WO2008088279A1

WO2008088279A1 - Device and method for washing and dewatering cellulose pulp

Info

Publication number: WO2008088279A1
Application number: PCT/SE2008/050001
Authority: WO
Inventors: Jörgen T LUNDBERG
Original assignee: Metso Paper, Inc.
Priority date: 2007-01-16
Filing date: 2008-01-02
Publication date: 2008-07-24
Also published as: SE0700090L

Abstract

A pulp washing arrangement (10) comprises a rotatable drum (2) and an enclosing stationary casing (1 1). The drum has a plurality of compartments (4), at least partially defined by a perforated surface (7), for pulp to be washed arranged at a periphery. Seals (12) between the stationary casing and the drum define a forming zone (F) whereby the compartments pass the forming zone from an entrance part (16) to an exit part (17) upon rotation of the drum. The arrangement further comprises a pulp inlet (19) arranged for entering pulp to be washed into the compartments within the forming zone and pressurizing means (30) for applying a pressure difference over the perforated surface, causing filtrate liquid from the pulp to pass the perforated surface. The pressurizing means is arranged for applying a smaller pressure difference over the perforated surface in the entrance part of the forming zone than an average pressure difference. A method for pulp washing is also disclosed.

Description

Device and method for washing and dewatering cellulose pulp

TECHNICAL FIELD

The present invention relates in general to cellulose pulp processing, and in particular to methods and devices for washing and dewatering of cellulose pulp.

BACKGROUND

When processing fibers into pulp, the process typically comprises steps where different liquors are added or removed. For instance, the digestion liquor or different types of bleaching liquors have to be removed from the pulp in different stages. In connection with such dewatering or deliquoring, the pulp is typically also washed by adding and subsequent removing of a washing liquor. There are a number of different types of washing equipments operating according to different principles.

A well-known type of washing arrangement is the drum washer, where the pulp is dewatered on a rotary filter drum after the addition of washing liquid, which displaces the liquor remaining on the pulp web after preceding process stages, for example a digestion stage or bleaching stage. An underpressure within the drum causes the displaced liquid to pass through a perforated metal sheet located at a periphery of the rotary drum. A further development of the original drum washer is the pressurized displacement washer, where the filtrate at over-pressure is caused to pass through the perforated metal sheet. The increased pressure difference leads to an improved displacement of the filtrate.

According to a known design of a pressurized displacement washer, the drum is provided with compartments, extending in the axial direction of drum and intended to be filled with pulp. The compartments are defined by walls in the form of bars arranged axially along the entire drum shaft, as well as a bottom that consists of the perforated metal surface. The subdivision in compartments of the drum ensures that the pulp cake does not break up and move, but instead maintains the form produced upon application of the pulp. The perforated metal surface, on which the pulp deposits, is located at a distance from the main surface of the drum, so that filtrate channels or conduits are formed in a space between the actual drum and the perforated metal surface.

In a drum washer, a plurality of different washing stages can be carried out, with separate addition of washing liquid to the different stages. In order to be able to separate different washing stages, carried out in one or more washing zones of the drum, and forming stages, carried out in the forming zone of the drum, and discharge stages, carried out in the discharge zone of the drum, the respective zones are sealed by longitudinal seals. These longitudinal seals are placed between the rotary drum and the surrounding casing defining the different zones. The filtrates from the respective zones are separated by seals in a peripheral end valve arranged at one or both of the end walls of the drum. When the drum rotates, the compartments successively pass the different zones.

Pulp to be washed is entered into the forming zone through a pulp inlet. The pulp is allowed to fill the compartments when the compartments are rotated from an entrance part to an exit part of the forming zone. The pressure difference over the perforated metal sheet extracts filtrate liquid from the pulp, thereby forming a pulp web in the respective compartments before the compartments pass into a next zone.

A general problem with prior art drum washers is that the result of the washing often exhibits large variations, even within the pulp of one and the same compartment. Different fractions of the pulp web are washed differently, and the overall washing efficiency becomes low. SUMMARY

A general object of the present invention is to improve the homogeneity of pulp washing. A further object of the present invention is to reduce pulp density gradients in a drum washer.

The above object is achieved by arrangements and methods according to the enclosed patent claims. In general words, in a first aspect, a pulp washing arrangement comprises a rotatable drum and a stationary casing enclosing the rotatable drum. The rotatable drum has a plurality of compartments for pulp to be washed arranged at a periphery. The compartments are at least partially defined by a perforated surface. Seals between the stationary casing and the rotatable drum, define at least a forming zone in a space between the casing and the rotatable drum whereby the compartments pass the forming zone from an entrance part to an exit part upon rotation of the rotatable drum. The arrangement further comprises at least one pulp inlet arranged for entering pulp to be washed into the compartments within the forming zone and pressurizing means for applying a pressure difference over the perforated surface, causing filtrate liquid from the pulp to pass the perforated surface. The arrangement is characterized in that the pressurizing means is arranged for applying a smaller pressure difference over the perforated surface in the entrance part of the forming zone than a pressure difference over the perforated surface averaged over the entire forming zone.

In a second aspect, a method for washing of pulp comprises forming of a pulp web on a periphery of a rotatable drum, rotating of the rotatable drum bringing the formed pulp web through at least one washing stage and removing of the pulp web from the rotatable drum. The forming in turn comprises providing of pulp in contact with a perforated surface at the periphery of the rotatable drum and applying of a pressure difference over the perforated surface, causing filtrate liquid from the pulp to pass the perforated surface. The method is characterized in that the pressure difference application is performed by applying a smaller pressure difference over the perforated surface at an initial stage of the forming than an applied pressure difference averaged over the entire forming step.

One advantage with the present invention is that the radial pulp density variations within each compartment are reduced, which in turn allows for a better control of the pulp washing. The washing efficiency can thereby be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an embodiment of a rotatable drum;

FIG. 2A is an axial cross-section through an embodiment of a washing arrangement;

FIG. 2B is an enlargement of a part of Fig. 2A;

FIGS. 3A-C are embodiments of pressure arrangements causing differentiated pressure difference in a washing arrangement;

FIG. 4 is a part cross-section view of an embodiment of rotatable drum;

FIG. 5 is a detail view of a part of an embodiment of an axial seal;

FIG. 6 is a detail view of a part of another embodiment of an axial seal; FIG. 7 is a detail view of a part of yet another embodiment of an axial seal;

FIG. 8 is a cross-section view of another embodiment of a washing arrangement; and

FIG. 9 is a flow diagram illustrating steps of an embodiment of a method according to the present invention. DETAILED DESCRIPTION

In the drawings, corresponding reference numbers are used for similar or corresponding parts.

Fig. 1 is a schematic perspective view of an embodiment of a rotatable drum 2 that is one main part of a pressurized displacement washing arrangement. The rotatable drum 2 is provided with a plurality of compartments 4 arranged at a periphery 9 of the rotatable drum 2. The compartments are also commonly called pulp compartments or cells. The compartments 4 are intended to be filled by paper pulp to be washed. Each compartment 4 in the present embodiment is at least partially defined by a bottom 5 made of a perforated surface 7, preferably made by metal, placed at a distance from the main peripheral surface 21 of the rotatable drum 2. A space is thereby defined between the peripheral surface of the rotatable drum 2 and the perforated surface 7, which is used for extraction of filtrate liquid. This is described in more detail further below. The compartments 4 are further defined by two compartment walls 8 arranged axially with reference to a shaft 6 of the rotatable drum 2. The compartment walls 8 of the rotatable drum 2 of the present embodiment are evenly distributed along the circumference of the rotatable drum 2. The rotatable drum 2 is in general rotatably supported by a stationary support (not shown in the figure) in the washing arrangement.

In Fig. 2A an axial cross-section through an embodiment of a washing arrangement 10 with a compartmented rotatable drum 2 is shown. The rotatable drum 2 is rotatably arranged with respect to the shaft 6 within a stationary casing 1 1. The stationary casing 11 thus encloses the rotatable drum 2 and defines a space 13 between the casing and the rotatable drum 2. The washing arrangement 10 comprises a plurality of seals 12 placed between the rotatable drum 2 and the surrounding stationary casing 11 in a longitudinal axial direction. These seals 12 seal particularly between the stationary casing 11 and the compartment wall 8 and serve as separating members between different zones of the washing arrangement 10 in the space between the casing and the rotatable drum. One function of the seals 12 is to ensure that washing liquid intended for a specific washing stage is not moved to a subsequent washing stage, in particular since there may be a difference in pressure between different washing stages. More specifically, the seals 12 define at least a forming zone F for forming of the pulp in the compartments 4 of the rotatable drum 2.

The illustrated embodiment of a washing arrangement 10 is provided with four longitudinal seals 12 that consequently divide the space 13 of a substantially annular shape into four zones. Besides the forming zone F, a first and second washing zone Tl, T2 for washing the formed pulp, and a discharge zone U for discharge of the washed pulp are provided.

A washing arrangement 10 of the above described design operates with a continuously rotating rotatable drum 2. This implies that the compartments 4 successively pass the different zones F, Tl, T2, U. More particularly, each compartment 4 pass the forming zone F from an entrance part 16 to an exit part 17 of the forming zone upon rotation of the rotatable drum 2. Pulp for washing in a porous state is fed into the forming zone F by at least one pulp inlet 19. Typically, the entire forming zone is filled with pulp, whereby the pulp is entered into the compartments 4 of the rotatable drum 2. The pulp 18 is in the present embodiment placed as, in the axial direction of the rotatable drum 2, long and narrow rectangles against the perforated surface 7, in this embodiment a metal sheet that constitutes the compartment bottom 5. The division of the periphery of the rotatable drum 2 into compartments ensures that the shape of the pulp cake or pulp web is maintained during rotation of the rotatable drum 2.

Washing liquid can be supplied to the annular space 13 and filtrate liquid is squeezed out of the pulp by a pressure difference applied over the perforated surface 7. The filtrate liquid thereupon passes through the perforated surface 7 and leaves a more densely packed pulp. This occurs in the present embodiment at overpressure in order to achieve an improved dewatering of the pulp. Consequently, the washing arrangement 10 comprises a pressurizing means 30 for applying a pressure difference over said perforated surface 7. The pressurizing means 30 comprises in the present embodiment means 41 for providing an overpressure of the pulp entering the forming zone F and means 51 for providing an underpressure of the filtrate liquid having passed the perforated surface 7. In a most simple design, the overpressure is simply provided as the overpressure caused by the pump feeding the pulp into the forming zone F. The means 51 for providing an underpressure is thereby fluidly connected to the volume between the perforated surface 7 and the main peripheral surface of the rotatable drum 2, as indicated by the dashed line 52. The pressurizing means 30 thereby provides a pressure difference over the perforated surface 7.

In alternative embodiments, either the means 41 for providing an overpressure or the means 51 for providing an underpressure may be omitted. The pressure difference will in such case be a difference between the over/under-pressure and atmospheric pressure.

The washing may, as in the present embodiment, be repeated in two or more stages at different pressures and with separate washing liquids. Used liquid is normally brought back to a preceding washing stage or out from the washing apparatus 10 and to previous process stages. The washed pulp is discharged through an outlet opening 20.

The present invention is based on the understanding that the problem with uneven washing of pulp in drum washers to a large extent is related to large pulp concentration gradients, particularly in a radial direction, of the pulp web. These pulp concentration gradients are essentially provided during the formation of the pulp web in the formation zone. In the early stages of the formation, when the first porous pulp reach the perforated surface of the compartment, the entire pressure difference over the perforated surface is applied. The initial pulp is thereby very efficiently dewatered but becomes at the same time compressed into a high pulp concentration. Once being initially dewatered, the pulp does not undergo any large pulp concentration changes during the washing operations. When subsequent pulp is dewatered, it takes place via the first added pulp. A part of the pressure difference applied over the perforated surface is therefore removed by the flow constrictions formed by the initially concentrated pulp. The dewatering thereby becomes less efficient and the pulp concentration becomes somewhat less than closer to the perforated surface. In this way, as the pulp web builds up, the pulp web itself acts as a flow constriction, reducing the efficient pressure difference used for dewatering purposes. The result is a pulp web exhibiting a radial pulp concentration gradient.

Fig. 2B is an enlargement of the forming zone F of Fig. 2A. Here, the pulp filled forming zone F is easily seen, in which a pulp concentration occurs in the compartments 4.

An improved, more even pulp concentration distribution would be achieved if a similar effective pressure difference would be utilized for dewatering pulp over the whole forming zone F. In particular, reduction of the pressure difference in an initial stage of the forming step would therefore be advantageous. According to the present invention, the pressurizing means 30 is therefore arranged for applying a smaller pressure difference over the perforated surface 7 in the entrance part 16 of the forming zone F than a pressure difference over the perforated surface 7 at a later stage in the forming zone F. Preferably, the pressurizing means 30 is arranged for applying a smaller pressure difference over the perforated surface 7 in the entrance part 16 of the forming zone F than a pressure difference over the perforated surface 7 averaged over the entire forming zone F. In other words, the pressure difference over the perforated surface 7 is reduced in a compartment 4 when that particular compartment 4 is situated in the entrance part 16. Yet another way of expressing the present invention is that the maximum pressure difference within the forming zone F should occur at a position outside the entrance part 16. The differences in pressure difference can be obtained in different ways. Fig. 3A is a schematic sketch of the pressure related means in connection with the perforated surface 7, in particular parts of the pressurizing means 30. The illustrations are intended as purely conceptual illustrations and should not be view as true drawings of the physical arrangements. A single means 41 for providing a uniform overpressure in the forming zone F is provided. As mentioned above, the means 41 for providing an overpressure may be omitted if the forming zone is operating at atmospheric pressure. The entire forming zone F is in such case provided with one and the same pressure, e.g. as provided by a pulp inlet pump. The means 51 for providing an underpressure comprises in this embodiment two part means. A first underpressure provider 44 is connected to the bottom side of the perforated surface 7 in the entrance part 16 of the forming zone F. A second underpressure provider 45 is also connected to the bottom side of the perforated surface 7, but in the exit part 17 of the forming zone F. In this way, a pressure difference over the perforated surface 7 can be different in the entrance part 16 compared to the exit part 17. Possibly, the first underpressure provider 44 may be omitted, thereby letting the bottom side of the perforated surface 7 in the entrance part 16 of the forming zone F operate at atmospheric pressure.

In Fig. 3B, an alternative embodiment is sketched. Here, the means 51 for providing an underpressure comprises one single underpressure provider 46. However, a flow constriction 47 arranged between the underpressure provider 46 and the perforated surface 7 reducing the flow of filtrate liquid from the entrance part 16 will reduce the actual pressure difference over the perforated surface 7.

Fig. 3C illustrates another approach, where the differences in applied pressures are arranged at the pulp inlet side. In this case, the forming zone F is divided into two part zones Fl , F2, corresponding to the entrance and exit parts 16, 17, respectively. Pulp is introduced into the part zones separately, and the means 41 for providing an overpressure comprises in this embodiment two part means. A first overpressure provider 54 is connected to the entrance part 16 of the forming zone F, i.e. part zone Fl . A second overpressure provider 55 is connected to the exit part 17 of the forming zone F, i.e. part zone F2. Possibly, the first overpressure provider 54 may be omitted, thereby letting part zone Fl operate at atmospheric pressure. Also here, a different pressure difference is provided in the entrance part 16 compared to the exit part 17.

A combination of the principles of Figs. 3A and 3B or 3C can also be utilized.

Fig. 4 illustrates a part of a cross-section view of an embodiment of rotatable drum 2 taken along the shaft of the rotatable drum 2. The narrow rectangle 18 of pulp is easily seen. The perforated surface 7 is in this embodiment placed at a distance d from the main peripheral surface 21 of the rotatable drum 2 such that conduits 22 in form of filtrate channels are formed in the space between the peripheral surface 21 and the perforated surface 7. The conduits 22 are arranged for conducting filtrate liquid having passed the perforated surface 7 to a respective orifice 23. The orifices 23 are arranged in a plane 29 perpendicular to the axis of the rotatable drum 2. This enables the orifices to move along an axial seal 28A provided in a valve housing 24 of the stationary casing 11. In the present disclosure, an axial seal is intended to be a seal sealing against a surface perpendicular to the axis. The valve housing 24 comprises an opening arrangement 25 allowing filtrate liquid from the orifices 23 to exit the stationary casing 11 when the orifices 23 are in registry with an opening of the opening arrangement 25. Since the rotatable drum 2 rotates, the openings of the opening arrangement 25 are in registry with at least a part of said orifices at a time. The pressurizing means is in the present embodiment arranged for applying a pressure difference between the forming zone and the orifices 23.

Fig. 5 illustrates a detail view of a part of an embodiment of a valve housing 24, having two concentric arc-shaped axial seals 28A directed towards the rotatable drum, i.e. inwards into the paper of Fig. 5. The valve housing 24 further comprises two end axial seals 28B, defining a volume into which the orifices of the drum may lead. The opening arrangement 25 comprises in this embodiment a number of holes 26 located between the axial seals 28A and 28B, through which holes 26 the filtrate liquid is allowed to exit. (Only a few of the holes 26 are provided with reference numbers in order to increase the readability of the figure.) The pressurizing means is in this embodiment arranged for applying a same pressure difference between the entire opening arrangement 25 and the forming zone. The pressurizing means of this embodiment further comprises flow constrictions 31 in a first part 32 of said opening arrangement 25. The flow constrictions 31 are provided over the entrance part of the forming zone, which entrance part covers at least one of the compartments of the rotatable drum. Holes 26 in the first part 32 are in registry with orifices of conduits conducting filtrate liquid from the entrance part of the forming zone. The flow constrictions 31 comprises in the present embodiment a cover plate 33 having holes 36 of a smaller diameter covering at least a part a respective orifice 23. The flow constrictions 31 thus reduce the effective pressure difference between conduits 21 in registry with the first part 32 and the forming zone.

The flow constrictions 31 can be designed in many different ways. One possibility is to have a successively increasing opening for each hole 26. Another possibility is to start with totally blocked holes 26 and then have a number of holes 36 having the same diameter. The

In a possible embodiment, the pressurizing means is arranged such that the flow constrictions 31 are externally controllable. Such control may e.g. involve an aperture with controllable opening diameter in front of each hole 26. Another alternative is to provide alternative exchangeable cover plates having different sets of holes. Yet another alternative is to arrange the cover plate in a displaceable manner, thereby being able to move one and the same cover plate to cover different sets of holes 26. There are many alternative designs of the opening arrangement 25. Fig. 6 illustrates one alternative embodiment. Here the opening arrangement 25 comprises a few openings 27 extended in a tangential direction of the rotatable drum. A cover plate 33, having a central slit 34 is provided over the first part 32 of the opening arrangement 25, thereby forming a flow constriction. The magnitude of the flow constriction can be tuned by displacing the cover plate 33 along the tangential direction T of the rotatable drum.

There are also other means for providing the differentiated pressure difference. In Fig. 7, the opening arrangement 25 again comprises holes 26 of equal size. However, holes in the first part 32, i.e. the holes being in registry with orifices of conduits conducting filtrate liquid from the entrance part of the forming zone, are fluidly separated from the rest of the holes by a first 42 and second 43 chamber. Preferably, the entrance part of the forming zone covers at least one of the compartments of the rotatable drum in the direction of rotation. Different pressures can then be provided to the different set of holes by pressurizing the chambers differently. In other words, the pressurizing means 30 is arranged for applying a first pressure difference between the first part 32 of the opening arrangement and the forming zone and a second pressure difference between a second part 38 of the opening arrangement and the forming zone. The first pressure difference is smaller than the second pressure difference. This is implemented in the present embodiment by the first underpressure provider 44, connected to the first chamber 42, and the second underpressure provider 45, connected to the second chamber 43. Preferably, the pressurizing means 30 is arranged such that at least one of the first pressure difference and the second pressure difference is externally controllable, e.g. by controlling the first and/ or the second underpressure provider 44, 45.

The pulp entering the washing arrangement comprises cellulose fibres and different liquids. The dewatering properties in the drum washer will depend on properties of the pulp entered into the forming step. The pulp properties are in turn dependent on the type of cellulose fibres, the degree of digestion, the amount and composition of liquids in which the pulp is mixed, the amount and composition of any additional compounds, etc. Pulp properties may also be dependent on external factors, such as e.g. temperature. The dewatering properties will furthermore be dependent on other factors, such as dimensions of perforations, compartment sizes, drum velocities as well as on the pressure difference. In order to achieve an efficient washing procedure, many of these factors should be taken into account. If a fixed arrangement for providing different pressure differences at different parts of the forming zone, the number of variable process parameters is reduced. It is thus advantageous to have the pressurizing means adaptable at least to some degree.

Fig. 8 illustrates a cross-section view of an embodiment of a washing arrangement 10. Most parts are similar as in Fig. 2 and will not be further discussed. However, a sensor 50 is provided within the forming zone F or in any other volume where pulp characteristic properties can be measured, e.g. somewhere in the supply conduits to the forming zone F. The sensor 50 is arranged for measuring properties of pulp entering the forming zone F. Typical types of sensors could be density sensors, viscosity sensors, temperature sensors etc. The readout from the sensor 50 is connected to the pressurizing means 30. The pressurizing means 30 is thereby arranged for adapting pressure differences over different parts of the perforated surface in dependency of pulp properties measured by the sensor 50. Such adaptation can be performed by the means indicated in the different part embodiments discussed further above.

Such an arrangement can thus efficiently be used for very differing quality of pulp without need for reconstructions. The level of sophistication is more or less limited only by the skills of the operator.

Fig. 9 is a flow diagram illustrating steps of an embodiment of a method according to the present invention. A method for washing of pulp begins in step 200. In step 210 a pulp web is formed on a periphery of a rotatable drum and in step 220, the rotatable drum is rotated, whereby the formed pulp web is brought through at least one washing stage. In step 230, the pulp web is removed from the rotatable drum. The step 210 in turn comprises a number of substeps. In step 212, pulp is provided in contact with a perforated surface at the periphery of the rotatable drum. A pressure difference is applied the perforated surface in step 214, causing filtrate liquid from the pulp to pass the perforated surface. The application of a pressure difference is performed by applying a smaller pressure difference over the perforated surface at an initial stage of the forming than an applied pressure difference in a later stage of the forming. Preferably, the application comprises applying of a smaller pressure difference over the perforated surface at an initial stage of the forming step compared to an applied pressure difference averaged over the entire forming step. In other words, a differentiated pressure difference is applied over the perforated surface 7.

The procedure ends in step 299. Anyone skilled in the art realizes that the steps of Fig. 9 typically are performed in parallel and essentially continuously.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

1. A pulp washing arrangement (10), comprising: a rotatable drum (2) ; said rotatable drum (2) having a plurality of compartments (4) for pulp (18) to be washed arranged at a periphery (9); said compartments (4) being at least partially defined by a perforated surface (7); a stationary casing (11) enclosing said rotatable drum (2); seals (12) between said stationary casing (1 1) and said rotatable drum (2), defining at least a forming zone (F) in a space (13) between said stationary casing (1 1) and said rotatable drum (2); whereby said compartments (4) pass said forming zone (F) from an entrance part (16) of said forming zone (F) to an exit part (17) of said forming zone (F) upon rotation of said rotatable drum (2); at least one pulp inlet (19) arranged for entering pulp to be washed into said compartments (4) within said forming zone (F); and pressurizing means (30) for applying a pressure difference over said perforated surface (7), causing filtrate liquid from said pulp (18) to pass said perforated surface (7), characterized in that said pressurizing means (30) is arranged for applying a smaller pressure difference over said perforated surface (7) in said entrance part (16) of said forming zone (F) than a pressure difference over said perforated surface (7) in a later part of said forming zone (F) .

2. Pulp washing arrangement according to claim 1, characterized in that said pressurizing means (30) is arranged for applying a smaller pressure difference over said perforated surface (7) in said entrance part (16) of said forming zone (F) than a pressure difference over said perforated surface (7) averaged over the entire said forming zone (F) .

3. Pulp washing arrangement according to claim 1 or 2, characterized in that said entrance part (16) of said forming zone (F) covers at least one of said compartments (4) in a direction of rotation.

4. Pulp washing arrangement according to any of the claims 1 to 3, characterized in that said rotatable drum (2) further comprising conduits (22) arranged for conducting filtrate liquid having passed said perforated surface (7) to a respective orifice (23); and said pressurizing means (30) is arranged for applying a pressure difference between said forming zone (F) and said orifices (23) .

5. Pulp washing arrangement according to claim 4, characterized in that said orifices (23) are arranged in a plane (29) perpendicular to an axis

(6) of said rotatable drum (2); said stationary casing (11) comprises an opening arrangement (25) being in registry with at least a part of said orifices (23) at a time, allowing filtrate liquid from said orifices (23) in registry to exit said stationary casing (11).

6. Pulp washing arrangement according to claim 5, characterized in that said pressurizing means (30) being arranged for applying a same pressure difference between entire said opening arrangement (25) and said forming zone (F); said pressurizing means (30) comprises flow constrictions (31, 47) in a first part (32) of said opening arrangement (25), reducing a pressure difference between conduits (22) in registry with said first part (32) and said forming zone (F); and said first part (32) being in registry with orifices (23) of conduits (22) conducting filtrate liquid from said entrance part (16) of said forming zone (F)-

7. Pulp washing arrangement according to claim 6, characterized in that said flow constrictions (31) comprises a cover plate (33) covering at least a part said respective orifices (23).

8. Pulp washing arrangement according to claim 6 or 7, characterized in that said pressurizing means (30) is arranged such that said flow constrictions (31) are externally controllable.

9. Pulp washing arrangement according to claim 5, characterized in that said pressurizing means (30) is arranged for applying a first pressure difference between a first part (32) of said opening arrangement (25) and said forming zone (F) and a second pressure difference between a second part (38) of said opening arrangement (25) and said forming zone (F); said first part (32) being in registry with orifices (23) of conduits (22) conducting filtrate liquid from said entrance part (16) of said forming zone (F); and said first pressure difference being smaller than said second pressure difference.

10. Pulp washing arrangement according to claim 9, characterized in that said first part (32) and said second part (38) are fluidly separated.

11. Pulp washing arrangement according to claim 9 or 10, characterized in that said pressurizing means (30) is arranged such that at least one of said first pressure difference and said second pressure difference is externally controllable.

12. Pulp washing arrangement according to claim 8 or 11, characterized by further comprising a sensor (50) arranged for measuring properties of pulp entering said forming zone (F), and in that said pressurizing means (30) is connected to said sensor (50) and is arranged for adapting pressure differences over different parts of said perforated surface (7) in dependency of pulp properties measured by said sensor (50) .

13. Method for washing of pulp, comprising the steps of: forming (210) a pulp web on a periphery (9) of a rotatable drum (2); rotating (220) said rotatable drum (2) bringing said formed pulp web through at least one washing stage; and removing (230) said pulp web from said rotatable drum (2); whereby said step of forming (210) in turn comprising the steps of: providing (212) pulp in contact with a perforated surface (7) at said periphery (9) of said rotatable drum (2); and applying (214) a pressure difference over said perforated surface (7), causing filtrate liquid from said pulp to pass said perforated surface (7), characterized in that said step of applying (214) comprises applying of a smaller pressure difference over said perforated surface (7) at an initial stage of said forming than an applied pressure difference in a later stage of the forming (210) step.

14. Method according to claim 13, characterized in that said step of applying (214) comprises applying of a smaller pressure difference over said perforated surface (7) at an initial stage of said forming than an applied pressure difference averaged over the entire forming (210) step.