Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
^~ 1088Z61
A. AIII.STR~M OSAKEYHTI~, Noormarkku
750799
Process for the delignification of a cellulose-containing
material with an oxidizing gas in an alkaline milieu
The present invention relates to a process for the
removal of the lignin from or the reduction of~ its concentra-
tion in wood, wood pulp or some other cellulose-containing
material which contains lignin, by means of oxygen gas under
alkaline conditions. Delignification denotes here not only the
digestion but also the bleaching of a cellulose-containing
material.
Possibilities for the delignification of wood and wood
pulp by means of oxygen and alkali have been studied inten-
sively in recent years. Oxygen is used for the bleaching of
; wood pulp on an industrial scale, but oxygen bleaching has
a drawback, a certain decomposition of hydrocarbons, which
cannot be prevented effectively enough. This decomposition
is manifested in hydrocarbon losses and in a lowering of the
polymerization degree of the cellulose, which for its part
results in a weakening of the mechanical properties of the
fibers. When attempts have been made to use oxygen for the
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delignification of wood, even yreater difficulties have been
~ncountered, and the so-~alled oxygen digestion process has
therefore not yet reached the stage of industrial application.
It is obvious that the uses of oxygen are limited above
all by the poor solubility of oxygen gas, which is the reason
why oxygen cannot be caused to pass into the fibers to a
sufficient degree.
The object of the present invention is to eliminate the
above drawback and to provide a process for digesting or
bleaching a cellulose-containing material under alkaline
conditions in the presence of oxygen so that a sufficient
amount of oxygen can be caused to pass into the fibers.
The present invention resides in a process for the
delignification of a lignin-bearing material by means of an
oxygen-bearing gas in an alkaline aqueous solution at an
elevated temperature, comprising performing the deligni~ication
in the presence of a water-soluble, oxygen-transferring agent
in order to oxidize the lignin-bearing material, whereafter
the reduced oxygen-transferring agent becomes reoxidized
under the effect of oxygen-bearing gas, to re-react with the
lignin-bearing material.
It has been observed surprisingly that the passage of
oxygen into fibers is significantly promoted if a water-
soluble reducing-oxidizing agent which oxidizes the fibrous
material and then becomes reoxidized under the effect of the
oxygen present in the solution is added to the solution.
Such an agent is called an oxygen-transferring agent since it
promotes the transfer of oxygen to the fibers without being
necessarily consumed in the reactions.
In the process according to the invention, oxygen-
transferring agents are used which are characterized in that
they oxidize the fibrous material and thereafter, after be-
coming reduced, are rapidly reoxidized by the oxygen present
in the solution. In the presence of such agents the transfer
of oxygen from the gas phase to the liquid phase and further
to the fibrous material is crucially accelerated. Furthermore,
if a sufficiently water-soluble transferring agent is selected,
the concentration of the oxidizing agent can be multiplied.
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If the agent is stable enough it is not itself consumed in .
the reaction.
When the process according to the present invention was
developed, it was proven that quinones and their derivatives
are oxygen-transferring agents especially practicable for
this purpose. Anthraquinone-2-sulfonate (AMS) proved especially .
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10~8261
eEfective. This compound has previously been used for stabilizinghydrocarbons in sulfate digestion ~ Bacll , B. and Fiehn, G.:
Zellstoff und Papier 21 (1972) 3_ 7, but in this case, if oxygen
is not present, large amounts of material are needed to produce
a noteworthy effect. Besides, as is shown below, the effect
of AMS is very slight if an attempt is made to perform the
delignification by means of alkali alone at 135C. On the other
hand, it was shown that AMS and other suitable additives
decisively promote delignification in the presen~e of oxygen.
~o This invention opens completely new possibilities for the
practical application of oxygen delignification.
The process according to the invention is applicable to
alkaline delignification, and some alkalis which can be used,
besides sodium hydroxide, are sodium carbonate, sodium bi-
carbonate and magnesium alkali. The process further offers
an advantageous possibility for performing delignification in,
for example, a digester provided with liquid circulation, in
which case oxygen can be dispensed into the liquid circulation
system in the exact amount required for the oxidation of the
transferring agent, without producing a noteworthy oxygen
overpressure in the digester. Naturally air, for example, can
also be used instead of pure oxygen.
The process according to the invention is elucidated
in more detail in the following examples.
.
Example 1
Birch wood (~tul = ) was ground into coarse
powder and digested in small batches (10 g calculated as
absolutely dry material) under conditions which are given in
Table 1 together with the obtained results. The experiments
were performed in rotating autoclaves, and the heating was
performed by means of an air oven. AMS was dispensed while dry
into the pulp. The maximum temperature was 135C (the temperature
raising stage 30 min) and the alkali quantity was NaOH 60%
~f the wood [initial oxygen pressure p = 784 kPa (kil~cals) and liquid/
wood ratio 20:1 (ml/g)]. After a suitable digestion period the
autoclaves were opened, the pulp was washed first with water,
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then with an acetic acid solution and finally again with water,
and the conventional yield and kappa number determinations
were performed. As can be seen from Table 1, the AMS addition
does not much improve the delignification nor increase the
yield if only alkali is used. Instead, the delignification
velocity increases very strongly in the presence of AMS in
an oxygen-alkali system, and the absolute yield can increase
as much as approx. 25~,the kappa number remaining the same.
A significant effect can be achieved with even small AMS
doses, and if the addition is 1-1.5~ of the wood, a nearly
maximal effect is obtained.
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Table 1
02-NaOH digestions with birch
Additive Digestion period Yield Kappa number
AMS ~ h
of wood
- 1 53.5114
_ 1.5 47.5 95
- 2 45.0 85
_ 3 35.0 57
- 4 28.8 46
- 5 25.1 35
0.5 1 55.7109
0.5 1.5 52.1 91
0.5 2 49.6 73 '
0.5 3 41.8 44
0.5 4 38.0 31
1.0 1 57.0104
1.0 1.5 53.1 62
1.0 2 46.8 46
1.0 3 43.9 32
3.0 1 58.1100
3.0 2 47.4 30
3.0 3 41.8 17
- (without 2) 1 66.0134 ~-
- ( " " ~ 3 63.4126
- ( " " ) 5 61.1 86
3.0 (without 2) 1 65.9114
3.0 ( " " ) 3 62.1 90
3.0 ( " " ) 5 58.0 69
xample 2
Unbleached pine sulfate pulp with a kappa number of 28 ~'
was used in the experiments. The oxygen alkali bleaching was
performed under the following conditions: pulp density 25%,
àlkali dose NaOH 4~/abs. dry pulp, initial oxygen pressure
Pe = 588 kPa, duration of treatment 45 min in ~he first four
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series of experiments and 30 min in the following ones. The
inhibitor used was triethanol amine (TEA) mixed with alkali
solution.
Table 2
Bleaching experiments with pine sulfate pulp
Kappa number Addition, % Yield
TEA AMS
6.3 0.5 - 95.1
6.1 0.5 0.2 95.3
6.1 0.5 0.5 95.8
5.9 0.5 1.0 96.1
10.9 0.5 - 96.6
10.4 0.5 0.2 96.6
10.2 0.5 0.5 96.7
12.4 0.5 1.0 97.3
Example 3
Birch wood (Betula verrucosa) comminuted into coarse
powder was digested under the conditions and in the equipment
of Example 1 with the difference that the alkali used was
sodium carbonate (Na2C03 40% of dry wood) and the maximum
digestion temperature was 140C. The results are given in
:~ Table 3.
- Table 3
:~ 02-Na2C03 digestions with birch
Additive Digestion period Yield Kappa number
AMS ~ h %
~ of wood
: 2 5 58.2 26.2
2 6.2 56.4 18.4
- 5 57.6 32.1
- 6.2 57.0 26.7
Even though in this case AMS does not have as great an
effect as in the digestions according to Example 1, it can be
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seen clearly that the oxygen-transferring agent has increased
the delignification velocity and improved the yield.
Example 4
The digestions according to this example were performed
with pine wood (Pinus silvestris) in the form of normal indus-
trial chips. The digestion was performed in two stages, the
first one being only alkali digestion without oxygen. Approx.
6 kg. of chips, calculated as absolutely dry, were batched in
the first stage into a 25-liter digester. Alkali was added,
Na2O 15% of the amount of absolutely dry chips, and the liquid-
wood ratio was 3.5:1 (l/kg) and the maximum temperature
170C (15 min) (25-170C 2 h); that is, it took 2 hours to raise
the temperature from 25~C to 170C. The pulp was defibrated
slightly, washed, and dried (yield 67.0%, chlorine number 30.6).
The actual oxygen-alkali digestion was performed on the
pulp obtained from the previous stage, in two batches. An
oxygen-transferring agent was added to one of the batches
(AMS 2% of the dry pulp, i.e., 1.3% per original chips~, the
other one being a control digestion without AMS addition. The
alkali dose was Na2O 20% calculated from dry pulp, MgCO3
addition 1%, liquid-wood ratio 10:1 (l/kg), initial oxygen
pressure Pe = 784 kPa, and maximum temperature 130aC (60 min)
(15-130C 64 min); that is, it took 64 minutes to raise the
temperature from 15C to 130C. The results are given in
Table 4. As can be seen, the delignification has been
considerably promoted and the pulp yield improved as a result
of the AMS addition.
Table 4
O2-NaOH digestions with pine chips
AMS addition Yield b Chlorine number Lignin content,%
+ 46.2(69.0) 6.6 5.9
~ 43.3t64.6) 7-3 6.6
a + addition 1.3% of the dry weight of the original chips
- control digestion without addition
b figures given in parentheses indicate the yield of the oxygen-
alkali stage, the other figures indicate the total yield.
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