Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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USE OF SODIUM DITHIONITE IN A CELLULOSE PULPING PROCESS
Description
The present invention relates to a method of producing cellulose from
lignocellulosic material by
sulfite digestion or sulfate digestion, as defined in the claims.
Methods of obtaining cellulose from lignocellulosic material, such as wood,
are known and
described for example in Ullmanns Enzyklopadie der technischen Chemie, 4th
edition, volume
17, "Paper, fibrous raw materials", pp. 531-576, Verlag Chemie Weinheim, New
York (1979).
Typically, cellulose is obtained from the lignocellulosic material, for
example wood, by chemical
processes of destructurization. Examples of such chemical methods of
destructurization are
sulfite digestion as used for these purposes and the similarly familiar
process of sulfate
digestion. Sulfite digestion and sulfate digestion are described in the above-
cited Ullmann
reference for example.
Put simply, lignocellulosic material is treated in the two abovementioned
processes as follows to
obtain cellulose.
In the sulfite process (hereinafter also called "sulfite digestion"),
lignocellulosic material, typically
wood, is treated with a cooking liquor in an acidic or neutral medium in the
presence of sulfites
(salts of sulfurous acid H2S03), whereby the lignin is typically sulfonated
and water-solubilized
and thus can be removed from the fibers to leave behind the cellulose.
There are various types of the sulfite process in existence, which differ
inter alia in the pH of
their cooking liquor. Examples are:
a) the acidic bisulfite process with magnesium dihydrogensulfite (hereafter
also
"Mg(HS03)2") and sulfur dioxide, SO2, as well as water as a component of the
cooking liquor,
b) the bisulfite process with Mg(HS03)2 as a component of the cooking liquor,
c) the neutral-sulfite process with disodium sulfite (hereinafter also
"Na2S03") and
sodium carbonate (hereinafter also "Na2CO3"), as components of the cooking
liquor, and
d) the alkali-sulfite process with Na2S03 and sodium hydroxide (hereinafter
also
"NaOH") as well as water as components of the cooking liquor.
The acidic bisulfite process generally utilizes magnesium in the form of
magnesium oxide (MgO)
as a base, which is then converted to the dihydrogensulfite. Instead of
magnesium (Mg), the
acidic bisulfite process can also utilize calcium (Ca), sodium (Na) or
ammonium (NH4) as a
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base for the cooking liquor, which is then used similarly to magnesium in the
form of the
corresponding oxides/hydroxides. These metals except for calcium can typically
also be used in
similar fashion in the bisulfite process.
Of the sulfite processes, the acidic magnesium bisulfite process is currently
the most frequently
used.
Softwoods such as sprucewood, also firwood and the wood of the hemlock fir
generally come
into consideration as lignocellulose material for the sulfite process. Some
hardwood species
such as beech, poplar and birch are also suitable. Sprucewood is preferred for
the sulfite
process.
The abovementioned various types of the sulfite process typically each operate
at pressures
ranging from 0.1 to 10 bar and generally at certain pH ranges. The typical pH
is in the range
from 2 to 3 for the acidic bisulfite process a), in the range from 3 to 5 for
the bisulfite process b),
in the range from 6 to 9 for the neutral sulfite process c) and around 11 for
the alkali-sulfite
process d).
The digestion temperatures in the sulfite process differ in line with the pH
range. Thus, the
temperature range is generally from 120 C to 150 C for the acidic bisulfite
process a), from
150 C to 160 C for the bisulfite process b) and in the range from 160 C to 180
C for both the
sodium sulfite process c) and the alkali-sulfite process d).
The cooking liquor in the sulfite process typically comprises so-called free
sulfur dioxide (SO2),
which is present as SO2 and sulfurous acid (hereinafter also "H2S03") and
bound SO2, which is
bound to a cation (base). Free SO2 and bound SO2 are generally reported as
total SO2. The
cooking liquor in the sulfite process generally has the following composition:
M2S03 + H2S03 + SO2 + H2O,
of which the H2S03 and the SO2 are assigned to the free SO2 and the M2S03 to
the bound SO2.
M here is the respective so-called base, for example magnesium.
The proportion of base and of SO2 in the cooking liquor is reported in weight
percent. For
instance, a cooking liquor having a total SO2 content of 80 g per liter
comprises 8% of total SO2.
The base fraction is reported in the particularly corresponding oxide form for
the base, such as
MgO, CaO, Na20.
A cooking liquor is typically prepared via an absorption of SO2 on water and
base vehicles. The
equation hereinbelow shall serve as an example of the principle of cooking-
liquor production in
the bisulfite process using magnesium as base (magnesium bisulfite).
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Mg(OH)2 + 2 SO2 Mg(HS03)2
In the sulfate process (also known as "kraft digestion or "sulfate digestion"
among those skilled
in the art), cellulose is typically obtained from lignocellulosic material for
example the wood of
trees or else from annual plants, for example reed, grain (straw), sugarcane
(bagasse), corn.
Typically, in the sulfate process, chips of the lignocellulosic material, for
example wood or
comminuted stems of plants, are heated in pressure vessels for several hours,
for example 3 to
6 hours, at elevated pressure, for example in the range from 7 to 10 bar,
typically in a mixture of
aqueous sodium hydroxide solution (aqueous NaOH), sodium sulfide (Na2S) and
sodium sulfate
(hereinafter also "Na2SO4") and optionally sodium carbonate (hereinafter also
"Na2CO3").
This produces the so-called "black liquor" (soluble alkali metal lignin),
which is separated from
the cellulose by filtration.
Using the sulfite and sulfate processes mentioned, cellulose can be separated
from lignin, but it
continues to be desirable to increase the pulp yield and to achieve this more
particularly with a
simultaneously low lignin content for the pulp.
It is known, for example for the destructurization of wood into pulp, that
there is a relationship
between the so-called "degree of destructurization", as expressed by the
"kappa number" for
example, and the pulp yield.
The kappa number is a measure of the lignin content of the pulp.
A very low kappa number, i.e., a very low lignin content of the pulp,
typically correlates with a
low pulp yield. This is because, typically, it is not just more and more
lignin which is remote with
increasing destructurization, but increasingly also pulp (components)
(predominantly
hemicelluloses) being dissolved out of the wood into the cooking liquor. The
result is a lower
quantity of isolated cellulose relative to the wood used.
One disadvantage of pulp digestion by the sulfate process is the formation of
malodorants such
as mercaptans, especially methyl mercaptan.
The addition of sodium dithionite (hereinafter also "Na2S204") in the
pulpmaking operation is
known in principle from the following references.
G. Jayme and G. Warner describe an alkaline sulfite pulping process for
sprucewood at 170 C,
24 hours, wherein 100 cm3 of the pulping liquor comprised 3 g of NaOH, 1.56 g
of sodium
dithionite (Na2S204) and 4.69 g of Na2S03(G. Jayme, G. Warner, Papier, volume
6, No. 11,
pp. 220-222 (1952)). Relatively large quantities of sodium dithionite, based
on the wood to be
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treated, are described indirectly therein via the quantitative schedule of
chemicals as well as the
"liquor ratio" (page 221, left-hand column, numerical table and the subsequent
paragraph).
Jayme and Warner further describe in Holz als Roh- und Werkstoff10 (1952) 6,
pp. 244-249,
the use of relatively large amounts of sodium dithionite (Na2S204) in a
sulfate liquor (65%
NaOH, 25% Na2S and 10% Na2CO3) in the sulfate pulping of sprucewood. The
amount of
sodium dithionite, based on the wood to be treated, is also described here
indirectly via the
quantitative schedule of chemicals and also the "liquor ratio" (page 246, left-
hand column from
"Effect of sodium hypodisulfite in sulfate pulping liquors" through table 2).
The 1 : 7.5 "liquor
ratio" described therein indicates that 7.5 parts by mass of cooking liquor
were used per 1 part
by mass of wood.
The problem addressed by the present invention was that of obtaining a high
pulp yield coupled
with a simultaneously low lignin content on the part of the pulp in the
digestion of lignocellulosic
material while reducing the creation of malodorous emissions in the sulfate
process in
particular.
The problem was solved by adding small amounts of a salt of dithionous acid
(hereinafter also
"H2S204") in the sulfite or sulfate process and otherwise as described in the
claims.
Cellulose is known and described for example in "Ullmanns Enzyklopadie der
technischen
Chemie", 4th edition, volume 17, "Paper, fibrous raw materials", Verlag Chemie
Weinheim, New
York (1979) in chapter 1. Lignocellulosic material herein is any material,
preferably natural
material, which comprises lignin and cellulose.
Preferred lignocellulosic material comprises wood, including comminuted woods,
such as wood
cuts from saw mills.
Softwoods, preferably spruce or pine or hardwoods such as beech are very
useful woods.
Lignocellulose material herein further comprehends grasses and annual plants,
for example
straw, reed, espartogras, bamboo and bagasse, although these are typically not
digested using
the sulfite process, but preferably using alkaline processes of digestion or
the neutral-sulfite
process.
Sulfite digestion and sulfate digestion to obtain cellulose are known and are
described at the
beginning and in more detail in Ullmanns Enzyklopadie der technischen Chemie,
4th edition,
volume 17, "Paper, fibrous raw materials", Verlag Chemie Weinheim, New York
(1979) pp. 535-
549 in chapter 1.4.
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Salts of dithionous acid (H25204) herein are any metal salts or substituted
(NR4+) or
unsubstituted (NH4) ammonium salts of this acid.
Alkali metal salts, alkaline earth metal salts, salts of metals of group 12 of
the periodic table and
5 also ammonium (NH4) salts are very useful salts of dithionous acid.
Preferred salts of dithionous acid are sodium dithionite (Na25204), potassium
dithionite
(K25204), calcium dithionite (Ca5204), zinc dithionite (Zn5204), ammonium
dithionite
((N H4)25204)=
Salts of dithionous acid, including the above-preferred ones, also comprise,
as will be
appreciated, those species which comprise water of crystallization and/or
additives, the latter for
stabilization for example.
The sodium dithionite product marketed by BASF SE as Blankit0 or BlankitOS is
a very useful
salt of dithionous acid.
Any version of the sulfite process as described at the outset and in Ullmanns
Enzyklopadie der
technischen Chemie, 4th edition, volume 17, "Paper, fibrous raw materials",
pp. 531-576, Verlag
Chemie Weinheinn, New York (1979) is in principle suitable for the method of
the present
invention.
Pulping temperature in sulfite digestion is typically in the range from 100 C
to 160 C.
The bisulfite process with Mg(HS03)2 as a component of the cooking liquor is a
very useful
sulfite process of the method of the present invention and will now be more
particularly
described.
The lignocellulosic material used comprises softwoods, preferably sprucewoods,
more
preferably as chips. Chips are typically used in the forest-fresh state (i.e.,
with a dry matter
content of about 50 wt%). The amount used is computed as oven-dry substance in
order that
the yield of pulp may subsequently be determined for example.
The cooking liquor can be prepared by suspending magnesium carbonate
(hereinafter also
"MgCO3") in water and then passing SO2 into the suspension, generally until
the suspension
has turned into a clear solution, which has a pH of about 3.8 for example. At
this stage, it is
typically the case that substantially the entire dissolved substance is
present as Mg(HS03)2. On
continued introduction of SO2, the pH would continue to decrease as the
proportion of sulfurous
acid increases.
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Cooking the so-called lignocellulosic material with the cooking liquor takes
place in the
customary cookers, batchwise or else continuously. Total cooking time is in
the range from 400
to 600 minutes.
A temperature profile is preferably used for cooking in the herein recited
bisulfite processes,
preferably the bisulfite process with Mg(HS03)2 as a component of the cooking
liquor.
A very useful temperature profile is as follows:
1st phase: heating up from a temperature in the range from 15 C to
30 C to a temperature in the range from 100 to 110 C, within
from 60 to 120 minutes;
2nd phase (impregnating phase): 60 to 90 minutes' pausing at a temperature in
the range
from 100 to 110 C;
3rd phase: heating up from a temperature in the range from
100 to
110 C up to a temperature in the range from 150 to 160 C,
within from 45 to 90 minutes;
4th phase (ready-cook time): 150 to 250 minutes' pausing at a temperature
in the range
from 150 to 160 C
5th phase: cooling down to a temperature in the range from
100 to
90 C.
The salt of dithionous acid, preferably sodium dithionite (Na2S204), calcium
dithionite (CaS204),
zinc dithionite (ZnS204), more preferably sodium dithionite, is added into the
mixture of
lignocellulosic material, preferably the chips of sprucewood and the cooking
liquor, as described
above, in an amount from 0.1 to 4.0 wt%, preferably 1.0 to 2.0 wt%, all based
on the oven-dry
lignocellulosic material, preferably the oven-dry chips of sprucewood.
In principle, the salt of dithionous acid can be added at any stage during the
cooking process or
else therebefore. The dosing regimens which follow are preferable, however:
a)at the start of the 2nd phase
b)at the start of the 4th phase
c)approximately halfway through the 4th phase
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Preferably, the salt of dithionous acid is added at the start of the 2nd
phase, i.e., the
impregnating phase.
A sulfate process which is very useful for the method of the present invention
will now be
described.
Woods, such as hard- or preferably softwoods, more preferably sprucewoods,
preferably in the
form of chips, are used as lignocellulosic material.
The cooking liquor used can in principle be the familiar sulfate-process
mixture of aqueous
sodium hydroxide solution (aqueous NaOH), sodium sulfide (Na2S) and sodium
sulfate
(hereinafter also "Na2SO4") and optionally sodium carbonate (hereinafter also
"Na2CO3"),
admixed with the salt of dithionous acid, preferably selected from the group
consisting of
sodium dithionite, zinc dithionite and calcium dithionite, in an amount from
0.1 to 4 wt%, based
on the amount of oven-dry lignocellulosic material.
A cooking liquor which is very suitable for the sulfate method of the present
invention will now
be described:
The cooking liquor for the sulfate process typically comprises NaOH and sodium
sulfide (Na2S)
as active cooking chemicals. The sum total of the two substances (expressed as
NaOH) relative
to the lignocellulosic material, preferably wood (reckoned oven-dry), is the
alkali ratio. This ratio
is typically in the range from 20 to 24 wt%.
The concentration in which these substances have to be present in the cooking
liquor is
typically dependent on the so-called "liquor ratio". This is understood by a
person skilled in the
art to refer to mass fractions of cooking liquor in relation to mass fractions
of lignocellulosic
material, preferably wood (reckoned oven-dry). In the case of softwoods, for
example spruce
and pine, this liquor ratio is generally in the range from 4 :1 to 4.5 : 1,
for example 4.2 : 1,
typically according to the fill density of wood in the cooker. Therefore, the
concentration of
active alkali in the cooking liquor is in the range from 45 to 60 g/I for
example.
The proportion of total active alkali which is accounted for by sodium sulfide
(Na2S) is the
sulfidity (reported in %). Sulfidity is generally in the range from 30 to 38%,
for example 30%.
The cooking liquor for the sulfate method of the present invention comprises a
salt of dithionous
acid, preferably selected from the group consisting of sodium dithionite, zinc
dithionite and
calcium dithionite, in an amount from 0.1 to 4 wt% based on the amount of oven-
dry
lignocellulosic material.
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The pH of the cooking liquor for the sulfate method of the present invention
is typically about 14
at the start of the cooking process.
Cooking the lignocellulosic material with the cooking liquor for the sulfate
method of the present
invention takes place batchwise or continuously in customary cookers.
Total cooking time for the sulfate method of the present invention is
typically in the range from
200 to 400 minutes, preferably 240 to 300 minutes.
The cooking temperature for the sulfate method of the present invention is in
the range from
160 to 185 C, for example 170 C.
The salt of dithionous acid, preferably selected from the group consisting of
sodium dithionite,
zinc dithionite and calcium dithionite, more preferably sodium dithionite, is
added in the sulfate
process of the present invention to the mixture of lignocellulosic material
and cooking liquor in
an amount from 0.1 to 4.0 wt%, preferably 1.0 to 2.0 wt%, all based on the
oven-dry
lignocellulosic material.
In principle, the salt of dithionous acid can be added at any stage during the
cooking process of
the sulfate method according to the present invention.
The salt of dithionous acid is preferably added in the impregnating phase, the
end phase of
digestion or the main phase of digestion in the sulfate method of the present
invention, more
preferably in the end phase of digestion or in the main phase of digestion in
the sulfate method
of the present invention.
The method which the present invention provides for producing cellulose from
lignocellulosic
material by sulfite digestion or sulfate digestion delivers pulp in high yield
combined with good
delignification of the lignocellulosic material. There is an improvement in
the brightness of the
unbleached pulp.
The addition of a salt of dithionous acid in the sulfate method of the present
invention reduces
the concentration of malodorants preferably mercaptans in the off-gas of the
sulfate cooking
process.
Examples
(I) Sulfite digestion by the bisulfite process with Mg(HS03)2 as a component
of the cooking
liquor.
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A) Lignocellulosic material:
Sprucewood chips presorted and predried in the ambient air for 2 to 3 days
before cooking,
water content ranging from 23.4 to 33.2 wt%, averaging about 30 wt%.
B) Cooking liquor:
Arithmetically 2.7 wt% of MgO per liter. pH before cooking (initial pH) 3.8.
1100 g of MgCO3 were suspended in 17 liters of deionized water to obtain an
arithmetic MgO
concentration of about 2.7 wt%. Gaseous sulfur dioxide (SO2) was passed into
the suspension
until the pH was 3.8.
C) Cooking
3200 g (reckoned oven-dry) of sprucewood chips having an original water
content as described
in A) and 16 liters of the cooking liquor from B) were filled into the 25-
liter capacity batch cooker,
corresponding to a 5 : 1 mixing ratio for cooking liquor: oven-dry wood. The
cooker was
equipped with a liquor recirculator, an electrical-type jacket heater, a
temperature controller, a
manometer, a temperature sensor, a pH electrode, and a connected electronic
data processing
system.
The following heating program was implemented:
1st phase: 105 min heat-up time from room temperature (23 C) to 105 C
2nd phase: 90 min hold time (impregnating phase) at 105 C
3rd phase: 60 min high-heat time from 105 C to ready-cook temperature of 155 C
4th phase: 195 min ready-cook time at ready-cook temperature of 155 C
5th phase: about 60 min off-gas time (heating off on reaching cooking time)
until
temperature below 100 C.
Digestion time totaled 510 min (8 h 30 min). The pressure in the cooker at the
end of the ready-
cook time was in the range from 8 to 9 bar.
Sodium dithionite (Blankit,OS from BASF SE) was added in the form of a
solution in water to the
mixture in the cooker within 10 min by metering pump, specifically at 32 g of
pure Na2S204
(1 wt% of Na2S204 based on employed wood reckoned oven-dry) and/or 64 g of
pure Na2S204
(2 wt% of Na2S204 based on employed wood reckoned oven-dry).
The times of addition for the sodium dithionite were as follows per
experiment:
At the start of the holding time (impregnating phase), about 105 min after
beginning the
experiment or from the start of the ready-cook time, about 255 min after
beginning the
experiment or halfway through the ready-cook time, about 360 min after
beginning the
experiment.
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In the case of experiment W 16, the chips were impregnated with sufficient
aqueous solution of
sodium dithionite (BlankitOS from BASF SE) to correspond to 1 wt% of pure
Na2S204 based on
employed wood reckoned oven-dry, immediately prior to digestion.
5
On completion of the digestions, the pulp was removed, admixed with water and
defiberized
with a stirrer. The defiberized pulp was filled into a sieve, washed with
water and dewatered in a
centrifuge.
10 D) Inventory and evaluation
Table 1 shows the experiments:
Table 1:
Experimental Auxiliary addition Time of addition min [1]
Temperature at
series [2] addition
W7 1% Beginning of ready-
cook 255 min 155 C
time
W8 1% Start of impregnating 105 min 105 C
phase
W9 1% Halfway through
ready- 360 min 155 C
cook time
W10 2% Start of impregnating 105 min 105 C
phase
W11 2% Beginning of ready-
cook 255 min 155 C
time
W12 1% Start of impregnating 105 min 105 C
phase
W14 none
W15 1% Start of impregnating 105 min 105 C
phase
W16 1% Before digestion
process
[1] The values denote numbers of minutes after beginning the experiment, i.e.,
commencement
of the first heat-up
[2] sodium dithionite Na2S204
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The results of the experiments in table 1 are collated in table 2:
Table 2:
Experimental Auxiliary Time of addition Accepts
kappa Viscosity
series addition yield (%) number/brightness
[ml/g]
[ok]
W7 1% Beginning of 54.2 24.5/60.7
676.1
ready-cook time
W8 1% Start of 54.9 19.0 62.9
640.6
impregnating
phase
W9 1% Halfway through 55.3 27.9/59.1
689.0
ready-cook time
W10 2% Start of 54.5 25.5 59.1
679.8
impregnating
phase
W11 2% Beginning of 53.1 17.2/63.6
660.8
ready-cook time
W12 1% Start of 54.7 16063.3
645.1
impregnating
phase
W14 none 55.6 36.1/58.6
708.8
W15 1% Start of 54.3 20.6/611
677.4
impregnating
phase
W16 1% Before digestion 55.0 29.0/57.1
694.9
process
The following definitions apply therein:
Accepts yield is the amount of pulp obtained (without rejects/shives) as a
proportion of the wood
used; it was determined by weighing and dry matter content measurement.
The kappa number indicates the hardness of the pulp and was determined
according to ISO
302. Put simply, the potassium permanganate consumption (KMn04 consumption) is
measured
to determine the kappa number in an aqueous pulp suspension in an acidic
medium under
defined conditions. The higher the lignin content of the pulp, the higher the
potassium
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permanganate consumption and thus the higher the kappa number. The higher the
kappa
number, the higher the residual lignin content of the pulp and the harder the
pulp generally is.
Brightness (R457) denotes reflectance at 457 nm and was determined on an
Elrepho from
Datacolor in accordance with ISO 2470.
Viscosity was determined in accordance with ISO 5351/1 (International Standard
ISO 5351/1,
Cellulose in dilute solutions ¨ Determination of limiting viscosity number,
Part 1: Method in
cupri-ethylene-diamine (CED) solution, First edition 1981-12-01).
A solution of cellulose in copper-ethylene-diamine solution is prepared. The
concentration of the
solvent is a fixed value. The concentration of cellulose in the solution is
decided according to
the sample to be determined. What is measured is the flow time of both the
solvent and the
cellulose solution through a capillary viscometer at 25 C. The limiting
viscosity number is
computed from the results of the determination and the known concentration of
the cellulose
solution according to the Martin equation.
The measurement was carried out according to alternative A of the method of
determination
(International Standard ISO 5351/1, Cellulose in dilute solutions ¨
Determination of limiting
viscosity number, Part 1: Method in cupri-ethylene-diamine (CED) solution,
First edition 1981-
12-01). A low concentration is employed for the cellulose and the same
capillary is used for
measuring the flow times of the solvent and of the cellulose solution.
It is apparent that adding the auxiliary, particularly when it is added at the
start of the
impregnation phase, delivers an improved combination of accepts yield with
kappa number/
brightness.
(II) Sulfate digestion
A) Lignocellulosic material:
Mixed spruce-pine chips having a 7:3 spruce:pine mixing ratio, undried, water
content 57%.
B) Cooking liquor:
The cooking liquor was prepared from aqueous sodium hydroxide solution (NaOH)
and sodium
sulfide (Na2S) by incorporating commercial laboratory-grade chemicals in
water. The amount of
chemicals used was determined such as to apply an alkali ratio of 23% coupled
with a sulfidity
of 20%.
C) Cooking:
Sufficient chips were introduced into a 10 I cooker to ensure that at the
given dry matter content
of the wood 1300 g of oven-dry wood matter were used. The cooker was filled
with cooking
liquor. This cooking liquor comprised 239.2 g of NaOH and 59.8 g of Na2S
(reckoned as NaOH)
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for a desired alkali ratio of 23% and a sulfidity of 20%. The cooker contents
were then heated to
170 C and maintained at 170 C until the desired digestion time was reached.
The so-called H-factor was used to calculate the desired digestion time. The
calculation was
made on the basis of the temperature dependence of the relative reaction rate
for the alkaline
digestion. An H-factor of 3500 was realized for all cookings.
In the case of selected cookings, 2 wt% of sodium dithionite were added in
each case relative to
the introduced quantity of wood (reckoned oven-dry). The time of addition was
during the main
phase of digestion in one cooking and during the end phase of digestion in a
further cooking.
On reaching the H-factor of 3500, the cookings were discontinued by ending the
heating and
cooling down in conjunction with depressurization ("off-gassing"). The pulp
was defiberized by
vigorous stirring and washed.
D) Inventory and evaluation
Table 3 presents the experiments:
Table 3
Cooking No. Auxiliary addition [1] Time of addition Temperature at
addition
1 none
2 2% main phase 170 C
3 2% end phase 170 C
[1] Sodium dithionite Na2S204
Table 4 presents the accepts yield and the kappa number.
Table 4:
Cooking No. Auxiliary addition [1] Accepts yield % kappa number
1 none 43.3 20.1
2 2 % in main phase 46.4 19.4
3 2 % in end phase 47.1 19.9
[1] sodium dithionite Na2S204
Accepts yield and kappa number are as defined under (I), above.
It is apparent that adding the sodium dithionite is associated with a distinct
increase in yield (by
3 to 4 percentage points) and even a slight reduction in the kappa number.
E) Reduction of methyl mercaptan emissions
A sulfate digestion of softwood was carried out as described above.
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During the release of gases from the cooker ("off-gassing"), off-gas samples
were taken at
different times using a detection pump. The concentration of methyl mercaptan
in these
samples was measured using gas testing tubes specific to methyl mercaptan.
The first measurement in each case was carried out immediately after
terminating cooking; the
temperature in the cooker was 172 C. Subsequent measurements were carried out
at further
decreased cooker temperatures, see table 5. The results are complied in table
5.
Table 5: Methyl mercaptan concentrations
Test wt% [1] of Sulfidity Cooker temperature Methyl mercaptan-
Na2S204 [Ok] at time of concentration [ppm]
measurement [ C]
lb 0 30 162 890
2b 0 20 162 410
3b 2 30 162 400
4b 2 20 162 180
[1]: based on oven-dry wood
Sulfidity is: Na25 fraction in active alkali
The methyl mercaptan concentration is highest at high sulfidity. Using Na2S204
results in a
decrease of the methyl mercaptan in the off-gas.
