Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Pulp washer
This invention relates to an apparatus for the liquid
treatment of cellulose pulp, for example washing and dewatering.
The invention, more precisely, refers to an apparatus where the
pulp from a head box is allowed to flow out and form a layer on
a plane, endless liquid permeable web. The web runs over two
end rolls, and ~reatment liquid i9 supplied on the upper surface
of the pulp layer at the same time as liquid is sucked away from
the lower surface of the pulp layer into vacuum vessels beneath
1~ the web.
In such apparatuses the web usually is a "wire" manu-
factured of plastic wires. The wire is driven by one end roll
and slides over the perforated covers on the underlying vacuum
vessels. Owing to the friction between the wire and vessel
covers, great forces are required for driving the wire. This
gives rise, at the same time, to high tensile stresses in the
wire, which thereby is restricted as to its possible length, be-
cause the tensile stress increases with the length of the con-
tact surface between wire and vessel covers. When it is desired
in an existing mill to increase the capacity, it is therefore
necessary in most cases to increase the wire width instead of
the wire length. This implies substantial re-construction of
the mill. If a lengthening of the wire were possible, this
could be effected by relatively simple measures at reasonable
cost.
Due to the fact that the wire is manufactured of
interplaited plastic wires, any damage in the form of broken
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wires calls for very troublesome repair work, which in most cases
is so expensive that it is more economical to exchange the entire
wire than attempt to repair it.
The present invention has the object of eliminating the
aforesaid problems and at the same time making further advantages
possible.
According to the invention, the plastic wire referred to
above is replaced by a perforated steel web. A st~el web has a
substantially higher tensile strength than a wire. It is,
further, relatively, simple and inexpensive to repair damages in
the ~Yeb. Since a steel web also has a higher resistance to
pressure, the dewatering of the pulp layer according to the
invention can be carried out much more effectively. A further
advantage is that the open area in the covers of the vacuum
vessels can be increased due to the inherent carrying capacity of
the steel web.
According to the present invention there is provided an
apparatus for liquid treatment of cellulose pulp comprising:
a pair of spaced-apart end rolls one of which is a drive roll;
~0 an endless perforated steel web arranged about the pair of end
rolls so as to provide a substantially horizontally disposed upper
portion, the perforated steel web being adapted to be driven by
the drive roll to move the upper portion in a longitudinal
direction extending from one of the end rolls toward the other of
the end rolls, and the endless steel web having a thickness of
between 0.5 and 1.5 millimeters and being perforated so as to have
an open area of between 8 percent and 25 percent; h0ad box means
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for supplying cellulose pulp to be treated directly onto the upper
portion of the endless steel web;liquid collection means arranged
between the pair of end rolls for collecting liquid displaced from
the cellulose pulp through the endless steel web as the endless
web is driven by the drive roll relative to the liquid collection
means, the liquid collection means having a substantially
horizontally disposed surface arranged beneath and in direct
contact with the upper portion of the perforated steel web; liquid
treatment means arranged above the upper portion of the web for
supplying a treatment liquid to the cellulose pulp supplied onto
the upper portion of the endless steel web as the steel web is
driven by the drive roll; means for applying a pressure
differential across the upper portion of the web to force liquid
from the cellulose pulp on the upper portion of the web through
the endless steel web into the liquid collection means as the
steel web is moved relative to the liquid collection means; and
removal means for removing treated cellulose pulp from the steel
web after the cellulose pulp has been subjected to the liquid
treatment.
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The invention is described in the following, with
reference to a preferred embodiment thereof and to the accom-
panying drawings wherein:
Figure l shows a liquid treatment apparatus according
to the invention, and
Figure 2 is a cross-section taken along the line II-II
in Figure l.
The embodiment of the invention shown in Figure l com-
prises an endless perforated steel web l, which runs about a
first end roll 2 and a second end roll 3. The rolls 2 and 3 are
mounted in a frame 4 so that the first roll 2 is movable by
means of a stretching device 5, preferably a hydraulic one, for
adjusting the web tension. The driving is effected via the
second roll 3, and both rolls preferably are coated with a fric-
tion-increasing surface layer.
A head box 6 is located adjacent roll 2 for distrib-
uting the cellulose pulp in the form of a uniform layer on the
web l. In the head box 6 a rotor 7 is provided for defloccu-
lating the pulp. Beneath the web a plurality of vacuum vessels
8 are arranged one after another in the longitudinal direction
o~ the web. The vessels 8 are provided with perforated covers
acting as a support for the web l.
Outlets ~ and l0, (see Figure 2) are connected to the
vessels 8 for the discharge of liquid and gas, respectively.
Above the web, means ll for the supply o~ treatment liquid are
located. Over 1:he pulp layer a pressure difference is main-
tained for effecting liquid transport crosswise through the
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pulp layer. The width of the pulp layer on the web is limited
by two endless deckle straps 12 which follow along with the web
1. Other Xnown means for limiting the width can, of course, be
employed. A hood 13 is placed over the web 1.
At the second roll 3 a transfer device 14 for the pulp
layer, after its completed treatmlent, is located, which may, for
example, be in the form of a doctor blade. Beneath the transfer
device a conveying screw 15 is provided for conveying the pulp
to subsequent processing steps. Second roll 3 also co-operates
with means for pressing liquid from the pulp layer. According
to the illustrated embodimentt this pressing means is a liquid-
permeable web 16 which is held pressed against the steel web 1.
The web 16 runs over two small rolls 17,18 and a tensioning roll
19 .
The second end roll 3 is provided with grooves extend-
ing all about the roll to permit liquid to be pressed through
the steel web 1. The liquid which has been pressed off is col-
lected in a trough 20 located beneath the end roll 3. It is, of
course, also possible to use other types of pressing means.
Through the pressing-off, a high dry content of the
pulp can be obtained. The pulp concentration can, for example,
be increased from about 8% to about 30% by this pressing-off.
This means that a greater part than normal of the chemical con-
tent of the pulp can be removed through the liquid treatment.
It is possible from a chemical content of 6-10% during a conven-
tional fourdrinier wire washing to be brought down to 1-3%. It
is thereby possible, for example, to pass the pulp
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from a treatment plant directly to a bleach plant without
further dewatering in special thickeners.
The steel web used preferably has a thickness of 0.5 -
1.5 mm. The diameter of the end rolls should commensurate with
the thickness of the web because ~he bending stress in the web
is directly proportional to the web thickness and reciprocally
proportional to the roll diameter~ A suitable diameter for the
rolls 2,3 is about 2 metres.
The hole si7e in the web should take into account
nature of the pulp to be treated, but should be in the range
0.5 - 1.5 mm in diameter.
It is also possible, of course, to form the holes as
slits. The open area must under all conditions be between ~ and
25%.
The length and width of the web are not critical. Any
desired width can be achieved by welding a number of narrow webs
longitudinally to form a wider web, because it is not presently
possible to directly manufacture steel webs having width as
great as desired for oùr novel apparatus, widths of about 6
metres. Steel webs normally are not manufactured in widths ex-
ceeding much more than one metre.
The invention is not restricted to the embodiment
shown, but can be varied within the scope of the invention idea
as expressed above on page 2 hereof and in the appended claims.
