SCREENING OF FIBROUS SUSPENSIONS

TAMISAGE DE SUSPENSIONS FIBREUSES

English Abstract

A method of separating a fiber fraction by screening from a liquid containing
fibers and small particles, where the separation takes place in that the
ingoing liquid is allowed to flow along a screen member with screen apertures
from an inlet (2) to a first outlet (3) for a first fraction which passes
through the screen apertures, and a second outlet (4) for a second fraction,
which does not pass through the screen apertures. 50-80 % of the flow of the
second fraction is recycled from the second outlet (4) to the inlet (2) for
increasing the concentration of network forming fibers in the ingoing liquid.

French Abstract

Cette invention concerne un procédé qui permet de séparer par tamisage une fraction fibreuse d'un liquide contenant des fibres ainsi que des particules de petite taille. La séparation se fait en faisant s'écouler le liquide entrant le long d'un élément tamis comportant des ouvertures de tamisage. L'écoulement se fait depuis une entrée (2) vers une première sortie (3) pour une première fraction passant à travers les ouvertures du tamis, et vers une seconde sortie (4) pour la seconde fraction ne passant pas à travers ces ouvertures. 50 à 80 % du débit de la seconde fraction sont renvoyés de la seconde sortie (4) vers l'entrée (2) afin d'accroître la concentration des fibres formant un réseau dans le liquide entrant.

Claims

5
Claims
1. A method of separating a fiber fraction by screening from a liquid
containing fibers and small particles, where separation takes place in that
the ingoing liquid is
allowed to flow along a screen member with screen apertures from an inlet (2)
to a first outlet
(3) for a first fraction, which passes through the screen apertures, and a
second outlet (4) for a
second fraction, which does not pass through the screen apertures,
characterized in
that 50 - 80% of the flow of the second fraction is recycled from the second
outlet (4) to the
inlet (2) for increasing the concentration of network forming fibers in the
ingoing liquid.
2. A method as defined in claim 1, characterized in that the ingoing
liquid consists of contaminated process water, from which usable fibers are
recovered in the
second fraction.
3. A method as defined in claim 1 or 2, characterized in that the
ingoing liquid consists of fibrous black liquor from pulpmaking, whereby
usable fibers in the
black liquor are recovered in the second fraction.

Description

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Screening of fibrous suspensions
This invention relates to separation of a fiber fraction from a liquid
containing fibers and
small particles.
At the manufacture of papermaking pulp, both from wood- and recycled fiber raw
material,
the treatment is carried out in several steps, at which the process water is
supplied and
drained. At the manufacture of paper in an integrated pulp- and paper mill,
the papermaking
pulp is transported as fiber suspension at a pulp concentration of 3 to 4 %
from the pulp mill
to the paper mill. The process water in the paper mill, thus, originates
mainly from the pulp
mill. The surplus water from the papermaking is re-used later on at the
pulpmaking. This
implies that at the making of pulp and paper very great amounts of water are
used which
mainly circulate in the manufacturing process. From this complicated process
results totally a
small surplus of water, which must be cleaned before it can be discharged to
the recipient.
In order to reduce the need of external water cleaning and to decrease the
pressure on the
environment, the water flow in the mills for the making of pulp and paper has
been closed to
the greatest possible extent. As a result thereof, disintegrated (dispersed)
impurities, fiber
fragments and loose substance circulate about to an ever increasing extent and
thereby disturb
the production process. The normal way of reducing the build-up of internal
circulation is at
the discharge of the surplus water of the process to choose a heavily
contaminated water. This
water is led to external water cleaning, for example microflotation or
membrane filtration.
The problems with heavily contaminated circulating water flows are especially
great at the
manufacture of paper products based on recycled fibers.
It becomes increasingly more difficult from an environmental and economic
point of view to
handle contaminated water flows from recycled fiber plants and paper mills.
Inreased re-circulation of water in the mils results in an increased build-up
of contaminations

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in the process. These contaminations, such as size lumps, particles of
printing ink, ash and
other matter (dispersed or solved) affect the process negatively. Even if it
was possible to
remove a large portion of the heavily contaminated process water, it is
normally not
defensible economically, because it often contains too many prime fibers.
It is one object of the precent invention to solve the asforesaid problems by
making it possible
to effectively recover usable fibers from contaminated process water in
connection with the
making of paper-making pulp.
Another object of the present invention is to increase the fiber separation
efficiency of liquor
screens in chemical pulp mills. These screens separate fibers from the waste
liquor, before it is
to be combusted. The separation efficiency of these screens is often much too
low, especially
in the case of short-fibrous hardwood pulps.
The characterizing features are apparent from the attached claims.
The method according to the invention is described in greater detail in the
following with
reference to the Figures, of which
Fig. 1 is a flow chart showing an embodiment of the invention;
Fig. 2 shows the effect of re-circulation of fibers according to the
invention.
The embodiment shown in Fig. I comprises a conventional screen means I with an
inlet 2 and
a first outlet 3 for a first fraction and a second outlet 4 for a second
fraction. There is further a
return line 5 with a central valve 6 between the second outlet 4 and inlet 2.
In Fig. 1 the
volume flow is indicated by the following definitions:
V e= inject flow
V ~ = recovered flow
V ~ = return flow

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A liquid, which can be process water containing fibers and contaminations, is
fed as inject
(V~ through the inlet 2 to the screen means 1. The liquid is allowed to flow
along a screen member
with screen apertures whereby a first fraction (Vf- Vr) passes through the
screen apertures and is
taken out through the first outlet 3, and a second fraction (V ~ - V ~, which
does not pass
through the screen apertures, is taken out through the second outlet 4.
The first fraction (V f - V ~) contains mainly small particles in the form of
dispersed
contaminations and fine material from the papermaking pulp, and the second
fraction
(V ~ - V ~ contains mainly long usable fibers. At this fractionation the fiber
concentration of
the ingoing liquid is very low and thereafter increases along the screen
member during the
course of fractionation. This implies that the strength of the fiber network
increases much
along the screen member, from the inlet to the outlet for concentration
fibers. The energy
supply from the rotor of the screen produces turbulence, which breaks up the
weak fiber
network at the beginning of the screening zone, which results in deteriorated
separation of
fibers. At the end of the screening zone the strength of the fiber network is
much higher,
because the fiber concentration has been multiplied, and the efficiency of the
separation
process is high. The fiber network formed on the surface of the screen member
due to the
flocculation tendency of the fibers has a very important role in the
separation mechanism. It
is, thus, not only the apertures of the screen member which separate the
fibers from the liquid.
By increasing according to the invention the flow of the second fraction (V ~ -
V ~ and
returning a portion (V ~) of the second fraction from the second outlet 4 via
the line 5 to the
inlet 2, the concentration of this fraction can be increased at the inlet end
of the screen
member. The flow of the second fraction preferably is increased 2-S times, and
50 - 80% of
this fraction is recycled to the inlet. The re-circulation factor (CF = V ~V
~) varies in these
cases from I-4. By this circulation of mainly long fibers the concentration
gradient of this
fiber fraction decreases along the length of the screening zone. Instead of
that the
concentration of the actual fraction to be separated varies with a power of
ten along the length
of the screening zone, the differences in concentration decrease strongly
along the length of
the screening zone with the help of the long fiber re-circulation according to
the invention.
This renders it possible to optimize the supplied de-flocculation energy along
the screening
zone so that the efficiency of the process increases, i.e. it is thereby
possible to substantially

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decrease the share of long usable fibers in the first fraction. An increased
fiber concentration
in the ingoing liquid increases the network formation in the ingoing fraction,
and the
probability that network forming fibers shall pass through the apertures of
the screen barrier
decreases.
Example
Fig. 2 shows by means of a theoretical model based on mill data the effect of
recirculation of a
varying portion (V ~} of the second fraction (V ~ + V ~ to the inlet when the
totally separated
flow is 5 and 10 %, respectively. The flow division (FS = V,/ V ~ is 0,05 and
0,10,
respectively. It appears from Fig. 2, that it is possible to increase the
efficiency of the recovery
of usable fibers from about 80 % to about 90 % by using a re-circulation
factor (CF) of 3-4.
The fiber concentration in the inlet has simultaneously trebled from 0,2 to
0,6.
The invention can also be used for increasing the efficiency of liquor screens
in processes for
the production of chemical pulp. Liquor screens are used, for example, in a
sulphate mill for
recovering fibers from black liquor before its combustion.
The invention, of course, is not restricted to the embodiment shown, but can
be varied within
the scope of the claims.

Representative Drawing