Note: Descriptions are shown in the official language in which they were submitted.
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Process for producing chemi-mechanical pulp
The present invention relates to a method of producing chemi-mechanical pulp
from a wood
raw material comprising wood chips, according to which method the wood chips
are
contacted with an alkaline impregnation solution at an impregnation stage
under conditions
in which the impregnation solution penetrates into the wood chips, and the
wood chips
treated with the impregnation solution are refined to a desired drainability
in order to
produce the pulp, characterized by subjecting the wood chips to deaeration,
and
impregnating the thus obtained, deaerated wood chips with an alkaline
impregnation
solution at 1.5-15 bar absolute pressure whereby the impregnation solution is
efficiently
absorbed into the wood chips before they are refined.
In a method such as this, the wood chips comprising the wood raw material are
brought into
contact with an alkaline impregnation solution in conditions where the
solution penetrates
into the wood chips to impregnate the wood chips, and the wood chips which are
impregnated with the impregnation solution are refined to produce a mechanical
pulp.
The present invention also relates to novel chemi-mechanical pulp prepared by
the method
of the invention.
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Chemi-mechanical pulp is produced by refining wood chips which have been
prepared
with chemicals, typically with an alkaline liquid, to a desired drainability.
The chemical
treatment facilitates the refining and improves the properties of the
mechanical pulp. In a
chemical cooking, too, the wood chips are prepared with alkaline solutions to
facilitate the
defibring.
In the production of mechanical as well as chemical pulp it is essentially
important that the
chemicals are both well absorbed and evenly absorbed into the pulp. An uneven
absorption
results in a high percentage of slivers in the pulp and in turn a decreased
strength potential.
Two mechanisms, namely penetration and diffusion, enable the absorption of the
liquid
and the chemicals into the wood. Penetration takes place as a result of the
differences in
pressure, and diffusion, correspondingly, as a result of the differences in
concentration. At
the beginning of the impregnation stage, the difference in the pressures
prevailing inside
the wood chips and, correspondingly, those outside them is the factor which
drives the
penetration, to the point at which the differences in pressure are evened out.
When a whole
wood chip becomes filled with liquid, diffusion takes over as the means by
which the
chemicals move into the wood chips and the reaction products out of them.
There are two kinds of penetration: natural and forced. In natural
penetration, the pressure
difference is generated by capillary forces, whereas in forced penetration the
pressure
difference is generated by an outside pressure or by generating a vacuum
inside the wood
chips.
Alkaline impregnation solutions swell the fibre walls and thus narrow the
capillaries. This
decreases the longitudinal penetration. However, at the same time, alkaline
solutions also
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dissolve the components of the fibre, which accelerates the penetration.
Diffusion is considerably slower than penetration. Raising the temperature
increases the
diffusion speed, but it is known that the speed of the reaction increases
considerably faster
than does the speed of the diffusion. Accordingly, the literature mentions,
for example, that
during the process of sulphate cooking, calculations using diffusion
coefficients and
activation energy show that the diffusion speed at 170 C is approximately 4
times higher
than at 100 C. At the same time, there is a 900-fold increase in the reaction
speed (Talton,
J., The diffusion of sodium hydroxide in wood at high pH as a function of
temperature and
degree of pulping, M.Sc. Thesis, North Carolina State University, 1986, 45 s).
When the reaction speed increases, the consumption of alkali increases as
well.
The alkali dose and the size of the wood chip, in turn, have an effect on the
diffusion. In
order to avoid generating more reject, the alkali dose must be increased when
the thickness
of the wood chip is increased. When the alkali dosage is raised, the alkali
gradient between
the wood chip and the surrounding solution increases as well (liquid/wood-
ratio the same),
and thus the diffusion speed increases.
It should also be noted that the dosage of the alkali which is dosed for the
impregnation is
the most important factor affecting the yield of the cherni-mechanical pulp
(CTMP). The
fibre loss mainly results from the breaking down of the acetic acid, and the
dissolving of
the lignin and the dissolving of acidic polysaccharides, all of which are
caused by the
alkali. The influence of the alkali dosage is greater for hardwood than for
softwood, and
for aspen it is especially large. When the alkali dose (for instance the
amount of NaOH) is
raised from 0.5 % to 3 %, the yield from aspen wood decreases linearly from
approximately 95 % to 89 % in a conventional process.
Part of the yield loss takes place already during impregnation, part not until
the following
refining stage at a high temperature.
In practice, a large alkali dose must be used to ensure maximum impregnation.
The
penetration of the alkali solution into the wood chip is slow and the alkali
tends to be
consumed in the outer parts of the wood chip, leaving insufficient alkali for
the inner part
of the chip. In the refining stage, this is expressed as a higher percentage
of slivers.
The effect of the alkali on the scattering qualities of the chemi-mechanical
pulp is
especially problematic. The qualities of the scattering fall with increasing
alkali dosage
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and, when level of refining is the same, the scattering is always smaller when
the alkali
dosage is larger. Also, bleaching decreases the scattering further. To make
high-class
printing paper it is prerequisite that the scattering and the brightness
properties of the pulp
are good. It is possible to produce hardwood CTMP pulps to a high brightness,
even up to
an ISO brightness of 88 %.
The purpose of the present invention is to eliminate at least some of the
disadvantages
associated with the known technology, and to provide a novel solution for
producing
chemi-mechanical pulp.
The present invention is based on the principle that when producing chemi-
mechanical
pulp, the impregnation of the chips is carried out at overpressure.
Richardson and LeMahieu have presented a method of producing a super-
groundwood
pulp, in which method aspen chips are impregnated at a temperature of
approximately 75
C and a pressure of 4 bar, using a mixture of sodium hydroxide and sodium
sulphite,
before the refining of the chips [Tappi 1965 (48), no 6, p. 344-346].
According to the
article, increasing the concentration of the alkali, the strength of the pulp
could be
increased and, at the same time, the energy consumption reduced. However, a
problem in
the known solution is that the addition of alkali decreases the bleachability
of the pulp.
Associated with the present invention we have unexpectedly discovered that
when
combining an effective deaeration of the chips, which is most suitably carried
out by
steaming, and a pressurized impregnation, it is possible to make the
impregnation more
effective and reduce the alkali dose significantly. When compared at the same
freeness
level, it is possible to use a significantly smaller alkali dosage to achieve
the same or even
lower sliver percentage and, at the same time, the scattering of the pulp is
rendered higher
than in a conventional unpressurized impregnation.
The method according to the present invention is carried out by arranging in
series
equipment comprising the following: the chip deaeration unit, the chip
impregnation unit
and the chip refining unit, in which case the impregnation unit comprises a
closed vessel
within which the impregnation treatment can be carried out at overpressure.
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3a
More specifically, the method according to the present invention comprises a
method of
producing a chemi-mechanical pulp from a wood raw material comprising wood
chips,
according to which method the wood chips are contacted with an alkaline
impregnation
solution at an impregnation stage under conditions in which the impregnation
solution
penetrates into the wood chips, and the wood chips treated with the
impregnation solution
are refined to a desired drainability in order to produce the pulp,
characterized by subjecting
the wood chips to deaeration, and impregnating the thus obtained, deaerated
wood chips
with an alkaline impregnation solution at 1.5-15 bar absolute pressure whereby
the
impregnation solution is efficiently absorbed into the wood chips before they
are refined.
The invention also includes a novel pulp prepared by the method of the
invention.
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Considerable advantages are obtained by means of the invention. Thus, with
pilot
experiments it has been demonstrated that using pressurized impregnation it is
possible to
decrease the alkali dose by 50 % or more. In addition, the invention increases
the scattering
of the pulp: in the experiments the scattering has unexpectedly been even
higher than in the
TMP reference (0 % of alkali). Also noteworthy is that it has been possible to
increase the
scattering without increasing the sliver percentage. At the same level of
freeness, the bulk
is improved, too.
Pressurized impregnation has produced a degree of penetration exceeding 95 %
in
laboratory experiments. In the reference points used in the experiments, the
maximum
degree of penetration has been 63-74 %. In pilot test runs, it has also been
possible to
decrease the alkali concentration from the conventional level of 0.8-1.2 %/Adt
(air dried
tonne of pulp) even to a level of 0.25 %/Adt without the sliver percentage
rising. In all test
points, the scattering has been clearly higher than in the reference.
Pressurized impregnation makes it possible to improve the quality of cherni-
mechanical
refiner pulp for different end-use applications.
In the following, the invention will be examined more closely with the help of
a detailed
explanation and with reference to examples of applications that are presented
below.
The accompanying figure presents a simplified drawing of the equipment used in
the
method according to the present invention
As explained above, the method according the present invention comprises three
stages,
which are
- first, at least the most part of the air included in the wood raw
material comprising
wood chips is removed (i.e. the wood chips are deaerated),
- second, the raw material thus obtained is impregnated with an alkaline
solution at
overpressure in order that the alkali is efficiently absorbed into the wood
chips, and
third, the treated wood chips are refined to a predetermined drainability.
As can be seen from the above, the pulp is produced by means of a chemi-
mechanical
process. In the present invention, chemi-mechanical pulp production means in
general a
process which comprises both a chemical and a mechanical defibring stage, as
described
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above. The CMP and the CTMP processes are chemi-mechanical processes. In the
CMP
process, the wood raw material is refined at normal atmospheric pressure,
whereas in the
CTMP process a pressure refiner mechanical pulp is produced. Because a higher
dosage of
chemicals is used in the CMP, its yield is generally smaller than that of the
CTMP process
5 (less than 90 %). In both cases, the chemical treatment of the wood is
traditionally carried
out with sodium sulphite (sulphonation treatment), in which case hardwood can
be treated
with sodium hydroxide, too. In this case, a typical chemical dosage in the
CTMP process is
approximately 0-4 % of sodium sulphite and 1-7 % of sodium hydroxide, and the
temperature is approximately 60-120 C. By contrast, in the CMP process, the
chemical
dosage is 10-15 % of sodium sulphite and/or 4-8 % of sodium hydroxide
(calculated from
dry wood), and the temperature is 130-160 C and, correspondingly, 50-100 C.
In a chemi-mechanical process, the chips can be impregnated with an alkaline
peroxide
solution, too (APMP process). The dosage of peroxide is generally 0.1-10 % (of
the dry
pulp weight), typically approximately 0.5-5 %. The amount of the alkali feed,
such as
sodium hydroxide, is about the same, i.e. approximately 1-10 weight %.
The present invention relates especially to the CTMP process, in which the
chips coming
from the impregnation are defibred using the pressure refiner mechanical pulp
method.
The initial material in the process according to the present invention is
chips comprising
soft- or hardwood material. In particular, hardwood chips are used in the
production, the
chips of which are prepared from birch (generally, a wood species of the
Betula genus) or a
wood species of the Populus genus or a mixture of them. Examples of suitable
wood
species of the Betula genus are B. pendula and B. pubescens, and of the wood
species of
the Populus genus especially the following: P. tremula, P. tremuloides, P
balsamea, P.
balsamifera, P. trichocarpa, P. heterophylla, P. deltoides and P.
grandidentata. Aspen (the
European aspen, P. tremula; Quaking aspen, P. tremuloides), aspen species
crossbred from
different stock aspens, hybrid aspens (for instance P. tremula x tremuloides,
P. tremula x
tremula, P. deltoides x trichocarpa, P. trichocarpa x deltoides, P. deltoides
x nigra, P.
maximowiczii x trichocarpa) and other species generated by gene technology,
along with
poplars, are considered to be particularly preferable.
Besides wood species of the Betula and Populus genera, other hardwood species,
too, can
be used as raw material, such as eucalyptus and mixed tropical hardwood. Among
the
coniferous trees, spruce (Picea abies) and pine (Pinus silvestris) and other
wood species of
the Picea and Pinus genera, should be mentioned.
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According to one application, chemi-mechanical pulp comprising up to 100 %
softwood
fibres is produced. However, with the present invention it is possible to
produce a chemi-
mechanical pulp which consists of a mixture of hard- and softwood fibres, and
which pulp
comprises at least 5 % softwood fibres, for instance it may comprise 50-99 %
hardwood
fibres and 1-50 % softwood fibres. It is possible to increase the bulk, the
strength
properties and the stiffness of the pulp, by using softwood fibres, especially
by using
spruce fibres.
The size of the wood chips of the wood raw material is generally approximately
20-50 mm
x 1-10 mm, typically approximately 35-40 mm x 3-5 mm.
First, as much of the air in the wood chips as is possible is removed.
Generally, the aim is
to remove at least 70 %, especially approximately 80-100 % of the air
contained in the
chips. The air is typically present in gaseous form. As will appear from the
accompanying
flow sheet, this deaeration can be carried out by steaming the chips in an
evaporator 1. In
the process according to the figure, the chips of the initial material are fed
in by the screw
conveyor 2 into the steaming silo 1, into which steam is fed either from one
feed nozzle or,
as the figure shows, from several nozzles 3a-3c, in order to distribute the
steam evenly into
the chips in the silo.
The purpose of the steaming is to remove the air from the wood chips. At the
same time,
steam remains in the wood chips.
The steaming can be carried out for instance in a continuous evaporator 1,
shown in the
figure, in which the wood chips move through the steaming silo 1, where they
are brought
into contact with the saturated or almost saturated steam for a period of
approximately 0.5-
20 minutes, especially approximately 1-10 minutes. The steaming can be carried
out at
overpressure but generally steaming at normal atmospheric pressure is fully
adequate. In
particular, an elevated temperature is used, for instance approximately 50-100
C,
especially approximately 80-100 C, depending on how saturated the steam used
is.
Instead of using steaming, the deaeration can be carried out at low
pressure/in a vacuum or
the steaming can be made more effective with vacuum treatment.
The treated chips are removed from the steaming silo through the outlet nozzle
4, after
which the chips are most suitably compressed in the plug screw 5. After this
stage,
typically at least 95 %, preferably at least 98 %, of the air has been
removed, and at the
same time, part of the steam has also been removed.
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For the sake of completeness, it should be mentioned that steaming of the
chips is used in
the production of both sulphate pulp and chetni-mechanical refiner pulp.
However, until
the present invention, it had never before been suggested that a combination
of the
steaming of wood chips and pressurized impregnation could be used in the
production of
chemi-mechanical pulp.
After the deaeration stage, the wood chips are conducted to the impregnation
treatment
stage 6. According to a preferred embodiment of the present invention, the
steamed wood
chips are brought, essentially still at the temperature of the steaming stage,
to the
impregnation stage which is carried out in the absorber 6. The temperature of
the
impregnation solution used in the impregnation stage is kept lower than the
temperature of
the steam in the steaming stage.
In practice, the impregnation stage is carried out in a closed vessel, i.e. a
pressure vessel,
which is arranged downstream from the steamer. The absorber illustrated in the
figure
basically comprises an elongated absorber, the longitudinal axis of which is
essentially
arranged vertically, and which has an upper and a lower part, in which case
the wood chips
coming from the deaeration unit can be fed into the upper part of the absorber
and removed
via the lower part of the absorber. In the absorber, according to the present
invention, it is
generally possible to create an absolute pressure of at least 1.5 bar,
preferably
approximately 1.5-15 bar.
When the wood chips are fed rapidly into the impregnation vessel, their
temperature may
fall at maximum approximately by 10-20 C before the impregnation stage
starts.
In the absorber 6, there is an upper separator 7. Through its input nozzle 7a
the chips are
fed into the absorber and in the separator, liquid is separated from the wood
chips. This
liquid is recirculated into the reject flow of the steaming silo 1.
According to a preferred embodiment, the steamed wood chips are fed into the
impregnation stage together with the impregnation chemicals, in which case the
impregnation chemicals are fed in from separate input nozzles 10a-10c to the
pipeline 11
which connects the outlet nozzle 4 of the steamer 1 and the input nozzle 7a of
the absorber
6. In order to create pressure, most suitably pumps 20, 21 or similar devices
are arranged in
the pipeline.
At the impregnation stage, an impregnation solution is used which comprises an
aqueous
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solution of an alkaline material, which solution optionally comprises
sulphonation
chemicals. Typically, an aqueous solution of either an alkali metal hydroxide,
such as
NaOH or KOH, or an alkaline earth metal hydroxide, such as magnesium
hydroxide,
Mg(OH)2, or calcium hydroxide, or mixtures thereof, are used. If desired, this
solution
comprises for instance also sulphite compounds, such as sodium sulphite. The
dosage of
alkali hydroxide is typically approximately 2-12 kg/Adt (air dried tonne of
pulp),
preferably, however, at maximum approximately 6 kg/Adt, more preferably at
maximum
approximately 4 kg/Adt. Alkaline earth metal hydroxides are used in (molarly)
corresponding dosages. The pH value of the solution is approximately 9-11. The
consumption of the sulphite compound is approximately 1-20 kg/Adt, for
hardwood most
suitably at maximum 3 kg/Adt.
Besides pure solutions, aqueous solutions of compounds of alkaline materials,
too, can be
used for impregnation, such as cooking liquor obtained from pulp cooking, for
instance
white or green liquor. The temperature of the impregnation stage is
approximately 30-95
C, preferably approximately 40-90 C, and which can be achieved at least in
part by the
heat brought in with the chips. Generally, the temperature of the impregnation
stage is
lower than the temperature of the deaeration stage. According to the present
invention, the
pressure of the impregnation stage is approximately 1.5-15 bar, preferably
approximately
2-10 bar absolute pressure. As a result, an overpressure of at least
approximately 0.5 bar is
used in the impregnation. The ratio between wood and liquid (pip) is generally
approximately 1:20...1:4, especially approximately 1:15...1:6.
The dosage of the impregnation chemicals can be adjusted, depending on the
chips to be
treated and, if necessary, it can be increased.
The duration of the impregnation treatment is approximately 1-240 minutes,
preferably
approximately 5-120 minutes, especially approximately10-60 minutes.
During the impregnation stage, the wood chips are impregnated with alkali to
the
maximum degree possible. Generally, at least 85 %, preferably at least 90 %,
more
preferably at least 95 % of the volume of the pores of the wood chips should
be filled with
the impregnation solution.
The impregnation can be carried out in one or in several stages, in which case
at least one
impregnation stage is carried out at overpressure. According to a preferred
embodiment,
the hot chips are first impregnated at overpressure under the conditions
mentioned above,
after which the impregnation process is still continued in an open container
or vessel at the
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same or at a different temperature. Approximately 10-80 % of the temporal
duration of the
impregnation treatment can be carried out under pressurized conditions. In the
application
described below, the duration times of the pressurized and the unpressurized
treatments
were equally long, the total duration time being 40 minutes.
The wood chips coming from the impregnation stage are removed through the
outlet
nozzle 6a. The reference number 6b refers to the discharger of the absorber.
With this
discharger the residual fraction that has collected at the bottom of the
device can be
removed. After that, the chips are fed to a refming stage 12 of conventional
chemi-
mechanical pulp, which can be carried out for instance in refiners equipped
with grooved
refining blades. The wood raw material is refined to a predefined
drainability, which is 50-
500 ml CSF, more preferably approximately 90-150 ml CSF.
The basic drawing 1 shows in practice how the chip flow generated from the
impregnation
stage can be further processed before the refining. Accordingly, the excess
impregnation
solution is first removed in the screw press 13, after which the reaction of
the chemicals
can be continued in the reaction silo 14, before the chips are transported
with the screw
conveyors 15a and 15b to the refining. The reaction time in the reaction silo
14, if such a
silo is used, is typically approximately 0.1-10 hours.
In the screw press 13, it is possible to separate impurities and fibre
material which are unfit
for refining, and they are removed through the screen 16 into the reject
channel. The liquid
phase 17 generated from the screw press can be recirculated into the pipeline
10, possibly
in combination with the fresh water feed.
The reference numerals 22 and 23 refer to the pumps arranged for the feeding
of the liquid
phase. As the figure shows, the impregnation solution is most suitably
recirculated in the
process and its alkali concentration can be adjusted (increased) with fresh
alkali feed.
It should be pointed out that the two essential factors in the present
invention are that a
good deaeration is achieved before the impregnation solution and the chips are
put
together, and that the impregnation is pressurized. These two factors together
allow an
efficient penetration of the impregnation solution into the wood chips. The
retention time
and the temperature in the pressure vessel are chosen in such as way that it
is possible to
adjust the diffusion time and the reaction time. There must be enough time for
the diffusion
to take place, the speed of the reaction must not be too high.
The chemi-mechanical pulp described above has extraordinarily good properties.
As
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described in the introduction, the light scattering qualities of the pulp are
improved, and
has been achieved without increasing the shives percentage. Consequently, at
the same
freeness level, the light scattering qualities of the pulp according to the
present invention
are at least 5 %, even 10 %, better compared to the high alkali reference. At
the same time,
5 the sliver percentages of the pulps according to the present invention
are lower than the
sliver percentage of the TMP reference and, unexpectedly, even lower than the
sliver
percentage of the high alkali reference. At the same freeness level, the bulk
is improved,
too, by as much as 5 %.
10 A noteworthy example is that the scattering qualities of CTMP pulp
prepared from aspen
may be higher than 45 m2/kg and the sliver percentage lower than 0.3 %.
Correspondingly,
from birch it is possible to generate a pulp exhibiting scattering qualities
which are higher
than 45 m2/kg and a sliver percentage lower than 1.5 %. These are only
examples of the
properties of pulp, and it should be pointed out that for a pulp producer,
within the limits of
the present invention, it is possible to choose freely either a desired level
of the scattering
or the sliver percentage and, with the help of the present invention, to
achieve a substantial
improvement in one of the parameters.
The pulp according to the present invention can be used for the production of
paper and
cardboard products.
Consequently, following the defibring described above, the pulp is generally
bleached,
using for instance hydrogen peroxide under alkaline conditions, to a
brightness of
approximately 75-88 %.
If desired, it is possible to tailor the properties of the initial material by
mixing the pulp
with a chemical pulp in such a way that a slushable initial material is
achieved, one which,
however, comprises a significant amount (at least 30 weight %) of chenai-
mechanical pulp.
Preferably, softwood pulp is used as chemical pulp and, in that case, its
percentage is 1-50
% of the dry weight of the fibres of the raw material. However, it is also
possible to use
only chemi-mechanical aspen pulp.
The paper pulp is first slushed to a suitable consistency in a way which is
known per se
(typically to a solids percentage of approximately 0.1-1 %), after which it is
spread onto
the wire, where it is webbed to form the paper or cardboard web. It is
possible to add a
filler such as calcium carbonate, generally approximately 1-50 weight % of the
weight of
the fibres, into the fibre slush.
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The paper web can be surface sized and/or equipped with a coating layer and,
if desired,
calendered. Coating pastes can be used for single coating, for precoating and
for surface
coating. Triple coatings are possible, too. Generally, a coating according to
the present
invention contains 10-100 parts by weight of at least one pigment or a mixture
of pigments,
0.1-30 parts by weight of at least one binder, and 1-10 parts by weight of
other additives
known per se.
In the way described above, it is possible according to the present invention
to produce
from the pulp material webs which have excellent printability properties, good
smoothness
and high opacity and brightness. Examples of applications are fine papers,
coated printing
papers and brochure papers, and liners of multilayer cardboards.
The following non-restrictive examples illustrate the present invention.
Example 1
CTMP pulp of aspen was prepared in the laboratory under the following
conditions:
Aspen wood chips, which had been steamed at 100 C for a period of 2-5 minutes,
were
impregnated with different amounts of sodium hydroxide at a pressure of 5 bar
(a), at 80
C for a period of 20 minutes in a closed container. After that, the
impregnation was
continued for another 20 minutes in an open reaction silo at 80 C.
The following methods, among others, were used to determine the properties of
the pulp:
¨ bulk cm3/g: EN 20534
- scattering m2/kg: ISO 9416
- CSF ml: ISO 5267-2
- shives: "Pulmac shives", sample quantity 3 g and a gap plate of 0.08 mm
for 150
ml of CSF pulp and of 0.10 mm for 325 ml of CSF pulp.
Table 1 shows what happens when the aspen chips treated in this way were
refined to a
drainability level of 150 ml CSF.
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Table 1
NaOH dose, % NaOH, g/1 NaOH, mo1/1 Scattering, m2/kg Shives,
1.2 _4.8 0.12 53.5 0.43
_0.42 1.68 0.042 58.5 0.07
0.33 1.32 0.033 56.4 0.04
0.23 0.92 0.023 59.0 0.14
0 0 0 57.3 0.62
As the table shows, using the present invention it is possible to decrease the
alkali
hydroxide dosage, in which case the scattering clearly increases without
increasing the
sliver percentage. Compared with the case in which a conventional amount of
alkali was
used, the scattering increases by more than 10 %. Unexpectedly, the sliver
percentage was
even lower than in the 1.2 % alkali reference.
Table 2 shows the bulk of the pulps described above.
Table 2
NaOH dose, % Bulk, cm3/g
1.2 2.66
0.42 2.70
0.33 2.81
0.23 2.74
_0 2.83
It can be seen that the bulk, too, is slightly better at the same CSF level.
The experiments were repeated with birch chips. Pressurized impregnation of
birch (80 C,
5 bar, 20 min) at the CSF level of 325 ml produced the following scattering
values:
Table 3
NaOH dose, % NaOH, g/1 NaOH, mo1/1 Scattering, Shives, %
m2/kg
1.1 4.4 0.11 41.3-42.5 2.24
0.42 1.68 0.042 45.1 1.40
In this case, too, a significant improvement in the scattering was thus
achieved, although
the sliver percentage remained high in the experiment. However, it was one
third lower
than in the reference.
In the case of birch, a reduced alkali dose did not have any considerable
effect on the bulk.
CA 02631767 2008-06-02
WO 2007/063171 PCT/F12006/000399
13
Example 2. Laboratory impregnations
In the laboratory, impregnations using aspen and birch were carried out at
normal
atmospheric pressure, using alkali doses of 2.5, 5 and 10 kg Na0H/Adt chips.
The
wood/liquid ratio was 1:8, and the temperature 80 C.
Samples were taken from the impregnation solution at the following times: 15
min, 30 min,
1 h and 3 h from the beginning of the impregnation process. The organic
material which
was dissolved in the impregnation solution increased drastically with
increasing alkali dose
and as a function of the impregnation time. At the same time, the dissolved
COD
increased drastically.
The table below shows the results, achieved in the example, of the effect of
the alkali dose
on the fibre loss, in the 40 minute laboratory impregnation of aspen.
It should also be pointed out that the fibre loss was determined from the
impregnation
solution before the refining. Refining increases the amount of dissolved
material and,
correspondingly, it increases the fibre loss, the larger the alkali dose the
greater the loss.
Table 4
NaOH dose, % NaOH, g/1 NaOH, moUI Fibre loss, %
1 1.43 0.035 2
0.5 0.71 0.017 1.1
0.25 0.36 0.009 0.65
