WO1998020959A1

WO1998020959A1 - Method and device for separating fine particles from white water

Info

Publication number: WO1998020959A1
Application number: PCT/SE1997/001763
Authority: WO
Inventors: Roine Andersson; Lars-Göran RUNDQVIST
Original assignee: Alfa Laval AB
Current assignee: Alfa Laval AB
Priority date: 1996-11-08
Filing date: 1997-10-22
Publication date: 1998-05-22
Anticipated expiration: 1999-05-08
Also published as: SE509553C2; SE9604088D0; SE9604088L

Abstract

Fine particles are separated from white water by the use of a disc filter, which comprises at least one rotating vertical filter disc (2), partly immersed in the white water to be separated, thereby sections (27) of the filter medium (5) of the filter disc are alternately displaced up from and down into the white water. When a section is displaced up from the white water it is covered by a cake containing fine particles which are detached in a detaching zone (16) before the section again is displaced down into the white water. According to the invention, each section is provided with a layer, solely containing fibres, in a forming zone (20) during each revolution and the section with its fibre layer is displaced through the white water in a separation zone (30), so that fine particles are deposited on the fibre layer and together with the latter form said cake.

Description

Method and device for separating fine particles from white water

The present invention relates to a method of separating fine particles from white water by the use of a disc filter, which comprises at least one substantially vertical hollow filter disc with two opposite walls of a filter medium, a container for white water, the filter disc being arranged partly immersed in the white water in the container, means adapted to create a pressure difference between the white water and the interior of the filter disc so that a filtrate is forced from the white water in the container through the filter medium into the interior of the filter disc while a cake containing fine particles is deposited on the filter medium, and a detaching means for detaching the cake from the filter medium above the white water. The filter disc is rotated about a substantially horizontal axis so that sections of the filter medium are alternately displaced up from and down into the white water, each section of filter medium having an exterior side.

The invention also relates to a disc filter for separating fine particles from white water, comprising a container for the white water to be separated, at least one substantially vertical hollow filter disc, which has two opposite walls of a filter medium and which is disposed in the container, each wall of filter medium having an exterior side, and a drive means adapted to rotate the filter disc about a substantially horizontal axis. The disc filter further comprises a level control means adapted to control the level of the surface of the white water in the container so that the filter disc is partly immersed in the white water, and means adapted to create a pressure difference between the white water and the interior of the filter disc, so that during operation a filtrate is forced from the white water through the filter medium into the interior of the filter disc while a cake containing fine particles is deposited on the exterior sides of the walls of filter medium. There is an inlet means for supplying the white water to be separated to the container, an outlet means for discharging filtrate from the interior of the filter disc, and a detaching means for detaching the cake from the filter medium in a removal zone, through which the filter medium passes above the white water during rotation of the filter disc.

White water is obtained when dewatering paper pulp, for instance in connection with paper production, and contains fine particles such as short fibres, fibre fragments and filler. It is desirable to separate the fine particles from the white water for at least two important reasons, namely to recover fine fibres and to produce clean water which can be reused in the pulp forming process. Uncleaned white water is also unsuitable to drain with respect to the environment. Since the fine particles easily may pass through the filter media used, usually consisting of cloth, white water is con- ventionally mixed with a fibre suspension substantially containing relatively long fibres, which cannot pass through the filter medium, before the white water is separated by means of the disc filter. During the separation of the white water said relatively long fibres form a fibre network on the filter medium, which efficiently captures the fine particles.

A problem however is that an operative fibre network is developed on each section of the filter medium only when the section initially has been displaced a distance down into the white water during the rotation of the filter disc. During the displacement of the section this distance, a prefiltrate of relatively uncleaned white water passes through the section. Therefore, the pre- filtrate is recirculated to the white water. During the further displacement of the section through the white water as the filter disc rotates, the fibre network on the section becomes tighter and captures the fine particles more efficiently. Consequently, also the filtrate passing through the section becomes cleaner as the section is displaced through the white water. The filtrate which is obtained during the remaining displacement of the section through the white water is usually divided into a clear filtrate, which can be reused as spray liquid, for instance for detaching pulp cakes from filter media on rotating filters, and an ultraclear filtrate (i.e. substantially clean water), which can be reused for for instance cleaning of filter media.

The prefiltrate which has to be recirculated constitutes up to 30 % of the total filtrate obtained, which results in the drawback that the capacity of the disc filter is substantially reduced. Nor is it possible to compensate the reduced capacity by increasing the rotational speed of the filter discs, since an increased rotational speed has the consequence that an efficient fibre network is developed on a section of the filter medium only when said section has been displaced farther into the white water. As a result, also the prefiltrate share of the total filtrate obtained increases.

Another drawback is that the disc filter has to be provided with a complicated and expensive valve device which is capable of separately conducting prefiltrate, clear filtrate and ultraclear filtrate, respectively, out from the disc filter.

The object of the present invention is to provide a method of separating white water which results in a substantially clean water without need for recirculating filtrate to the white water.

A further object of the invention is to provide a disc filter for separating white water which in comparison with known disc filters has a simpler design and an improved capacity.

The first mentioned object is achieved by the method initially described, which is characterized by providing the exterior side of each one of said sections of filter medium with a layer solely containing fibres during each revolution of the filter disc, and displacing the section with its said fibre layer through the white water so that fine particles are deposited on the fibre layer and together with the fibre layer form said cake. Hereby a filtrate is obtained having such a high cleanliness degree that any pre- filtrate does not need to be recirculated in most applications.

At applications having high demands on the cleanliness of the filtrate, a prefiltrate may however alternatively be recirculated to the white water, which results in that the filtrate obtained becomes a particularly clean ultraclear filtrate, which previously only was possible to achieve by coupling two conventional disc filters in series. The ultraclear filtrate of poorer grade obtained from the first conventional disc filter in this case was mixed with long fibrous pulp and was separated by means of the second disc filter, whereby an ultraclear filt- rate of an improved grade ( less contaminants ) was obtained. Thus, according to the invention such an improved ultraclear filtrate can be obtained by means of one single disc filter.

Preferably said section of filter medium is provided with the fibre layer as it is displaced without contacting the white water, so that the fibre layer is fully operative during the entire following displacement of the section through the white water. In this case the filtrate flowing through the section becomes the kind of grade described above as ultraclear filtrate, i.e. substantially clean water, regardless of where the section is in the white water.

Thus, development of unclear filtrate which has to be recirculated is eliminated.

The section of filter medium is suitably provided with the fibre layer when it is displaced downwardly, since normally the cake has to be detached from the filter medium when the latter is displaced upwardly and above the white water.

The section of filter medium is preferably provided with the fibre layer with the aid of a fibre suspension which substantially contains fibres that cannot pass through the filter medium and which has a fibre concentration of maximally 4 %, preferably 3-3,5 %. For instance, the section may be displaced through a liquid body of the fibre suspension at the same time as a pressure difference is created between the fibre suspension and the interior of the filter disc. As an alternative, the fibre suspension may be extruded or sprayed onto the filter medium. To prevent the fine particles in the white water from rapidly forming a substantially liquid impervious layer on the fibre layer, the white water is suitably mixed with a fibre suspension substantially containing fibres which cannot pass the filter medium.

Said further object of the invention is achieved by means of a disc filter of the kind initially described, which is characterized in that a means is adapted to form a layer, which solely contains fibres, on the exterior side of each wall of filter medium in a forming zone, through which the filter medium passes during rotation of the filter disc, the forming zone being situated after said detaching zone and ahead of a separation zone, through which the filter medium passes in the white water during rotation of the filter disc, as seen in the direction of rotation of the filter disc, whereby fine particles are captured by said fibre layer during the displacement of the filter medium in the white water, as the filter disc rotates.

According to a preferred embodiment of the invention said fibre layer forming means comprises a stationary wall, which defines a pocket with the filter disc in said forming zone, the pocket communicating downwardly with the interior of the container via a passage, which is formed between the wall and the filter disc and which is situated below the level of the surface of the white water, and a supplying means adapted to supply said pocket with a fibre suspension containing fibres that cannot pass the filter medium. A level adjustment means is suitably adapted to adjust the level of the surface of the fibre suspension such that the latter is at least as high as the level of the surface of the white water. According to other embodiments of the invention the fibre layer forming means may comprise an extrusion means, a spray means, a distribution means with an overflow for the fibre suspension or a distribution means adapted to distribute the fibre suspension onto the surface of the white water such that the fibre suspension contacts the filter medium in the forming zone.

Other advantageous features of the invention are defined in the attached claims.

The invention is described more closely in the following with reference to the accompanying drawings, in which

figure 1 shows a view of a cross-section through a disc filter according to a preferred embodiment of the invention,

figure 2 shows a partly sectionized side view of the disc filter according to figure 1,

figure 3 schematically shows filtrate zones of a conventional disc filter,

figure 4 schematically shows a cake detaching zone, a fibre layer forming zone, a separation zone and an ultraclear filtrate zone of a disc filter according to the invention,

figure 5 shows a modification of the disc filter according to figure 1, figure 6 shows a fibre layer forming means comprising a pocket with fibre suspension contacting the filter medium,

figure 7 shows a fibre layer forming means comprising an overflow for fibre suspension,

figure 8 shows a side view of a fibre layer forming means comprising a spray means for spraying fibre suspension onto the filter medium,

figure 9 shows the embodiment according to figure 8 from above,

figure 10 shows a side view of a disc filter with a fibre layer forming means comprising a distribution means for distributing fibre suspension onto the surface of the white water,

figure 11 shows the embodiment according to figure 10 from above, and

figure 12 shows a fibre layer forming means comprising an extrusion means for extruding fibre suspension onto the filter medium.

In the figures identical components have been provided with the same reference numerals.

In figures 1 and 2 there is shown a disc filter according to the invention comprising a container 1 for white water to be separated, five vertical annular hollow filter discs 2, each of which has two opposite walls 3,4 of a filter medium 5, and a horizontal tubular shaft 6, on which the filter discs 2 are mounted coaxially with the shaft 6. The shaft 6 is by means of bearings 7 journalled in the container 1 so that the filter discs extend downwardly into the container 1. A drive motor 8 is connected to the shaft for rotating the latter.

The container 1 is provided with an inlet 9 for supplying white water to the interior of the container 1 at the ascending side of the filter discs 2. The interior of each filter disc 2 communicates with the interior 6A of the tubular shaft 6 through a number of radial holes 10 in the shaft 6. The distal end of the shaft 6 as seen from the drive motor 8 is connected to a stationary filtrate outlet pipe 11 via an annular sealing means, so that the interior 6A communicates with the interior of the filtrate outlet pipe 11.

A level control means comprises a control means 12 with a pressure sensor 13, which senses the pressure head of the white water in the container 1. The control means 12 is adapted to control the rotational speed of the drive motor 8 in response to the pressure sensor 13 such that the surface of the white water is at a desired level 14 somewhat above the centre of the filter discs 2.

Since the hollow filter discs 2 are partly immersed in the white water from the level 14 the resulted hydrostatic pressure difference between the white water and the interior of the filter discs will force a filtrate through the filter medium 5 of the filter discs 2 while cakes containing fine particles are deposited on the outside of the filter medium 5. A detaching means comprising a number of spray members 15 is adapted to remove the cake from the filter medium 5 of each filter disc 2 by means of liquid jets in a detaching zone 16, through which the filter medium 5 passes during rotation of the filter discs 2. The detaching zone 16 is situated at the ascending side of the filter discs 2 and above the level 14. The detachment means further comprises a number of hoppers 17, which are adapted to receive released cakes and which have a common conveyor screw 18 for discharging received cakes .

At each outer side of each filter disc 2, there is a fibre layer forming means 19 which is adapted to form a fibre layer on the filter medium 5 in a forming zone 20, through which the filter medium 5 passes during rotation of the filter disc 2. The forming zone 20 is situated at the descending side of the filter disc 2 and extends through the level 14 of the surface of the white water. The fibre layer forming means 19 comprises a stationary wall 21 having U-shape in a horizontal section through the wall 21 and extending close to the filter disc 2 without contacting the latter, whereby a pocket 22 is defined by the wall 21 and the filter disc 2. The pocket 22 communicates downwardly with the interior of the container 1 via a passage 23, which is defined between the wall 21 and the filter disc 2 and which is situated below the level 14. In a vertical plane extending perpendicular to the filter disc 2 and through the pocket 22 the distance between the wall 21 and the filter disc successively decreases towards the passage 23. A supply means for supplying a fibre suspension to the pocket 22 comprises a feed conduit 24, which opens into the pocket 22, a pump 25 in the feed conduit 24 and an adjustable valve 26 in the feed conduit 24 situated downstream of the pump 25.

To facilitate the understanding of the function of the disc filter according to figures 1, 2 and 6, in figure 1 the filter medium 5 has been divided into fifteen iden- tical sections 27, which are evenly distributed in the circumference of the filter disc 2. In figure 1 the filter disc 2 is rotated clockwise. The white water, which may have been premixed with fibres, is supplied to the container 1 via the inlet 9 and fills the container 1 up to the desired level 14, whereby the hydrostatic pressure difference between the white water and the interior of the filter discs 2 forces a filtrate through the filter medium 5 into the filter discs 2 and further into the tubular shaft 6 via the holes 10. The filtrate is discharged from the shaft 6 via the filtrate outlet pipe 11. The speed of the drive motor 8 is controlled by the control means 12 in response to the pressure sensor 13, such that the surface of the white water is at the desired level 14. I.e. if the pressure sensor 13 senses a decreasing pressure indicating a decreasing level of the surface of the white water the control means 12 reduces the speed of the drive motor 8, so that the capacity of the disc filter is reduced and the white water rises in the container 1, and vice versa.

When a clean section 27 of the filter medium 5 is displaced clockwise through the forming zone 20, which is situated at about the three o^'clock position in figure 1, the section 27 will be in contact with the fibre suspension in the pocket 22, whereby a fibre layer 28 is formed on the exterior side of the section 27. The thickness of the fibre layer is determined by the width of the passage 23 as seen axially relative to the filter disc 2. In order to prevent white water from penetrating into the pocket 22 via the passage 23 and disturbing the formation of the fibre layer, the valve 26 is adjusted so that the level 29 of the surface of the fibre suspension in the pocket 22 is at least as high as, preferably higher than, the level 14 of the surface of the white water. When the section 27 with its fibre layer 28 is further displaced clockwise in the white water through a separation zone 30, which in figure 1 extends from the three o'clock position to the nine o'clock position, fine particles are deposited in a further layer 31 on the fibre layer, whereby a cake of the layers 28 and 31 is formed on the section 27. When the section 27 is displaced upwardly from the separation zone 30 and the white water, it passes through the detaching zone 16, which in figure 1 is situated at about the ten o'clock position, and the spray means 15 sprays liquid jets, for instance ultra- clear filtrate, against the cake on the section 27, so that the cake is rolled off and drops into the hopper 17, from which the cake is discharged by means of the conveyor screw 18. Then the above described procedure is repeated .

Since each section 27 of the filter medium 5 is provided with a finished fibre layer 28 already before it is displaced down into the white water, all filtrate that passes the section 27 during the displacement of the section 27 in the white water is of the grade called ultraclear filtrate, i.e. substantially clean water. The advantage of this is evident at a comparison between figure 3, which schematically shows a filter disc 32 of a conventional disc filter, and figure 4, which schematically shows the filter disc 2 of the disc filter according to the invention. The filtrate obtained in the conventional filter disc 30 consists of an unclean prefiltrate developed in a prefiltrate zone A and a relatively clean clear filtrate developed in a subsequent zone B as seen in the direction of rotation of the filter disc. Often the even more clean ultraclear filtrate is desired which is obtained by dividing the zone B into a clear filtrate zone Bl and an ultraclear filtrate zone B2. The division into said filtrate zones A, Bl and B2 requires in the first place that the interior of the filter discs is divided into separate cells corresponding to the sections 27 of the filter disc 2 in figure 1, in the second place that the cells in each axial row of cells in the filter discs are connected to an axial channel, which is situated in the tubular shaft and which is rotated with the filter discs about the centre of the filter discs, and in the third place that a distribution valve is arranged at one end of the shaft for distributing the filtrate flowing through the axial channels to a prefiltrate outlet, when the row of cells passes the prefiltrate zone A, to a clear filtrate outlet, when the row of cells passes the clear filtrate zone Bl and to an ultraclear filtrate outlet, when the row of cells passes the ultraclear filtrate zone B2. The unclean prefiltrate obtained in the prefiltrate zone A has to be returned to the white water, and for this reason the filter medium is actually only operative during displacement through the zone B.

As is evident from figure 4 the filter medium 5 of the disc filter according to the invention is operative during substantially the entire displacement through the separation zone 30, which increases the capacity as compared with the conventional case according to figure 3. Furthermore, all filtrate that passes through the filter medium 5 in the separation zone 30 is of the high cleanliness grade called ultraclear filtrate, which is obtained in the ultra filtrate zone C. Nor do the filter discs need to be provided with said cells, channels and distribution valve. In addition to this, the capacity of the disc filter according to the invention may be further increased by increasing the rotational speed of the filter discs, since the operative fibre layer exists on the filter medium already before the filter medium goes down into the white water. The speed may be increased until other factors become limiting, but at least up to twice the speed as compared with conventional disc filters.

If it is desired to improve the capacity of the disc filter according to figures 1, 2 and 6 and to increase the dryness of the cakes, the disc filter may be modified as illustrated in figure 5. To increase the pressure difference between the white water and the interior of the filter discs an external vacuum source, for instance in the form of a suction leg 33, is connected to the interior of the filter discs. In this case each filter disc has to be provided with radial walls 34, so that cells 35 of the type described in connection with the conventional disc filter according to figure 3 are formed. In addition to this, axial channels 36 have to be arranged connected to axial rows of cells 35, respectively. The axial channels 36 are connected via a distribution valve, not shown, to the suction leg 33 when the channels 36 are in a turning area indicated by a bow 37 marked at the centre of the filter disc in figure 5.

Thus, a cell 35 is subjected to underpressure as the cell approximately is in the three o'clock position in figure 5 and as the cell rotates through the white water to approximately the half past nine o^'clock position in figure 5, in which the cell is above the white water so that air can be sucked into the cake and replace the filtrate leaving the cake. Figure 7 shows an alternative fibre layer forming means comprising a distribution means 36 for fibre suspension with two overflows 37, which are situated above the level 14 and adjacent to two filter discs 2. The distri- bution means 36 has a container 38, in which a supply pipe 39 for fibre suspension opens. When the supply pipe 39 fills the container 38 with fibre suspension, the fibre suspension flows across each overflow 37 and comes into contact with the respective filter medium 5 so that said fibre layer 28 is formed. Of course, the distribution means 36 may have one single overflow 37 for serving one single side of a filter disc 2.

Figures 8 and 9 show another alternative fibre layer forming means comprising a spray means 40, having a radial distribution pipe 41 for fibre suspension with a number of spray nozzles 42 directed towards two adjacent filter discs 2. The spray nozzles 42 are adapted to spray fibre suspension onto the filter medium of the respective filter disc above the level 14, so that coherent fibre layers 28 are formed. Of course, the distribution pipe 41 may be provided with spray nozzles 42 directed at the same direction for serving one single side of a filter disc 2.

In figures 10 and 11 there is shown a further alternative fibre layer forming means comprising a distribution means 43 with a number of spray nozzles 44, which are directed radially relative to the filter discs 2 for distributing fibre suspension onto the surface of the white water, so that the fibre suspension contacts the filter medium of the respective filter disc 2 and forms a coherent fibre layer on the filter medium. In figure 12 there is shown a further alternative fibre layer forming means comprising an extrusion means 45 with a distribution pipe 46 for fibre suspension extending substantially radially along the filter disc 2 above the level 14. The distribution pipe 46 has a nozzle 47 with a notch having an extension corresponding to the radial extension of the filter medium. The nozzle 47 is directed towards the filter medium for extruding fibre suspension in a coherent mat, which is applied onto the filter medium and forms said fibre layer 28.

The fibre suspension forming the fibre layer 28 shall contain fibres which cannot pass the filter medium. Such a fibre suspension suitably is constituted by fresh pulp with a fibre concentration of 3-3,5 %, which usually is available in most pulp and paper mills.

Claims

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3. A method according to claim 1, c h a r a c t e r i z e d i n that each one of said sections ( 27 ) of the filter medium (5) is provided with said fibre layer (28) when it is displaced downwardly.

4. A method according to any one of claims 1-3, c h a r a c t e r i z e d i n that each one of said sections (27) of the filter medium (5) is provided with said fibre layer ( 28 ) by displacing it through a liquid body of a fibre suspension substantially containing fibres which cannot pass through the filter medium, at the same time as a pressure difference is created between said fibre suspension and the interior of the filter disc.

5. A method according to any one of claims 1-3, c h a r a c t e r i z e d i n that each one of said sections ( 27 ) of the filter medium ( 5 ) is provided with said fibre layer ( 28 ) by extruding a fibre suspension substantially containing fibres, which cannot pass through the filter medium, onto the section.

6. A method according to claim 2, c h a r a c t e r i z e d i n that each one of said sections ( 27 ) of the filter medium ( 5 ) is provided with said fibre layer (28) by spraying a fibre suspension substantially containing fibres, which cannot pass through the filter medium, onto the section.

7. A method according to any one of claims 1-6, c h a r a c t e r i z e d i n that before the fine particles are separated from the white water by means of the disc filter the white water is mixed with a fibre suspension substantially containing fibres which cannot pass through the filter medium (5).

8. A method according to any one of claims 4-7, c h a r a c t e r i z e d i n that said fibre suspension has a fibre concentration in the range of 1-4 %, preferably 3-3,5 %.

9. A disc filter for separating fine particles from white water, comprising a container (1) for the white water to be separated, at least one substantially vertical hollow filter disc (2), which has two opposite walls (3,4) of a filter medium (5) and which is situated in the container, each wall of filter medium having an exterior side, a drive means (8) adapted to rotate the filter disc about a substantially horizontal axis, a level control means (8,12,13) adapted to control the level (14) of the surface of the white water in the container such that the filter disc is partly immersed in the white water, means (33) adapted to create a pressure difference between the white water and the interior of the filter disc, such that during operation a filtrate is forced from the white water through the filter medium into the interior of the filter disc while a cake containing fine particles is deposited on the exterior sides of the walls of filter medium, an inlet means ( 9 ) for supplying the white water to be separated to the container, an outlet means (11) for discharging filtrate from the interior of the filter disc, and a detaching means (15,17,18) for detaching the cake from the filter medium in a detaching zone (16), through which the filter medium passes above the white water during rotation of the filter disc, c h a r a c t e r i z e d i n that a means ( 19; 36; 40; 43; 45 ) is adapted to form a layer ( 28 ) , which solely contains fibres, on the exterior side of each wall of filter medium ( 5 ) in a forming zone ( 20 ) , through which the filter medium passes during rotation of the filter disc, the forming zone being situated after said detaching zone (16) and ahead of a separation zone (30), through which the filter medium passes in the white water during rotation of the filter disc, as seen in the direction of rotation of the filter disc, whereby fine particles are captured by said fibre layer (28) during the displacement of the filter medium in the white water, as the filter disc rotates.

10. A disc filter according to claim 9, c h a r a c t e r i z e d i n that said forming zone (20) is situated above said level ( 14 ) .

11. A disc filter according to claim 9, c h a r a c - t e r i z e d i n that said forming zone (20) is situated at the descending side of the filter disc ( 2 ) .

12. A disc filter according to claim 9, c h a r a c t e r i z e d i n that said forming zone (20) is situated at the descending side of the filter disc ( 2 ) and extends through said level (14).

13. A disc filter according to claim 12, c h a r a c t e r i z e d i n that said fibre layer forming means (19) comprises a stationary wall (21), which defines a pocket (22) with the filter disc (2) in said forming zone ( 20 ) , the pocket communicating downwardly with the interior of the container via a passage ( 23 ) , which is formed between the stationary wall and the filter disc and which is situated below said level (14), and a supply means (24,25) adapted to supply said pocket with a fibre suspension containing fibres, which cannot pass the filter medium ( 5 ) .

14. A disc filter according to claim 13, c h a r a c t e r i z e d i n that a level adjustment means (26) is adapted to adjust the level (29) of the surface of the fibre suspension such that the latter is at least as high as the level (14) of the surface of the white water.

15. A disc filter according to claim 13 or 14, c h a r a c t e r i z e d i n that the distance between the stationary wall (21) and the filter disc decreases successively in the direction towards said passage ( 23 ) , as seen in a vertical plane extending perpendicular to the filter disc (2) and through said pocket ( 22 ) .

16. A disc filter according to any one of claims 9-12, c h a r a c t e r i z e d i n that said fibre layer forming means comprises an extrusion means (45) adapted to extrude a fibre suspension onto the filter medium ( 5 ) in said forming zone (20), the fibre suspension containing fibres which cannot pass through the filter medium.

17. A disc filter according to claim 16, c h a - r a c t e r i z e d i n that the extrusion means

(45) comprises a distribution pipe (46), which extends substantially radially along the filter disc (2) and which is provided with at least one nozzle ( 47 ) directed towards the filter medium ( 5 ) for extruding the fibre suspension, such that a layer (28) of fibres is applied onto the filter medium during operation.

18. A disc filter according to claim 10, c h a r a c t e r i z e d i n that said fibre layer forming means comprises a spray means (40) adapted to spray a fibre suspension onto the filter medium ( 5 ) in said forming zone ( 20 ) , the fibre suspension containing fibres which cannot pass through the filter medium.

19. A disc filter according to claim 18, c h a r a c t e r i z e d i n that the spray means ( 40 ) comprises a distribution pipe (41), which extends substantially radially along the filter disc (2) and which is provided with a number of spray nozzles (42) directed towards the filter medium ( 5 ) for spraying the fibre suspension, such that a coherent layer of fibres is formed on the filter medium during operation.

20. A disc filter according to claim 11, c h a r a c - t e r i z e d i n that said fibre layer forming means comprises a distribution means (36), which has an overflow (37) above said level (14) and which is adapted to distribute a fibre suspension via the overflow such that the fibre suspension contacts the filter medium ( 5 ) in said forming zone (20), the fibre suspension containing fibres which cannot pass through the filter medium.

21. A disc filter according to claim 12, c h a r a c t e r i z e d i n that said fibre layer forming means comprises a distribution means (43) adapted to distribute a fibre suspension onto the surface of the white water such that the fibre suspension contacts the filter medium ( 5 ) in said forming zone ( 20 ) , the fibre suspension containing fibres which cannot pass through the filter medium.

22. A disc filter according to any one of claims 9-21, c h a r a c t e r i z e d i n that said inlet means ( 9 ) for supplying white water is arranged to supply the white water at the ascending side of the filter disc (2).