Note: Descriptions are shown in the official language in which they were submitted.
METHOD FOR DEWATERING A FIBER SUSPENSION IN A DEWATERING
APPARATUS
The present invention relates to a method for dewatering
a suspension of cellulose fibers in a dewatering apparatus
working under pressure, by regulating the working conditions of
the dewatering apparatus so that it operates under optimum
conditions.
In cleaning fiber 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 and then thicken it again. The
fiber suspension taken to a hydrocyclone or a hydrocyclone plant
usually has a content of fibers which is lower than 1%. The
cleaned, i.e. the accepted fraction from a hydrocyclone may have
a 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
form, 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
25 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%.
Cleaning fiber suspensions with hydrocyclones occurs
before taking the pulp to a wet machine, for 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 invention there is
provided a method of dewatering a suspension of cellulose fibers
in a dewatering apparatus working under pressure and having an
inlet for fiber suspension, a supply conduit connected to the
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inlet, a first outlet for dewatered fiber-rich fraction and a
second outlet for substantially fiberfree fraction, a discharge
conduit being connected to each of the outlets, characterized in
that the suspension is partially taken into the dewatering
apparatus and partially through a branch conduit past the
dewatering apparatus and into the discharge conduit for fiber-
rich fraction.
By partially taking the fiber suspension past the
dewatering apparatus and partially through it, there is achieved
a larger quantity of fiber suspension being dewatered than that
passing through the apparatus. In this way the fiber suspension
in the dewatering apparatus will be dewatered to a higher fiber
concentration than what is desired or needed, and the fiber-rich
fraction coming from the apparatus can be diluted with the by-
passed fiber suspension to a suitable concentration.
According to one embodiment of the invention, thepressure of the suspension taken to the dewatering apparatus is
regulated by means of a valve disposed in the branch conduit.
The pressure in the supply conduit is preferably maintained
constant. A particular advantage is obtained in that the
pressure in the supply conduit is determined by an accept
pressure of a hydrocyclone arranged before (upstreams of) the
exit of the branch conduit. By hydrocyclone there is
particularly intended a hydrocyclone plant having many
hydrocyclones connected in parallel.
In accordance with another embodiment, the fiber-rich
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fraction obtained from the dewatering apparatus and the by- ;
passing suspension, which are united by the branch conduit
opening out into the discharge conduit, are taken to a fiber
processing apparatus, the pressure of the suspension or the
level in the fiber processing apparatus being regulated by a
valve in the dewatering apparatus discharge conduit for the
substantially fiberfree fraction. In the present description
and claims, level (sometimes called head in the art) and
pressure are equivalent expressions in relation to the sus-
pension in the fiber processing apparatus.
According to a further embodiment, at least one
physical parameter of the fiber fraction is measured in the
dewatering apparatus first outlet for fiber-rich fraction, or
in the associated discharge conduit. The measured paramete-
value is compared with a predetermined first criterion valuefor the parameter and the flow is then adjusted, by means of
a valve disposed in the discharge conduit for fiber-rich
fraction, in relation to the criterion value in response to
the deviation of the parameter value from the criterion value.
As parameter are oreferably selected the fiber concentration,
pressure and/or flow.
The method in accordance with the invention is de-
scribed in more detail in the followinq with the aid of the
drawings, on which
Fig. 1 schematically illustrates parts of a processing
line for fiber suspensions, in which there is included a
dewatering appa atus, and
Fig. 2 schematically illustrates the same parts of
the processing line as in Fig. 1 but a thickener has been
exchanged for a wet machine with a headbox.
The method in accordance with the invention is de-
scribed with the aid of Fig. 1. A fiber suspension which is
to be cleaned in a hydrocyclone and then dewatered and
thickened as well as given other treatment, e.g. dispersion
in a pulp press, comes via a conduit 21 from an unillustrated
source of fibers~ is diluted to suitable fiber concentration
and is pumped by a pump 17 to a protective sieve 1 in front
of a hydrocyclone 2. The protective sieve 1 is a pressure
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sieve, in which contaminants are removed which could damage
the hydrocyclone if thev were supplied to it. It is prefer-
able to extract rejected material discontinuously 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 hydrocvclone 2.
The hydrocvclone 2 is preferably a hydrocyclone plant
having many hydrocyclones connected in parallel in at least
two steps. In Figs. 1 and 2, three steps 2, 3 and 4 are
illustrated. The hydrocyclone steps are often cascade-con-
nected. Re]ect from the first or primary hydrocyclone step 2
goes to a second or secondary hvdrocyclone 3, the accept of
which is recycled to the primary hydrocyclone 2, in this case
via the pump 17 and sieve 1. The accept from the primary
hydrocyclone 2 is taken via a conduit 22 to a dewatering
apparatus 5 working under pressure.
The incomin~ fiber suspension is divided into a
fiber-rich fraction and a substantiallv fiberfree fraction
in the dewatering apparatus 5. The fiber-rich fraction is
taken to a thickener 6, via a conduit 23. The substantially
fiberfree fraction from the dewatering apparatus 5 is taken
to the conduit 21 before the pump 17 via the conduits 24 and
26, where the substantially fiberfree fraction is used for
diluting the suspension coming from the fiber source to a
suitable fiber concentration. The thickened fraction, thick-
ened in the thickener 6, goes to the next treatment step,
not more closely described here. The weak fraction, i.e. the
fiberfree fraction, from the thickener 6 is collected in a
level vessel 8 with an overflow. Via the respective conduits
30 26, 27 and 28, the level vessel 8 is connected to the suction
side of all pumps 17, 18 and 19 upstream of the thickener 6.
In the case where the quantity of the substantially fiberfree
fraction from the dewatering apparatus 5 is not sufficient
for diluting the fiber suspension coming from the fiber
source, the fiberfree fraction from the thickener 6 is usedfor 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 dewatering apparatus 5, there
departs a branch conduit 25 which opens out into the conduit
23 after the dewatering apparatus 5, the conduit 23 being
used to take the fiber-rich fraction from the dewatering
apparatus 5 to the thickener 6. There is a control valve 11
in the branch conduit 25.
The thickener 6 is provided-with a trough 7 from
which the fiber suspension su~plied to the thickener 6 goes
to the thickening section of the thickener 6.
When cleaning fiber suspension in a hydrocyclone, the
pressure drop above the hydrocyclone must be kept constant to
obtain a uniform and acceptable separation of contaminants.
The hydrocyclone 2 is fed with fiber suspension by the pump
17 via the sieve 1 under constant pressure. For this reason
the accept pressure of the hydrocyclone 2 must be maintained
constant while also keeping a constant pressure of the sus-
pension taken to the dewatering apparatus 5. In order to
maintain the accept counter-pressure of the hydrocyclone 2
constant, the pressure in the conduit 22 is regulated by
means of the valve 11 in the branch conduit 25. The pressure
in the conduit 22 is sensed, preferably before or upstreams
the branching of the conduit 25 from the main conduit 22,
with the aid of a pressure sensor 31. A signal, proportional
to the pressure sensed in the conduit 22, is taken to a
control and actuating means 14 which compares the intensity
of the signal with a first criterion value and, if there is
a difference between the intensity of the signal and the
criterion value, alters the setting of the valve 11 in a
direction and~to an extent such that the pressure in the
conduit 22 approaches the criterion value.
Accordinq to one embodiment of the invention, a
physical parameter of the fiber-rich fraction from the de-
watering apparatus 5 is sensed, in the vicinity of the out-
let of the apparatus 5 for the fiber-rich fraction, with the
aid of a transducer 32. A signal proportional to the magni-
tude of the parameter value is taken to a second control and
actuating means 15, in which the magnitude of the incoming
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signal is compared with a second criterion value and which,
if there is a difference between the magnitudes of the
criterion value and that of the signal, actuates the setting
of a valve 12 dispos~d before the transducer 32 in the
conduit 23 to such a direction and extent that the value of
the parameter sensed in the conduit 21 approaches the second
criterion value. Examples of suitable parameters are fiber
- concentration, pressure and flow. The criterion value-for the
second parameter is of course dependent on the property
measured. Measuring with the aid of the pressure sensor 31,
which emits signals to the control and actuating means 14,
and with the aid of the transducer 32, which emits signals
to the control and actuating means 15, is to advantage
continuously done. The measuring cycle can also be repeated
with short intervals, i.e. periodical measuring.
In order to regulate the quantity of dewatered fiber
suspension taken to the thickener 6, the level or liquid
pressure in the trough 7 is measured~ and if the level (in
the following, reference is made only to level and not to
equivalent pressure) deviates from a third criterion value,
the valve 13 in the conduit 24 is reset so that the quantity
of suspension supplied to the thickener 6 is altered in
relation to the criterion value for the level. The adjustment
is made via a sensor disposed in the trough 7 and a control
means 16 actuating the valve 13.
In Fig. 2 the fiber processing apparatus, which is a
thickening apparatus in Fig. 1, is a wet machine for paper
pulp. A headbox 46 and a wire 47 are included in the machine.
Remaining parts of the fiber processing line are identical
with those in Fig. 1.
The fiber-rich fraction from the dewatering apparatus
5 goes to the headbox 46 and from there onto a wire 47 for
dewatering. The substantially fiberfree white water, sepa-
`rated on the wire 47, is collected in the container 8 which
has an overflow. In the same way as in Fig. 1, this substan-
tially fiberfree white water is used as a pressure reference
for all the pumps 17, 18 and 19 before the dewatering appara-
tus. A constant excess pressure above the fiber suspension
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is maintained in the headbox 46, which ex~esspressure corre-
sponds to a given liquid level so that the suspension de-
parting from the lid of the headbox 46 is given the desired
speed, which is adjusted to the speed of the wire. If the
pressure in the headbox 46 deviates from the predetermined
value, the setting of the valve 13 is affected via the
control and actuating means 16 in relation to the criterion
value.
