Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A method and an apparatus for removing extractives from fibrous
suspension
Field of the invention
The invention relates to a method for removing extractives from fibrous
suspension. The invention also relates to an apparatus for implementing the
aforementioned method.
Background of the invention
In the production of cellulose pulp, the chips are cooked in an alkaline
water solution. As a result of the cooking, the fibers and lignin of the
wood separate from each other, and lignin dissolves in the cooking
solution. After cooking the pulp is washed, wherein the fibrous pulp can
be separated from the lignin-containing cooking solution.
Wood also contains a number of other substances besides lignin and
fibers, such as fatty acids and resins, which are commonly called ex-
tractives. Most of the extractives dissolve in the cooking solution during
the cooking, but a large amount of them still remain in the finished pulp.
Especially in hardwood pulps contain large amounts of neutral extrac-
tives after the cooking. The extractives in hardwood pulps are mainly
formed of unsaturated fatty acids, which are for the most part located in
the small-sized parenchymatous cells among the pulp. The extractives
can also appear on the surface of the fibers or freely in the solution.
Extractives cause numerous problems in the further processing of pulp.
They cause yellowing of the pulp in the following pulp processing
stages and odour and taste nuisance in paper made of pulp. The free
extractives in the solution can accumulate and form deposits, sticki-
ness and precipitations in the pulp processing equipment and its parts.
In addition to disturbing the function of the process equipment, the de-
posits can suddenly disintegrate into the pulp and cause soiling of the
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pulp. Furthermore, the agglomerated extractives disturb paper manu-
facturing, because they cause greasy spots in the paper, which hinder
the surface-sizing and printing of paper because they do not absorb
glue and reject printing ink.
To eliminate these problems, the aim is to remove the extractives from
the pulp at some stage of the pulping process, before pulp is conveyed
to drying or production of paper or paperboard. For this purpose, addi-
tives have been used, which are added in the pulping process. They
function as surfactants, and make the extractives soluble to the so-
lution in the pulp suspension. As a result, it is possible to remove the
extractives from the pulp with the solution, and they do not remain in
the pulp. In the process of cooking hardwood pulp, especially birch-
wood pulp, resin acids obtained from the process of cooking softwood,
such as tall oil and tall oil soap have been used as additives. These
additives have been added into the cooking solution of the pulp. At
present, synthetic resin soap is mainly used as an additive in the
process of removing extractives, the ratio of the resin acid and fatty
acids therein being selected in such a manner that they dissolve
extractives efficiently. Such methods are disclosed for example in the
Finnish patent application No. 20022270
(corresponding WO
publication 2004/057107), in which a mixture of resin acid/fatty acid is
added in the cooking solution of pulp, especially pulp made of
birchwood, and in the Finnish patent application No. 20030846
(corresponding European patent 1484375) in which the addition of
resin acid/fatty acid is made either in the cooking solution or washing
solution of pulp.
The above-mentioned methods for removing extractives from the pulp
do work as such. However, they have the drawback that because they
affect the entire amount of pulp travelling through said process stage,
they consume a large amount of additives used for removal of extrac-
tives. Thus, the additive costs are very high.
Furthermore, by means of these known methods it is not possible to
remove the extractives in the parenchymatous cells of hardwood suffi-
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ciently well, but they remain in the pulp causing problems in later
process stages.
Brief description of the invention
Consequently, it is an aim of the present invention to provide a method
that avoids the above-mentioned problems and by means of which it is
possible to remove the extractives from fibrous pulp more efficiently
and with considerably smaller amounts of additives than before when
producing hardwood pulp. Furthermore, it is an aim of the invention to
provide an apparatus implementing the aforementioned method.
The other, dependent claims will present some preferred embodiments of the
invention.
The invention is based on the idea that a suspension containing fine
particles,
i.e. fines containing suspension is separated from the pulp obtained from the
cooking, and it is treated by means of an additive for removing the
extractives
therein.
It has been noticed in test runs that the extractives mainly accumulate in the
fines of the pulp, even up to 90% of the extractives in the pulp can
accumulate in the fines. The amount of fines in the total amount of pulp is
typically 3 to 10 %. When the fines-containing suspension is separated from
the pulp flow and only the small fraction, in which most of the extractives is
present is treated with an additive removing the extractives, it is possible
to
significantly reduce the amount of additives necessary for the treatment.
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The removal of the extractives can be intensified further for example by
refining or dispersing the fines suspension so that the size of the par-
ticles contained therein can be reduced. As a result of the treatment
the parenchymatous cells in the fines disintegrate, wherein the additive
can affect the extractive and increases its solubility to the solution
component of the suspension.
According to an embodiment of the invention, a solution contained in
the fines-containing suspension is removed from it before treating it,
wherein the treatment, such as refining or dispersion is more efficient.
The fines-containing suspension separated from the pulp substantially
contains all the fines existing in the pulp. Typically it contains noil
(fines), parenchymatous cells, pieces of fiber, and sometimes even
sand. The fines-containing suspension can be defined in such a man-
ner that it is a suspension in which the particles therein have such a
size that they penetrate through a 200 mesh wire, when the suspen-
sion has been analyzed by means of methods suitable for fractionating
analysis of fibers. One such method is the Bauer-McNett analysis.
As an additive it is possible to use a side product obtained from the
cooking of softwood, such as tall oil, tall oil soap or a fatty acid/resin
acid mixture. The fatty acid and the resin acid can also be present in
the additive in the form of their salts, i.e. in a saponified form. It is also
possible to use a synthetic fatty acid/resin acid mixture as an additive.
It is an advantage of the invention that in the production of softwood
pulp the amount of additive necessary for removing the extractives is
reduced, wherein savings can be attained in the additive costs.
Furthermore, by treating the fines containing suspension by breaking
the parenchymatous cells containing extractives and contacting the
extractive contained in them with the additives makes the removal of
the extractives far more efficient. These advantages can be attained
only by separating the fines-containing suspension from the pulp ac-
cording to the invention, because by means of the invention the addi-
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tives treatment can be targeted to the part of the pulp in which the ex-
tractives actually exist. It would be impossible to refine or disperse the
entire amount of pulp to reduce the particles. Furthermore, the refining
of the pulp would change the fiber properties, thus affecting the quality
5 of pulp and the final product produced therefrom. The refining of the
fines-containing suspension does not have such a strong effect on the
quality of pulp, and especially it does not have a significant effect on
the strength properties of fibers
Brief description of the drawings
In the following, the invention will be described in more detail with ref-
erence to the appended drawings, in which
Fig. 1 shows schematically an embodiment of the invention for
removing extractives from a fiber-containing suspension and
Fig. 2 shows schematically another embodiment of the invention
for removing extractives from a fiber-containing suspension.
In Figs. 1 and 2, the same numerals refer to corresponding parts and
they will not be explained separately later on, unless required by the
illustration of the subject matter.
Detailed description of the invention
In this context the term pulp refers to the pulp treated in different
stages of producing hardwood pulp. The term fines-containing sus-
pension refers to a suspension separated from pulp by means of a
separator, said suspension containing a solution typical for the pulp
producing stage preceding the separator and fine particles. The so-
lution can be for example cooking liquor or pulp washing solution and
in addition to fine particles it may also contain for example lignin, cellu-
lose, hemicellulose and inorganic material dissolved from wood. Fine
particles are for example noil (fines), parenchymatous cells and pieces
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of fiber. The term line refers to any pipe, duct or channel suitable for
transferring a solution or a suspension.
Figure 1 shows schematically an apparatus according to the invention
for removing extractives from a suspension containing fibers. The pulp
from the pulping process is conveyed via a line 1 to separating means,
i.e. a separator. The separator is a screen or a precipitator, for example
a screw precipitator that functions as a pre-precipitator that is capable
of separating the fines-containing suspension from the pulp. As a sepa-
1 0 rator it is possible to use for example a screen, a sieve, or a filter.
From
the separator 2 the fines containing suspension is conveyed via a line 3
to a tank 4. The main pulp flow containing coarser material is conveyed
forward in the pulping process via a line 5. Before the tank 4 an addi-
tive is led to the line 3 via first means for adding additive, i.e. an addi-
tive line 6. The additive makes the extractives in the fines containing
suspension more easily soluble to the solution contained in the sus-
pension. The tank 4 functions as a retention tank that guarantees suffi-
cient reaction time for the additive and the extractives. From the tank 4
the fines-containing suspension that has reacted with the additive is
conveyed by means of a pump 7 via a line 8 to the main pulp flow. Part
of the suspension that has reacted with the additive can be conveyed
to the pulp in line 1, before the separator 2.
The solubility of the extractives to the solution contained in the fines-
containing suspension can be improved by treating the fines containing
suspension so that the size of the particles contained therein is re-
duced and adding additive to the very fine particles containing sus-
pension thus obtained. The treatment is effected by a treatment device
9 succeeding the first point of additive addition through an additive line
6. As a result of the treatment the parenchymatous cells contained in
the fines break, wherein the extractive comes in contact with the addi-
tive. The treatment device 9 can be a refiner or a disperser shown
schematically in Figs 1 and 2 with broken lines. After the treatment, an
additive is conveyed to the suspension via second means for adding
additive, i.e. a second additive line 10, which is also marked with bro-
ken lines in Figs 1 and 2.
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If the apparatus comprises the above-described treatment device 9, the
additive is fed both via the first additive line 6 and the second additive
line 10 to the fines-containing suspension travelling in the line 3. How-
ever, the amount of the additive does not increase when compared to
the above-described method of removing extractives without the treat-
ment of the fines-containing suspension, because the amount of addi-
tive is divided in such a manner that a part of the additive is fed via the
first additive line 6 and the rest of the additive is fed via the second
additive line 10. It is also possible that the entire amount of additive is
fed only via the second additive line 10 and no additive is supplied to
the fines-containing suspension before its treatment. If desired, it is
also possible to feed the entire amount of additive to the fines-con-
taining suspension via a first additive line 6 before the treatment of the
suspension.
Fig. 2 shows another embodiment in which the dry matter content of
the fines-containing suspension is increased before it is conveyed to
treatment. In this embodiment, the fines-containing suspension ob-
tained from the separator 2 is conveyed via a line 11 to a solution
separator 12 which is a device removing solution from the suspension,
such as a filter or a drum filter. The purpose of the solution separator is
to increase the dry matter content of the fines-containing suspension
led to the treatment device 9, wherein its treatment with the treatment
device 9 is easier and more efficient. As a result of the reduction in the
solution volume it is also possible to reduce the dimensions of the
pipeworks and tank used in treating the fines-containing suspension.
From the solution separator 12 the fines-containing suspension having
higher dry matter content is conveyed via a line 13 to the treatment de-
vice 9 and further to the tank 4. To remove the extractives, an additive
is conveyed to the suspension before the treatment device 9 via the
additive line 6 and/or after the treatment device 9 via the additive line
10. From the tank 4 the suspension that has reacted with the additive is
conveyed by means of a pump 7 via a line 8 to the main pulp flow. The
solution separated from the fines-containing suspension and obtained
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from the solution separator 12 is conveyed via a line 14 out of the so-
lution separator 12. According to the need, the solution separated from
the fines-containing suspension can be conveyed either into the line 5
to the coarser material containing main pulp flow that is obtained from
the separator 2 or via the line 15 to the pulp flowing in the line 1, before
the separator 2. It can also be conveyed via the line to the tank 4 as
dilution water.
The purpose of the tank 4 disclosed in all the above-described em-
bodiments of the invention is to guarantee sufficient retention time for
the extractives in the fines-containing suspension to react with the
additive added to the suspension. The reaction time varies from a
couple of seconds to a couple of minutes. The size of the tank 4
depends on the amount of suspension to be treated and the point of
adding additive before the tank. If the adding of the additive takes place
far enough before the tank 4, the reaction with the additive and the
extractives takes place already before the tank, wherein the tank
functions as a storage or dilution tank for the rest of the process.
The separation of the fines-containing suspension from the hardwood
pulp and the addition of the additive thereto can be done at any stage
of the pulping process. It can be conducted either after the cooking,
after the blow tank following the digester, before or after the bleaching
stages, or in between bleaching stages. It can also be conducted
before or after the secondary classification of the pulp. The most
advantageous point to separate the fines-containing suspension from
the pulp is after the knot screening following the cooking. The most
essential aspect is that the reactions of the extractives after the
separation take place in alkaline conditions. Thus, the fines-containing
suspension must be alkaline, which is typically not a problem in pulping
processes, in which process solutions are often alkaline. If the
apparatus for removing extractives according to the invention is placed
at that stage of the pulping process in which the alkalinity of the fines-
containing suspension would be too low for extractive removal, it is
possible to increase the alkalinity of the suspension by adding sodium
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hydroxide or white liquor in the fines-containing suspension before
adding the additive.
The invention can be applied in pulps produced of hardwood such as
birch, asp, alder, maple, or a mixture of these, and eucalyptus, acacia,
and so-called mixed tropical hardwood. The invention can also be ap-
plied in pulps produced of annual plants, such as straw, reed canary
grass, reed and bagasse.
The invention is not intended to be limited to the embodiments pre-
sented as examples above, but the invention is intended to be applied
widely within the scope of the inventive idea as defined in the ap-
pended claims.
