Note: Descriptions are shown in the official language in which they were submitted.
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Method and apparatus for processing pulp stock derived from a pulp or paper
mill
The present invention relates to a method and apparatus according to the
preambles
of the independent claims presented below for processing pulp stock,
particularly
silted broke in a pulp or paper mill.
TECHNICAL BACKGROUND
Broke is generated in many steps in a paper manufacturing process, and the
broke
contains fibres and possible fillers, adhesives, coating materials, etc. Thus
the broke
contains both useful raw material and waste, which must be removed from the re-
cycled raw material. The task of the broke processing system is to return the
valu-
able raw material components of the broke into the process.
Broke pulping
The first processing step is pulping of the broke. There are pulpers at
several loca-
tions under the paper machine and the coating machine. In break situations the
pro-
duction of the machine can be directed to these pulpers. There are further
pulpers
adjacent to the glazing calanders and the slitter winders.
The edge trimmings of the slitter winders can be directed to any of the above
men-
tioned pulpers or to a separate edge trimmings pulper. Most often there is a
separate
pulper for pulping broke rolls.
The system generally also contains a two-stage screening before the pulp
deposition
and defibration, because the mill broke contains plastics and other
impurities.
Pulping means to mix broke in web form with water, so that there is created a
pulp
stock which can be pumped and where the fibres are at least partly apart from
each
other.
Broke equalisation is carned out in the pulper. In a dry cellulose or paper
sheet the
fibres are closely bonded to each other. In order to break the bonds between
the fi-
bres the pulp must be soaked in water. Then the fibres also will swell when
water
penetrates the pores of the fibres. External forces are also required in order
to sepa-
rate the fibres. They are generated by strongly agitating the pulp stock. This
is pro-
vided by a rotor located in the pulper. It operates like the impeller of a
pump; the
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pulp flies outwards from the periphery and at the same time a strong suction
is cre-
ated in the centre of the rotor.
The dispersion properly of broke paper depends on the paper quality. The
dispersion
is reduced by paper adhesives, surface treatment and coating. The most easily
dis-
persed paper qualities are newsprint and other uncoated wood-containing paper
qualities. Calandering makes it more difficult to disperse the above mentioned
paper
qualities. Paper qualities which are particularly difficult to disperse are
for instance
wet strength papers and coated paper qualities as well as kraft liner and
kraft paper.
Broke storage
A production line making coated paper qualities has usually separate broke
storage
towers for uncoated and coated broke. This is because it is desired to portion
out the
coated broke in a controlled way back to the paper machine, for instance due
to ash
control of the base paper.
Broke screening and defibration
Screening of the broke stock is the more necessary the higher quality the
manufac-
tured final product shall have.
Poorly dispersed broke, or impurities accompanying the broke (for instance
plastics,
shives, sand), cause problems in the process and breaks at the paper machine.
A modern screening and defibration line for broke typically comprises
- a protective screen, generally a pressure screen provided with a perforated
drum,
- broke defibrators,
- a pressurised screen, which is a pressure screen provided with a slit drum
or a per-
forated drum, and
- a broke reject screen, which is either an oscillating strainer or a pressure
screen.
The purpose of the protective screen is to remove large solid impurities from
the
broke, for instance pieces of tape, pieces of threading rope, shives.
The pressurised screen returns to the defibration such fibre clusters which
were not
dispersed.
The purpose of the broke reject screen is to separate from the pulp such
refuse and
other solid particles, which must be removed, and to return approved pulp to
the
screening circulation and back to the process.
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If the papers in question contain fillers and corresponding non-organic
components,
the mineral containing fraction is supplied further to a separate recovery
plant (filler
recovery).
As noted above, it is very difficult to process the broke from paper
qualities, which
are provided with adhesives, surface treated and coated. The organic
components in
the adhesives and the coating slip cause agglomeration of the fibre material
and the
mineral pigments. The generated agglomerates do not disperse in a satisfying
way in
known dispersing devices, so most part of the processed material is useless in
sense
that it can not be returned to the process. Therefore such materials often
must be
disposed of, for instance by driving them to the dumping area. This is an
environ-
mental disadvantage, but primarily it is a substantial loss of fibrous raw
material and
valuable non-organic filler. A machine producing coated paper can produce such
material in an amount of about 10 tons per day. A recovery and reuse of such
mate-
rial in the process would mean great cost savings on a yearly basis.
The object of the invention is to eliminate the above mentioned problems and
to
provide a method in a pulp or paper mill for processing pulp stock containing
fibres
and/or any other components in order to disperse agglomerates contained in the
pulp
stock.
With the method it is also possible to process difficult broke qualities, and
thus to
recover useful raw materials contained in them, and to return the recovered
materi-
als to the process. This processing can also substantially improve the optical
prop-
erties of the produced paper, in other words increase both the lightness and
the
opacity.
With the method it is also possible to process pulp stocks containing
extractives in
an adhering form (agglomerate form) and to disperse the agglomerates contained
in
these pulp stocks, in order to transform the extractives into a non-adhering
form.
These objectives are achieved with the method and the apparatus according to
the
invention, which are characterised in what is presented below in the
characterising
parts of the claims.
The term "pulp stock" covers any stock made of a wood-based pulp. The pulp can
be mechanical pulp, semi-chemical pulp or chemical so called virgin stock, or
it can
be any broke flow present in the paper making, or a mixture of said
components.
The term "broke processing" covers in this invention also the deinking of
printed
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paper, where the ink is removed from the fibrous pulp.
Devices operating with the double action impact mill principle are previously
known for instance from the Danish patent publication DK 104778, the Finnish
pat-
ent applications FI 945945, FI 946048 and FI 955474. These devices are
character-
s ised in that two coaxial rotors provided with blades are arranged in a
housing,
whereby the rotors are arranged within each other in the housing and arranged
to
rotate in opposite directions within the housing.
An apparatus according to the invention comprises
- a first rotor provided with blades,
- a second rotor, which is coaxial with the first rotor and provided with
blades, the
second rotor being arranged to rotate in the opposite direction regarding the
first
rotor, so that the rings of the first rotor and the rings of the second rotor
are arranged
in an intermeshed fashion, and
- a feeding opening which opens to the nave of the rotors.
1 S The material flow to be processed (the pulp stock) is supplied into the
feeding
opening which opens out to the rotor naves and the material flow is caused to
flow
via the blades of the rotors arranged within each other, to the outermost
rotor ring,
and to be discharged from there as a discharge flow from the apparatus.
With this method it is for instance possible to process very different silted
broke
fractions, both very highly fibrous broke and broke containing a high degree
of
filler. Thus the silted broke fraction can be coated broke from the pulpers, a
flow
containing filler and directed to the filler recovery plant, reject from the
filler recov-
ery plant, or reject discharged from the screening and defibration plant.
The method is also particularly well applicable for the reduction of
extractives ad-
hering to different virgin pulps.
The solid contents of the pulp stock can vary in a wide range, suitably from 3
to
60 %.
According to a suitable embodiment the suspension obtained as the discharge
flow
from the double action impact mill is recirculated either wholly or partly,
one or
more times, to the double action impact mill. Another suitable embodiment
utilises
two double action impact mills connected in series.
An apparatus according to an embodiment is provided with a housing of its own.
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S
The number of rings of the apparatus is at least two. The energy consumption
of the
apparatus increases when the number of the rings increases, and therefore an
opti-
mal number of the rings is probably 2 to 5.
The invention is described in more detail in the enclosed figures, in which
Figure lA shows in a vertical cross section the double action impact mill used
in the
method according to the invention;
Figure 1B shows an alternative solution to that of figure lA;
Figure 2 shows an apparatus of the type in figure 1, in a horizontal cross
section;
Figure 3 shows an equipment for processing broke with a method according to
the
invention;
Figure 4 shows another equipment, where the apparatus according to the
invention
is used for processing broke;
Figure 5 shows a third equipment, where the apparatus according to the
invention is
used for processing broke;
Figure 6 shows a fourth equipment, where the apparatus according to the
invention
is used for processing broke;
Figure 7 shows a fifth equipment, where the apparatus according to the
invention is
used for processing broke;
Figure 8 shows schematically the broke processing system in a paper mill
producing
coated paper;
Figures 9a to 9d, 1 Oa to 1 Od and 11 a to 11 d show microscope photographs of
a
sample taken from the reject of a filler recovery plant; and
Figure 12 shows the effect of the method according to the invention on the
particle
size distribution of the reject from a filler recovery plant.
Figure lA shows in a vertical cross section the double action impact mill 20
used in
the method according to the invention, whereby the mill comprises a housing
10,
within which there is disposed a rotor 11 provided with blades la, lb, ...,
3a, 3b, ...,
and so on (the individual blades are better seen in figure 2). Within the
housing
there is also disposed another rotor 12, which is coaxial with the first rotor
11. The
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second rotor 12 is also provided with blades 2a, 2b, ..., 4a, 4b, ..., and so
on. The
blades Ia, lb, ..., 2a, 2b, ... 3a, 3b, ... of the first rotor 11 and the
second rotor 12 are
disposed in coaxial rings 1, 2, 3, ... so that the rings 1, 3, 5 of the first
rotor 11 and
the rings 2, 4 of the second rotor 12 are arranged in an intermeshed fashion.
Then
the rotors 11 and 12 with their blades can freely rotate in opposite
directions. At the
end of the housing there is disposed an opening 14 which opens out to the
naves of
the rotors 11 and 12, whereby this opening acts as the feeding opening of the
pulp
stock. This feeding arrangement is possible as the axes of the rotors are
disposed
within each other, as in the solution of the patent application FI 946048. In
the wall
of the housing there is arranged an opening 1 S which opens out to the
outermost
blade ring, whereby this opening acts as the discharge opening.
The rotors rotating in the opposite directions generate strong centrifugal
forces
which effectively keep the flow-through in motion, which could not be possible
with a stator/rotor-system.
Figure 2, showing an apparatus of the type in figure 1 A in a horizontal cross
section
(however, modified so that each rotor 11, 12 has one ring more than in the
apparatus
of figure 1 ), presents the direction of rotation of the rotors. Each rotor
can of course
also rotate in the opposite direction.
It is not necessary that the peripheral wall of the housing of the double
action impact
mill is immediately adjacent to the rotor pair, but it can be farther away,
whereby
the housing can be quite spacious. Then the purpose of the housing is mainly
to act
as a recovery container for the processed material. Figure 1 B shows a
solution in
which the apparatus used in the method according to the invention is not
provided
with a fixedly mounted housing of its own. The apparatus comprising the rotor
pair
11, 12 is disposed in a container 30 having an opening 14a arranged in
connection
with the feed opening 14, and having a discharge opening 15a for the processed
material flow.
According to the solution of figure 2 the horizontal distance L between the
rings 1,
2, 3, ... is about 3 mm and equal between all rings. When required the
apparatus can
be constructed, or the apparatus can be adjusted, so that the distance L
between the
adjacent rings increases or decreases towards the outermost ring 7 (not shown
in the
figure).
According to one embodiment the apparatus can be constructed so that the
distance
S between the blades of the outermost rings is smaller or larger than the
distance
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between the blades of the inner rings.
The above mentioned measure can present a possibility to ensure that also
coarser
material can be supplied to the apparatus (for instance reject pulps
containing large
agglomerates), but despite that provide a final product which is sufficiently
well
dispersed. An essential advantage is that the number of blades on the rotor
rings and
the distances between the rings (tightness) are selected according to the
require
ments. The distance between the rings, as well as the distance between the
blades in
the rings, can be arranged so that they decrease towards the outer ring. Then
the dis
persed agglomerates will pass into ever tighter spaces before the obtained
suspen
sion is discharged from the apparatus.
According to figure 2 the blades, which have a rectangular profile in cross
section,
are turned so that the impact surfaces of the blades are directed radially.
For exam-
ple, in order to increase the impact effect, the blades of one or more rings
can also
be turned so that the direction of their impact surfaces are different from
the radial
direction. Of course the blade profile in the cross section can also be
replaced by a
triangular cross section, whereby the impact surfaces of the blades are not
parallel,
but they form a certain angle between them.
Figure 3 shows the apparatus used in the invention, where the double action
impact
mill 20 is connected to a recovery container 16 with a volume of 300 litres in
the
tests. The recovery container 16 contains a mixer driven by a motor M. The
refer-
ence numeral 17 is the feed piping and 18 is the discharge piping. The
discharge
pump is marked by the reference numeral 19.
Figure 4 shows another apparatus, which contains only the double action impact
mill 20 and the feed piping 17 and discharge piping 18.
Figure 5 shows a third embodiment of the invention where the material flow
accu-
mulated in the recovery container 16 is partly returned to the double action
impact
mill 20 with the aid of the recirculation piping 21. Of the material supplied
to the
container 16 this arrangement suitable returns 1/4 ... 1/2 to the double
action impact
mill 20. The rest is returned to the process via the overflow pipe 18.
Figure 6 shows an apparatus solution which in other respects is similar to
that of
figure 5, except that two double action impact mills 20 and 20' are connected
in se-
ries. Instead of returning the material flowing in the overflow pipe 18 to the
process
it is directed to the second double action impact mill 20', from which the
material is
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discharged via the pipe 18' .
Figure 7 shows an apparatus solution which in other respects is similar to
that of
figure 6, except that there is no material recirculation to the first double
action im-
pact mill 20.
Tests
A factory test was made where samples were taken from the broke stock
processing
line of a paper mill producing coated paper, and these samples were supplied
to a
double action impact mill for processing. The test was made under operating
condi-
tions, so that small side flows were supplied to the double action impact mill
from
the sampling points.
Sampling points:
The figure 8 shows the marked sampling points 1 to 4 in a very schematically
pre-
sented broke process in a paper mill.
No. 1 at the flow input to the filler recovery plant from the broke processing
and screening section before the dispergator (the point is before the Supraton
dis-
pergator, or before the double action impact mill, respectively);
No. 2 after the broke recovery container (broke tower);
No. 3 fibrous reject after the broke screening and defibration;
No. 4 reject from the filler recovery plant.
Samples were taken at each test point both before the double action impact
mill and
after it. At the sampling point no. 1 a sample was taken also after the
Supraton dis-
pergator. This sample acted as a reference for the double action impact mill,
because
the Supraton dispergator and the double action impact mill were connected in
paral-
lel.
The test run variables were:
1 ) the feed volume to the double action impact mill,
2) the rotational speeds of the rings in the double action impact mill,
3) the effect of the recirculation in the double action impact mill on the
decomposi-
tion result,
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4) series connection of a number of double action impact mills.
The following laboratory analyses were carried out from the samples:
All samples were photographed through a microscope. The ash and consistency re-
garding the flows coming to the double action impact mill (input samples) were
de-
termined.
For some samples were further determined the SR numbers (drainage resistance,
Schopper-Riegler) and the particle size distribution (Sedigraf). Further, test
sheets
with a laboratory sheet mould, using as partial components the samples
obtained at
different test points were produced.
Results
The solid contents (consistency):
test point 1: 40
test point 2: 4,3
test point 3: 7,2
test point 4: 17
Figures 9a to 9d show microscope photographs (enlargement 90x) of a sample
taken
at the test point 4, which was unprocessed (9a), once processed in the double
action
impact mill (9b), twice processed in the double action impact mill (9c), and
proc-
essed three times in the double action impact mill (9d). In the figures it can
be seen
that the unprocessed sample (9a) contained large filler agglomerates, and that
every
cycle through the double action impact mill caused a substantial reduction of
the
particle size.
Figures l0a to lOd show another similar test result, however, taken at another
fac-
tory, where the sample corresponded to the above mentioned test point 4. The
en-
largement is 90x. Figure l0a represents an unprocessed sample, figure lOb
repre
sents a sample which is processed once in the double action impact mill,
figure lOc
represents a sample which is processed twice in the double action impact mill,
and
figure lOd represents a sample which is processed three times in the double
action
impact mill. Also these figures show the effect of the double action impact
mill in
dispersing the pigment agglomerates.
A test was also made, where the effect of the double action impact mill on the
sam-
ple from the test point 4 was observed. The double action impact mill was con-
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nected before the tank before the cyclone cleaner equipment of the filler
recovery
plant. The figures 11 a to 11 d show microscope photographs (90x) of the
samples
taken at the test point 4: before ( 11 a) the double action impact mill was
connected to
the process, 1.5 hours later { 11 b), 3 hours later ( 11 c), and 4.5 hours
later ( 11 d).
5 Figure 12 shows the effect of the method on the particle size distribution
of the re-
ject from a filler recovery plant. The x axis of the figure represents the
particle size
and the Y axis represents the undercut, i.e. the proportion of material having
a par-
ticle size which is below the particle size marked on the X axis. The light
columns
represent material which is not processed in the double action impact mill.
The dark
10 columns represent material which is processed once in the double action
impact
mill. Material with a particle size smaller than 15 ~m is in practice accept,
useful for
the process. The accept ratio of untreated material is only about 27 %, while
the ac-
cept ratio of material processed once increased to about 91 %.
In the above described tests was used a double action impact mill with three
pin
1 S rings in the upper rotor and two pin rings in the lower rotor. The
rotational speed of
the motor for the upper rotor was 1500 rpm, and the rotational speed of the
motor
for the lower rotor was 2000 rpm. The maximum flow fed into the double action
impact mill was 6 m3/hour.
It is of course possible to obtain better results than those presented above
by opti-
mising the running conditions.
According to a particularly recommendable manner to run the apparatus the rota-
tional speed of its outermost ring is 40 to 80 m/second.
A factory test was also performed, where the pulp after the broke tower was
proc-
essed only in the manner according to the invention, in a double action impact
mill,
ZS after which the pulp was run in the paper machine. The paper's technical
properties
were at least as good as in normal operating conditions. The retention was
improved
and the filler consumption decreased, because the pigment originating from the
coating replaced fresh filler.
On the basis of the tests we can see that the invention provides essential
advantages.
When the apparatus according to the invention is connected to a broke
processing
equipment it would make it possible to substantially reduce the size of the
cyclone
cleaning equipment, which at present comprises 6 stages. The broke processed
with
an apparatus according to the invention, i.e. the feed to the cyclone cleaning
plant,
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would require only a 2-stage cyclone cleaning.
Also laboratory sheets were made with the aim to simulate the quality of the
paper
made in the paper machine in a case where the pulp after the broke tower is
proc-
essed only by a double action impact mill, after which the pulp is supplied to
the
paper machine.
Thus about 15 % pulp after the broke tower was used for the laboratory sheets.
The
rest was material supplied normally to the paper machine (virgin pulp,
pigments,
etc). For comparison we made three series, in which the pulp after the broke
tower
was a) unprocessed, b) once processed with the double action impact mill (" 1
run"),
and c) twice processed with the double action impact mill ("2 runs"). For each
series
the consistency, ash, charge and turbidity of the broke pulp were measured.
The re-
sults are presented in table 1. For each series were made sheets containing 15
broke pulp, and then we measured the lightness and opacity. The results are
pre-
sented in table 2.
Table 1: Properties of the broke pulp
Unprocessed 1 run 2 runs
Consistency, % 11.74 8.44 18.54
Ash, % 84.1 84.3 84.8
Charge -57.3 -46.3 -53.6
Turbidity 114 18 26
Table 2: Optical erties of the a function of the
prop laboratory proc-
sheets as
essing of the broke
pulp component
Unprocessed 1 run 2 runs
Lightness, % (ISO)
- sheet's upper 84.42 85.85 86.30
side
- sheet's lower 83.84 85.65 86.16
side
Opacity, % 71.72 76.31 75.53
From the results presented in table 2 it can be found that the processing of
the broke
pulp component in the double action impact mill will have a surprisingly
advanta-
genus effect on the optical properties of the paper. An increase in the
lightness by 1
to 1.5 percent units is usually regarded as a significant achievement.
Traditionally
an increased lightness is obtained by adding bleached cellulose, which again
lowers
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the opacity, or by adding an expensive titanium oxide pigment or corresponding
pigment.
On the basis of the results presented in table 1 it is also proofed that the
lightness
and opacity of the paper can be increased simultaneously by processing the
broke
pulp one or more times in a double action impact mill, and by supplying the so
processed pulp flow to the paper making.
From the results presented in table 1 it can also be observed that the
turbidity of the
water phase of the pulp stock decreases quite significantly as a result of the
process-
ing in a double action impact mill. The high turbidity of the water phase of
an un-
processed pulp stock is due to materials in colloidal form which later adhere
particu-
larly to the drainage equipment of the paper machine. Such materials are
typically
extractives, such as resin and latex. The extractive in colloidal form, i.e.
the adher-
ing resin or the like extractive, is an agglomerate made up of small
particles, and
any latex particles will accumulate on the surface of this agglomerate. Tests
made
I S have shown that the processing in a double action impact mill removes
these adher-
ing agglomerates, either by bonding them to fibres or pigment particles, or by
dis-
persing them into small particles which will not adhere. Also the fact that
the charge
of the pulp stock remains almost constant speaks in favour of an effect
mechanism
of this kind.
On the basis of the above mentioned results it is obvious that the amount of
adher-
ing extractives of any pulp stock containing extractives can be reduced by
process-
ing the pulp stock with the method according to the invention. The pulp stock
can be
coated broke, as in the test described above, another reject flow, or a fresh
pulp
(mechanical, semi-chemical, or chemical), which is processed in connection
with
the pulp making (for instance before the drying of the cellulose) or in
connection
with the paper making, before said pulp is supplied to the paper machine. It
is also
possible to supply talc or some other dispersing agent together with said pulp
stock
into the double action impact mill, so that the dispersing agent will
"capture" the
dispersed particles and prevents the creation of inconvenient agglomerates.
Particu-
larly mechanical pulps, such as thermomechanical pulp, have high contents of
ex-
tractives. It would be very difficult to feed talc or dispersing agent into a
pressurised
process equipment, but the problem could be solved when the pulp is processed
in a
separate process step according to the invention.
The above mentioned embodiments of the invention are only examples of how the
inventive idea can be realised. To a person skilled in the art it is obvious
that differ-
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13
ent embodiments of the invention may vary within the scope of the claims
presented
below.
