Note: Descriptions are shown in the official language in which they were submitted.
CA 02285440 1999-10-O1
Process and a device for the formation of fiber board
The invention relates to a process and a device for the formation of fiber
board.
The classic fiberboard production process is the so-called "wet process".
This is the preferred ecological production method because the fiber
s board can be produced with virtually no chemical bonding agents. The
alternatives are a dry process for production of particle board and the
MDF (medium-density fiberboard) process, which is also a dry process.
The current wet process technology is several decades old. It is
ineffective in many process stages and has virtually no means of being
regulated. The fibers are reduced in size in a pressurized refiner. Since
no (or only very small quantities o~ bonding agents are used, the fibers
must develop sufficient bonding capacity during the refining process.
The principle of web formation used to date on Fourdrinier machines does
have some disadvantages. The present headbox technology does not
15 meet the requirements in terms of formation and calibration of the board
thickness. Furthermore, web formation according to the Fourdrinier
principle requires a vacuum as driving force for the greater part of the
water to be removed. This means that the energy consumption of a
typical plant is approximately 150 kW. Since the web dewaters to one
2o side, the machine required has to be very long.
In order to avoid the disadvantage of the chemical bonding agents used in
dry processes, the aim is to improve the wet process used to date.
The invention is thus characterized by the fiber stock suspension being
dewatered on two sides between wires or felts. The energy consumption
2s can be greatly reduced as a result because there is no vacuum required.
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A further development of the invention is characterized by dilution water
being added locally in order to regulate the board thickness across the
web running direction. This is a simple means of evening out the board
thickness without requiring a great deal of design work. The thickness is
s then regulated on the basis of the final board thickness measured.
A favorable configuration of the invention is characterized by headbox lips
being set locally to regulate the board thickness across the web running
direction. The board thickness can be set approximately using these
headbox lips, with a more precise board thickness being obtained in
~o combination with the dilution water.
A favorable further development of the invention is characterized by pre-
dewatering taking place in a wedge zone. Due to the rising pressure in
the wedge zone, even dewatering to a high consistency can be achieved
quickly over a short length and without applying a vacuum, where the
15 board thickness can be influenced accordingly by using an adjustable
wedge zone.
An advantageous further development of the invention is characterized by
the stock being distributed over the working width by means of a cross-
flow distributor and by part of the fiber stock suspension flow being
2o returned to the headbox. The optimum basis weight cross-profile can be
set as a result of this suspension being returned to the headbox.
An advantageous configuration of the invention is characterized by the
process dewatering to a dry content of more than 40%, preferably more
than 45%. As a result, there is a drop in subsequent pressing time in the
2s hot press (providing a higher throughput), in the amount of heavily loaded
filtrate produced during hot pressing, and in the amount of steam needed
to evaporate the residual water.
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A favorable configuration of the invention is characterized by a top layer
being applied after pre-dewatering, with vacuum extraction possibly also
being provided in the area where the top layer is applied. Thus, it is
possible to obtain good fiberboard surface properties by using small
s amounts of high-grade fibers without having to alter the composition or
quality of the rest of the board.
A favorable further development of the invention is characterized by
further dewatering in a wedge zone after the top layer has been applied.
As a result, the entire web, including the top layer, can be dewatered well.
o An advantageous further development of the invention is characterized by
several points being provided with line pressure, for example two to six,
preferably three to five. A particularly high final dry content can be
achieved as a result.
The invention also refers to a forming device for fiberboard.
15 This device is characterized by a top wire being provided in the main
dewatering zone. With the top wire added, the web can dewater on two
sides, virtually halving the dewatering paths, which can also substantially
shorten the length of the machine.
An advantageous further development of the invention is characterized by
2o the top wire forming a wedge zone together with the bottom wire. As a
result, high and controlled pressing force can be applied to the pulp web,
which means that no vacuum is required later for dewatering purposes.
A favorable configuration of the invention is characterized by the wedge
zone being of adjustable design. With this adjustable wedge zone, the
2s board thickness can be set particularly well.
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An advantageous configuration of the invention is characterized by the
bottom wire running essentially horizontally as this limits any
disadvantageous effects of gravity.
A favorable further development of the invention is characterized by the
wire or felt in the wedge zone being supported by perforated plastic or
'steel plates, foil strips or table rolls. Only by using a top wire according
to
the invention is it possible to exploit the advantages of the variants
mentioned.
A favorable configuration of the invention is characterized by a second
headbox being provided in order to apply a top layer.
An advantageous configuration of the invention is characterized by the
wedge zones being suitable for pressure loading at the end, which means
that the maximum dry content can be obtained after pre-dewatering,
regardless of the board thickness to be produced and thus, the web
~5 undergoes optimum preparation for the subsequent press zone.
A favorable further development of the invention is characterized by
several press nips being provided, particularly two to six, preferably
between three and five, with the press rolls being arranged almost
vertically above one another. This achieves a particularly high final dry
2o content.
A favorable further development of the invention is characterized by the
press rolls in the press nips being suitable for individual pressure loading.
As a result, the final dry content and the dewatering curve can be
controlled effectively.
25 A favorable configuration of the invention is characterized by the machine
frame being suitable for cantilevering. This allows the use of endless
woven wires, which provide a longer service life at high pressing forces.
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The invention will now be described in examples and referring to the
drawings, where
Figure 1 shows a side view of a state-of the-art plant, Fig. 2 shows a
horizontal projection of Fig. 1, Fig. 3 shows a side view of a plant
s according to the invention, Fig. 4 shows a section through the line marked
IV-IV in Fig. 3, Fig. 5 a section through the line marked V-V in Fig. 3, and
Fig. 6 provides a sectional view of a headbox.
Figure 1 shows a side view of a Fourdrinier plant 1, with a gravity
dewatering zone 2, where the web is guided over rolls 3. Adjoining this is
a zone with vacuum rolls 4, where the greater part of the water is
extracted from the web. After this zone, more water is pressed out of the
web by rolls 5 and 6 mounted in pairs. A typical Fourdrinier plant
according to the state of the art is approximately 14 m long for a
throughput of 180 tonnes/day.
15 Figure 2 shows the horizontal projection, illustrating the large number of
rolls 3 required, as well as the vacuum rolls 4 and the press rolls 5 and 6.
This figure also shows the drive motor 7 with the gearbox 8.
Figure 3 illustrates a fiberboard plant according to the invention. It
comprises a headbox 22 and an initial dewatering zone 9, where the main
2o dewatering process takes place. This dewatering zone 9 has a bottom
wire 10, which runs through the entire plant. It also has a top wire 11 so
that the web can dewater in both directions. If no secondary pulp is to be
added, there is only one top wire, which also runs through the entire plant.
As a result, the web is dewatered evenly over its entire thickness, which is
2s particularly important in fiber board production. The first dewatering zone
9
is adjoined by a further dewatering zone 12, where more water is
extracted from the web by vacuum boxes 13. Since only a comparatively
very thin layer is fed in here, the amount of water extracted is very small
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compared with state-of the-art plants. At the end of this zone 12, a top
wire 14 is applied again for further dewatering. This wire 14 also runs
through the subsequent press zone 15 with the rolls 16 mounted in pairs.
Here, a final dry content of more than 40% is achieved, preferably more
s than 45%, due to the mechanical dewatering process. The headbox 22
used here can be a cross-flow distributor with a diffuser block and a
perforated roll to break up the flocks forming in the suspension. The
bottom wire 10 runs through the entire plant in an essentially horizontal
position. A wedge 17 is formed in the first dewatering zone 9 together
with the top wire 11. The wires 10, 11 run over perforated plates 18 here
made of plastic or steel. As an alternative, foil strips or table rolls can be
used. The gap height can be set at the end 19 of the wedge zone 17 or
the wedge zone 17 can be pressure-loaded. A roll 20, driving against the
top wire 11, forms the end of the wedge zone. At the dewatering zone 12,
a further headbox 23 can be provided for a top layer. In order to dewater
the top layer, a further top wire 14 is provided. Dewatering is assisted by
extraction using vacuum (through boxes 13). The top wire 14 and the
bottom wire 10 form a further wedge zone 21, which is also adjustable and
can be designed for pressure-loading at the end of the zone if necessary.
2o In order to increase the final dry content, the press zone 15 contains two
to six, preferably three to five, press nips, i.e. pairs of rolls pressed
against
one another. The present example shows four such roll pairs 16 which
form press nips. A plant of this type has an overall length of
approximately 11.5 m for a throughput of approximately 320 tonneslday,
2s i.e. although production is increased by approximately 80%, only some
80% of the length required in a state-of-the part plant is needed here.
This provides a specific output of approximately 220% compared with
state-of the-art plants.
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Figure 4 shows a section through the line marked IV-IV in Fig. 3. In this
section viewed against the web running direction, the bottom wedge
plate 18, a vacuum box 13 and the roll 20 at the end of the wedge
zone 17 are all clearly shown. The so-called front side is marked FS and
s the rear side, where all of the drives and other leads and lines are
located,
is marked TS.
Figure 5 shows a section through the same point as Fig. 4, however
viewed in the web running direction. Here, the rear and front sides are
reversed compared with Fig. 4. In addition to the vacuum box 13 and the
wedge plate 18, this illustration also shows the second headbox 23 for the
top layer. The suspension to the headbox 23 is fed through a connection
pipe 24 coming from the rear side. The water extracted is removed from
the vacuum box 13 through a pipe 25.
Figure 6 shows the headbox 22 in detail. The suspension is fed in
15 through a cross-flow distributor 26 and flows through a diffuser block 27
to
a perforated roll 28, which breaks down any flocks that have formed in the
suspension. From here, it is brought into the wedge zone 17 formed by
the top 10 and bottom 11 wires.
The invention is not limited to the examples described. It would also be
2o possible to combine the individual dewatering zones in different ways,
depending on the given requirements.
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