Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS AND SCREEN FOR SCREENING MECHANICAL
FIBRE PULP
[0001] The invention relates to a method for screening a mechani-
cally manufactured fibre pulp mixture containing fibres of varying lengths, in
which method the fibre pulp mixture obtained by mechanically defibering wood
in the presence of water is screened into at least two fractions containing fi-
bres of mainly varying lengths.
[0002] The invention also relates to an apparatus for screening a
mechanically manufactured fibre pulp mixture containing fibres of varying
lengths, the apparatus comprising screening means for screening the fibre
pulp mixture obtained by mechanically defibering wood in the presence of wa-
ter into at least twa fractions containing fibres of mainly varying lengths.
[0003] The invention further relates to a screen for screening a me
chanically manufactured fibre pulp mixture containing fibres of varying
lengths
into two fractions containing fibres of substantially varying lengths, the
screen
comprising at least one screening surface through which water is removed
from the fibre pulp mixture and some of the shorter fibres with the water and
the remaining fibre pulp mixture is removed from the screen from the same
side of the screening surface to which the fibre pulp mixture was fed.
[0004] Mechanical fibre pulp is manufactured by grinding or ther-
momechanically refining wood into fibre in the presence of water, resulting in
a
fibre pulp mixture. Fibres having different freeness values are typically
created
in prior art manufacturing processes. The division of the fibres is based an a
defibering process and the material used, and the ratio between the fibres
having different freeness values cannot be significantly adjusted during the
actual defibration. The result is therefore a fibre pulp mixture, in which the
por-
tion of fibres having different freeness values and operating properties is by
no
means necessarily the most favourable as regards use. After this the fibre ma-
terial is screened in the prior art methods using screens comprising various
screening surfaces so that fibres, which are smaller than a particular size
and
which have passed through a screening surface, are directed for use, and the
coarser fibres are removed from the process and returned again to the proc-
ess through reject refining. However, the final result is a fibre pulp
mixture,
where the lengths of the fibres and thus their freeness values vary widely,
and
the structure of the fibre pulp mixture and the properties thereof do not
corre-
spond with the desired ones. When making a particular type of paper, it is
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preferable to use such mechanical fibre pulp that contains long fibres of high
quality and short fibres provided with a binding ability, but nat
substantially
mid-size fibres. Such fibre pulp cannot be achieved using conventional me-
chanical fibre pulp screening methods.
(0005 It is an object of the present invention to provide a method,
an apparatus and a screen that enable to achieve fibre fractions containing
fibres of mainly varying lengths to be appropriately used for different
purposes.
[00061 The method of the invention is characterized in that in at
least one screening phase the fibre pulp mixture or a part thereof is screened
to separate short fibres from the fibre pulp mixture or from a part thereof
using
a gap screen comprising a convergent gap and at least one wire restricting
said gap, whereby the fibre pulp mixture to be screened or a part thereof is
fed
into said convergent gap so that it flows in the same direction as the wire to-
wards an output port at the convergent end of the gap, and the short fibres
and some of the water fed into the gap exit through the openings in the wire
whereas the water and some of the remaining fibre pulp mixture exit through
the output port of the gap.
(0007 The apparatus of the invention is characterized in that the
apparatus comprises a gap screen to separate short fibres comprising a con
vergent gap and at least one wire restricting said convergent gap, whereby the
fibre pulp mixture to be screened or a part thereof is fed into said
convergent
gap so that it flows through the gap screen in the same direction as the wire
towards the convergent end of the gap so that the short fibres and some of the
water exit through the openings in the wire and the remaining part of the
fibre
pulp mixture exits from the gap screen through the output port at the conver-
gent end of the gap.
[0008 The screen of the invention is characterized in that the
screen is a gap screen comprising a convergent gap restricted by a wire and
into which the fibre pulp mixture is fed to move in the same direction as the
wire, whereby water and shorter fibres exit through the openings in the wire
and the remaining fibre pulp mixture exits from the output port at the conver-
gent end of the gap.
[00091 An essential idea of the invention is that an extremely fine
and short-fibered fibre pulp material is separated from the fibre pulp mixture
using a gap screen, preferably one formed by means of twin wires, whereby a
very short-fibred material having a high freeness value is obtained that
cannot
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be so distinctly distinguished as a specific pulp by means of other screening
methods. In accordance with a preferred embodiment of the invention, a part
of a fibre pulp mixture containing medium-length and shorter fibre materials
preferably already screened by separating long fibres therefrom is screened
anew using said gap screen formed by means of twin wires. According to a
second preferred embodiment of the invention, the medium-sized fibre mate-
rial containing some fine fibres separated from the gap screen is directed fur-
ther to be refined. As a result a fraction containing medium-sized and short
fibres with the desired properties is obtained.
(0010 The invention provides such an advantage that a fibre frac-
tion that is shorter than a fraction representing a particular length can be
very
accurately separated from the fibre pulp mixture containing fibres of varying
lengths. An advantage provided by a preferred embodiment of the invention is
that as the long-fibred fibre material is at first screened and thereafter a
very
short fibre material having a high freeness value is separated using the gap
screen from the remaining fraction containing medium-sized and short-fibred
material, then the final result obtained is three fibre pulp fractions with
different
properties that can be mixed with one another or with other fibre materials in
order to achieve a desired fibre web quality. Hence, long fibres can be com-
bined with very short fibres in order to obtain a particular paper quality,
which
is not possible when medium-sized fibres are included in the fibre pulp.
(0011 Gonsequently, fibre pulp fractions of varying properties and
lengths can be obtained as desired from the mechanically manufactured fibre
pulp, and thereby the desired fractions can easily be combined with the manu-
factoring of fibre web, and only the fibre pulp fractions required can be used
to
produce a particular type of fibre web. Thus, a fibre web that contains only
long or short fibres can be manufactured and the medium-sized fibre obtained
can be used to produce another type of fibre web.
(0012 In the following the invention is described in greater detail in
the appended drawings, in which
Figure 1 schematically shows the method and apparatus of the in-
vention,
Figure 2 schematically shows a gap screen to be used in the appli-
cation of the method of the invention, and
Figure 3 schematically shows a screening result obtained using the
gap screen of the invention.
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(0013] Figure 1 schematically shows the method and apparatus of
the invention, in which freeness values of a particular fibre pulp are
provided
by way of example, on the basis of which values the division into different
fibre
pulp fractions can be carried out.
[0014] Figure 1 schematically shows how a material 1 to be defi-
bered is directed to a defibrator 2, which may be a grinder known per se or a
refiner, typically a thermomechanical pulp refiner. The fibre pulp mixture ob-
tained from defibration is directed to screening means, or at first to a first
screen 3, where the mixture can be divided for example in accordance with
freeness value "16" so that the fraction containing long fibres and has passed
through the first screen 3 proceeds to be further processed and the fraction
containing even longer fibres and shives and has not passed through said
screen is directed to be refined to a first recirculation refiner 4, from
where the
fraction is returned to the first screen 3 for screening. Consequently, a
fibre
that is longer than the desired one continuously circulates until it passes
the
first screen and then continues onwards in the procedure. The first refiner
may
either be a separate refiner, or the fraction to be returned can also be
returned
to the actual defibrator 2 as indicted by the dashed line.
[0015] The fibre pulp fraction containing long fibres and has passed
through the first screen is directed forward to a second screen 5, where the
fraction is screened again according to length on the basis of the fibre
length
provided by way of example with freeness value "28", The fibre pulp fraction
that has passed through the second screen 5 continues forward to be further
processed and the longer fibre pulp fraction that is unable to pass through
said
screen is directed to a second refiner 6, which is preferably a refiner known
per se. The fibre pulp obtained from the second refiner 6 is directed to a
third
screen 7, from where the fibre that is shorter than a particular length and
that
has passed through said screen, for example a fibre provided with freeness
value "40" is directed forward for use, and correspondingly the fibre that is
un-
able to pass through the screen 7 is returned again to the second refiner 6 to
be refined. Hence, the long fibre circulates the same loop until it is
shortened
to a desired size in such a manner that it passes through the third screen 7
and is then ready for use. The fibre pulp fraction that has passed through the
second screen 5 is directed in accordance with the invention to a fine screen-
ing 8, where a fine fibre pulp fraction is separated from the fibre pulp using
a
separate gap screen, for example fibres provided with a freeness value of at
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least "200" and correspondingly fibres that are longer than that are directed
to
a third refiner 9, which preferably is a refiner known per se. The fibre pulp
frac-
tion obtained from the output of the third refiner 9 contains high-quality
fines
and medium-sized fraction.
5 [0016 The gap screen is either a single wire or twin wire structured
screen. Such a screen can be used to efficiently remove a desired amount of
fibre pulp of a particular length or shorter than that from the fibre pulp
mixture,
thus making the division between the fractions drastic and providing a desired
ratio. The other screens are screens known as such either so that each screen
represents a separate individual screen or that two or more screens form one
multi-phase screen. Then again it is necessary to use a gap screen to sepa-
rate short fibres.
[0017 Figure 2 schematically shows gap a twin wire structured
screen 10. The gap screen preferably allows to screen the fibre pulp into two
fractions in accordance with the invention. The gap screen comprises two
movable wires 11 and 12 forming opposing screening surfaces that move
through the gap screen and return as closed loops so as to form a convergent
gap 13 within the gap screen in the direction of movement thereof. Supporting
structures 14 and 15 are placed at the back of the wires 11 and 12, i.e. on
the
opposite side of the gap 13. The wires 11 and 12 are supported against the
supporting structures 14 and 15, which naturally include openings for
receiving
water and the fine fibre material removed through the openings of the wires 11
and 12 and removing the same from the gap screen as schematically shown
by arrows A. The wires 11 and 12 circulate around rollers 16 and 17 shown by
way of example, which as such is known in the art. The sides of the gap 13 at
the edges of the wires 11 and 12 are naturally closed, so that the fibre pulp
mixture can be fed from a feeding channel 18 to the gap screen appropriately
under pressure so as to remove the water and fine fibre material as desired.
What is also schematically indicated in the Figure is engines 19 and 20, which
are connected to rotate one of the rollers 16, and 17 respectively, although
in
a normal situation the engines can be used to circulate each roller separately
or a required amount of the rollers in a manner known as such in the art. The
engines 19 and 20 are, in turn, connected to a control unit 21 included in the
control means that allows to adjust, for example, the rotational speed of the
engines so as to make the wires 11 and 12 move at a desired pace, which
may deviate from the flowing rate of the fibre pulp through the screen and
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most preferably exceeds the flowing rate of the fibre pulp mixture.
[0018] During the screening the fibre pulp mixture is fed from the
feeding channel 18 to the gap screen, and the mixture thus flows through the
gap 13 of the screen and exits from a narrower output port 22 of the gap 13,
i.e. the smaller output port in cross section as regards the feeding opening,
from the same side of the screen surfaces or the wires, to which the fibre
pulp
mixture was originally fed. At the same time, owing to the structure of the
wires
11 and 12, water is removed from the openings in the wires as well as short
fibre material. Depending on the amount of fibre material to be removed the
speed of motion in the wires can be adjusted in relation to the feeding rate
of
the fibre pulp mixture fed into the screen until the desired screening result
is
obtained. At the same time, more water can be removed from the fibre pulp
mixture with the short fibre pulp fraction in comparison with the amount of wa-
ter that remains with the fibre pulp fraction that does not pass through the
wires, i.e. exits through the output port of the gap 13. Consequently the
longer
fibre pulp fraction can simultaneously also be thickened, and the consistency
may rise up to 10%. In this application and claims the output part 22 refers
to
the part in the screen, where water and fibres are no longer removed through
the wire or wires and the remaining fibre pulp mixture is transferred from the
screening phase to the following possible processing phases either directly or
controlled by various conveyor means and control means such as wires and
the like.
[0019] What can naturally also affect the quality of the fraction is to
appropriately select the dimensioning and penetration of the wires 11 and 12,
whereby wires provided with different penetration properties can be used for
different purposes. What can also affect the screening is to set one or both
of
the wires to move towards the gap along a convex supporting structure, in
which case water and the fine fibres are at the same time removed more effi-
ciently from the fibre pulp fraction at the feeding end of the screen, and
corre-
spondingly more slowly towards the output end of the gap screen. If the aim is
to use a single wire structure, then one of the wires 11 and 12 can be
replaced
with a fixed wall surface for example, and a combination can preferably be
employed in which the wire 11 moves towards an arched supporting structure
14 and the specific wall surface replaces the wire 12 moving directly within
the
area of the gap.
[OOZO] In practice, the adjustment of the screening is mainly based
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on the fact that the gap screen adjusts the basis weight of the fibre pulp mix-
ture including the long-fibred fibre material that flows through said gap
screen.
This can be implemented either by adjusting the consistency of the fibre pulp
mixture to be fed into the screening or the velocity of the wires in relation
to
the feeding rate of the fibre pulp mixture. When a diluted fibre pulp mixture
is
fed into the gap screen, more short fibres can be removed from the fibre pulp
mixture than if a more consistent fibre pulp mixture is fed under similar
circum-
stances. Correspondingly, if the velocity of the wires is increased in
relation to
the feeding rate of the fibre pulp mixture, then more short fibres can corre-
spondingly be removed from the fibre pulp mixture. Thus, the separation value
of the short fibres can be set at a desired level by adjusting the consistency
of
the feed and/or the velocity of the wires, so that the result obtained is
distinctly
difiFerent fibre pulp fractions, where the fraction that passes through the
wires
contains only very short fibres and correspondingly the fraction that flows
through the gap of the gap screen contains longer fibres but very few short
fibres. The velocity of the wires can be adjusted in various ways, so that it
re
mains below the feeding rate of the fibre pulp, exceeds the feeding rate of
the
fibre pulp or equals the feeding rate of the fibre pulp. The choice of
velocity is
based on the properties of the fibre pulp mixture to be screened and on the
desired screening result.
[0021 Figure 3 schematically shows the result of the screening ob-
tained by means of a gap screen shown in Figure 1 in accordance with the
method of the invention. Figure 3 shows the screening result, or the portion
of
the fibres having different freeness values, in the fibre pulp mixture fed and
correspondingly in the fraction that passes through the gap and exits through
the output end of the gap screen. As Figure 3 shows the portion of the fibres
having different freeness values is presented in such a manner that the
portion
of different fibres in the fibre pulp mixture fed of the entire fibre pulp
mixture is
indicated by the black column and correspondingly the portion of fibres having
a corresponding freeness value included in the fibre pulp fraction that exits
through the output end is indicated by the white column. As Figure 3 shows
the portion of fibres having a freeness value 30 or greater than that is
nearly
30°l0, the portion of fibres having a freeness value ranging from 50 to
30 is
nearly 16%, the portion of fibres having a freeness value ranging from 100 to
50 is roughly 13%, the portion of fibres having a freeness value ranging from
200 to 100 is approximately 7% and the amount of very fine fibres having the
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freeness value 200 is 34%. In such a situation, the average freeness value of
the fibre pulp mixture fed is 100.
[0022] The fibre pulp that has completed the screening shows that
the portion of coarse fibres has risen proportionally by nearly 50%, the
portion
of the following fibres approximately 25°I°, whereas the portion
of the shorter
fibres has increased only slightly. The extremely short fibres have, in turn,
been removed to a great extent through the wires with the water, and the por-
tion of the short fibres has therefore decreased proportionally by 60%. In
prac-
tice, this means that only the extremely short-fibred material is
substantially
removed in the gap screen from the fibre pulp mixture fed, the freeness value
of said short-fibred material being in this example approximately 200 or
higher.
Hence, the extremely short fibre can efficiently be removed from the fibre
pulp
mixture to be screened and thus obtain a specific fibre pulp fraction that can
be used for various purposes by mixing a desired amount thereof to the long
fibres for example, in order to manufacture a particular type of fibre web pro-
vided with specific properties. Correspondingly the average freeness value of
the fibre pulp mixture that passes through the gap screen has increased.
[0023] Figure 3 shows that the amount of long fibres in the fraction
that passes through the wires is substantially 0 and correspondingly the
amount of short fibres in the long-fibred fraction that passes through the gap
screen is very low. This indicates that the gap screen operates fairly
efficiently
and divides the fibre pulp mixture drastically into two fractions of
distinctly
varying lengths. An appropriate selection of wire structures in the gap screen
allows to determine the applicable screening conditions for the fibres of vary-
ing lengths in accordance with use and to adjust the efficiency of the
screening
by controlling the velocity and/or the consistency of the fibre pulp mixture.
[0024] The invention is described above in the specification and the
drawings by way of example but is not merely restricted thereto. What is es
sential is that mechanically manufactured fibre pulp is screened using a gap
screen in such a manner that a short fibre fraction is separated therefrom and
a desired amount of fibre pulp fractions of predetermined fibre lengths is ob-
tained and preferably so that the short fibre fraction is separated from such
a
fibre pulp from which the long fibres have already been separated and the
short fibres can therefore be as efficiently as possible separated from the
rest
of the fibre material.
