FI85887B - Method for spreading thick pulp into a web - Google Patents

Description

1 85887

Method for applying a thick mass to a web Förfarande för utbredning av tjock pulp account en bana

The present invention relates to a method for applying a thick mass 5 to a web.

The object of the invention is to provide a method for applying the pulp to a web which is suitable for use on a pulp of 6-15%, in particular 8-12%.

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Today, a web-forming method is commonly used in which pulp is dispensed from a headbox through a narrow gap extending over the entire width of the machine as a jet onto or between the forming wires. In order to obtain a satisfactory web, the pulp jet must be uniform in thickness and direction and homogeneous. With a low consistency, up to about 1%, this does not cause problems, but with a higher consistency, where the fibers tend to flocculate, the situation is different.

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Applying the thick mass to a flat web is very difficult because, for example, the mass in the 10% consistency range is so solid that it can form a "snowball" without removing water. The number of bonds 25 in the pulp mixture is directly proportional to the consistency of the pulp. The shear stress required to release the bonds increases according to the following equation (Gullichsen & Härkönen, Tappi Pulping Conference, 1980): 30 S ~ = kC K, where k and (/ are constants specific to each mass grade.

For example, the shear stress required to loosen the bonds between bleached pine sulfate fibers increases, with 10% pulp requiring about 100 times the shear stress to loosen the fiber bonds than 1% pulp to loosen the fiber bonds.

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Under certain conditions, the thick mass can be momentarily made to have water-like flow properties. This phenomenon is called fluidization.

5 In fluidization, energy is introduced into the pulp to break the bonds between the fibers. The energy required is proportional to the consistency of the pulp and the number of bonds between the fibers.

10 The thick mass flocculates and easily clogs the flow channel as it flows in tapered conical structures. In tapered cones, the fibers of the thick mass are densely packed, which easily obstructs the flow and clogs the flow channel. Leaching is the result of condensation of the “flow lines” 15 without turbulence.

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It is known from Finnish patent 51116 that if a thick mass is subjected to high shear forces, an even distribution of the fibers over the entire width of the machine can be achieved.

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Finnish patent application 1284/73 discloses a device in which a high-density fiber suspension is divided into several parallel channels and injected at high speed into a chamber in which the direction of flow is abruptly changed.

Swedish Offenlegungsschrift 7807097-6 discloses a method and apparatus for forming a web from a fibrous suspension, in particular from high-density pulp suspensions, in which the suspension is made to flow in parallel jets tangentially into a mixing chamber having a ____: rotationally promoting cross-section .

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The above-mentioned devices in which the fiber suspension, after flowing through the chamber, is led to the forming wire through an outflow channel in which a cohesive fiber network is formed, are usable at a mass of up to 6%.

When moving to the highest consistencies, mechanical fluidizers or high flow rates are required to introduce sufficient turbulent energy into the pulp to be applied to the web.

The fluidized state is a reversible state. The fluidized pulp thus quickly recovers into a dense fibrous web. Therefore, the locations and dimensions of the flow channel lengths 15 must be carefully considered. It is also important in thick mass webing that the resulting web is smooth and in good shape. Thus, the web must be formed when the mass is fluidized. Fluidization and application of the pulp to the web must be performed as close as possible. To achieve this goal, in the method according to the invention, the pulp in the fluidized state is applied by combining free-discharge pulp jets in the space between the headbox and the web-forming device.

The method according to the invention is mainly characterized in that the pulp jets are fed towards the web-forming device at such a high speed that, due to the high impact force and the consequent fluidization, the pulp is spread into a flat web.

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The invention makes it possible to form a pulp web by simple means. According to the invention, the pulp is applied to the web by spraying the pulp from a plurality of nozzles onto a web-forming surface or into a gap 35 between two web-forming surfaces, where the pulp jets stop. The mass jets coalesce when they meet or before the forming surface. The method according to the invention for forming a web has many advantages: 4 85887 - The method can be implemented by simple means.

- The method is suitable for different masses and can be used over a wide consistency range. At the lower limit of the range (6-15%), the invention is suitable for web formation without mechanical accessories, especially in the case of a pulp with weak interfiber bonds. As the consistency increases, the accessory can be used during start-up to prevent clogging. A mechanical accessory, e.g. a rotating fluidizing member, can be mounted in front of the nozzle in high consistency ranges, or when there are many interfiber bonds in the pulp. The purpose of the accessory is to lower the pressure level and break the bonds between the fibers.

One application of the invention is the use of compressed air to promote the fluidization of the pulp jets and to separate the fibers from each other. Compressed air can be conducted to the mass jet-20 moon either from the outside or from the inside. Air flowing much faster than the pulp jet tears the fibers apart and transports them to a wire or other web-forming member through which compressed air escapes. The advantage of this method is that when carried by compressed air, the individual fibers do not form flocs in the air. Compressed air supplied from the inside dissipates the mass jet efficiently.

In addition, chemical additives can be used to aid web formation to reduce the bonding of the fibers. However, the use of additives is limited by the properties required of the final product.

The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 schematically shows an embodiment of the invention in a cut-away side view, Fig. 2 shows the device of Fig. 1 in a top view.

Fig. 85887 Fig. 3 shows on a larger scale a nozzle suitable for carrying out the invention in a sectioned top view, Fig. 4 shows a section along the line AA in Fig. 5, Fig. 5 shows a second embodiment of the invention in a top view, Fig. 6 shows a third embodiment of the invention in a sectional side view and Fig. 7 shows embodiment 10 of the invention in a sectional side view.

Figures 1 and 2 show a web-forming section of a paper machine in which two rolls 1 and 2, over which the upper wire 3 and the lower wire 4 are guided, form a converging groove 5 between which a pulp is fed. A headbox 6 is arranged in front of the kite, which comprises a pressure chamber 7, into which the mass is led through the piping 8, and in which a plurality of parallel nozzles 9 in the transverse direction of the machine are arranged at a distance from each other. The nozzles are directed towards the gap 5 between the forming rollers 20. The mass is discharged from the nozzles as separate showers. Due to the high flow rate caused by the pressure maintained in the chamber 7, the mass flows from the nozzles at such a high rate that it stagnates and spreads when the jets stop, forming a uniform web between the forming wires 3 25 and 4. Depending on the shape of the nozzle, the mass can be discharged from the nozzle in a substantially constant or expanding jet in cross section. The mass jets can be combined before they arrive at the interlayer between the wires. The surface of the rollers and the jacket may be open, e.g. perforated or grooved, or impermeable to water.

Figures 3 and 4 show a nozzle 30 from which the mass discharges as a thin jet. The nozzle consists of a hemispherical portion 31 with a transverse slot 32 of the machine.

The embodiment shown in Fig. 5 differs from that shown in Fig. 2 mainly in that the nozzles are movable in the transverse direction of the machine.

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In the embodiment shown in Fig. 6, a fluidization rotor 14 is arranged in front of the nozzle opening 13 in the wall of each chamber in the pressure chamber 12 of the headbox 6, which is rotated from outside the chamber 12 by means of a motor 15 (not shown). The mass fed to the pressure chamber is fluidized by means of rotors 14 in front of the orifice row 13 and flows at a rate determined by the pressure difference in the space 17 bounded between the pressure chamber and the forming rollers 15 and 16. As forming rolls, over which the forming wires 18 and 19 20 pass, it is possible to use e.g. suction rollers as schematically shown in the figure. Compressed air is supplied to the pulp jets from the inside through a hole 23 drilled in the shaft of the fluidizer. Compressed air can also be introduced into the pulp jet from the outside immediately after the nozzle opening. In one embodiment, it is conceivable that the pulp nozzle is tubular and strong air or gas flows are directed to the rest or discharge point in several directions, disintegrating the pulp flow into an air-to-water mixture in which the unitary fiber network is at least partially decomposed and the bonds between the fibers reduced, that it is possible to combine the jets from wire to web.

A commonly used way to disperse liquid jets is by ejector dispersion. This method of decomposition can also be used to decompose the mass coming from the nozzle by means of air or other gas. The jets of pulp from the nozzles are led to an ejector, where the air mixes with the pulp stock and a mixture is formed with air in addition to the pulp and water. Air weakens the bonds between the fibers and improves the chances of forming a good path when the jets meet the web-forming surface.

In the embodiment shown in Fig. 7, the pulp is fed to the forming wire 20 by means of nozzles 9 arranged in parallel in the transverse direction of the machine in the pressure chamber 7 of the headbox 6 at a distance from each other. The pulp, which is fed at a high speed, in separate jets forming an angle with the forming surface, spreads therein into a uniform web, and the formed web passes, supported by a wire, to the nip between the press rolls 21 and 22. When the jets meet at the high speed of the wire, such high mixing and shear forces are generated at the moment of impact that the orbit formed is uniform. The pulp jets can be combined before they meet the forming wire.

Example:

The test paper machine used in the experiments had five nozzles at a distance of about 10 cm 20, from which the pulp jets were discharged at a pressure of about 3 bar. During the test runs, it was possible to make cardboard about 700 g / m2 thick. Fluidizers were used on the inlet side of the nozzles and the consistency of the pulp was 10%. The pulp was pine sulphate pulp.

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The invention is not limited to the exemplary embodiments, but various modifications and applications are possible within the scope of the inventive idea defined by the claims.

Claims (11)

A method of feeding pulp having a consistency of 6-15% and propagating it to a web of a paper machine web forming device by supplying the web forming device in the form of nozzle-free pulp streams, characterized in that the pulp streams is measured against the web forming device at such a high velocity that the mass, due to the large impact force and the resulting fluidization, propagates to a smooth web. 25 Method according to claim 1, characterized in that the pulp jets are joined before they hit the forming surface (s). 3. A method according to claim 1, characterized in that the pulp jets are joined as they hit the forming surface or surfaces. 4. A method according to claim 1, 2 or 3, characterized in that the pulp is discharged onto the web of the web forming device. 35 5. A method according to claim 1, 2 or 3, characterized in that the pulp is fed between the webs or rollers of the web forming device. 10 85887 6. A method according to claim 1, 2 or 3, characterized in that the pulp beam (s) forms an angle with the wire. 5 7. Procedure according to any of claims 1-6, characterized in that the mass is measured in the form of rays extending in the transverse direction of the web. 10 Method according to any of the claims 1 to 7, characterized in that the pulp jets are moved back and forth in the transverse direction of the web to be formed. Method according to any of claims 1-8, characterized in that the fluidization of the pulp jets is promoted by conducting air to them. Method according to any of claims 1-9, characterized in that the air is fed into the pulp jets from within. 20 Method according to any of claims 1-10, characterized in that the air is fed into the pulp jets from outside.