Note: Descriptions are shown in the official language in which they were submitted.
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Method and apparatus for treatment of fiber suspension
The present invention relates to a method and apparatus for the treatment of
fiber
suspensions in the pulp and paper industry. The invention relates in
particular to
intensifying the so-called post-screening or secondary screening of bleached
pulp or pulp
which is in the bleaching stage.
There are prior art systems used for the corresponding purpose according to
which pulp in
the bleaching stage is guided to a hydrocyclone plant in which heavier
impurities such as
sticks, large fiber accumulations, and so on, still remaining in the pulp are
separated from
the pulp. As is known, a hydrocyclone plant consists of dozens of vortex
cleaners, i.e.
hydrocyclones, in which the cleaning effect is brought about by causing the
material to be
cleaned to circulate at a high speed along the cylindrical or conical wall of
the cleaner.
Then the coarse fraction to be rejected is collected by the centrifugal force
to the surface
of the cyclone and the surface is soon worn out; then the cleaning capacity of
that cleaner
decreases at first when extra friction rapidly decreases the circulation speed
of the
material. In the end, a whole is worn through the wall of the cleaner and the
material to
be cleaned is discharged to the floor of the screen room. Further, the
hydrocyclone plant
requires quite a large space, mainly due to the large number of cyclones. The
use of
cyclones further has a third drawback, i.e. a remarkable increase in the
volume of the
internal circulation waters and the effluent load of the pulp mill. As is
known, the
consistency of the pulp suspension to be cleaned must be of the order of
tenths of a
percent in order to enable a hydrocyclone to work efficiently and separate
well enough
the acceptable fiber fraction from knot particles and sticks.
A further drawback of a hydrocyclone cleaning plant should be mentioned, i.e.
the fact
that bleaching plants usually employ washers, the washing boxes of which
require a
consistency of about 1.2 %, at least 1.0 %, of the pulp to be fed into the
boxes. Also the
drying machine, which is the subsequent treatment phase in the process
following the
bleaching, requires the same feed consistency of about 1.0 - 1.2 %. However, a
hydrocyclone plant does not work efficiently enough at the consistency
mentioned, it
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requires a consistency of about 0.5 %. This means that pulp must be thickened
after a
hydrocyclone plant from a consistency of 0.5 % to a consistency of 1.0 -1.2 %.
The drawbacks of the apparatus of the old technology described above may be
eliminated
by replacing the hydrocyclone plant with, for example, narrow-slot screening
whereby the
task of dozens of hydrocyclones is taken care of by one single screen. At the
same time
the thickener required after a hydrocyclone plant is dispensed with. This
approach has
been described for example in U.S. patents no. 5,470,432 and 5,571,384. The
saving of
space as well as the reduction of the liquid load of the plant are obvious as
there is no
need to dilute the suspension to be treated to the consistency required by a
hydrocyclone
plant but the consistency may be the one required by the washer or the drying
machine
feed box. Thus the volume of dilution liquid needed is reduced remarkably. The
U.S.
patents describe an approach in which the reject from a pressure screen is
treated in a
hydrocyclone plant and the accept from the hydrocyclone plant is returned to
the feed of
the pressure screen and the reject is either discharged from the process
altogether or
forwarded to a previous treatment stage.
A disadvantage or a kind of default of the above U.S. patents is that they do
not disclose
what happens to the reject of the pressure screen. The publications mention
only that the
reject is treated further in a hydrocyclone plant and the accept from the
hydrocyclone
plant is returned for reuse and the reject is discharged. However, practice
has proved that
the approach according to the invention allows particles to pass both to the
accept and to
the reject which do not belong there. Thus, it has been necessary to develop
this so-called
post-screening or secondary screening concept further.
It is known that in conventional screens employing screen drums having either
holes or
slots, particles are divided into accept or acceptable fraction and reject or
rejectable
fraction mainly based on their size. Particularly when pressure screens are
used, some
compressible particles, for example expanded polystyrene bits, are know to be
pressed
through the openings in the screen plate and thus end up to the accept. The
only means of
preventing this in screen-type apparatus is to use an adequately small screen
aperture,
preferably a narrow slot, whereby the pressure prevailing in the screen cannot
press the
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pieces through the aperture. This may be done in a primary screen but at the
same time
also some of the fiber material which has already been accepted at this stage
is entrained
to the reject and it must then be recovered in a further screening stage. If a
screen must
be used in a secondary stage for screening, the screen must have larger screen
openings
for recovering fiber material; then for example the expanded polystyrene
pieces
mentioned will pass through the secondary screen as such. Thus, it is almost
impossible to
separate non-desirable material of for example the type mentioned above from
the
accepted fiber material.
It is also known that the screening action in a hydrocyclone is based mainly
on the
specific gravity of the particles whereby in conventional applications {for
example
separation of sand) heavy particles are rejected and the light fraction is
recovered. Thus,
also a conventional hydrocyclone accepts light reject, for example expanded
polystyrene
bits and also other light pieces, such as plastics particles, with the fiber
material.
A so-called reverse hydrocyclone disclosed for example in U.S. patent no.
4,155,839 has
been developed for solving the problem mentioned. A characteristic feature of
the
cleaner in question, and of its function in particular, is that it is capable
of separating
light rejects, for example plastics, and acceptable fiber material from each
another
efficiently and precisely although the specific gravities of the materials do
not differ
much. A cleaner of this type works so that it discharges the heavier fiber
fraction via its
narrower end at the tip of a cone which in the conventional position of the
apparatus is the
lower end, and the lighter rej ect via a discharge opening located centrally
at the top end.
Reversed hydrocyclones have, however, been erected also the other way around,
i.e. the
tip of the cone upwards. Further, it is typical of reversed hydrocyclones that
they may be
used to thicken pulp. For example the U.S. patent 4,155,839 mentioned above
indicated
that at a feed consistency of 1.0 -1.5 % the consistency of the accept was 1.6
- 2.6 %.
A severe problem which may be solved with the method and the apparatus
according to
the present invention is connected with the so-called secondary screening
plant described
above which is capable of preventing by means of the state-of the-art narrow
slot
screening for example plastics pieces from ending up to the end product.
However, since
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the state-of the-art secondary screening plant comprises a hydrocyclone as a
device
subsequent to the narrow-slot screen, it accepts also light reject, i.e. for
example plastics
particles, with the fiber material. The light reject mentioned remains
circulating in the
screen circulation until it is ground into so small pieces that the narrow-
slot screen
accepts it. As a consequence the light reject ends up to the end product and
in the worst
case causes clearly perceivable quality problems.
According to a preferred embodiment of the method and the apparatus of the
invention,
reject from the so-called primary screen is taken to a pressure screen, and
reject from this
pressures screen is further treated in a so-called first hydrocyclone step.
The method and the apparatus described above may still be developed further so
that
accept from the first hydrocyclone step mentioned is taken either to a second,
reversed
hydrocyclone step or to a pressure screen.
The method and the apparatus described above may still be developed further so
that the
accept from the reverse hydrocyclone step or the pressure screen mentioned is
brought to
the feed of the secondary screen and the reject is removed to a second reverse
hydrocyclone step.
According to another preferred embodiment of the method and the apparatus of
the
invention, the reject from the so-called primary screen is introduced into the
first
hydrocyclone step, the accept from which is treated in the reverse
hydrocyclone step, the
accept from which is returned to the feed of the primary screen and the reject
is removed
from the process or guided to a suitable further treatment point.
The methods and the apparatus according to the embodiments described above
improve
the cleaning result of the screening plant remarkably because different
screening
apparatus, such as pressure screen, hydrocyclone and reverse hydrocyclone, are
used in a
3 0 way to utilize their best properties, only. The apparatus in question
provide excellent
supplement to each other so that the final reject contains only particles,
which really
should be rejected.
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Further, the apparatus according to the invention is essentially less
expensive for the pulp
mill to purchase and to maintain; thus, considering also the improved
efficiency/accuracy
of the screening plant, a superior product is provided compared with both
prior art
5 methods and apparatus.
The characteristic features of the method and the apparatus of the invention
are disclosed
in the appended patent claims.
The invention will be described below more in detail with reference to the
accompanying
drawing figures of which
Fig. 1 illustrates schematically a prior art apparatus;
Fig. 2 illustrates schematically another prior art apparatus;
Fig. 3 illustrates schematically an apparatus according to a preferred
embodiment of the
invention;
Fig. 4 illustrates schematically an apparatus according to another preferred
embodiment
of the invention;
Fig. 5 illustrates schematically an apparatus according to a third preferred
embodiment of
the invention; and
Fig. 6 illustrates schematically an apparatus according to a fourth preferred
embodiment
of the invention.
According to Fig. 1, a prior art arrangement comprises mainly a bleaching
tower 2, a
subsequent hydrocyclone plant 4, a thickener 6 and a washer, most usually a
drum washer
8, and a bleaching tower 10 of the subsequent bleaching stage. Fiber
suspension to be
treated is discharged from the tower 2 to a pipe line in which the suspension
is diluted to a
consistency of about 0.5 % and supplied further to the hydrocyclones 4. After
the
hydrocyclones the accepted material is fed directly to the thickener 6, which
thickens the
pulp to the feed consistency of the subsequent washer 8. From the thickener 6
the pulp is
supplied to the washer and the washed pulp is discharged from the washer at
the
bleaching consistency of the bleaching tower. The rej ect from the
hydrocyclones is
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removed from the apparatus either to be destroyed or to be guided to a
previous treatment
stage.
Figure 2 illustrates another prior art arrangement (U.S. 5,571,384) comprising
a bleaching
tower 20, a screen 22, a hydrocyclone/cleaning plant 24, a washer 26 and a
bleaching
tower (not illustrated) of the subsequent, usually the last bleaching stage.
According to
the invention, the pulp bleached in the tower 20 is discharged from the
bleaching tower
20 to a pipeline. The pulp either has been diluted while being discharged or
it is diluted
in the pipeline to a screening consistency of > about 1.0 - 1.2 %; from the
pipeline the
pulp is supplied to the screen 22. The accept from the washer 22 is fed
directly into the
washer 26 and the reject is supplied to a hydrocyclone plant 24, which is
remarkably
smaller than prior art hydrocyclone plants, at a consistency diluted to be
suitable for the
cyclones. The accept from the hydrocyclones 24 is recirculated to the feed of
the screen
22 mentioned above or directly to the washer 26. The reject in turn is either
destroyed or
returned to a previous suspension treatment stage.
The apparatus according to a preferred embodiment of the invention illustrated
in Fig. 3
comprises a so-called primary screen 30 and the accept from this is directed
by prior art
means in a so-called main treatment line 30a to the process for further use.
The reject
from the primary screen 30, i.e. the coarse fraction, is in this embodiment
transported in a
line 30r to a second step pressure screen 32, a so-called secondary screen;
the accept from
the secondary screen is returned in line 32a to the feed of the primary screen
30, and the
reject, the coarser fraction, is directed in line 32r to a first hydrocyclone
step 34. In some
cases, particularly when the secondary screen has been constructed in the
required way,
the accept from the secondary screen may be returned to the main line 30a to a
location
after the primary screen (illustrated with a broken line). The accept, i.e.
the lighter
fraction, from the first hydrocyclone step 34 is guided in line 34a to a
pressure screen 36
and the reject, i.e. the heavier fraction, is taken in line 34r to a second
hydrocyclone step
38. The accept from the second hydrocyclone step 38 is returned in line 38a to
the feed of
the first hydrocyclone step 34 and the reject is discharged from the process.
The accept
from the pressure screen 36, i.e. the fraction which has passed through the
screen surface,
is guided in line 36a to the feed of the pressure screen 32 of the second
screening step and
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the reject, i.e. the coarser fraction, in line 36r to a reverse hydrocyclone
40. The heavier
fraction from the hydrocyclone 40 is recirculated in line 40a to the feed of
the pressure
screen 36 as the accept from the reverse hydrocyclone step 40, whereas the
lighter
fraction obtained from the cleaner 40 is removed (line 40r) as reject from the
process, or
is guided to a suitable further treatment.
The operation of the apparatus illustrated in Fig. 3 is described below in a
more detailed
fashion by way of an example. In the example case, the screening member
serving as the
primary screen 30 is a slotted cylinder having a slot width of 0.15 mm. A
cylinder of this
kind accepts, or allows passage therethrough of, only fiber material suitable
for further
use. All larger fiber particles, sand, sticks, and other impurities, such as
for example
plastics, entrain to the reject. The reject is conveyed to the secondary
screen 32
preferably comprising a slotted cylinder having a slow width of 0.18 mm. In
principle
also this cylinder should accept only acceptable fiber fraction but, only to
be sure, the
accept from this screen is returned to the feed of the primary screen for
rescreening. The
reject from the secondary screen is conveyed to the first hydrocyclone step 34
comprising
a required number of cleaners connected in series. The task of the first
hydrocyclone step
is to separate from the reject of the secondary screen, on one hand, the
acceptable fiber
fraction to the accept and, on the other hand, the heavy fraction, such as
sand, to the
reject. The heavy reject is treated in the secondary hydrocyclone step 38 in
order to
recover usable fiber material and to return it to the first hydrocyclone step;
after this
almost only sand or other heavy impurities are removed from the step 38 as the
reject.
The accept from the first hydrocyclone step 34 is conveyed at a very low
consistency (of
the order of 0.4. - 0.6 %), which is typical of the accept from cleaners, to
the pressure
screen 36 which has a very small slot size, for example 0.12 mm. The purpose
is to
separate from the accept of the first hydrocyclone step almost all the usable
fiber fraction
and to return it to the feed of the secondary screen. Reject from the pressure
screen 36,
which may be taken either continuously or intermittently depending on the
amount of the
reject, is treated in the reverse hydrocyclone 40 the main task of which is to
separate the
light reject from the fiber-containing material. The fiber flow obtained as
the accept from
the reverse hydrocyclone step 40 at the tip of the cone is conveyed in a
thickened state to
the feed of the pressure screen 36 through which the fiber fraction is further
taken into
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use. When slotted cylinders are used in the different screens the slot width
of the
secondary screen 32 drum is less than 0.25 mm, preferably less than 0.20 mm,
and in the
screen 36 less than 0.20 mm, preferably less than 0.15 mm.
The apparatus according to another preferred embodiment of the invention
illustrated in
Fig. 4 comprises, like the one in Fig. 3, a primary screen 30, a secondary
screen 32, and a
subsequent first and a second hydrocyclone step 34 and 38. In fact, the only
difference
between the embodiment illustrated in Fig. 3 and the one in Fig. 4 is that the
pressure
screen 36 of Fig. 3 has been replaced in Fig. 4 with a reverse hydrocyclone
step 46, the
accept of which, i.e. the heavier fraction, is returned in line 46a to the
feed of the
secondary screen 32 and the lighter fraction discharged as reject is conveyed
in line 46r to
a second reverse hydrocyclone step 48. The accept from the second reverse
hydrocyclone
step is conveyed in line 48a to the feed of the first reverse hydrocyclone
step and the
reject is discharged (line 48r) from the process. It is possible also in this
embodiment,
and particularly when the secondary screen has been constructed in the
required way, to
return the accept from the secondary screen also to the main line 30a to a
position after
the primary screen (illustrated with a dash line). When slotted cylinders are
used in the
different screens, the slot width of the cylinder in the primary screen 30 is
less than 0.20
mm, preferably about 0.15 mm and the slot width of the cylinder in the
secondary screen
32 is less than 0.25 mm, preferably less than 0.20.
The apparatus according to a third preferred embodiment of the invention
illustrated in
Fig. 5 comprises, like the previous embodiments, a primary screen 30, a
secondary screen
32, and a first hydrocyclone step 34. It differs from the previous embodiments
in that the
accept from the secondary screen 32 is conveyed in line 32a, instead of the
feed of the
primary screen 32, to a reverse hydrocyclone 57 and the accept, i.e. the
heavier fraction,
from the hydrocyclone 57 is conveyed in line 57a to the feed of the primary
screen. This
means that the secondary screen 32 removes large particles to the hydrocyclone
step 34
which classifies them according to their specific gravity either to reject
(line 34r) or to be
returned in line 34a to the feed of the secondary screen 32. The accept of the
secondary
screen contains light material which is taken to the reverse hydrocyclone 57
in which the
light reject (line 57r) is separated from the process and the heavier, the
fiber fraction is
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returned to the feed of the primary screen 30. Also in this embodiment, and
particularly
when the secondary screen 32 and the hydrocyclone step 34 have been
constructed in the
required way, the accept from the hydrocyclone step 34 may be recirculated to
the main
line 30a to a point after the primary screen (illustrated with a dash line).
The apparatus according to a fourth preferred embodiment of the invention
illustrated in
Fig. 6 comprises a primary screen 30, the reject from which is conveyed by
prior art
methods in line 30r to a first hydrocyclone step 62, the reject from which is
removed by a
know method in line 62r from the process or it is conveyed to a suitable
further treatment
step. The new approach in the apparatus of this embodiment is to convey the
accept from
the first hydrocyclone step 62 in line 62a to a reversed hydrocyclone step 64
the heavier
fraction from which is returned in line 64a to the feed of the primary screen
30 and the
lighter fraction is rejected from the process. A second new feature is that
the filtrate from
the thickening device 66 following the primary screen in the line 30a is
treated by
flotation in order to recover fiber material.
The invention preferably employs a screen cylinder disclosed in Finnish patent
application no. 955724, comprising a plurality of screen wires provided side
by side and
secured firmly to support wires provided on the accept space side. In the
approach of the
present invention, narrow slots are preferably used the width of which is of
the order of <
0.25 mm, preferably < 0.20 mm, sometimes even nearly of the order of 0.10 mm.
It
should be mentioned here that, in addition to cylindrical, the screen element
may have the
form of a cone, a double cone, or even a plane.
The rotor in the pressure screen mentioned may be for example the rotor
illustrated in
U.S. patent no. 5,000,842 in which the surface of the rotor cylinder has been
provided
with so-called projections to generate turbulence which disrupts fiber flocs
in the
suspension. Also so-called foil rotors may be used in some cases. Further, a
screen
cylinder either inside or outside the rotor may be used, or even cylinders
provided on both
sides of the rotor. Further, in some cases, a rotating screen cylinder or
cylinders
may be used.
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As may be understood from the above description, a method and apparatus for
screening
suspension has been developed which, compared with prior art methods and
apparatus, is
very user-friendly and advantageous for both the process economy and the
energy
economy. However, it should be noted that the invention has been described
above by
5 way of reference to only a few embodiments thereof although the real scope
of protection
of the invention is much broader and covers more. Thus, the invention is
defined by the
appended patent claims, only.
