METHOD AND APPARATUS FOR SPREADING PULP TO A WEB

Abstract: Method and apparatus for spreading pulp of a consistency of 6 - 15 % to a web. Pulp discharged from the feed chamber of a head box through a space between a rotating roll (22) and a wall of the feed chamber (21) is supplied onto a web forming device (20) whereby a high energy content fluidizes the pulp in a space (23) and the pulp is evenly distributed to a web onto the web forming device (20).

Description: Method and apparatus for spreading pulp to a web The present invention relates to a method of spreading high-consistency pulp to a web and an apparatus for carrying out the method. The object of the invention is to provide a method and an apparatus for spreading pulp to a web in the consistency range of 6 - 15 %, in particular in the range of 8 - 12 %. The method of the invention is especial¬ ly well applicable in the head boxes of paper machines. Another example where this invention can be applied are pro¬ cesses in which high consistency pulp must be spread to an even mat-like layer. A web-forming method is now-a-days commonly employed in which pulp is supplied from a head box in a stream through a narrow slice extending over the whole width of the machine onto a web forming wire or between web forming wires. In order to form a good web the thickness and the direction of the pulp stream must be even and homogenous. When the consistency is low, i.e. up to 1 %, this does not cause problems but at higher consistencies when fibers tend to floe the situation is quite different. It is very difficult to spread out high consistency pulp to an even web-like layer as pulp of for instance at the consistency of 10 % is very solid, in other words the bondings between the fibers " "are so tight that "a snow ball" can be formulated of it without removing water. The amount of fiber bondings in the pulp mixture is directly proportional to the consistency of the pulp. The shear stress T " required to disrupt the bondings increases accord¬ ing to the following formula (Gullichsen & Harkδnen, Tappi Pulping Conference, 1980) τ= kC* where k and <. are constant factors typical of that par¬ ticular consistency range. For example the shear stress required for disrupting the bondings between the fibers in bleached pine sulfate pulp increases so much that the stress required for disrupting the bondings in a pulp of the consistency of 10 % is approximately 100 times the stress required for disrupting the bondings at the consistency of 1 %. At certain conditions, high consistency pulp can be caused to behave momentarily like water as regards its flow properties. This phenomenon is called fluidizing. In fluidization, energy is introduced to the pulp in order to disrupt the bondings between fibers. The amount of energy required is proportional to the consistency of the pulp and the amount of bondings between the fibers. Fluidizing does not produce continuously fluid pulp or loose fibers and the higher the consistency of the pulp is the faster the fibers contained in the pulp tend to attach to each other. At adequately high consistencies the fibers attach to each other or resume their original network structure in a centesimal or even millesimal of a second. Tests have shown that the conventional channels tapering in the flow direction employed in prior art head boxes can not be used with high consistency pulp. High consistency pulp floes and easily clogs the flow channel when it flows in the conical structures. In the tapering cones the fibers in the high consistency pulp are packed tight which easily prevents the flow and clogs the flow channel. Clogging results from packing of the "flow lines" without tur- bulence. In this respect high consistency pulp differs as to its flow properties from water and dilute pulps. Finnish patent no. 51116 teaches that if high consistency pulp is subjected to great shear forces the fibers can be distributed evenly over the whole width of the machine. Finnish patent application no. 1284/73 discloses an apparatus in which high consistency fiber suspension is distributed to a plurality of adjacent channels and sprayed at a high speed to a chamber where the direction of the flow is suddenly changed. Swedish patent specification no.- 7807097-6 discloses a method and an apparatus of forming a web of fiber suspen¬ sion, in particular of high consistency paper pulp suspension according to which method the suspension is caused to flow tangentially in parallel streams to a mixing chamber which has a cross section favouring turbulence and from which chamber the pulp is discharged after a change of direction of 90 - 180°. Also structures are known in which a rotating means, for example a perforated roll, is disposed in the head box in transverse direction in relation to the machine. These structures are not employed to fluidize high consistency pulp but to break floes in dilute pulp or to add turbulence to the pulp. The purpose is rather to homogenize the pulp and to even out pressure variations from short circulation apparatus and pumps. High consistency pulps easily clog a perforated roll and thus make web formation difficult. Examples of structures of this kind are discussed below. German patent specification no. 10 58 355 discloses an apparatus in which part of the water molecules contained in the paper pulp are displaced by pressure air and a revolving mixer. The number of revolutions of the mixer must be so low that no separation of phases occurs. The high revolution speeds required by fluidization would be harmful for the operation of the apparatus described here as they could separate the pressurized air from the rest of the pulp which would eliminate the benefit of adding the pressurized air. German patent specification no. 22 39 080 discloses a paper machine head box comprising a narrow annular duct. In the center of the duct there is a movable roll disposed transverse to the direction of the machine. According to this publication, turbulence can be created by supplying pulp to the narrow annular duct in which the pulp is divided in two and pulp flows to the slice duct from two directions. German patent specification no. 12 94 177 discloses a paper machine head box comprising a stationary perforated roll in front of the slice opening in the transverse direction in relation to the machine. Pulp is supplied to a channel inside the perforated roll and distributed therefrom to the slice opening. A rotating shaft expanding conically is provided inside the perforated roll to equalize the flow from the head box. The perforated roll filters the non-flocculated fibers to the slice. The shaft expand- ing conically tapers the flow channel conically. Struc¬ tures which taper conically are not applicable for pulps of the consistencies of 6 - 15 %. Further, the holes of a perforated roll are easily clogged at this consistency range. Also a device disclosed in Finnish patent application no. 843463 is known. The device comprises a conically expanding shaft provided with a feed screw, the shaft being disposed transverse to the machine direction in front of an elongated opening. A space having an annular cross section area continuously tapering in the feed screw is defined between the core of the screw and the surface of the back of the screw. This invention does not relate to fluidizing or even breaking floes but to a feed screw unit used for pushing pulp of the consistency of even 9 - 12 % out through an elongated opening. The quality of the web thus formed is not even or controlled. A basic precondition for the formation of an even and controlled web is that the fiber network is broken and the fibers are loose to form a new network in the forming web and that the formed web can flow on. A conically tapering flow duct can for these reasons not be used in the flow of high consistency pulps. U.S. patent no. 3,563,853 discloses a method of spreading the flow supplied to a head box to a flow of the width of the web to be formed. The object of the invention is to provide an even, wide and smooth flow by means of rotating elements and changes in the volume of the channel. The rotating elements are disposed transverse to the machine direction; they do not fluidize the pulp. U.S. patents no. 3,119,734 and 3,255,074 disclose a head box containing a rotating element located transverse to the machine direction. In U.S. no. 3,119,734 the rotating element is a roll the shell of which is provided with thin toothed plates in an inclined position in relation to the center of the shaft. The purpose of the structure is to control the inlet flow and to distribute the pulp evenly. The pulp flows to the slice in a conically tapering duct. The object of the invention of U.S. patent no. 3,255,074 is to minimize flocculation. The apparatus of this patent, as also that of the previous patent, is meant to be used as an .alternative for a perforated roll. Also structures comprising a vibrating deflocculator inside a head box are known. Devices of this kind are described for example in U.S. patent publication no. 3,562,108 and CA 1,173,681. An element vibrating in the head box can break floes in a dilute pulp but creates harmful pulses. Finnish patent no. 73764 discloses an apparatus for dis¬ tributing pulp of the consistency of up to 6 % onto the wire of a paper machine. The head box according to this invention comprises a chamber in which there is a bar provided with plate-like members which during vibration of the bar exert to-and-fro pressure shocks to the pulp which disrupt bondings between the fibers and thus facilitate the flow of the pulp through the slice onto the wire of the paper machine. The above devices in which the pulp is distributed through a chamber and an outlet duct onto a forming wire where a coherent network is formed are applicable for pulps of the consistency of 6 % at the most. When higher consistencies are treated efficient mechanical fluidizers or high flow velocities are required at the boundary of the head box and the web forming device, which in most cases is a wire, or immediately close to it, in order to generate sufficient turbulence and shear energy in the pulp so that the pulp can be spread out to a web before the pulp forms fiber networks which would disturb web formation. Mr Johan Gullichsen has presented (Tappi Pulping Conference, 1980) a principle drawing of a paper machine head box operating with a consistency of approximately 10 %. The head box comprises a large roll in the machine direction employed to fluidize the pulp in a flow duct between the head box and the roll before the pulp is discharged from the head box through a narrow slot onto a wire or between two wires. The high consistency pulp forms a strong and homogenous network. Considerably more energy is required for disrupting this fiber network than for breaking the fiber floes in a dilute pulp. Fluidizing the entire pulp volume in the head box of a mill-scale paper machine by a large roll, as suggested by Gullichsen, would require a roll with the diameter of over 0,5 m and would consume too much energy. A solution in which the entire volume of the head box is fluidized and supplied to the slice channel is too expensive. As the drawbacks of the example described above are to be avoided and as the fluidized state quickly transforms to a fiber network the locations, lengths and dimensions of the flow ducts supplying high consistency pulp suspension to the wire must be carefully considered. If large quantities of high consistency pulp must be fluidized at the same time the advantage gained by using a pulp of the consistency of 10 % instead of 1 %, in other words the reduced volume of water required, is lost. Then the use of high consistency pulp has no practical meaning. A high consistency head box must therefore be designed to fluidize a small volume of pulp at a time and even then the fluidizing has to be carried out exactly in the right and necessary place. One of the advantages gained in this way are for instance a remarkably reduced size of the head box as the required volume of water is only 10 % of the one required before. Also the size of all the short circulation devices can be remarkably reduced. Consequently the effluent load of a paper mill is reduced. It is important also in high consistency web forming that the formation of the produced web is homogenous and good. Therefore the web must be formed while the pulp is in a fluidized state. Fluidizing and distributing the pulp to a web must be carried out in as close succession as possible. To achieve this, the head box according to the present invention is provided with a pulp handling section, or the first phase of the head box, and with a web forming section. In the pulp handling section or the first phase as much energy is supplied to the pulp that it purges out at least in a partially fluidized stats through holes, slots or nozzles to the web forming section. In the web forming section, or the second phase, the pulp impinges against a moving surface and becomes fluidized whereby a web is formed in a closed chamber, in a space between wires or in a free space on a wire. The method of the invention described above is characterized in that the high consistency pulp in the head box is kept moving by supplying energy to it and that the pulp is supplied onto a web former in one or several pulp streams the flow velocity and the impingement force of which, together with the kinetic energy of the web former, are maintained great enough to be able to fluidize the pulp at the web forming moment when the pulp meets one or several of the surfaces of the web former. It is a characteristic feature of the head box according to the present invention that at least one rotating element revolving at a high velocity is provided immediately by the outlet of the pulp discharge channel which element facilitates the fluidization of the high consistency pulp at the moment of the web formation. The invention makes possible formation of a web by simple means. According to a preferred embodiment of the invention pulp is distributed to a web by spraying pulp through a narrow slot onto a forming surface or to a gap between two forming surfaces where the pulp stream stops on a wire. Fluidization of the pulp in the slot can be intensified if needed by arranging a rotor or a corresponding fluidizer prior to the slot. In another embodiment of the invention, a fluidizer is arranged immediately close to the forming surface so as to prevent floes from being formed in the pulp before it hits the wire. The method and the apparatus of the present invention have many advantages: - the method can be carried out by simple means - the method is applicable for various pulps and for a wide range of consistencies. At the lower limit of the consistency range (6 - 15 %) the invention can be applied in web formation without mechanical auxiliary devices, in particular with pulps with weak bondings between the fibers. When the consistency increases auxiliary devices can be used in connection with start-up to avoid clogging. A mechanical auxiliary device, for example a rotating fluidizing means, can be provided in front of a nozzle, or a slot, at high consistency ranges or when there are plenty of bondings between the fibers in the pulp. The auxiliary device is employed to decrease pressure and to break fiber bondings. Further, chemical additives can be used to decrease the tendency of the fibers to attach to each other and thus to aid web formation. However, the properties required of the end product limit the use of additives. The invention is decribed in detail, by way of examples, below with reference to the accompanying drawings in which: Fig. 1 is a chematical sectional side illustration of a preferred embodiment of the invention; Fig. 2 is a top view of the apparatus in Fig. 1; Fig. 3 is a sectional side illustration of another preferred embodiment of the invention; Fig. 4 is a sectional side illustration of a third preferred embodiment of the invention; and Fig. 5 illustrates a fifth preferred embodiment of the invention. Figures 1 and 2 illustrate the web forming section of a paper machine in which two rolls, 1 and 2, around which a top wire 3 and a bottom wire 4 are guided, define between each other a tapering gap 5 into which pulp is supplied. Immediately in front of the gap, a head box 6 is disposed which comprises a feed chamber 7 into which pulp is supplied via pipe line 8 and which is provided with a narrow slot or a plurality of parallel nozzles 9' at a distance from each other transverse to the machine direction. The slot 9 or the nozzles 9' are directed towards the gap 5 between the forming rolls. The pulp is discharged through the slot 9 in an even at least partially fluidized high-pressure jet or through the nozzles 9' in separate jets which are united in the gap 5. The high pressure in the chamber 7 causes the pulp to flow through the slot 9 or the nozzles 9' at such a high velocity that it becomes fluidized when it is discharged through the slot or the nozzles and remains fluidized until it forms a uniform web between the forming wires 3 and 4. Depending on the configuration of the slot or the nozzle, the pulp can be discharged from the nozzle as a jet the cross section of which is substantially unchanged or widening. The pulp jets can be united before they reach the gap between the wires. The surface of the rolls can be open, for example perforated or grooved, or also water impervious. In the embodiment illustrated in Figure 3, a fluidizing rotor 14 is disposed in front of each nozzle outlet 13 in the wall of a feed chamber 12 of a head box 6. The rotors are driven by a motor which is provided outside the chamber and is not illustrated in the figure. The pulp supplied to the feed chamber is at least partially fluidized by the rotors 14 in front of the row of outlets 13 and flows at a velocity determined by the pressure difference to a space 17 defined by the feed chamber and rolls 15 and 16 between the rolls. The forming rolls, over which forming wires 18 and 19 are arranged to run, can be for instance suction rolls, as the figure schematically illustrates. Pressurized air can be supplied to the pulp jets from inside through a hole 11 drilled in the shaft of the rotor of the fluidizer. Pressurized air can be introduced to the pulp jets also from outside immediately after the nozzle outlet. One embodiment of the invention could comprise a tubular pulp nozzle whereby strong air or gas flows would be guided from several directions to the end portion or discharge point of the nozzle which would disperse the pulp flow to a mixture of air, pulp and water in which the fiber network is at least partly disrupted and the bondings between the fibers reduced to the extent that uniting the jets on a wire to a web is possible. A commonly used way of dispersing a liquid jet is dispersion with an ejector. This method can be used also in dispersing with air or other gas the pulp flowing from the nozzles. The pulp jets coming from the nozzles are guided to an ejector in which air is mixed to the pulp and a mixture containing pulp, water and air is produced. Air weakens the bondings between the fibers and improves possibilities of a good web formation when the jets reach the web forming surface. When the above embodiment was tested the pilot paper machine was provided with five nozzles at the distance of approx. 10 cm through which the pulp jets were discharged at the pressure of approx. 3 bar. Board of approx. 700 g/m 3 could be produced in the test runs. Fluidizers were used at the inlet side of the nozzles and the consistency of the pulp was 10 %. The pulp was pine sulfate pulp. Figure 4 illustrates a third embodiment of the invention in which a roll 22 in a chamber 21 of a head box is disposed on top of a forming cylinder 20 so that the shell 25 of the roll and the surface 24 of the forming cylinder define between each other a space 23 in which the pulp is subjected to shear forces between two moving surfaces forming a narrow slot. Fluidization can be carried out very efficiently in the narrow slot, .substantially to a fiber- o-fiber state, due to great shear forces. A wall 26 of the chamber the lower end of which has an adjustable guide plate 27 and the outer surface of the roll define a flow duct 28 leading to the space 23 while the roll and the cylinder rotate in different directions. The revolving movement of the roll 22 facilitates the flow of the pulp in the duct 28. The position of the guide plate 27 determines the size of the slot 29 between the roll 22 and the guide plate and the sl ' ot 30 between the forming cylinder 20 and the guide plate. At the lower end of the other wall 31 of the chamber there is a second adjustable guide plate 32 which serves as a sealing between the forming cylinder and the chamber. The distance between the roll 22 and the forming cylinder can be changed by moving the chamber which adjusts the degree of fluidization in the slot to the desired level. The closer to the forming cylinder 20 the roll 22 is the more efficient the fluidization is and the higher the consistencies of the pulp to be treated can be. The rotating roll 22 feeds pulp to the space 23 between the roll 22 and the forming cylinder 20 where part of the pulp flows in a fluidized state to the slot 30 between the guide plate 27 and the forming cylinder 20 and forms a web in which the fibers are evenly distributed. A duct 33 between the chamber wall 31 and the roll 22 returns the remaining portion of the pulp to the chamber 21. The revolving movement of the roll 22 facilitates the flow in the duct 33. When the roll and the cylinder rotate in the same direction pulp is supplied to the web forming space 23 via duct 33. In this case the duct 28 serves as the return duct and the flow in it may be zero if the return flow is adjusted by the guide plate 27 to be zero. The surface of the rotating roll may be smooth or grooved. Even the use of a rotating fast rotor provided with separate fluidizing vanes can be considered. In the embodiment illustrated in Figure 5 the feed chamber 21 of a head box is provided with a slot or a row of holes 30, which is transverse to the machine direction and close to which there is a fluidizer 31 the shaft of which is parallel with the slot or the row of holes. The fluidizer 31 is used to fluidize the pulp to the extent that it can be discharged from the feed chamber 21 through the slot or the holes to a web forming space 32 defined between the feed chamber 21 and a moving wire 33 of a web forming device disposed under the chamber. An adjustable sealing 34 is provided between the lower portion of the feed chamber and the forming surface in the direction of the machine which limits the space 32 under the slot or the holes in the direction opposite to the running direction of the wire 33. The feed chamber must be able to create a fluidizing rate which is adequate to keep the fibers in the web forming point loose and to form a fiber network of the thickness of the web. The fluidizing energy is provided by the pulp streams discharged through the slot or the holes to the web forming space 32 and the moving forming surface 33. The thickness of the web is determined by the dimentions of the outlet of the web forming space, in other words by the distance between the bottom 35 of the feed chamber and the forming surface. The thickness of the web is adjusted by changing the height of this opening. It is of course possible to arrange the feed chamber of the head box so that it defines a web forming chamber together with two rotating rolls and adjustable sealings provided between the rolls and the feed chamber. The web is formed in the point where the webs running over the rolls meet and the thickness of the web is determined by the distance of the webs between the rolls. The web forming chamber of all the embodiments described above should be as small as possible. In the most advantageous case the fluidizing space serves as the web forming chamber as illustrated in Figure 4. Then no floes can be built in the high consistency pulp before the pulp reaches the web forming surface. It should be noted that when the method and the head box of the invention are applied for pulps of the higher consistencies of the given range the velocity of the rotating means should be so high that it together with the movement of the web forming device is able to fluidize the pulp to be treated. It has been found out that the peripheral speed of the rotating means should be at least 25 m/s, preferably higher than 30 m/s, to keep the high consistency pulp in the head box in motion and to secure adequate fluidization in the web forming point. It is also possible to intensify fluidization by fluidizing at a time as small a volume as possible. This is gained by arranging the distance between the rotating means and the web forming device adequately small. An appropriate distance could be 10 - 20 times the thickness of the paper or product to be produced or on the other hand a distance of less than 10 mm. The invention is not limited by the embodiments presented here as examples only but several modifications and applications of it are possbile within the scope of protection defined by the accompanying patent claims.

Claims: We claim: 1. A method of distributing pulp of the consistency of 6 - 15 % to a web, characterized in that the high consistency pulp in a head box is kept in motion by supplying energy to it; and in that pulp is supplied to a web forming point in one or several pulp streams the flow velocity and the impingement force of which together with the kinetic energy of the web forming device are maintained high enough at the time of the pulp hitting one or several forming surfaces to fluidize the pulp at the moment of web formation. 2. A method as claimed in claim 1, characterized in that kinetic energy is supplied to the high consistency pulp in the first phase of the head box by which the pulp is kept in motion; and that high consistency pulp is fed towards the web forming device and the amount of the energy supplied to the pulp is increased in such a way that the maximum amount of energy is supplied at the point where web formation takes place whereby the high consistency pulp is fluidized at the moment of web formation. 3. A method as claimed in claim 2, characterized in that the amount of the energy supplied to the pulp is increased by reducing the volume of pulp to which the energy is supplied. 4. A method as claimed in claim 1, 2 or 3, characterized in that the high consistency pulp is supplied to the web forming device in a pulp jet discharged through a narrow slot. 5. A method as claimed in claim 1, 2 or 3, characterized in that the high consistency pulp is supplied to the web forming device in one or several pulp jets discharged through slots or holes which jets are united at the moment they hit the forming surface or surfaces whereby the pulp is fluidized. 6. A method as claimed in claim 1, 2, 3, 4 or 5, characterized in that the pulp is supplied to a wire serving as the web forming device. 7. A method as claimed in claim 1, 2, 3, 4 or 5, characterized in that the pulp is supplied between wires or rolls serving as the web forming device. 8. A method as claimed in any of claims 1 to 7, characterized in that fluidization of the pulp is facilitated by supplying air to the pulp stream. 9. A method as claimed in any of claims 1 to 8, characterized in that air is supplied to the streams from inside. 10. A method as claimed in any of claims 1 to 8, characterized in that air is supplied to the pulp streams from outside. 11. A method as claimed in claim 1, characterized in that the flow velocity and the impingement force of the pulp is kept high enough by an adequately high feed pressure and by an as small distance between the pulp discharge opening and the web forming device as possible. 12. A head box for the distribution of pulp of the consistency of 6 - 15 % to a web, the head box comprising a feed chamber; an inlet duct for the pulp to be supplied to the feed chamber; a discharge duct for the pulp to be discharged from the feed chamber; and one or more rotating means disposed in the chamber; characterized in that at least one means (14,22, 31) rotating at a high velocity is provided close to the opening of the discharge outlet (13, 23, 30) which means facilitates the fluidization of the high consistency pulp at the moment of web formation. 13. A head box as claimed in claim 12, characterized in that the rotating means (22, 31) together with the web forming device (20, 33) define a discharge duct (33) for the pulp from the feed chamber (21). 14. A head box as claimed in claim 12, characterized in that the rotating means is a roll (22, 31) the surface of which is located close to the surface of the web forming device (20, 33) whereby a pulp flow is discharged through a slot (28/33) between the rotating means (22, 31) and the wall (26/31) of the feed chamber (21) onto the web forming device (20, 33), the flow being at least partially fluidized and remaining fluidized at the moment of web formation because of the strong shear forces subjected to the pulp flow by the web forming device and the rotating means. 15. A head box as claimed in claim 12, characterized in that the rotating means (14, 22, 31) is a special fluidizer comprising one or more vanes or a rotor having non-flat surface the rotation of which at a high speed causes fluidization of the high consistency pulp. 16. A head box as claimed in claims 12 - 15, characterized in that the peripheral speed of the rotating means (14, 22, 31) is higher than 25 m/s. 17. A head box as claimed in claim 12, characterized in that an adjustable plate (27) or a corresponding means is disposed close to the rotating element (22) the position of which plate or means determines the thickness of the web produced. 18. A head box as claimed in claim 12, characterized in that the discharge outlet of the pulp is defined by two rotating means which fluidize the pulp. 19. A head box as claimed in claim 12, characterized in that at the moment of web formation the distance between the rotating means (22, 31) and the web forming device (20, 33) is approximately 10 - 20 times the thickness of the dried paper or corresponding product to be produced. 20. A head box as claimed in claim 12, characterized in that at the moment of web formation the distance between the rotating means (22, 31) and the web forming device (20, 33) is less than 10 mm.

Inventors: HAKULIN BERTEL KRISTIAN, HENRICSON KAJ OLOF, KEJONEN MINNA KANERVA, NISKANEN TOIVO, HAKULIN, BERTEL, KRISTIAN, HENRICSON, KAJ, OLOF, KEJONEN, MINNA, KANERVA, NISKANEN, TOIVO
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1988
Main classification: D21F1
IPC codes: D21F1