Note: Descriptions are shown in the official language in which they were submitted.
1 333965
Method and apparatus for spreading pulp to a web
The present invention relates to a method of spreading
high-consistency pulp to a web and an apparatus for carrying
out the method. The ob~ect of the invention is to provide
a method and an apparatus for spreading pulp to a web in
the consistency range of 6 - 15 ~, in particular in the
range of 8 - 12 %. The method of the invention is especial-
ly well applicable in the head boxes of paper machines.
Another example where this invention can be applied are pro-
cesses in which high consistency pulp must be spread to an
even mat-like layer.
A web-forming method is now-a-days commonly employed in
which pulp is supplied from a head box in a stream through
a narrow slice extending over the whole width of the machine
onto a web forming wire or between web forming wires. In
order to form a good web the thickness and the direction
of the pulp stream must be even and homogenous. When the
consistency is low, i.e. up to 1 %, this does not cause
problems but at higher consistencies when fibers tend to
floc the situation is quite different.
It is very difficult to spread out high consistency pulp
to an even web-like layer as pulp of for instance at the
consistency of 10 % is very solid, in other words the
bondings between the fibers are so tight that "a snow
ball" can be formulated of it without removing water.
The amount of fiber bondings in the pulp mixture is directly
proportional to the consistency of the pulp. The shear
stress ~ required to disrupt the bondings increases accord-
ing to the following formula (Gullichsen & Harkonen, Tappi
Pulping Conference, 1980)
r= kC
where k and ~ are constant factors typical of that par-
ticular consistency range.
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For example the shear stress required for disrupting thebondings between the fibers in bleached pine sulfate pulp
increases so much that the stress required for disrupting
the bondings in a pulp of the consistency of 10 % is
approximately 100 times the stress required for disrupting
the bondings at the consistency of 1 %.
At certain conditions, high consistency pulp can be caused
to behave momentarily like water as regards its flow
properties. This phenomenon is called fluidizing. In
fluidization, energy is introduced to the pulp in order to
disrupt the bondings between fibers. The amount of energy
required is proportional to the consistency of the pulp
and the amount of bondings between the fibers. Fluidizing
does not produce continuously fluid pulp or loose fibers
and the higher the consistency of the pulp is the faster
the fibers contained in the pulp tend to attach to each
other. At adequately high consistencies the fibers attach
to each other or resume their original network structure
in a centesimal or even millesimal of a second. Tests have
shown that the conventional channels tapering in the flow
direction employed in prior art head boxes can not be used
with high consistency pulp. High consistency pulp flocs
and easily clogs the flow channel when it flows in the
conical structures. In the tapering cones the fibers in
the high consistency pulp are packed tight which easily
prevents the flow and clogs the flow channel. Clogging
results from packing of the "flow lines" without tur-
bulence. In this respect high consistency pulp differs asto its flow properties from water and dilute pulps.
Finnish patent no. 51116 teaches that if high consistency
pulp is subjected to great shear forces the fibers can be
distributed evenly over the whole width of the machine.
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Finnish patent application no. 1284/73 discloses an
apparatus in which high consistency fiber suspension is
distributed to a plurality of adjacent channels and sprayed
at a high speed to a chamber where the direction of the
flow is suddenly changed.
Swedish patent specification no. 7807097-6 discloses a
method and an apparatus of forming a web of fiber suspen-
sion, in particular of high consistency paper pulp
suspension according to which method the suspension is
caused to flow tangentially in parallel streams to a mixing
chamber which has a cross section favouring turbulence and
from which chamber the pulp is discharged after a change of
direction of 90 - 180.
Also structures are known in which a rotating means, for
example a perforated roll, is disposed in the head box in
transverse direction in relation to the machine. These
structures are not employed to fluidize high consistency
pulp but to break flocs in dilute pulp or to add turbulence
to the pulp. The purpose is rather to homogenize the pulp
and to even out pressure variations from short circulation
apparatus and pumps. High consistency pulps easily clog a
perforated roll and thus make web formation difficult.
Examples of structures of this kind are discussed below.
German patent specification no. 10 58 355 discloses an
apparatus in which part of the water molecules contained
in the paper pulp are displaced by pressure air and a
revolving mixer. The number of revolutions of the mixer
must be so low that no separation of phases occurs. The
high revolution speeds required by fluidization would be
harmful for the operation of the apparatus described here
as they could separate the pressurized air from the rest
of the pulp which would eliminate the benefit of adding
the pressurized air.
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German patent specification no. 22 39 080 discloses a
paper machine head box comprising a narrow annular duct.
In the center of the duct there is a movable roll disposed
transverse to the direction of the machine. According to
this publication, turbulence can be created by supplying
pulp to the narrow annular duct in which the pulp is
divided in two and pulp flows to the slice duct from two
directions.
German patent specification no. 12 94 177 discloses a
paper machine head box comprising a stationary perforated
roll in front of the slice opening in the transverse
direction in relation to the machine. Pulp is supplied to
a channel inside the perforated roll and distributed
therefrom to the slice opening. A rotating shaft expanding
conically is provided inside the perforated roll to equalize
the flow from the head box. The perforated roll filters
the non-flocculated fibers to the slice. The shaft expand-
ing conically tapers the flow channel conically. Struc-
tures which taper conically are not applicable for pulps
of the consistencies of 6 - 15 %. Further, the holes of
a perforated roll are easily clogged at this consistency
range.
Also a device disclosed in Finnish patent application no.
843463 is known. The device comprises a conically expanding
shaft provided with a feed screw, the shaft being disposed
transverse to the machine direction in front of an elongated
opening. A space having an annular cross section area
continuously tapering in the feed screw is defined between
the core of the screw and the surface of the back of the
screw. This invention does not relate to fluidizing or
even breaking flocs but to a feed screw unit used for
pushing pulp of the consistency of even 9 - 12 % out through
an elongated opening. The quality of the web thus formed
1 333965
is not even or controlled. A basic precondition for the
formation of an even and controlled web is that the fiber
network is broken and the fibers are loose to form a new
network in the forming web and that the formed web can
flow on. A conically tapering flow duct can for these
reasons not be used in the flow of high consistency pulps.
U.S. patent no. 3,563,853 discloses a method of spreading
the flow supplied to a head box to a flow of the width of
the web to be formed. The object of the invention is to
provide an even, wide and smooth flow by means of rotating
elements and changes in the volume of the channel. The
rotating elements are disposed transverse to the machine
direction; they do not fluidize the pulp.
U.S. patents no. 3,119,734 and 3,255,074 disclose a head box
containing a rotating element located transverse to the
machine direction. In U.S. no. 3,119,734 the rotating
element is a roll the shell of which is provided with thin
toothed plates in an inclined position in relation to the
center of the shaft. The purpose of the structure is to
control the inlet flow and to distribute the pulp evenly.
The pulp flows to the slice in a conically tapering duct.
The object of the invention of U.S. patent no. 3,255,074
is to minimize flocculation. The apparatus of this patent,
as also that of the previous patent, is meant to be used as
an alternative for a perforated roll.
Also structures comprising a vibrating deflocculator inside
a head box are known. Devices of this kind are described
for example in U.S. patent publication no. 3,562,108 and
CA 1,173,681. An element vibrating in the head box can
break flocs in a dilute pulp but creates harmful pulses.
Finnish patent no. 73764 discloses an apparatus for dis-
tributing pulp of the consistency of up to 6 % onto the
1 333965
wire of a paper machine. The head box according to this
invention comprises a chamber in which there is a bar
provided with plate-like members which during vibration of
the bar exert to-and-fro pressure shocks to the pulp which
disrupt bondings between the fibers and thus facilitate
the flow of the pulp through the slice onto the wire of
the paper machine.
The above devices in which the pulp is distributed through
a chamber and an outlet duct onto a forming wire where a
coherent network is formed are applicable for pulps of the
consistency of 6 % at the most.
When higher consistencies are treated efficient mechanical
fluidizers or high flow velocities are required at the
boundary of the head box and the web forming device, which
in most cases is a wire, or immediately close to it, in
order to generate sufficient turbulence and shear energy
in the pulp so that the pulp can be spread out to a web
before the pulp forms fiber networks which would disturb
web formation.
Mr Johan Gullichsen has presented (Tappi Pulping Conference,
1980) a principle drawing of a paper machine head box
operating with a consistency of approximately 10 ~. The
head box comprises a large roll in the machine direction
employed to fluidize the pulp in a flow duct between the
head box and the roll before the pulp is discharged from
the head box through a narrow slot onto a wire or between
two wires. The high consistency pulp forms a strong and
homogenous network. Considerably more energy is required
for disrupting this fiber network than for breaking the
fiber flocs in a dilute pulp. Fluidizing the entire pulp
volume in the head box of a mill-scale paper machine by a
large roll, as suggested by Gullichsen, would require a roll
with the diameter of over 0,5 m and would consume too much
1 3339~
energy. A solution in which the entire volume of the
head box is fluidized and supplied to the slice channel is
too expensive.
As the drawbacks of the example described above are to be
avoided and as the fluidized state quickly transforms to a
fiber network the locations, lengths and dimensions of the
flow ducts supplying high consistency pulp suspension to
the wire must be carefully considered.
If large quantities of high consistency pulp must be
fluidized at the same time the advantage gained by using
a pulp of the consistency of 10 % instead of 1 %, in other
words the reduced volume of water required, is lost. Then
the use of high consistency pulp has no practical meaning.
A high consistency head box must therefore be designed to
fluidize a small volume of pulp at a time and even then
the fluidizing has to be carried out exactly in the right
and necessary place. One of the advantages gained in
this way are for instance a remarkably reduced size of the
head box as the required volume of water is only 10 % of
the one required before. Also the size of all the short
circulation devices can be remarkably reduced. Consequently
the effluent load of a paper mill is reduced.
It is important also in high consistency web forming that
the formation of the produced web is homogenous and good.
Therefore the web must be formed while the pulp is in a
fluidized state. Fluidizing and distributing the pulp to
a web must be carried out in as close succession as
possible. To achieve this, the head box according to the
present invention is provided with a pulp handling section,
or the first phase of the head box, and with a web forming
section. In the pulp handling section or the first phase
as much energy is supplied to the pulp that it purges out
at least in a partially fluidized state through holes,
-- 8 1 333965
slots or nozzles to the web forming section. In the web
forming section, or the second phase, the pulp impinges
against a moving surface and becomes fluidized whereby a
web is formed in a closed chamber, in a space between
wires or in a free space on a wire.
The method of the invention described above is characterized
in that the high consistency pulp in the head box is kept
moving by supplying energy to it and that the pulp is
supplied onto a web former in one or several pulp streams
the flow velocity and the impingement force of which,
together with the kinetic energy of the web former, are
maintained great enough to be able to fluidize the pulp
at the web forming moment when the pulp meets one or several
of the surfaces of the web former.
It is a characteristic feature of the head box according
to the present invention that at least one rotating element
revolving at a high velocity is provided immediately by
the outlet of the pulp discharge channel which element
facilitates the fluidization of the high consistency pulp
at the moment of the web formation.
The invention makes possible formation of a web by simple
means. According to a preferred embodiment of the invention
pulp is distributed to a web by spraying pulp through a
narrow slot onto a forming surface or to a gap between
two forming surfaces where the pulp stream stops on a
wire. Fluidization of the pulp in the slot can be
intensified if needed by arranging a rotor or a
corresponding fluidizer prior to the slot. In another
embodiment of the invention, a fluidizer is arranged
immediately close to the forming surface so as to prevent
flocs from being formed in the pulp before it hits the
wire.
1 333965
g
The method and the apparatus of the present invention
have many advantages:
- the method can be carried out by simple means
- the method is applicable for various pulps and for a
wide range of consistencies.
At the lower limit of the consistency range (6 - 15 ~) the
invention can be applied in web formation without
mechanical auxiliary devices, in particular with pulps
with weak bondings between the fibers. When the consistency
increases auxiliary devices can be used in connection with
start-up to avoid clogging. A mechanical auxiliary device,
for example a rotating fluidizing means, can be provided
in front of a nozzle, or a slot, at high consistency ranges
or when there are plenty of bondings between the fibers in
the pulp. The auxiliary device is employed to decrease
pressure and to break fiber bondings.
Further, chemical additives can be used to decrease the
tendency of the fibers to attach to each other and thus to
aid web formation. However, the properties required of
the end product limit the use of additives.
The invention is decribed in detail, by way of examples,
below with reference to the accompanying drawings in which:
Fig. 1 is a chematical sectional side illustration of a
preferred embodiment of the invention;
Fig. 2 is a top view of the apparatus in Fig. 1;
Fig. 3 is a sectional side illustration of another preferred
embodiment of the invention;
Fig. 4 is a sectional side illustration of a third preferred
embodiment of the invention; and
1 333q65
Fig. 5 illustrates a fifth preferred embodiment of the
invention.
Figures 1 and 2 illustrate the web forming section of a
paper machine in which two rolls, l and 2, around which a
top wire 3 and a bottom wire 4 are guided, define between
each other a tapering gap 5 into which pulp is supplied.
Immediately in front of the gap, a head box 6 is disposed
which comprises a feed chamber 7 into which pulp is supplied
via pipe line 8 and which is provided with a narrow slot
or a plurality of parallel nozzles 9' at a distance from
each other transverse to the machine direction. The slot 9
or the nozzles 9' are directed towards the gap 5 between
the forming rolls. The pulp is discharged through the slot
9 in an even at least partially fluidized high-pressure
jet or through the nozzles 9' in separate jets which are
united in the gap 5. The high pressure in the chamber 7
causes the pulp to flow through the slot 9 or the nozzles
9' at such a high velocity that it becomes fluidized when
it is discharged through the slot or the nozzles and remains
fluidized until it forms a uniform web between the forming
wires 3 and 4. Depending on the configuration of the slot
or the nozzle, the pulp can be discharged from the nozzle
as a jet the cross section of which is substantially
unchanged or widening. The pulp jets can be united before
they reach the gap between the wires. The surface of the
rolls can be open, for example perforated or grooved, or
also water impervious.
In the embodiment illustrated in Figure 3, a fluidizing
rotor 14 is disposed in front of each nozzle outlet 13 in
the wall of a feed chamber 12 of a head box 6. The rotors
are driven by a motor which is provided outside the chamber
and is not illustrated in the figure. The pulp supplied
to the feed chamber is at least partially fluidized by the
,~
1 333965
11
rotors 14 in front of the row of outlets 13 and flows at a
velocity determined by the pressure difference to a space
17 defined by the feed chamber and rolls 15 and 16 between
the rolls. The forming rolls, over which forming wires 18
and 19 are arranged to run, can be for instance suction
rolls, as the figure schematically illustrates.
Pressurized air can be supplied to the pulp jets from
inside through a hole 11 drilled in the shaft of the rotor
of the fluidizer. Pressurized air can be introduced to
the pulp jets also from outside immediately after the
nozzle outlet. One embodiment of the invention could
comprise a tubular pulp nozzle whereby strong air or gas
flows would be guided from several directions to the end
portion or discharge point of the nozzle which would
disperse the pulp flow to a mixture of air, pulp and water
in which the fiber network is at least partly disrupted
and the bondings between the fibers reduced to the extent
that uniting the jets on a wire to a web is possible.
A commonly used way of dispersing a liquid jet is dispersion
with an ejector. This method can be used also in dispersing
with air or other gas the pulp flowing from the nozzles.
The pulp jets coming from the nozzles are guided to an
ejector in which air is mixed to the pulp and a mixture
containing pulp, water and air is produced. Air weakens
the bondings between the fibers and improves possibilities
of a good web formation when the jets reach the web forming
surface.
When the above embodiment was tested the pilot paper
machine was provided with five nozzles at the distance of
approx. 10 cm through which the pulp jets were discharged
at the pressure of approx. 3 bar. Board of approx. 700
g/m3 could be produced in the test runs. Fluidizers were
used at the inlet side of the nozzles and the consistency
of the pulp was 10 ~. The pulp was pine sulfate pulp.
~.
12 1 3~3~
Figure 4 illustrates a third embodiment of the invention
in which a roll 22 in a chamber 21 of a head box is disposed
on top of a forming cylinder 20 so that the shell 25 of
the roll and the surface 24 of the forming cylinder define
between each other a space 23 in which the pulp is subjected
to shear forces between two moving surfaces forming a narrow
slot. Fluidization can be carried out very efficiently in
the narrow slot, substantially to a fiber-to-fiber state,
due to great shear forces. A wall 26 of the chamber the
lower end of which has an ad;ustable guide plate 27 and the
outer surface of the roll define a flow duct 28 leading to
the space 23 while the roll and the cylinder rotate in
different directions. The revolving movement of the roll
22 facilitates the flow of the pulp in the duct 28. The
position of the guide plate 27 determines the size of the
slot 29 between the roll 22 and the guide plate and the
slot 30 between the forming cylinder 20 and the guide
plate. At the lower end of the other wall 31 of the chamber
there is a second ad;ustable guide plate 32 which serves
as a sealing between the forming cylinder and the chamber.
The distance between the roll 22 and the forming cylinder
can be changed by moving the chamber which adjusts the
degree of fluidization in the slot to the desired level.
The closer to the forming cylinder 20 the roll 22 is the
more efficient the fluidization is and the higher the
consistencies of the pulp to be treated can be.
The rotating roll 22 feeds pulp to the space 23 between
the roll 22 and the forming cylinder 20 where part of the
pulp flows in a fluidized state to the slot 30 between the
guide plate 27 and the forming cylinder 20 and forms a web
in which the fibers are evenly distributed. A duct 33
between the chamber wall 31 and the roll 22 returns the
remaining portion of the pulp to the chamber 21. The
1 333~5
13
revolving movement of the roll 22 facilitates the flow in
the duct 33.
When the roll and the cylinder rotate in the same direction
pulp is supplied to the web forming space 23 via duct 33.
In this case the duct 28 serves as the return duct and
the flow in it may be zero if the return flow is adjusted
by the guide plate 27 to be zero. The surface of the
rotating roll may be smooth or grooved. Even the use of a
rotating fast rotor provided with separate fluidizing
vanes can be considered.
In the embodiment illustrated in Figure 5 the feed chamber
21 of a head box is provided with a slot or a row of holes
30, which is transverse to the machine direction and close
to which there is a fluidizer 31 the shaft of which is
parallel with the slot or the row of holes. The fluidizer
31 is used to fluidize the pulp to the extent that it can
be discharged from the feed chamber 21 through the slot or
the holes to a web forming space 32 defined between the
feed chamber 21 and a moving wire 33 of a web forming
device disposed under the chamber. An adjustable sealing
34 is provided between the lower portion of the feed chamber
and the forming surface in the direction of the machine
which limits the space 32 under the slot or the holes in
the direction opposite to the running direction of the
wire 33.
The feed chamber must be able to create a fluidizing rate
which is adequate to keep the fibers in the web forming
point loose and to form a fiber network of the thickness
of the web. The fluidizing energy is provided by the pulp
streams discharged through the slot or the holes to the web
forming space 32 and the moving forming surface 33. The
thickness of the web is determined by the dimentions of
the outlet of the web forming space, in other words by the
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14
distance between the bottom 35 of the feed chamber and the
forming surface. The thickness of the web is adjusted by
changing the height of this opening.
It is of course possible to arrange the feed chamber of
the head box so that it defines a web forming chamber
together with two rotating rolls and adjustable sealings
provided between the rolls and the feed chamber. The web
is formed in the point where the webs running over the
rolls meet and the thickness of the web is determined by
the distance of the webs between the rolls. The web forming
chamber of all the embodiments described above should be
as small as possible. In the most advantageous case the
fluidizing space serves as the web forming chamber as
illustrated in Figure 4. Then no flocs can be built in the
high consistency pulp before the pulp reaches the web
forming surface.
It should be noted that when the method and the head box
of the invention are applied for pulps of the higher
consistencies of the given range the velocity of the
rotating means should be so high that it together with
the movement of the web forming device is able to fluidize
the pulp to be treated. It has been found out that the
peripheral speed of the rotating means should be at least
25 m/s, preferably higher than 30 m/s, to keep the high
consistency pulp in the head box in motion and to secure
adequate fluidization in the web forming point. It is
also possible to intensify fluidization by fluidizing at a
time as small a volume as possible. This is gained by
arranging the distance between the rotating means and the
web forming device adequately small. An appropriate
distance could be 10 - 20 times the thickness of the paper
or product to be produced or on the other hand a distance
of less than 10 mm.
~ ~3~ 5
The invention is not limited by the embodiments presented
here as examples only but several modifications and
applications of it are possbile within the scope of
protection defined by the accompanying patent claims.
