the precipitation of calcium sulfite. A further drawback of the use of
calcium as a cation for the acid sulfite process is the formation of calcium sulfate
during the course of the recovery process. The conversion to calcium sulfite is not
practical, since a temperature above 1200 °C is required to achieve its complete
decomposition to calcium oxide and sulfur dioxide. At this high temperature the
crystal structure of calcium oxide changes, thus reducing its reactivity. Furthermore,
calcium sulfite anhydride tends to disproportionate to calcium sulfate and
calcium sulfide. For these reasons calcium has been replaced by more soluble
bases, and is now reserved for a few pulp mills with complete by-product recovery.
Today, the dominating base used in sulfite pulping is magnesium. The corre-
392 4 Chemical Pulping Processes
sponding aqueous magnesium bisulfite solutions are soluble in a pH range up to
5–6, so that acid bisulfite and bisulfite (magnefite) pulping processes in both oneand
two-stage operations can be carried out. The big advantage of the magnesium
bisulfite process compared to the calcium base system lies in its thermochemical
behavior [1]. In contrast to the calcium system, the thermal decomposition of MgSO3
occurs at a rather low temperature, generating only a small amount of sulfide. The
magnesiumsulfate obtained from the combustion of magnesiumsulfite spent liquor
can be decomposed thermally in the presence of carbon from the dissolved organic
substances to give gaseous SO2 and magnesium oxide according to Eq. (156) [2]:
2 MgSO 4 C __ 2 SO 2 2 MgO CO 2 _156_
In order to avoid secondary oxidation of SO2 to SO3 in the absorption unit, the
flue gas must not contain free oxygen in excess of 3%, so that the surplus of air in
the combustion process must not exceed 1.5–2.0% [2].
As alternatives to calcium and magnesium, sodium and ammonium cations are
also used in sulfite pulping. Since both monovalent cations are soluble over the
entire pH range, they can be used in acid, bisulfite (magnefite), neutral and alkaline
sulfite processes. The prevailing sulfite processes are defined according to the
pH range of the resultant cooking liquor, as shown in Tab. 4.51.
Tab. 4.51 Assigament of sulfite pulping processes according to
the different pH ranges.
Nomenclature Initital pH range at 25 °C Base alternatives Acitve reagents
Acid bisulfite 1–2 Ca2+, Mg2+, Na+, NH4
+ H+, HSO3
–
Bisulfite (Magnefite) 3–5 Mg2+, Na+, NH4
+ (H+), HSO3
–
Neutral sulfite 6–9 Na+, NH4
+ HSO3
–, SO3
2–
Alkaline sulfite 10–13.5 Na+ SO3
2–, OH–
The use of monovalent cations, especially ammonium, tends to increase the
rate of delignification at given process conditions [3]. Mill experience indicates
that maximum temperature could be decreased by 5 °C when changing from calcium
to ammonium base while keeping the cooking cycle constant [4]. The reason
for the more rapid delignification in the cooks on soluble cations is not entirely
known. According to the Donnan law, it appears that acidity in the solid phase
decreases as the affinity of the cation to the solid phase increases. It is assumed
that the concentration of the lignosulfonate groups in the solid phase equals about
0.3 N, corresponding to a pH level below 1.0 in the absence of cations other than
protons [5]. The affinity for the solid phase is increasing in the order [6]:
H+ < Na+ < NH4
+ < Mg2+ < Ca2+ < Al3+
4.3 Sulfite Chemical Pulping 393
The acidity of the solid phase should therefore be lower in the presence of aluminum
ions, and highest in the presence of sodium ions. It is likely that the higher
acidity of the solid phase in the case of monovalent bases contributes to a slightly
higher extent of carbohydrate hydrolysis and somewhat greater velocity of delignification.
Although the differences in rate and selectivity of delignification are not
significant, mill application has revealed several advantages, such as higher pulp
yield, viscosity and alphacellulose content at a given kappa number and a lower
amount of rejects [7,8]. These advantages can be attributed to better penetration
with cooking chemicals and a more uniform cook when changing from calcium
to magnesium, ammonium, or sodium base. The brightness of the unbleached
pulps is, however, clearly impaired in the case of ammonium-based pulps. There,
the lower brightness is probably due to a selective reaction between the ammonium
ion and carbonyl groups of lignin. This reaction is also responsible for a
much darker color of the ammonium-based spent liquors. However, no differences
in the bleachability of ammonium-based pulps in comparison to other sulfite
pulps can be observed.
Despite some clear advantages of the monovalent over the bivalent bases with
respect to flexibility (entire pH range available) and pulping operations (more
homogeneous impregnation, higher rate of delignification), their use in sulfite
cooking processes has been limited to a few applications, mainly due to deficiencies
in recovery of the cooking chemicals. For ammonium sulfite waste liquor no
economically feasible solution exists to recover the base. Ammonia recovery processes
based on ion exchange have been developed to the mill level, but have not
gained acceptance in praxis because of high costs. The use of ammonium base in
particular has been shown to be advantageous for the production of highly reactive
dissolving pulp where mill scale operations still exist. Sodium base is predominantly
used in neutral and alkaline sulfite processes. The recovery of sodiumbased
sulfite processes combines the use of a kraft-type furnace and the conversion
of the resulting sodium sulfide to sodium sulfite using carbonation processes
(e.g., liberation of hydrogen sulfide from the smelt by the addition of CO2 from
the flue gas, oxidizing hydrogen sulfide to SO2, reaction of SO2 with sodium carbonate
to give sodium sulfite). The technology employing carbonation of green
liquor was developed in the 1950s and 1960s, but since then no decisive improvements
of this recovery concept have been made. Thus, the recovery of the sodiumbased
sulfite cooking chemicals is significantly less efficient than the sodiumbased
kraft process, and this may be the main reason for the comparatively limited
application of the sodium-based sulfite processes.
During the first 50 years of chemical pulp production, the sulfite process was
the dominating technology, due mainly to the high initial brightness and the easy
bleachability of the sulfite pulps. With the developments of both a reductive recovery
boiler for the regeneration of kraft spent liquor by Tomlinson and chlorine
dioxide as a bleaching agent to ensure selective bleaching to full brightness in the
mid-1940s, the kraft pulping technology became the preferred method because of
better energy economy, better paper strength properties, and lower sensitivity
towards different wood species and wood quality. In the meantime, efficient
394 4 Chemical Pulping Processes
chemical recovery systems have been developed especially to use magnesium as a
base. The high sensitivity to the wood raw material still constitutes a problem in
the case of acid sulfite pulping. Most softwoods except spruce, such as pines,
larches and Douglas fir, are considered less suitable for sulfite pulping. A certain
part of the extractives of phenolic character such as pinosylvin, taxifolin (Douglas
fir) as well as the tannins of bark-damaged spruce and oaks give rise to condensation
reactions with reactive lignin moieties in the presence of acid sulfite cooking
solutions.
Since the 1960s the basic and applied research has been directed almost exclusively
towards alkaline pulping technologies, with kraft pulping as the key technology,
due to the higher overall economic potential. Consequently, the kraft process
has become increasingly important and is now the principal pulping process,
accounting for far more than 90% of world pulp production. For the production of
most paper-grade pulps, the strength properties are of utmost importance. Kraft
pulps show clearly better strength properties, especially with regard to the tear
strength as compared to sulfite pulps. Consequently, new installations for the production
of paper-grade pulps are almost exclusively based on kraft pulping technology.
Unlike paper-grade pulping, the acid sulfite process is the dominant technology
for the production of dissolving pulps and accounts for approximately 70%
of the total world production. Although a clear niche product, the dissolving pulp
production is a firmly established pulp market with a predicted annual growth
rate of about 5% within the next five years. The strong position of sulfite technology
in dissolving pulp production of low-purity grades sufficient for regenerated
fiber manufacture is based on a favorable economy, because of higher pulp yield,
better bleachability and higher reactivity as compared to a corresponding prehydrolysis-
kraft pulp. Therefore, the following presentation of acid sulfite pulping
technology is predominantly oriented toward dissolving pulp production.
4.3.2
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