Note: Descriptions are shown in the official language in which they were submitted.
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Metho~ ~nd pl~nt for tre~ting ~ cont~min~te~
~ lls~n~ion
The present invention relates to a method of treating a
pulp suspension containing relatively light and heavy
contaminants, the contaminants being separated ~rom the
pulp suspension with the aid of hydrocyclones. The
invention also relates to a plant for treating such a
pulp suspension comprising a first multihydrocyclone
unit having a multiplicity of parallelly coupled hydro-
cyclones designed for separating light contaminants from
the pulp suspension under substantial thickening of ~he
pulp suspension, and a second multihydrocyclone unit
having a multiplicity of parallelly coupled hydrocyclo-
nes designed for separating heavy contaminants from thepulp suspension. The plant further comprises at least
one pump for pumping the pulp suspension to be separated
to the multihydrocyclone units. With the expression
"relatively light and heavy contaminants" is meant such
contaminants which are light and heavy relative to the
~ibres of the pulp suspension. In this connection,
relatively light cont~m;n~nts also comprise particles
which in themselves are heavier than fibres but because
of their shape behave as lighter fibres in the hydro-
cyclones.
Conventionally, light and heavy contAmin~nts areseparated from pulp suspensions by first removing the
heavy contaminants by means of a "regular" type of
hydrocyclone, whereafter the light contaminants are
separated by means of a "reverse" type of hydrocyclone.
A hydrocyclone plant comprising such a regular
hydrocyclone and such a reverse hydrocyclone arranged
downstream of the ragular hydrocyclone is disclosed in
for instance CA l 203 778.
-
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Since the density of light cont~m~n~nts, such as plastic
and glue fragments, is close to the density of wood
fibres, the concentration of fibres of a pulp suspension
which is separated from such light cont~m;n~nts by a
reverse hydrocyclone has to be relatively low, about 0.4
- 0.7 %, (the fibre concentrations mentioned throughout
the text all relate to weight percentage) in order to
make the separation efficiency good. For this reason,
the regular hydrocyclone is conventionally fed with a
pulp suspension having a fibre concentration in the
range of 0.4 - 0.75 %, which gives an outgoing pulp
suspension which is suitable for feeding the reverse
hydrocyclone, since the regular hydrocyclone somewhat
dilutes the pulp suspension. Thus, the flows of pulp
suspension which are supplied to the regular
hydrocyclone and the reverse hydrocyclone are of about
the same sizes, which makes it advantageous to utilize
one single pump to pump the pulp suspension both through
the regular hydrocyclone and the subsequent reverse
hydrocyclone, see for instance EP-B-0 422 314.
The investment and operation costs ~or plants of the
kind described above are significant, since the multi-
hydrocyclone units have to be dimensioned for very large
flows, between 40 000 and 180 000 litres/minute is
usual.
The object of the present invention is to provide a
method and a plant of the kinds here presented, which in
comparison with the above described conventional tech-
ni~ue results in substantially reduced flows of the pulp
suspension which is treated, whereby investments and
operation costs are substantially reduced.
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This object is obtained by the method stated initially,
which is characterized in that at first light contami-
nants are separated from the pulp suspension and the
fibre concentration of the pulp suspension is substan-
tially increased by means of a first multihydrocycloneunit comprising a multiplicity of parallelly coupled
hydrocyclones designed for separating light contami-
nants, and therea~ter heavy contaminants are separated
from the pulp suspension with the increased ~ibre con-
centration by means of a second multihydrocyclone unitcomprising a multiplicity of parallelly coupled
hydrocyclones designed for separating heavy contami-
nants. As a result, the number of hydrocyclones in the
second multihydrocyclone unit can be substantially
reduced, since the flow of pulp suspension, which is
separated from light contaminants, from the first multi-
hydrocyclone unit is substantially smaller than the flow
of pulp suspension which is fed into the first multi-
hydrocyclone unit. The reduced flow through the second
multihydrocyclone unit also results in a reduced need
for energy to pump the flow. For instance, it is quite
possible to increase the fibre concentration of a pulp
suspension from 0.4 ~ to 0.8 ~ by means of the first
multihydrocyclone unit, which reduces the necessary
number of hydrocyclones in the second multihydrocyclone
unit by 50 %, since the flow through the second multi-
hydrocyclone unit is halved.
The pulp suspension with the increased fibre concentra-
tion is advantageously conducted from the first multi-
hydrocyclone unit via a pump to the second multihydro-
cyclone unit.
Each hydrocyclone of the second multihydrocyclone unit
is preferably provided with turbulence increasing means
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adapted to counteract the formation of fibre net-work in
radially outer liquid layers in the hydrocyclone, and
the fibre concentration of the pulp suspension is
increased to at least 0.9 ~ by the first multihydro-
cyclone unit before the pulp suspension is separated bythe second multihydrocyclone unit. It has surprisingly
been proved that hydrocyclones with such turbulence
creatin~ means are capable of separating a pulp suspen-
sion from relatively heavy contaminants without the
separation efficiency becoming unacceptably low. As a
matter of fact the separation efficiency can be satis-
factorily maintained with increasing fibre concentra-
tion, up to about 1.5 %. Hydrocyclones with particularly
efficient turbulence creating means in the form of
radially outwardly directed steps in the separation
chambers of the hydrocyclones, as shown in W0 93/10908,
are marketed by Alfa Laval Celleco AB under ~he designa-
tion Step~eleaseTM.
The object of the present invention is also achieved by
means of the plant described initially, which is charac-
terized in that the pump is adapted to pump the pulp
suspension to be separated through the first multihydro-
cyclone unit, and that the first multihydrocyclone unit
is connected to the second multihy~.rocyclone unit for
supplying thickened pulp suspension., which is separated
from light contaminants, to the second multihydrocyclone
unit.
Preferably, each hydrocyclone of th.e second multihydro-
cyclone unit is provided with turbulence creating means
adapted to counteract the formation of fibre net-work in
radially outer liquid layers in the hydrocyclone, and
the first multihydroc~clone unit is adapted to thicken
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the pulp suspension to a fibre concentration of at least
0.9 %, preferably at least 1.0 ~ and m;~xim~lly 1,5 96.
The first multihydrocyclone unit is advantageously con-
~ 5 nected to the second multihydrocyclone unit via a con-
nection conduit with an additional pump, for transfer-
ring said thickened pulp suspension from the first to
the second multihydrocyclone unit. The energy consump-
tion of said two pumps will be substantially less than
the energy comsumption of a single pump which is uti-
lized to operate the first and the second multihydro-
cyclone unit.
Upstream of said additional pump and downstream of the
first multihydrocyclone unit, said connection conduit
can advantageously be provided with a counter pressure
device adapted to maintain a constant counter pressure
in an outlet for accept fraction of the first multi-
hydrocyclone unit. The counter pressure device enables a
return flow with recovered fibres to be transferred from
the reject fraction of the second multihydrocyclone unit
back to the second multihydrocyclone unit without loa-
ding the first multihydrocyc~one unit with said return
flow.
Further advantageous features of the plant according to
the invention are defined in the attached claims.
The invention is described more closely in the ~ollowing
with reference to the accompanying drawings, in which
figure l is a diagram showing how the cleaning effi-
ciency depends on the fi~re concentration of the pulp
suspension at hydrocyclones with and without turbulence
creating means,
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figure 2 shows a flow chart of a plant according to a
first embodiment of the invention, and
figure 3 shows a flow chart of a plant according to a
second embodiment of the invention.
~n the figures identical components have been provided
with the same reference numerals.
In figure 1 there is shown as an example a diagram, in
which the dependence of the cleaning efficiency ~ upon
the fibre concentration C of a pulp suspension for an
older conventional regular hydrocyclone is illustrated
by a continuous curve line and for a newer regular
hydrocyclone, which is provided with turbulence creating
means of the kind shown in W0 93/10908, is illustrated
by a dotted curve line. The pressure difference dP
between the inlet and the accept outlet of each hydro-
cyclone is in this case 120 kPa. As is evident from the
diagram the cleaning efficienc~ of a conventional
hydrocyclone decreases from about 92 % at a fi~re con-
centration of 0.5 % to about 87 % at a fibre concen-
tration of 0.9 ~, whereas the cleaning efficiency of the
newer hydrocyclone still is as high as about 92 % at a
fibre concentration of O.g %. This means that only 8 ~
contaminants are left in a pulp suspension, which has a
fibre concentration of 0.9 % and which has been separa-
ted by the newer hydrocyclone, whereas 13 % contaminants
are left in the same pulp supension which has been sepa-
rated by the older hydrocyclone. Conseguently, the olderhydrocyclone allows about 60 % more contaminants to pass
through than the newer hydrocyclone when separating pulp
suspensions with 0.9 % fibre concentration.
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As is evident from the diagram, the cleaning efficiency
of the newer hydrocyclone at a higher fibre concentra-
tion of 1.0 ~ is still about 92 %, whereas the cleaning
efficiency of the older hydrocyclone has decreased
further to about 85 %. At higher ~ibre concentrations
than 0.9 ~ the risk of clogging of the apex outlet of
the older hydrocyclone with fibre net-work is drama-
tically increased, and for this reason in practice the
older hydrocyclone is only utilized for pulp suspensions
with a fibrè concentration less than 0.9 % The newer
hydrocyclone, on the other hand, can be utilized for
separating pulp suspensions with a fibre concentration
of up to 1.5 %, without risk for clogging. The limiting
factor here is the poorer cleaning efficiency, not the
1~ risk of clogging.
It should be noted that the diagram accordin~ to ~igure
1 relates to a certain kind of pulp suspension. Other
kinds of pulp suspensions of course give other curve
lines in the diagram, but the principal differences
which are evident from the example above are also valid
for such other kinds of pulp suspensions.
In figure 2 there is shown a plant according to a first
embodiment of the invention comprising a feed conduit 1
for a pulp suspension, which contains relative light and
heavy cont~m;n~nts, a machine 2 ~or receiving and dewa-
tering separated pulp suspension, e.g rotary filter, bow
sieve or paper machine, and a container 3 ~or receiving
water from the machine 2. From the container 3 a conduit
4 with a pump 5 extends to a multihydrocyclone unit 6,
which comprises a multiplicity of parallelly coupled
conical hydrocyclones of reverse type, e.g. Tripac 90
ReverseTMwhich are marketed by Alfa Laval Celleco AB.
The multihydrocyclone unit 6 has an inlet 7 for pulp
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suspension, an apex outlet 8 for an accept fraction
containing pulp suspension separated from relatively
light contaminants and a base outlet 9 for a light
reject fraction containing relatively light conta-
minants. From the apex outlet 8 a connection conduit 10with a pump 11 extends to an inlet 12 of a multihydro-
cyclone unit 13, which comprises parallelly coupled
conical hydrocyclones with turbulence creating means of
the kind shown in W0 93/10908. The multihydrocyclone
unit 13 has an apex outlet 14 for a heavy reject
fraction containing relatively heavy contaminants and a
base outlet 15 for an accept fraction containing pulp
suspension separated from relatively heavy contaminants.
From the base outlet 15 a transport conduit 16 extends
directly to the machine 2.
A return conduit 17 for separated pulp suspension
extends from the transport conduit 16 to the conduit 4.
The return conduit 17 is provided with a valve 18 for
adjusting the return flow in the return conduit 17.
The heavy reject fraction flowing through the apex out-
let 14 of the multihydrocyclone unit 13 contains some
fibres which are recovered by means of a multihydro-
cyclone unit 19 comprising parallelly coupled hydro-
cyclones of the same kind as in the multihydrocyclone
unit 13. (The fibres in the reject fraction are usually
recovered by several stages of multihydrocyclone units
coupled in cascade, but for reasons of simplicity only
one such a stage is shown here).
A conduit 20, which is connected to the conduit 4
upstream of the connection between the latter and the
conduit 17~ extends to the multihydrocyclone unit 19. Tn
the conduit 20 there is a pump 21. The apex outlet 14 of
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the multihydrocyclone unit 13 is connected via a conduit
~2 to the conduit 20 upstream of the pump 21. A base
outlet 23 in the multihydrocyclone unit 19 is connected
via a conduit 24 to the conduit 4 downstream of the
connection between the conduit 4 and the conduit 20, for
supplying recovered fibres to the pump 5. An apex outlet
25 of the multihydrocyclone unit 19 is connected to a
container, not shown, for separated heavy contaminants.
The light reject ~raction flowing through the base out-
let of the multihydrocyclone unit 6 contains some fibres
which are recovered by a multihydrocyclone unit 26
comprising parallelly coupled conical hydrocyclones of
the same kind as in the multihydrocyclone unit 6. (The
1~ fibres in the light reject fraction are usually re-
aovered by several stages of multihydrocyclone units
coupled in cascade, but for resons of simplicit~ only
one such a stage is shown here). A conduit 27 with a
pump 28 extends from the conduit 20 upstream of the
connection between the conduits 20 and 22 to the multi-
hydrocyclone unit 26. A drain conduit 29 from the base
outlet 9 is connected to the conduit 27 upstream of the
pump 28. An apex outlet 30 of the mu~tihydrocyclone unit
26 is r.~n~e.nted via a conduit 31 to the conduit 4 down-
stream of the connection between the conduit 4 and theconduit 20, for supplying recovered fibres to the pump
5. A base outlet 32 of the multihydrocyclone unit 26 is
connected to a container, not shown, for separated light
contaminants.
During operation of the plant according to figure 2 the
pulp suspension which comes in via the feed conduit 1 is
deluted with water from the container 3 so that the
fibre concentration of the pulp suspension supplied to
the multihydrocyclone unit 6 becomes about 0.6 %, which
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gives a fibre concentration of about 1.2 % at the accept
fraction of the pulp suspension flowing through the apex
outlet 8. The light reject fraction from the base outlet
9 is deluted with the water in the conduit 27 and is
pumped by the pump 28 to the multihydrocyclone unit 26,
which gives an accept fraction with recovered fibres
through the apex outlet 30 and a light re~ect fraction
with light contaminants through the base outlet 32. The
accept fraction is condueted via the conduit 31 back to
the pump 5.
From the base outlet 15 of the multihydrocyclone unit 13
an accept fraction, which contains pulp suspension sepa-
rated from relatively heavy contaminants and whieh in
this case has a fibre concentration, of about 1.15 ~, is
conducted to the machine 2. Depending on the capacity of
the machine 2 the valve 18 is adjusted so that a part
flow of the flow in the transport conduit 16 is returned
to the pump 5.
The heavy reJect fraction from the apex outlet 14 of the
multihydrocyclone unit 13 is deluted with water in the
conduit 20 and is pumped by the pump 21 to the multi-
hydroeyclone unit l9, which gives an accept fraction
with reeovered fibres through the base outlet 23 and a
reject fraetion with heavy cont~m1n~nts through the apex
outlet 25. The aecept fraction is conducted via the
conduit 24 baek to the pump 5.
In figure 3 there is shown a plant according to a second
em~odiment of the invention, which is identical to the
plant according to figure 2, except that means are
arranged to bring back the accept fraction with reco-
vered fibres from the multihydrocyclone unit 19 without
loading the pump 5 and the multihydrocyclone unit 6, and
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11
th~t suitable control devices are implemented. Conse-
quently, the connection conduit 10 comprises a first
part lOa extending upwardly from the apex outlet 8 of
the multihydrocyclone unit 6 to a first open container
33 with an overflow 34 and a second part lOb extending
from a second open container 35 to the pump 11, the
container 35 being arranged to receive pulp suspension
from the container 33 via the overflow 34.
The contalners 33 and 35 and the overflow 34 constitute
a counter pressure device adapted to maintain a constant
counter pressure in the apex outlet 8.
A level control means 36 is adapted to control a control
valve 37, which is arranged in the drain conduit 29 from
the base outlet 9 of the multihydrocyclone unit 6, in
response to the level of the liquid surface in the
second container 35, so that said level is below the
overflow 34. A return conduit 38 extends from the con-
duit 24 via a valve 39 to the container 35. A valve 40
is arranged in the conduit 24. By the valves 39 and 40
desired part flows of the accept fraction from the
multihydrocyclone unit 19 can be adjusted in the con-
duits 38 and 24. For instance, a major part flow or the
entire accept fraction may be conducted via the return
conduit 38 to the container 35, whereby energy can be
saved for the operation of the pump 5 and the number of
hydrocyclones in the multihydrocyclone unit 6 can be
reduced.
A control device 41 is adapted to control the capacity
of the pump 5 in response to the pressure in the inlet 7
of the multihydrocyclone unit 6. Since it exists a
constant counter pressure in the apex outlet 8 of the
multihydrocyclone unit 6, because of the arrangement
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with the open container 33, which is located at a
certain height above the apex outlet 8, the eontrol
device 41 only needs to control the pressure in the
inlet 7 in order to maintain a desired differenee
pressure between the inlet 7 and the apex outlet 8.
A control device 42 is adapted to control the capacity
of the pump 11 in response to the difference pressure
between the inlet 12 and the base outlet 15 of the
multihydroeyclone unit 13. A control valve 43 is
arranged in the eonduit 22 and is eontrolled by a
control device 44 in response to the pressure difference
between the accept outlet 15 and the apex outlet 14 o~
the multihydroeyelone unit 13.
Since the accept fraetion with recovered fibres from the
multihydrocyclone unit 19, during operation of the plant
according to figure 3, is supplied to the pulp suspen-
sion from the multihydrocyclone unit 6 via the container
35 the fibre coneentration of the pulp suspension is
increased to about 1.3 ~ before the pulp suspension is
pumped into the multihydrocyelone unit 13. As a result,
the fibre eoncentration of the pulp suspension separated
from relatively heavy contaminants and flowing through
the base outlet 15 of the multihydroeyelone unit 13
beeomes about 1.15 ~.
In addition to this, the plant aeeordin~ to figure 3 is
operated in the same manner as the plant aceording to
figure 2.
As an alternative, one of or both of the eontrol devices
42 and 44 of the plant aeeording to figure 3 may also be
installed in the plant according to figure 2.
