Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1335191
Screenlng method and apparatus for llght reject removal
The present inventlon relates to a screening method and
apparatus for separatlng solld particles, such as fibre
bundles and llght partlcles from flbrous slurries used in
paper making. The apparatus malnly comprises a substantially
vertical cylindrical screen, a substantially cylindrical
rotor concentrically arranged thereln, said rotor and
said screen defining a screenlng zone for the pulp, an
accept chamber surrounding the outer surface of said screen,
an inlet for supplying the pulp to be screened into the
upper end of said screening zone, a re~ect chamber connected
to the lower end of sald screenlng zone and means for
separating the heavy and the light impurities from each
other inside said rotor. The method comprises introducing
dilution liquid close to the top cover of the rotor inside
the rotor for separatlng and washing the light reject off
from the heavier reJect, flber bundles, knots etc.
When screening paper pulps contalnlng a variety of
impurities in a screen described e.g. in U.S. patent no.
3,363,759, the re~ect contalns also partlcles which can be
separated therefrom by employlng centrifugal force. The
particles separated in this way, the specific gravity of
which is lower than that of the average particles, i.e.
the light reject, contains e.g. various plastics. The
remaining reject contains e.g. fibre bundles and pieces of
knots. It is an ob~ect of the present invention to provide
a screening apparatus which in a simple manner separates
light impurities from the reject. The reason for removing
light rejects, e.g. plastics from the reject flow is the
fact that plastics etc. are very harmful in paper making
processes. The reject from screening apparatuses is usually
recirculated through grinding devices back to the screening
apparatuses for recovering all the fibers from fiber bundles
in such a way that the light re~ects, like plastics, are
~'
2 1335191
also ground whereby they wlll be small enough to be accepted
by the screens and they end up to the paper machine and
thus lower the quality of the end product.
The screening apparatus in which the present invention is
a remarkable improvement has been developed from a known
screen disclosed e.g. in U.S. patent 3,363,759. An improved
version of the screeni ng apparatus of the above patent has
later been modified for removal of plastics, said apparatus
in turn being disclosed in U.S. patent 3,912,622. In this
version the separation of the light particles from the
rejected pulp is arranged through a pipe one end of which
is fixed to the inlet end of the screen and the other end
extended through an opening in the cover of the casing. A
conical extention is provided in the other end of the pipe
which extends to the open upper end of the rotor. Due to
the vortex movement of the reJected pulp the light reject
concentrated in the lnner part of the vortex flows out
through a discharge pipe while the heavier reject gathered
in the outer part is recirculated into the screening zone.
In order to achieve efficient separation the reject flow
must be ample which decreases the capacity of the machine.
The screening apparatus on which the present invention is
based comprises a substantially vertical cylindrical screen,
a substantially cylindrical rotor which is concentrically
disposed inside the screen and the substantially closed
outer surface of which together with the interior surface
of the screen defines a screening zone for the pulp, an
accept chamber surrounding the outer surface of the screen,
an inlet for supplying the pulp to be screened into the
upper end of the screening zone, a reject chamber connected
to the lower end of the screening zone, and means for
separating the heavy and the light impurities from each
other inside the rotor.
Said means for separating the heavy and the light impurities
i.e. reject fractions is a duct that extends from outside
- 3 1335191
the apparatus through the bearing support lnslde the rotor
close to the top cover of the rotor. Sald screening
apparatus is more closely described in US patent 4,634,521.
It is an object of the present invention to provide an
arrangement ln connectlon with the device of said US patent
4,634,521 for further improving the separation of light
rejectable material from the re~ect. It has been found
important to decrease the amount of light rejects as much
as possible as usually the llght rejects are discharged from
the screening apparatus to a tank for further processing.
The further processlng ls necessary for recovering the
fiber materlal being discharged with the light rejects as
the rejected lighter materlal normally contains about 90 %
fiber material that could be recirculated back to the
system. Sald further processlng usually means a separate
sorting arrangement wlth screening devices and perhaps
even cyclones for separatlng the llght rejects, most often
plastics, from the acceptable flber material. By using the
improvement of the present lnventlon lt is possible, in
the most advantageous clrcumstances, to discharge the
llght reJect, malnly plastlcs, totally without any need to
reclrculate lt to further screenlng. Thls is achieved by
lmprovlng the ablllty of the apparatus to separate light
reject from flbre bundles, sllves, knots etc. in such a
way that the fibre bundles, knots etc. do not enter the
discharge duct for the light rejects. It has been found
out that the light reJects, mainly plastics, are able to
separate themselves from the cellulosic material if the
consistency of the suspension ls low enough. Now, by
lntroducing the dllution liquid close to the top cover of
the rotor inside the rotor, the consistency of the
suspension is lowered just where it is needed. Now that
the consistency is lowered the plastics and like rejects
tend to float on the liquid/suspension thus filling the
space under the top cover of the rotor. The simplest way
of doing that ls to lead the dilution liquid through the
4 1335191
same duct upwards whlch duct iQ used for removing the
llght re~ects.
Thls ls made possible by arranging a branch pipe having a
valve in
connection with the duct leading inside the rotor for
removing the light re~ect. Said branch pipe is preferably
arranged outside the screening apparatus housing close to
the valve in the duct for discharging the light reject
between said valve and the screening apparatus housing.
The present invention provides a reliable way for removing
light re~ect in a pro-environmental manner.
The apparatus according to the invention is illustrated by
the accompanying drawing which ls described in detail below.
Figure 1 is a vertical section of an embodiment of the
lnventlon.
The maln components of the screen lllustrated in Figure 1
are a multi-compartment houslng l, a substantially
cylindrical screen drum 2, a substantially cyolindrical
rotor 3 and a shaft 4. The shaft is rotatably mounted
inside the rotor on bearings on a cylindrical support 5
which is fixed to the housing by a sealing ~oint. The
rotor is mounted on the upper end of the shaft and a belt
pulley 6 on the lower end of the shaft for rotating the
rotor. A disc 7 is fixed to the upper end of the support
through which the shaft extends. The rotor outer surface
9 provided in a preferred embodiment with e.g. half-spheri-
cal protrusions 8 and the inner surface 10 of theconcentric screen drum define a ring-shaped screening zone
ll where the pulp to be screened ls fed into. The upper
end of the rotor is closed by a top cover 12 into which
the shaft is fixed.
The upper part of the housing constitutes of a feed chamber
13 provided with an inlet connection 14 for the pulp to be
screened. There is a re~ect chamber 15 provided with a
;~
1335191
dlscharge connection 16 in the lower part of the housing.
The upper end of the screening zone 11 is connected to the
feed chamber and the lower end to the reject chamber.
There is an accept chamber 17 outslde the screen drum.
An inlet 18 for dilution liquid may be arranged in the
reject chamber. A discharge duct l9, which extends through
the support and the disc 7 in the upper end of the support,
is disposed inside the support; the upper end of the duct
is disposed adjacent to the inner surface 21 of the rotor
top cover 12.
The pulp to be screened is supplied into the device through
the inlet connection 14 and at first it circulates in the
feed chamber tangentially. The feed chamber acts at the
same time as a scrap catcher, where stones and other heavy
particles are removed. From the feed chamber the pulp
flows into the ring-shaped channel ll between the rotor 3
and the screen drum 2 functioning as a screening zone.
The accept flows from the screening zone 11 through the
screen surface 10 to the accept chamber 17. From the
accept chamber the pulp is discharged through a discharge
connection 20.
The reject, i.e the portion of the pulp flow which does
not pass through the screen surface, ends up in the reject
chamber 15. In the reject chamber the reject may be diluted
by dilution liquid supplied through the opening 18 and is
discharged through the discharge connection 16.
Several mill scale test have shown that e.g. the plastics,
which is a typical constituent in the light reject, remains
on the reject side. E.g. by employing a 0.45 mm slot
screen as the screening surface, 85 to 95 ~ of the plastics
in a typical sulphate cellulose pulp can be removed, which
yields accept with a plastics content of 5 to 15 ~ , only,
of the plastics content of the feed flow. The problem
that led to the invention of US 4,634,521 was how to
1335191
separate the plastics and other llght partlcles from the
reJect flow that ls to be reclrculated as explalned above.
The solution ls slmple and efflclent. The llght reject
flows lnto a space 22 inslde the rotor through the rotor
bottom end 23. The high revolution speed, which is typical
to the rotor, brings the pulp in the screening space, in
the re~ect chamber and in the space 22 inside the rotor
into a rotating motlon which creates the centrifugal force
necessary for the separation of the light reJect. Dilution
liquid may be supplied into the re~ect chamber in order to
regulate the consistency.
In the apparatus in accordance with said US patent the
light reject is gathered inside the rotor 3 in the space
under the top cover 12 wherefrom lt may be removed through
duct l9 either continuously or intermittently. The only
two problems with the gathered llght reject when uslng the
apparatus in accordance wlth sald US patent are that the
separated light re~ect contalns plenty of fiber material
that could well be used and that the duct 19 easily become~
blocked when the conslstency of the re~ect is too high.
The solution to the flrst problem ls dllutlng the reject
lnside the rotor in such a way that the light rejects have
space enough to rise upwards and separate from the fibrous
particles. The most effective way of dllutlng ls to lead
the dilution llquid close to the top cover of the rotor so
that the light rejects will remaln floatlng on the liquid
and the coarser re~ects, flber bundles etc. will be washed
off from the light re~ects whereby the liquid flowing slowly
downwards takes the fiber bundles and knot particles. The
solution to the second problem is arranging the dilution
through duct 19 in such a way that the dilution liquid
slowly flows through said duct to the top of the rotor
compartment malntalning the duct clean. For regulating the
flows of the diluting liquid and the light rejects the
branch pipe 30 is provided with valve means 31 and the
~'
1335191
dlscharge duct 19 with valve means 32. Said valve means
are used in such a way that the valve 32 is for the most
of the time closed and the valve 31 open letting the
diluting liquid to flow throught the duct inside the rotor.
After e.g. four hours the valve 31 in the branch pipe is
closed and valve 32 in the duct 19 opened so that the
plastics etc. are able to flow away from inside the rotor.
After few minutes the valve 32 is closed and valve 31
opened whereby the dilution starts again. In a mill scale
test the reJects discharged through duct 19 without the
branch pipe arrangement contained 300 - 500 kgs fiber per
day. After the branch pipe with the valve arrangement having
been installed the amount of fibers in the re~ects dropped
to 30 - 50 kgs fiber per day.
It is of course clear that the pulp may also be partially
diluted at the bottom end of the screening zone by liquid
supplied through the dilution water ring. There may be
openings in the rotor through which the main portion of
the re~ect flow is supplied into the space inside the
rotor where the light re~ect is separated by centrifugal
force. The light reJect flows up lnto the space under the
rotor top cover, is diluted/washed there and is removed
therefrom through the discharge duct from time to time and
the remaining re~ect flows down lnto the re~ect chamber
from whlch lt is discharged through the opening.
The present invention ls not limited to the embodiments
above said embodiments being presented and described as
examples only, but several modifications may be made thereof
within the scope of protection defined by the appended
claims.
k~
