Note: Descriptions are shown in the official language in which they were submitted.
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The method relates to a method for processing
waste paper.
A processing method is known from German Patent
Specification 23 11 674, in which the inventive idea is to
conserve energy by just slight slushing in the pulper and
extensive slushing in the bleaching tower during a storage
period of 2 to 3 hours. However, in this case additional
apparatus in the form of a thickener is required in order to
bring the suspension to the consistency of approximately
25~, with which operations are normally carried out in the
bleaching tower. Therefore in this case there is a
relatively low consistency in the pulper and a relatively
high consistency in the bleaching tower. However this has
the disadvantage that the pulper has a relatively low
processing capacity in comparison with the bleaching tower.
The additional thickening apparatus, which involves high
costs, is also disadvantageous. Furthermore, the slushing
process in the pulper at the given consistency of
approximately 5% to 6% is no longer economical nowadays and
cannot be performed under the most technologically
favourable conditions. It has to be borne in mind that
impuritiss which are normally contained in waste paper are
not extensively broken down, but are meant to be removed at
an early stage.
As is known, nowadays pulpers are used with a
relatively high consistency o~ 12% to 17~ and with a
specific expenditure of energy of normally 25 Kwh/t and
above (per tonne of waste paper) until there is a low
residual flake content. For this purpose, there is used a
slushing rotor, which has screw conveyors as the circulating
components, which have an external diameter which
continually decreases substantially towards the top, at
least in its lower part, or which tapers conically at its
outer circumference. As a result, the waste paper is gently
broken up, predominantly by moderate shearing forces and
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fibre friction, i.e. the impurities are broken down as
little as possiblQ by the action of the slushing rotor.
According to the present invention, there is
provided a method for processing waste paper, wherein the
waste papex is moderately broken up into mainly rags in a
pulper at a consistency of between 8% and 12% with a maximum
expenditure of energy of 15 Kwh/t, then the broken down
waste paper rags are screened through a screening
perforation of between 18mm and 25mm diameter, the screened
waste paper rag suspension is conveyed by means of a pump
into a storage tank and is stored there at a consistency of
between 5% and 7% for the purpose of the further
disintegration of the waste paper rags and flakes, and
wherein the unscreened waste paper suspension is circulated
from and to the pulper via an intermittently operating
vortex flow appliance in a proportion of 6% to 20% of the
pulper throughput, whereby the waste paper rags in the
suspension are further broken up and the fibre content
therein undergoes further screening so as to return to the
pulper only waste paper constituents which can be slushed.
According also to the present invention, there is
provided apparatus for processing waste paper comprising a
pulper, a storage tank for the accept from the pulper and a
vortex flow and screening appliance providing a circulation
path from and to the pulper, wherein a rotor is disposed on
the floor of the pulper and having a vertical axis of
rotation, wherein said rotor has conveying screws having an
external diameter which gradually decreases at least in the
lower part of the rotor in the upward direction, or having
an external diameter which decreases conically, wherein an
accept chamber is housed on one side of the pulper, which is
separated by a substantially flat screen which deviates from
the vertical by max. 15 r has perforations of between 18mm
and 25mm, and wherein an impeller is mounted for rotation in
front of the wire to keep the latter clean and for mixing.
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It has been found in practice that there is no
additional deflaking at the slushing rotor or slushing wheel
of said rotor, i.e. by the rotating blades of said rotor
brushing past a screenO The result is an even more gentle
separation of the waste paper, in which the impurities can
be gradually removed from the pulper in a particularly
fa~ourable way through the circulation. After the paper
rags have been separated, the accept is screened out in the
circulating equipment and returned to the pulper again.
The specific slushing energy in the pulper is kept
low and is at most 10 Kwh/t to 15 Kwh/t waste paper,
depending upon the strength or wet strength of said paper;
in practice, slushing is only performed up to pumping
capability, in which case there is a flake content of
bstween 30% and 40% related to the whole (paper) fibre
content.
A delivery chamber having a screen separating it
from the rest of the pulper, parallel to which and along
which the arms or blades of a further rotor similar to an
impeller can be moved to keep it clear, is not provided in
the direct vicinity of the slushing rotor, but on one side
wall of the pulper.
The invention is explained below by means of an
exemplified embodiment shown in the f igures of the drawings
where
Figure 1 shows schematically a waste paper
processing apparatus partially in section; and
Figure 2 shows a horizontal section through the
apparatus of Figure 1.
The waste paper is supplied to the tank of a
pulper 1 in bales 15 or loose via a conveyor 14. Here it is
separated into shreds of rags by the action of screw
conveyors 3 of a conveying rotor 2, which becomes
continually narrower in the upward direction at its
circumference/ at least in its lower part, or becomes
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narrower in a conical shape as shown in Figure 1. In this
case, the consistency in the pulper is between 8% and 12%,
preferably 10% and 12%. The slushing rotor has a vertical
axis of rotation and is disposed on the floor of the pulper
and is driven by a motor 31 via a drive shaft 30. On one
side of the tank wall of the pulper, there is disposed a
screen 7 having substantially uniform perforations of 18mm
to 25mm diameter, which separates an accept chamber 8 from
the rest of the tank. In front of the screen an impeller 5
is located and rotates to keep the screen clear and to mix
the contents of the pulper in this region to a certain
extent by means of diluting medium supplied through line 13.
On the pulper there is an outlet 6 from which a line 26
leads to a vortex flow appliance 10. The appliance 10 has
an impeller 11 which rotates to draw suspension ~rom the
pulper at a rate oE 6% to 20% of its throughput and feed
this via line 16 to a screening appliance in the form of a
perforated screening drum 17. This drum is housed in
bearings 19 on its shaft 18, for example any non-utilizable
dirt is discarded through apertures at the front on the
left-hand side of the screening appliance. These apertures
are defined between radial spokes 20, by means of which the
drum is attached to its shaft 18. Of course it would also
be possible to support the screening drum on rollers over
its circumferences.
The necessary diluting medium is supplied to the
circulation, i.e. via a line 34 into the line 26a.
Heavy material is removed form the pulper as
normal via heavy particle sluice 27.
From Figure 2 it can be seen that the accept
chamber 8 and the screen 7 are disposed on a projection 28
of the pulper which is more or less on one side. As a
result the suspension does not flow tangentially past the
impeller 5, but is supplied at a steep angle thereto and to
the screen. This improves the throughput of the accept.
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The screen 8 is substantially vertical or at a maximum is
inclined at 15 to the vertical.
In the accept chamber 8, the waste paper
suspension achieves a consistency of approximately 5% to 7%
and this suspension is then pumped to a tower-shaped storage
tank 22 via a line 26 and pump 29. There it remains for
roughly 1 to 2 hours, at most 2 to 3 hours, for further
(chemical) slushing and it is then conveyed through a line
25 for a subsequent processing operation. Whilst in the
tank 22 the suspension can optionally be further diluted via
a pipe 24 with the help of propeller 23 to roughly 4% before
the subsequent processing operation.
of course, further screening stages are provided
below, in which waste paper bales which have not yet broken
up are partially broken up.
The storage tank 22 is preferably constructed so
that it is naturally aspirated whereby pumping can be
carried out even with non-avoidable larger amounts of
t:rapped air. It has been shown that this is perfectly
possible.
Consequently in contrast to systems with slushing
consistencies of 12% to 17% in the pulper, which are also
frequently used nowadays, a strong dilution associated with
subsequent thickeniny and thus a thickener in the "front'
part of the process is avoided.
It can also be seen from Figure 2 that the angle
"a" between the radial of the pulper drawn through the
cutting line of the central axis of impellor 5 and screen 7
with~ the wire surface is roughly an angle of 120. This
angle may preferably be between 115 and 130. The angle of
the ~3ide wall of the pulper in this upstream region with the
said radial may be between 20 and 50, i.e. it may be up to
5 smaller and up to 10 larger than the angle between the
shaft of the impeller and the radial.
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However it must be remembered that the slushing
rotor 2 does not rotate above a screen and is not close to
the screen as is customary, with the result that it does not
exert any considerahle tearing and deflaking action and it
consequently breaks up the waste paper in a particularly
gentle wayO The intermittent circulation of the fibre
suspension by means of vortex flow appliance 10 via line 16
and screening device 17 is known from USP 4,634,059.
The vortex flow appliance may be a pump working on
the vortex principle (see USP 4,370,172 or Papier, Carton et
Cellulose 1986, page 57, Figure 5) or an appliance as
specified in USP 4,634,059, in which the ratio of the
(axial) length to the diameter of the interior of the
housing may lie between 0.8 and 1.5. The motor 31 is
intermittently driven at between 10 and 120 cycles per hour
(i.e. 6 minutes to 30 seconds) with the high values applying
more to the vortex pump.
The slushing chemicals, i.e. 1.5 - 4 % NaOH or
Na2SO3 or a part thereof, may preferably already have been
added to the pulper, just like any chemicals required for
subsequent flotation.
The slushing in the pulper results in a relatively
high proportion of residual paper rages, but these can to a
large extent be screened through the large wire
perforations.
