Note: Descriptions are shown in the official language in which they were submitted.
1235~3 ~
ME.T_IOD IN Tl-lICKENING A FIBEF< SUSPENSION IN A ~I~1~ A~A~
The present invention relates to a method in thickening
a suspension of cellulose Eibers in a thickeniny apparatus
working under pressure, by regulating the work:ing conditions of
the thickening apparatus so that it operates under optimum
conditions.
In cleaning Eiber suspension stocks in sieves and
hydrocyclones, relatively diluted fiber suspensions are used in
order to obtain the best possible cleaning and good separation of
contaminating particles, e.g. shives, sand, bark and pieces of
plastic. It is therefore necessary first to dilute a fiber
suspension to a low fiber content of 0.4% or lower, depending on
the type of pulp and the starting material or desired cleaning.
For washing, refining or other treatment of the paper pulp, e.g.
dispersion of wax in the pulp in treating recycled fiber pulp, it
is necessary for the pulp to be present in a considerably more
concentrated from, e.g. with a content of at least 30%.
Thickening from 0.5% to 30-40%, for example, must be carried out
in several steps~ usually two. In the first step, a filter is
most often used which gives a thickening by a factor of 10 - 15,
e.g. from 0.5% to 5-7%. The thickened pulp is then taken to a
pulp press, e.g. a screw press, for final thickening to the
desired fiber content. The lowest fiber content in a suspension
for supplying to a pulp press is about 3%.
Cl~aning fiber suspensions with hydrocyclones occurs
before taking the pulp to a wet machine, Eor example. The fibers
must be diluted to a concentration suitable for cleaning in a
hydrocyclone, and once again be brought up to a higher fiber
content suitable for the wet machine, e.g. 2-3%.
In accordance with the present invéntion there is
provid~d a method of thickening an aqueous suspension containing
cellulose fibers in a thickening apparatus working under pressure
and having an inlet for fiber suspension, a supply conduit being
connected to the inlet, a first outlet for a fiber-rich fraction
and a second outlet for a substantially fiberfree fraction, a
discharge conduit being connected to each of the outlets,
characterized in that the flow through the outlet for the fiber-
rich fractisn is regulated in response to one or more physical
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parameters of the fiber-rich fraction.
In accordance with a preferred embodiment, the pressure
of the fiber suspension coming to the thickening apparatus is
regulated, and preferably mai.ntained constant. This is done by
sensing the pressure in the supply conduit, compariny the sensed
pressure with a criterion value, and altering the flow through
the second outlet :for the substantially fiberfree fraction
towards the criterion value in response to the difference between
the sensed value and the criterion value.
lo According to another embodiment, this pressure
regulation is achieved with the aid of a valve arranged in a
branch conduit, through which the fiber suspension is partially
taken past the thickening apparatus to the discharge conduit for
the fiber-rich fraction, by sensing in a corresponding way the
pressure in the supply conduit, comparing the sensed value of the
pressure with the criterion value, and if there is a difference,
altering the flow through the branch conduit so that the pressure
in the supply conduit approaches the criterion value.
In accordance with a still further embodiment, the
fiber-rich fraction obtained from the thickening apparatus and
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the by-passin~ suspension, which are unitecl b~ the hraneh
conduit openin~ out into the discharge eonduit, are taken to
a Eiber proeessin~ ~pparatus, the pressure o~ ~e sus~ension
or the level in the ~iber processin~ apparatus being regulated
by a valve in -the thickening apparatus disehar~e eonduit for
-the subs-tantially fiberfree fraction. In the present descrip-
tion and claims, level and pressure are equivalent expressions
in relation to the suspension in the fiber processing appara-
tus.
The method in accordance with the invention is de-
scribed in more detail in the following with the aid of the
drawings, on which
Fig. 1 sehematieally illustrates parts of a processing
line for fiber suspensions, in which there is included a
thickening apparatus,
Fig. 2 schematically illustrates the same parts of the
fiber processing line as in Fig. 1 but a thiekener has been
exehanged for a wet machine with a head box for paper pulp,
and
Fig. 3 illustrates another embodiment of the proeessing
line in Fig. 2.
The method in aeeordanee with the invention is de-
scribed with the aid of the drawings, and here there is first
a deseription of the proeessing line in Fig. 1. A fiber sus-
pension which is to be eleaned in a hydroeyelone and then de-
watered and thiekened as well as given other treatment, e.g.
dispersion in a pulp press, eomes via a eonduit 21 from an
unillustrated souree of fibers~ is diluted to suitable fiber
eoneentration and is pumped by a pump 17 to a proteetive
sieve 1 in front of a hydroeyelone 2. The proteetive sieve 1
is a pressure sieve, in whieh eontaminants are removed whieh
could damage the hydrocyclone if they were supplied to it. It
is preferable to extraet rejeeted material diseontinuously at
the sieve 1 and throw it away. Accept from the sieve 1, which
is mainly the entire suspension taken-to the sieve, goes
further to the hydrocyelone 2.
The hydrocyelone 2 is preferablv a hydrocyclone plant
having many hydroeyelones eonneeted in parallel in at least
t~o steps. In the fic~ures on the drawings, three steps 2,
ancl 4 are illustrated, The hydroc~clone 6teps ~re oft~n
cascade-connected. Reject from the first or pr:l.mary hydro-
cyclone step 2 yoes t~ a second or secondary hydrocyclone
step 3, the accept of which is rec~cled to the pri~ary hydro-
cycli.ng step 2, in this case via the pump 17 and sieve 1. The
accept from ~he primary hydrocyclone step 2 is taken via a
- conduit 22 to-a thickening apparatus 5 working under pressure.
The incoming fiber suspension is divided into a
fiber-rich fraction and a substantially fiberfree fraction in
the thickening apparatus 5. The suspension consisting of the
fiber-rich fraction is taken to a fiber processing apparatus,
in this case a thickening apraratus 6'.via a conduit 23. The
mainly fiberfree fraction, the weak fraction, is taken from
the thickening apparatus 5 to the conduit 21 before the pump
17 via the conduits 24 and 26, where the substantially fiber-
free fraction is used for diluting the suspension from the
fiber source to a suitable fiber content.
The thickened fraction, thickened in the thickening
apparatus 6, goes to the next treatment step, not more closely
described here. The weak fraction, i.e. the fiberfree fraction,
from the thickening apparatus 6 is collected in a level vessel
8 with an overflow.
The level vessel 8 is connected to the suction side
.~ 25 of all pumps 17, 18 and 19 downstream or upstream of the
thickening apparatus 5 via the respective conduits 26, 27 and
28. In the case where the quantity of the substantially fiber-
free fraction from the thickening apparatus 5 is not sufficient
for diluting the fiber suspension coming from the fiber source,
the fiberfree fraction from the fiber processing apparatus is
used for dilution via the conduit 26. The method of controlling
and regulating the dilution is well-known for one skilled in
the art and is not more closely described.
From the conduit 22, between the accept outlet of the
hydrocyclone 2 and the thickening apparatus 5, there departs a
branch conduit 25 which opens out into the conduit 23 after
the thickening apparatus, the conduit 23 being used to take
the fiber-rich fraction from the thickeninq ap~aratus 5 to the
thickening appa.ratus 6. There is a control valve~in the branch
conduit 25.
T~e thickening apparatus 6 is providecl with a trough 7
:Erom which the fiber suspension supplied to the apparatus 6
goes to the thickening section o~ the apparat-~s ~.
The properties o the fiber~rich fraction obtained
from the thickenin~ apparatus 6 such as pressure, fiber content
- and particularly flow, are kept in accbrdance with the inven-
tion to a predetermined, preferably constant value by sensing
a parameter value of the fraction with the aid of a txansducer
32, taking the signal proportional to this value to a con-
trolling and actuating means 15 in which the parameter value
is compared with a criterion value for the parameter in
question and, if there is a difference, actuating the setting
of a valve 12 which preferably is situated ~ of the
transducer 32 such that the measured propert~ of the fiber-rich
fraction approaches the criterion value. It is preferred to
regulate the fiber-rich fraction flow through the thickening
apparatus outlet for the fiber-rich fraction and that the
20 transducer 32 is a magnetic flow meter.
In order that the thickening apparatus 5 will work as
optimally as possible, the pressure of.the.incoming suspension
is also.regulated. The pressure in the conduit 22 is sensed by
a sensor 31. The measured pressure is compared with a criterion
25 value, and if there is a difference, the setting of the valve
11 in the branch line 25 is altered with the aid of a control
and actuating means 14 towards the criterion value for the
pressure. Constant pressure in the conduit 22 is not only
advantageous in relation to the uniformity of the thickening~
but is also necessary if there is a hydrocyclone before the
c3nduit 22, the accept of the hydrocyclone being the suspen-
sion taken to the thickening apparatus S, since the accept
counterpressure must be constant to.obtain a uniform and
acceptable separation in the hydrocyclone 2. The criterion
value of the suspension pressure in the conduit 22 is prefer-
; ably determined by the hydrocyclone accept pressure.
- In accordance with another embodiment, the flow
. through the second outlet of the thickening apparatus 5, for
~S~ 7
the substantially fiberEree Eraction, is also recJulated. 1'his
flow is set so that in the processing apparatus, which is a
thickener 6 in Fig. 1, a constant level is maintained in -the
trou~h 7. The li~uid level or correspondir~g h~drostatic
pressure is -thus sense~ in the trough 7, the measured value is
compared wit~ a criterion value, and if there i5 a difference,
the setting of the valve 13 is altered via a control and
actuating means 16 so that the level in the trough approaches
the criterion value for the level.
In Fig. 2 the fiber processing apparatus, which is a
thickening apparatus 6 in Fig. 1, is a wet machine for paper
pulp. A headbox 46 and a wire 47, on which the fiber suspen-
sion is dewatered, are included in the machineO Remaining
parts of the fiber processing line are identical with those
in Fig. 1.
The fiber-rich fraction from the thickening apparatus
5 goes to the headbox 46 and from there onto the wire 47 for
dewatering. The substantially fiberfree white water, separated
on the wire 47, is collected in the container 8 which has an
overflow. In the same way as in Fig. 1, this substantially
fiberfree white water is used as a pressure reference ~or all
the pumps 17, 18 and 19 be~ore the thickening apparatus 5. A
constant excess pressure is maintained-in the-headbox 46,
which corresponds to a given liquid-level so that the suspen-
sion departing from the lid of the headbox 46 is given thedesired speed, which is ad~usted to the speed of the wire.
If the pressure in the headbox deviates from the predeter-
mined value, the setting of the valve 13 is affected via the
control and actuating means 16 so that the pressure in the
headbox approaches the criterion value.
In Fig. 3, the fiber processing apparatus is a wet
machine as in Fig. 2. According to this embodiment, the
branch conduit 25 with the valve 11 is lacking, or the valve
11 is completely closed so that no suspension by-passes the
thickenin~ apparatus 5. The thickening apparatus 5 thus de-
waters the incoming fiber suspension to the desired fiber
concentration. Regulation of the pressure or level in the
headbox 46 is done with the aid of the valve 12 in the dis-
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ch~rc3e conduit 23 of the thickenlng appara-tuc) 5.
In cont~aclistinction to the embodiment :Ln Fiy. 2,
reglllation of the pressure of ~e supply co~duit 22 to the
thickenin~ apparatus 5 takes place with the a:Ld of the valve
13. The pressure in the condult 22 is thus sensed wi~h the
sensor 31 and pressure deviations in the conduit 22 are
regu~ated via the control and actuation means 14 with the
valve 13, so that the pressure approaches the~criterion value.
An alteration of the flow through the conduit 24 affects the
thickening and thereby the fiber content and the flow of the
suspension in the conduit 23, The properties of the fiber-
rich fraction are regulated with the aid of the valve 12, so
that a desired quantity of fibers, i.e. the desired fiber
concentration and flow, comes to the headbox 46.
The substantially fiberfree white water, separated
on the wire 47, is collected in the container 8 with its
overflow. In the same way as in Figs. 1 and 2, this substan-
tially fiberfree white water is used as a pressure reference
for all the pumps 17, 18 and 19 before the thickening appa-
ratus 5.
