Note: Descriptions are shown in the official language in which they were submitted.
CA 02205651 2005-O1-31
1
Screening arrangement
This invention relates to an arrangement for
separating light impurities from pulp suspensions. Light
impurities are understood to be particles or aggregates with
lower density than water, for example plastic particles.
Aggregates are found, for example, at flotation where a
heavy particle is bound to a gas, and the aggregate
particle/bubble behaves like a light particle.
Heavy impurities in the pulp are separated in
conventional screens by means of centrifugal force in
separate scrap taps, while coarse fiber fractions and
impurities are separated by a screen member whereby the pulp
is divided into accept and reject.
Light impurities are more difficult to separate,
especially at high pulp concentrations. The pulp
concentration, however, is desired to be high, for example
3-50, in order to achieve a high production capacity and to
avoid the transport of large liquid volumes in the screening
system.
Light impurities, therefore, usually are separated
in separate devices, such as hydrocyclones. This requires
not only separate equipment, but also low concentrations,
below 1%, in order to achieve a good effect. It is,
therefore, a less attractive method to vortex clean the
entire flow from a pulp line. It is, instead, possible to
carry out pressure screening through very fine slits at
highest possible concentration and thereafter to use vortex
cleaner for separating the light impurities from the reject
flow of the screens.
According to the present invention, there is
provided an arrangement for separating light impurities from
CA 02205651 2005-O1-31
1a
pulp suspensions, intended to be connected to a casing of a
fractionation device where the fiber network is broken up
and the pulp is caused to rotate for enriching the light
impurities, wherein the separation arrangement is designed
with a rotation symmetrical housing intended through a
passage to be connected centrally to the upper portion of
the casing, an inlet for dilution liquid is connected
tangentially to the housing, and an outlet for the light
impurities is located centrally at the top of the housing.
Embodiments of the present invention offer a
solution for the aforesaid problems, in that a device for
removing light impurities is designed to be directly
connected to the casing of a fractionation means, for
example a conventional screen arrangement.
CA 02205651 1997-OS-20
WO 96/17998 2 PCT/SE95/01340
The invention is described in greater detail in the following, with
reference to the accompanying Figure, illustrating an embodiment of
the invention where the separation device is combined with a fract- '
ionation device in the form of a pulp screen.
The arrangement shown comprises a pulp screen with an airtight cas-
ing 1, which has an inlet 2 for inject and outlets 3 and, respect-
ively, 4 for accept and, respectively, reject. A rotation symmetrical
screening member 5 with vertical axle is located in the casing 1. The
pulp inlet 2, which preferably is tangential, communicates with the
inside of the screening member 5 at the upper end thereof while the
reject outlet 4 communicates with the lower end of the screening
member. The accept outlet 3 is connected to a space 6, which is loc-
ated outside and extends about the screening member 5. In connection
to the upper pprtion of the casing, a space with outlet 7 for heavy
impurities, such as sand and scrap, is located.
A rotor 8 within the screening member 5 extends along the entire
screening member. The rotor'is concentric~with the a Greening member
in such a way that a screening zone 9 extending all about is formed
between the rotor and screening member. The rotor 8 and screening
member 5 can be cylindric or conic.
The rotor 8 preferably is provided with wing elements 10, which
are intended to bring about pulsations in the pulp in the screening
zone 9 in order to break up the fiber network and to render it poss-
ible to divide the pulp into accept and reject.
A member 11 for the separation of light impurities is connected c~~ht-
rally to the upper portion of the casing 1 and communicates with the
in+erior of the casing 1 via a passage 12. The said member is formed
with a rotation symmetrical housing, which preferably comprises a
conic upward diverging lower portion 13 and a substantially cylindric
upper portion 14. In the transition between the lower and upper hous-
CA 02205651 1997-OS-20
WO 96/17998 3 PCT/SE95/01340
ing portions 13, 14, an inlet 15 for dilution liquid is connected
tangentially, and an outlet 16 for light impurities is located centr-
ally at the top of.the housing 11'. In said outlet 16 a valve 17
is located, for example a sluice valve, for controlling the outflow
from the member 11.
A rotor axle 18 can possibly be provided to extend from the casing
1 through the passage 12 upward in the housing 13,14, and a rotor
element 19 with a carrier be attached to the axle 18 in the housing
13,14. The rotor axle 18 can be provided with a screw thread 20 for
upward feeding. The rotor axle 18 preferably is driven by being
attached to the rotor 8 of the fractionation device. The rotor
element 19 preferably shall be located on the same height as the
inlet 15 for dilution liquid so that correct flow geometry is obtained.
The rotor element supplies energy, which otherwise must have been
supplied by the inlet rate and amount of dilution liquid. By using
the rotor element, thus, the dilution amount supplied can be reduced
at maintained separation effectiveness.
The pulp suspension is supplied through the inlet 3 in the casing 1
where it is caused to rotate due to the tangential supply and the
rotation of the rotor 8. Scrap and other heavy impurities are coll-
ected in the space 7 by the effect of centrifugal force. The pulp is
introduced into the screening zone 9 and moves axially downward to
the reject outlet 4 while being simultaneously rotated. The accept
passes thereby through the apertures of the screening member 5. The
wing elements 10 bring about pulsations in the pulp which facilitate
the division into accept and reject. The reject is discharged through
the outlet 4.
Due to the rotation of the pulp in the casing 1, the., light impurities
a
are collected centrally at the top of the casing. The location and
design of the separation device 11 give rise to an upward directed
flow centrally in the passage 12 at the same time as a downward dir-
ected return flow is caused in the outward portion of the passag'e~ 12.
CA 02205651 1997-OS-20
WO 96/17998 PCT/SE95/01340
4
The light impurities are thereby moved up through the passage 12 to
the member 11 where they are collected centrally upwardly. By tang-
ential supply of dilution liquid through the inlet 15 energy is suppl-
ied which drives the rotation and produces suitable flow conditions to
enrich the impurities and promote the collection of light impurities
centrally upwardly in the device 11. These impurities can thereby be
taken out through the outlet 16. The discharge, which is controlled
by a valve 17, can be continous or intermittent, depending on the
amount of impurities. When the arrangement comprises a rotor axle 18
with a xntc~~ element 19, the rotation of the contents in the housing
13,14 is affected additionally in that the effect of the tangential
dilution liquid supply is increased.
The flow rate through the passage 12 shall be low, of the magnitude
0,02 m/s. The conical design of the lower portion 13 of the device 11
promotes in this portion circulation, which is directed upward at the
centre and downward along the conical walls, at the same time as there
is a horizontal rotation movement.in the entire device 11. Owing to
the location of the tangential inlet 15 for dilution liquid, an opp-
osed circulation movement is caused in the upper portion 14, i.e. a
movement directed downward at the centre and upward along the outer
walls, which results in a movement of the light impurities toward the
centre in this portion 14.
In the embodiment shown the separation device 11 for light impurities
is shown connected to a type of screen arrangement, but it is obvious
that also other types of fractionation means can be applied, for ex-
ample screens with rotating screen members and screens with other types
of rotors. The screening can take place from the inside outward or from
the outside inward through the screen member. The screening can also
take place during the passage from above downward or from below up-
ward through the screening zone. In all cases is common, that the pulp
suspension shall be rotated in the upper portion of the casing of the
fractionation device.
The invention, of course, is not restricted to the embodiments shown,
but can be varied within the scope of the invention idea.
