Note: Descriptions are shown in the official language in which they were submitted.
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SULPHITE PULPING PROCESS
The present invention relates to a sulphite pulping
process for the production of wood pulp, using alkaline
monosulphite solutions, organic solvents and quinone
derivatives.
The most significant chemical processes to produce
wood pulp are the sulphite pulping and the sulphate
pulping processes. In addition, processes using a
sodium hydroxide solution and chlorine, or using nitric
acid, are known, but are not very widely used on an
industrial scale.
In the sulphite pulping process, the raw materials
(comminuted wood) are substantially introduced into a
cooking acid, which is adjusted to be acid or neutral,
and these lignocellulose-containing materials are
cooked with solutions of hydrogen sulphites or
sulphites. When the hydrogen sulphite solutions
additionally also contain sulphur dioxide, the
processes are referred to as "acidic bisulphite
processes".
In this regard, processes which are frequently
applied are the calcium bisulphite process and, in
particular, the magnesium bisulphite process. A
detailed summary of the various known sulphite pulping
processes is found in Ingruber and Allard (1973)
"Alkaline Sulfite Pulping for Kraft Strength", Pulp and
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Paper Magazine of Canada 1974, Pages 354 - 369.
An alkaline sulphite pulping solution generally
comprises a sulphite and carbonates or alkaline
solutions; the specific pulping variant applied does,
however, always depend on the respective wood type
used.
It is known that, in a sulphite pulping process in
which a sodium sulphite solution, i.e. a pure
monosulphite solution, is used and in which furthermore
methanol and/or anthraquinone is added to the cooking
solution, it is possible to achieve an increased
delignification rate and an increased pulping
efficiency (Tappi-Journal 65 (10) (1982) 29)
A particularly advantageous process is described in
EP-Bl-0 205 778. There, in a sulphite pulping process,
carbonates and/or hydroxides, in addition to methanol
and a quinone derivative, are furthermore added to the
digestion solution which contains a predominant
quantity of monosulphite solution. This process is
referred to as the ASAM process (alkaline sulphite
anthraquinone methanol process).
EP-Al-0 538 576 and AT-B-398 992, in each case,
disclose processes to recover the chemicals, which are
used for wood pulp digestion, from the waste liquors
after the ASAM process. To this end, the waste liquor
is incinerated in a liquor incinerator, the slag or
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21 98485
ash, respectively, is withdrawn, dissolved in water,
converted to H2S and incinerated, sodium sulphite being
recovered from the waste gases produced during the
incineration.
In an article by R. Patt et al. in Das Papier, Vol.
43, Pamphlet lOA, 1989, pages V 108-115, a sulphite
digestion process for the production of pulps by using
alkaline monosulphite solutions, organic solvents and
quinone derivatives is described. A particular
impregnation step is, however, not disclosed.
In EP-O 498 330 A1, an impregnation step is
disclosed to be used in a strictly sulphur-free
digestion process. According to column 1, lines 10 to
12, it is stated that ORGANOSOLV processes - and the
process of this document is such a process - avoid the
use of sulphur compounds.
The object of the present invention is to provide a
sulphite pulping process which is further improved
relative to the prior art described and which permits a
more efficient digestion of the raw material, an
improved recovery of the substances used, as well as a
simplification of the process in terms of process
technology.
According to the invention, this object is met by a
sulphite process for lignocellulose-containing raw
material, using water-soluble alkaline monosulphite
solutions, organic solvents as well as at least one
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21 98485
quinone derivative, according to which process the raw
material, prior to the actual digestion of the raw
material, is subjected to an impregnation step, without
organic solvents, in the presence of the monosulphite
solution and, optionally, of the quinone derivative, at
a temperature below the temperature required for the
digestion reaction, the impregnating liquor being
composed of only a portion of the chemical constituents
required for the actual digestion step.
The quinone derivative may be present during the
impregnation of the raw material, or it is added,
together with the other components still required for
digestion, prior to the digestion step. As an
oxidation-reduction catalyst, the quinone derivative in
alkaline pulping processes accelerates the
delignification and stabilizes the reducing ends of the
carbohydrates against an alkaline "peeling-off". A
preferred quinone derivative is anthraquinone. The
organic solvent is added to the alkaline monosulphite
solution only after this impregnation step.
The said raw material is impregnated with the
alkaline monosulphite solution for a period ranging
from 5 to 360 min, preferably from 15 to 60 min. The
temperatures usually applied in such cases range
between 60 and 150~ C, preferably between 100 and 130~
C.
The impregnation with the alkaline monosulphite
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solution may be terminated by adding the organic
solvent or the solvent/water mixture into the
impregnating liquor. In this process, the ionic
strength of the impregnating liquor should be adjusted
to the concentration required for the digestion process
by adding the organic solvent or the solvent/water
mixture.
The solvent, e.g. low-boiling alcohols such as
methanol, may be added under pressure to the digestion
system after the impregnation of the matter to be
pulped. Subsequently, the matter to be pulped, together
with the complete digestion solution, is heated to the
temperature required for the reaction, or the
impregnation is replaced, by means of modern
replacement techniques, by a complete digestion liquor
which contains the solvent.
Particularly preferred in this modification of the
process is an impregnating liquor which has a higher
ion concentration than the actual digestion liquor.
This brings about a more rapid and more homogeneous
distribution of the alkaline and monosulphitic
digestion chemicals in the matter to be pulped.
It is possible for the impregnating liquor to be
used either with a surplus quantity of organic
chemicals and liquids, as compared to those quantities
required for cooking, or with that quantity of
chemicals required for digestion, with a reduced liquid
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volume, as compared to cooking.
In the first-mentioned case, a portion of the
quantity of liquid, together with the quantity of
chemicals still present therein after the impregnating
step, must either be drawn off or displaced from the
digestion system, before the solvent is added into the
digestion system. The solvent, or the solvent/water
mixture, may be used as the displacement medium. The
displaced or drawn-off displacement liquor may be
enriched to the starting concentration, using fresh
chemicals, and may be re-used.
Water may be added to the solvent, which is used
for the purpose of displacement, such that it is
possible for the concentration of chemicals and
solvents required for digestion to be variably
adjustable. An energy saving is made possible, due to
the relatively low demands with respect to purity and
concentration of the solvent recovered.
It is, alternatively, also possible for the
impregnation to be carried out using less liquid than
is required for the digestion itself. It must, however,
be ensured that all the matter to be pulped is
thoroughly impregnated. This may be achieved, e.g. by
circulating the impregnating solution by pumping. In
the impregnating step, the matter to be pulped absorbs
the inorganic digestion chemicals. At the end of the
impregnating step, the concentration of the chemicals
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in the digestion solution is adjusted to the
concentration required for digestion, by the subsequent
addition of the solvent or of the solvent/water
mixture, or by displacement of the impregnating liquor
by complete digestion liquor.
This design of the process has the technological
advantage that the solvent-containing liquor
circulation remains restricted to the areas of the
digestion system, the control of the black liquor up to
the separation of the solvent, and to parts of the
washing of the brown pulp. The white liquor remains
solventfree, thereby increasing the solubility of the
inorganic digestion chemicals.
A feature of this modification of the process is
that, in continuously operating pulping systems,
conventional feed systems for the matter to be pulped
may be used, since the impregnating liquid is solvent-
free.
~ haracteristic features of this aspect of the
invention are the acceleration of the digestion
procedure and the improvement of the homogeneity of
digestion, which becomes apparent, in particular, in a
reduction in the fragment quantities. The quality of
the wood pulps thus produced remains at the level of
the wood pulps produced according to the standard ASAM
process, with slightly higher yields of wood pulp and
lower fragment contents.
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The impregnation step may be terminated by
replacing (e.g. by displacement of) the impregnating
solution by the digestion solution.
After impregnating, an aqueous solution of the
inorganic digestion chemicals, possibly together with
the quinone derivative to be used, may be charged into
the digester, the digester heated to the maximum
temperature, and the digestion may be carried out. In
comparison to normal digestion, it is possible for the
heating time to be very considerably reduced, since the
inorganic digestion chemicals penetrate very rapidly
into the microstructure of the wood, as a result of the
improved perviousness of the wood. The overall cycle
time of the digester is not extended. In the case, for
example, of pine wood, it is possible for the maximum
digestion temperature to be reduced from 180 to 173 -
170~, as a result of which the digester pressure is
reduced, depending on the solvent content in the
digestion solution.
During the digestion reaction, the solvent should
preferably be present in a quantity of between 0.5 and
50 % by volume of the entire quantity of liquid.
In alkaline monosulphite solutions, sodium sulphite
is preferably used, namely in a quantity of between 5
and 40 % based on the raw material. Other bases, i.e.,
for example, potassium hydroxide or potassium
carbonate, are also suitable.
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According to a preferred embodiment of the process,
aqueous solutions of sodium hydroxide and/or sodium
carbonate are admixed with the monosulphite solutions,
and this in a quantity of between O and 15 %, based on
the raw material used.
The digestion solution and the matter to be
digested may be heated to the temperature required for
the digestion reaction directly after or simultaneously
with the addition of all the digestion chemicals still
required to be added.
The digestion process may ultimately be terminated
by displacement of the digestion solution or by cooling
the system.
The processes described may be operated
continuously or discontinuously. If the process is to
be carried out discontinuously, a preferred variant of
the process comprises that the impregnating step be
carried out already in upright discontinuous digesters
in which the liquids are agitated by means of pumps and
the raw material is added by means of feeders. In this
case, the impregnating liquor is preferably fed into
the digesters from a separate impregnating liquor tank.
When the process takes place continuously, the
impregnating step is preferably carried out in a
separate impregnating tower and the matter to be
digested is subsequently transported into the digester
together with a liquid.
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An example of a plant for a continuous process
sequence is illustrated in Figure 1.
Raw material (1) is heated by steam (3, 4). in a
container (2) and is transferred into the charging
system (5) in which the mixing with the impregnating
liquor (6) takes place, said liquor comprising NaOH,
Na2CO3, Na2SO3 and anthraquinone. The mixture is
directed, via a heat exchanger (7), into the
impregnating tower (8) in which the actual impregnating
step takes place. From the impregnating tower, the
impregnated raw material is directed, via further heat
exchangers (9, 10), into the digester tower (12), the
solvent methanol (11) additionally also being added
between the heat exchangers (9) and (10). Following on
the digestion phase, the wood pulp (13) as well as the
liquor (14) are discharged from the digester and are
subjected to further treatment or reprocessing.
The invention specifically permits a distinct
simplification of the process control of the ASAM
process and simplifies its conversion to industrial
practice, both for continuous and for discontinuous
digestion systems. The fragment content of the wood
pulps is reduced and the accepts yield is increased,
via an improved impregnation.
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Examples: 1. Pulping of spruce wood
Manner of pulpingStandard Methanol-free
(ASAM process) impregnation
Impregnation
temperature, ~C 120
heating time, min. 53
time at Tmax 60
MeOH, %/liquor bath O
liquor bath 3:1
AQ, %/wood 0.1
Digestion
temperature, ~C 180 180
heating time, min. 100 37
time at TmaX 120 150
MeOH, %/liquor bath 15 15
liquor bath 4:1 4:1
pressuremax, bar 14 14
total alkali (calcu-
lated as NaOH), ~ wood 25 25
Na 2 SO 3: NaOH 80:20 80:20
AQ, % wood 0.1 0.1
kappa number 22.2 22.4
accepts yield, 45.9 50.0
fragments, % 4.9 2.2
viscosity, ml/g 1327 1334
degree of whiteness, % ISO 45.7 39.6
TABLE 1
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2.: Standard pulping according to the ASAM process,
conventional and with white liquor impregnation and
surplus of chemicals during pre-impregnation
2.1.: ASAM - conventional
Raw material: spruce wood
Pulping was carried out in a 15 1 autoclave.
Chemicals: 20 % Na2SO3 and 5 ~ NaOH, calculated as
NaOH on oven-dry wood
0.075 % anthraquinone on oven-dry wood
15 % v/v methanol
Liquor bath: 4:1 (liquid:wood)
~echnique: Dissolving all the chemicals in the
liquor bath
20 min of steaming the digestion stock
90 min of initial cooking
180 min of final cooking at 180~C
20 min of replacing the digestion liquor
and
emptying of the digester
Result: Kappa number: 24.6
Accepts yield: 47
Fragments 1.2
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2.2.: ASAM - with white liquor impregnation
Raw material: spruce wood
The digestion was carried out in a 15 l autoclave.
Chemicals: 130 g/l Na2SO3 and 20 g/l NaOH (on oven-
dry wood)
0.075 ~ anthraquinone in pre-impregnation
15 ~ v/v methanol in digestion;
is added only after impregnation
Liquor bath: 6:1 (liquid:wood)
Technique: Dissolving the inorganic matter in the
liquor bath
20 min steaming
30 min impregnation at 110~C
10 min methanol addition and displacement
of liquor
30 min heating
100 min final cooking at 180~C
20 min displacement of digestion liquor
and
emptying of the digester
Result: Kappa number: 16.2
Accepts yield: 46.6
Fragments 0.1
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3.: Standard digesti-on according to the ASAM process
with white liquor impregnation without surplus of
chemicals during the pre-impregnation
Raw material: spruce wood
Digestion was carried out in a 10 m3 digester.
Chemicals: 17% Na2SO3, 3% NaOH and 14~ Na2CO3,
- based on wood
0.07 % anthraquinone during impregnation
10~ v/v methanol in the digestion,
is added only after the impregnation
Liquor bath: 3.3:1 (liquid:wood) during impregnation
4:1 (liquid:wood) during digestion
Technique: Preparation of the impregnating liquor
20 min steaming the digestion stock
15 min impregnation at 122~C
10 min methanol addition and liquor-
liquor displacement
60 min heating
200 min final digestion at 180~C
30 min displacing digestion liquor and
emptying of the digester
Result: Kappa number: 19.8
Accepts yield: 50.8
Fragments: 1.8
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