EP2470715A1

EP2470715A1 - Arrangement and method for washing cellulose pulp

Info

Publication number: EP2470715A1
Application number: EP10812405A
Authority: EP
Inventors: Magnus Danielsson; Jörgen Lundberg; Rickard Andersson
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2009-08-25
Filing date: 2010-08-23
Publication date: 2012-07-04
Also published as: WO2011025446A1; SE534149C2; CN102472005A; SE0901115A1; CN102472005B

Abstract

An arrangement (100) for washing cellulose pulp, comprising a pulp transport rotor (10) and a number of pulp chambers (12), is provided. Each pulp chamber is partly defined by a perforated surface (12a) of the rotor. The rotor is arranged to rotate stepwise so as to transfer the perforated surface of the respective pulp chambers between different pulp handling zones (Z1, Z2, Z3, Z4). The washing arrangement further comprises a piston device (20) arranged at one pulp handling zone, said piston device being arranged for piston movement (M) in a radial direction so as to, in operation and upon movement towards the pulp, increase the consistency of pulp, and means for displacement washing of the pulp with increased consistency using the piston device.

Description

ARRANGEMENT AND METHOD FOR WASHING CELLULOSE PULP

TECHNICAL FIELD

The present invention relates to treatment of cellulose pulp and more specifically to an arrangement and a method for washing cellulose pulp. BACKGROUND

Pulp washing is a key operation in the chemical pulping line. There are many different types of washers available, including wash presses, dewatering presses, diffusers and filters. One well-known washing principle is displacement, where dirty liquid (liquor) in the pulp is replaced by cleaner washing liquid. Another well-known washing principle is pressing, where dirty liquid is pressed (squeezed) out from the pulp. Displacement and pressing often occurs in the same washing machine, such as in a press washer of the general type disclosed in US Patent 3,980,518 for example.

Two important issues to consider regarding washing are washing efficiency and capacity. These factors often have to be balanced against each other in the sense that an increase of capacity leads to a decrease of efficiency, or vice versa. A washing solution providing an improved washing efficiency as well as high capacity would thus be very desirable.

SUMMARY

Briefly, a new kind of washing arrangement for cellulose pulp is provided. The washing arrangement comprises a rotor with a number of pulp chambers that are transferred between different pulp handling zones as the rotor rotates in a stepwise manner. The washing arrangement is adapted for increasing the consistency of the pulp and then perform displacement washing. This is achieved by means of a piston device, provided at one of the pulp handling zones and arranged for piston movement in a radial direction with respect to the rotor.

More specifically, an arrangement for washing cellulose pulp comprising a pulp transport rotor and a number of pulp chambers is provided. Each pulp chamber is partly defined by a perforated surface of the pulp transport rotor, whereby filtrate is allowed to pass through the perforated surface when there is pulp in the pulp chamber. The pulp transport rotor is arranged to rotate stepwise so as to transfer the perforated surface of the respective pulp chambers to a subsequent one of a number of pulp handling zones in the washing arrangement. The washing arrangement further comprises a piston device arranged at one of the pulp handling zones, said piston device being arranged for piston movement in a radial direction with respect to the pulp transport rotor so as to, in operation and upon movement towards the pulp, increase the consistency of pulp in the pulp chamber currently located in that pulp handling zone, and means for displacement washing, at the pulp handling zone associated with the piston device, of the pulp with increased consistency using the piston device.

The means for displacement washing can for example be arranged for displacement washing at a pressure of 5-20OkPa on a pulp having an increased consistency of 10-30%. The proposed washing arrangement allows for displacement washing at high consistency and high pressure. The washing efficiency is high, and yet the washing arrangement is associated with a comparatively large capacity.

The means for displacement washing generally comprises means for wash liquid addition to the pulp handling zone associated with the piston device. The wash liquid can, for example, be added via (i.e. from inside) the piston device, for instance from a wash liquid compartment within the piston device and through a perforated end wall of the piston device. This results in even wash liquid distribution throughout the width of the pulp chamber, i.e. between its side walls, and hence efficient displacement washing.

The means for displacement washing can with advantage comprise means for piston movement of the piston device. Piston movement towards pulp in the pulp chamber then generally causes wash liquid to be pressed into the pulp, where it replaces dirty liquid. Thus, in an advantageous embodiment, piston movement is used both for increasing the pulp consistency and for displacement washing.

According to one example embodiment, the piston device is a double piston device comprising an inner piston that is arranged inside an outer piston and movable in relation to the outer piston. Efficient dewatering and displacement can be achieved by first moving the whole piston device and then pressing only the inner piston further towards the pulp. The piston need not be reversed/backed outwards from the pulp transport rotor before the wash liquid addition of the displacement phase. Hence, the double piston device provides an efficient and sophisticated means for pressing and displacement. Another advantage of the double piston device is that the pulp cake will be "locked", i.e. held tightly, by the outer piston during the displacement washing, which inter alia reduces the risk of air being mixed into the pulp.

As an alternative to the double piston device, it is also possible to provide the increased pulp consistency and displacement washing by means of a single piston device. An advantage of using a single piston device is that the piston itself can be of a rather simple design, comparatively easy and cheap to manufacture.

According to one example embodiment with a single piston device, the single piston is adapted for "back and forth movement", typically such that the single piston, after being moved towards the pulp (pulp consistency increase), is released from the pulp and backed a certain distance out from the rotor to allow wash liquid to enter the pulp chamber. Then, the single piston is again moved towards the rotor for the displacement to be effected. According to another example embodiment with a single piston device, the single piston is connected to an external pressure device, such as a pump, arranged to press wash liquid through the pulp in the pulp chamber. An advantage is that the pulp cake will be "locked", i.e. held tightly, by the single piston during the displacement washing, which inter alia reduces the risk of air being mixed into the pulp. Another advantage is that the pressure device embodiment offers a comparatively simple piston device design and straightforward functionality. According to another aspect of the present invention, a method for washing cellulose pulp is provided. According to still another aspect of the present invention, a piston device for use in an arrangement for washing cellulose pulp is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a schematic cross-sectional view of a washing arrangement according to an exemplifying embodiment of the present invention;

Figs. 2A to 2G illustrate various operational stages of a washing arrangement according to an exemplifying embodiment of the present invention;

Figs. 3A to 3E illustrate various operational stages of a washing arrangement according to another exemplifying embodiment of the present invention; Figs. 4A to 4E illustrate various operational stages of a washing arrangement according to still another exemplifying embodiment of the present invention; and

Fig. 5 is a schematic flow chart of a method for washing cellulose pulp in accordance with an exemplifying embodiment of the present invention.

DETAILED DESCRIPTION

In the drawings, similar or corresponding elements are denoted by the same reference numbers. For the purpose of this disclosure, "piston movement" will refer to movement of a piston within an outer structure. The piston is typically a closed, although possibly perforated, piston structure with a connecting element, such as a piston rod, at one end, through which motion is transmitted. The outer structure can for example be the pulp chamber or the outer piston described below. The piston is arranged to travel or slide within the outer structure in a back and forward linear manner.

Fig. 1 schematically illustrates an example washing arrangement in accordance with the present invention. The washing arrangement 100 comprises a rotating structure, referred to as pulp transport rotor 10, with four pulp chambers 12 arranged to rotate in steps of about 90 degrees when the washing arrangement 100 is in operation. As illustrated, the pulp chambers 12 would normally be arranged on the outer portion of the pulp transport rotor 10. Each pulp chamber 12 has a perforated surface 12a, typically substantially the entire inner wall of the pulp chamber 12, onto which a layer of pulp is typically deposited in the initial position of the pulp chamber.

Filtrate is allowed to flow through the perforated surface 12a in a generally radially inward direction when there is pulp in the pulp chamber 12. The filtrate is typically collected by an inner filtrate collection structure 16, and can for example be led to valve(s) at one or both ends of the rotor 10. The pulp chamber 12 is further restricted by two side walls 12b, formed by dividing members 10a, which in the example of Fig. 1 are part of the rotor 10.

The stepwise rotation R of the pulp transport rotor 10 transports the respective pulp chambers 12 between pulp handling zones Zl, Z2, Z3, Z4 in the washing arrangement. These zones are stationary and will each generally enclose one of the pulp chambers, i.e. the pulp chamber currently present in that zone (when the rotor is not moving), as well as possible pulp handling equipment (piston device, cleaning means, etc., see below).

At pulp handling zone Z2, there is a piston device 20 arranged to cooperate with the pulp chamber 12 currently in zone Z2. The piston device 20 is arranged for piston movement M in a radial direction with respect to the pulp transport rotor 10 so as to, in operation and upon movement towards the pulp, increase the consistency of the pulp in the pulp chamber 12. In the illustrated washing arrangement 100, there is also provided means for wash liquid addition to zone Z2 via the piston device 20, and piston movement M of the piston device 20 provides for displacement washing.

To achieve the cooperation between the piston device 20 and the pulp chamber 12, the piston device 20 may comprise a piston arranged to fit into the respective pulp chambers when they are in the pulp handling zone Z2 associated with the piston device. The piston device 20 typically comprises a piston with a width substantially corresponding to the width of the pulp chamber 12 between the side walls 12b. The piston device 20 typically comprises a pressing surface, facing the perforated surface 12a of the pulp chamber 12 and the shape and orientation of which substantially corresponds to that of the perforated surface 12a. In Fig. 1, the pressing surface is the outer surface of the piston end wall 22a. The pressing surface is movable and adapted for pressing pulp against the perforated surface 12a in a generally radially inward direction with respect to the pulp transport rotor 10, whereby the pulp is dewatered and its consistency increases. Advantageously, the piston device 20 extends along substantially the entire axial length of the pulp transport rotor 10. Thereby, the washing arrangement can be used efficiently, with a displacement and dewatering zone Z2 along substantially the entire rotor 10 (and pulp chambers 12). The piston device may be hydraulic, e.g. using oil or wash liquid as hydraulic liquid, but alternatives are possible.

The illustrated example piston device 20 is a double piston device comprising an outer piston 22 and an inner piston 24. The inner piston 24 is arranged inside the outer piston 22 and movable in relation to the outer piston 22. The outer piston 22 cooperates with the pulp chamber 12. It may, for example, be arranged to travel in the pulp chamber 12, at least for a part of its radial movement. The outer piston 22 can then with advantage be of a corresponding shape and slightly smaller than the pulp chamber 12 so as to slide within the pulp chamber. The inner piston 24, on the other hand, may be arranged to travel in the outer piston 22, suitably for its entire radial movement. The inner piston 24 can then with advantage be of a corresponding shape and slightly smaller than the inside of the outer piston 22 so as to form a sliding relationship with the outer piston.

The means for wash liquid addition via the piston device 20 can for example involve arrangements for wash liquid addition from an internal wash liquid compartment in the piston device. In the double piston device of Fig. 1, such a wash liquid compartment 26 is defined between an end wall 22a of the outer piston 22 facing the perforated surface 12a, and a corresponding end wall 24a of the inner piston 24. (The end wall 22a of the outer piston 22 is in Fig. 1 opposite to the rod/shaft 22b of the outer piston 22, and the end wall 24a of the inner piston 24 is in Fig. 1 opposite to the rod/shaft 24b of the inner piston 24 .) The end wall 22a of the outer piston 22 is perforated so as to allow wash liquid into the pulp chamber 12. Thus, in Fig. 1 the wash liquid is added to pulp handling zone Z2 via the piston device, i.e. from within the piston device itself, which in other words has a "built-in" wash liquid addition. Alternatively, the wash liquid may be added to the space between the end wall 22a and the perforated surface 12a from a line, or other suitable means in the washing arrangement, which is external to the double piston. In such a case, the end wall 22a (i.e. the pressing surface of the piston device) need not be perforated.

A person skilled in the art realizes that Fig. 1 is a schematic view in order to illustrate the basic structure and principles of the washing arrangement. For example, there will generally be a housing (not shown) enclosing the pulp transport rotor 10 and the piston device 20. In a particular advantageous embodiment the housing forms a controllable pressurized environment. Such a housing comprises a closed space, which makes it easy to control and to operate.

The washing arrangement of the present invention can be used at many different positions in the pulping line. In particular, it can for example take the place of conventional wash presses in order to improve the washing capacity and efficiency.

Example: double piston device

Figs. 2A to 2G illustrate various operational stages of a washing arrangement according to an exemplifying embodiment of the present invention. In this example, the respective pulp chambers 12 rotate in steps to a pulp input zone Zl, a displacement and dewatering zone Z2, a pulp removal zone Z3, and a rotor cleaning zone Z4 of the washing arrangement 100. The piston device 20 is arranged at the displacement and dewatering zone Z2. In the pulp input zone Zl, cellulose pulp 14 is fed to the pulp chamber 12 (Fig. 2A). The pulp

14 input to the washing arrangement 100 suitably has a consistency of 3-12%, preferably 6- 12%. After rotating one step, the pulp chamber arrives at the displacement and dewatering zone Z2 (Fig. 2B), and this is where the actual washing takes place. Firstly, the consistency of the pulp 14 is increased by piston movement, suitably to a consistency of 10-30%, and preferably to a consistency of 15-25%. This is, in the double piston embodiment, accomplished by moving the outer piston 22 in a radially inward direction with respect to the pulp transport rotor 10, see

Fig. 2C. The outer piston 22 travels within the pulp chamber 12, its end wall 22a being pressed towards the pulp 14 in the pulp chamber 12. Filtrate flows through the perforated surface 12a and is in the illustrated example collected by an inner filtrate collection structure 16 arranged to transport the filtrate through the rotor shaft to one or both ends of the pulp transport rotor 10. (Naturally, the movement of the outer piston 22 also causes the inner piston 24, which is arranged inside the outer piston 22, to move with respect to the pulp chamber 12. However, there is no piston movement of the inner piston 24 with respect to the outer piston 22.)

Referring to Fig. 2D, displacement washing is performed of the pulp at its increased pulp consistency. This is, in the double piston embodiment, accomplished by moving the inner piston 24 in a radially inward direction with respect to the pulp transport rotor 10. The inner piston 24 slides within the outer piston 22, and there is, in other words, piston movement of the inner piston 24 with respect to the outer piston 22. The piston movement of the inner piston 24 causes the wash liquid compartment (26 in Fig. 2C) to be compressed, whereby wash liquid is pressed through the end wall 22a of the outer piston 22 and into the pulp 14 in the pulp chamber 12. Dirty liquid in the pulp is replaced by the cleaner washing liquid and filtrate is led away from the pulp chamber 12 through the perforated surface 12a.

The displacement can for example take place at 5-200 kPa, and it is advantageous if the pressure is 50-20OkPa during the displacement phase. Thus, the displacement can be performed at high consistency combined with high pressure, which is a considerable advantage of the present invention, leading to an efficient displacement and hence an enhanced washing performance. As illustrated by Fig. 2E, the displacement may with advantage be followed by a further consistency increase by pressing the pulp to a desirable discharge consistency. This is, in the example double piston embodiment, accomplished by moving the outer piston 22 in a radially inward direction with respect to the pulp transport rotor 10. As the outer piston 22 travels further within the pulp chamber 12, its end wall 22a compresses the pulp 14 in the pulp chamber 12, causing more liquid to flow through the perforated surface 12a.

After rotating another step, the washed pulp 14 is removed from the pulp chamber 12 in pulp removal zone Z3 (Fig. 2F). The removed pulp is output from the washing arrangement 100 and transported to other process stages in the pulping line. The pulp 14 suitably has a discharge consistency of 10-40%, preferably 15-30%.

After rotating yet another step, to the rotor cleaning zone Z4, the perforated surface 12a of the pulp chamber 12 is cleaned (Fig. 2G). For this, a stationary spray means 30 may suitably be arranged at zone Z4. Other means for cleaning the perforated surface 12a may also be used.

In one example embodiment, the means for cleaning the perforated surface is arranged to use the filtrate passing through the perforated surface 12a as cleaning liquid, which provides for efficient liquid utilization. Another example embodiment has separate addition of cleaning liquid, e.g. from en external source. Nevertheless, it should be understood that rotor cleaning is optional and could be left out.

As an alternative to the double piston device, it is also possible to provide the increased pulp consistency and displacement washing by means of a single piston device. Such a device comprises a single piston with an end wall facing the perforated surface 12a of the pulp chamber 12. Examples of single piston device embodiments are provided in Fig. 3 and 4, respectively. Example: single piston with "back and forth" piston movement

Figs. 3A to 3E illustrate various operational stages of an example embodiment of the invention using a single piston device. An apostrophe (') is used to indicate particular elements, which are more or less different in design and/or function as compared to the elements of the previous embodiment. The pulp consistency (input consistency, consistency after increase, discharge consistency) and/or pressure during displacement can for example fall within the ranges mentioned above in connection with Fig. 2. The example piston device 20' of Fig. 3 is a single piston device comprising a single piston 22' arranged to cooperate with the pulp chamber 12. It may, for example, be arranged to travel in the pulp chamber 12, at least for a part of its radial movement. The single piston 22' can then with advantage be of a corresponding shape and slightly smaller than the pulp chamber 12 so as to slide within the pulp chamber.

In the pulp input zone Zl, cellulose pulp 14 is fed to the pulp chamber 12 (Fig. 3A). After rotating one step, the pulp chamber arrives at the displacement and dewatering zone Z2 (Fig. 3B), where the actual washing takes place. Firstly, the consistency of the pulp 14 is increased by piston movement. This is, in this single piston embodiment, accomplished by moving the single piston 22' in a radially inward direction with respect to the pulp transport rotor 10, see Fig. 3C. The piston 22' travels within the pulp chamber 12, its end wall 22a' being pressed towards the pulp in the pulp chamber 12. Filtrate flows through the perforated surface 12a to be collected by the inner filtrate collection structure 16.

Referring to Fig. 3D, displacement washing is performed of the pulp at its increased consistency. This is basically accomplished by a two stage procedure, as illustrated by the upper (I) and lower (II) view of Fig. 3D, respectively. Firstly, the single piston 22' is "backed", i.e. moved in a radially outward direction with respect to the pulp transport rotor 10, whereby wash liquid flows out through the perforated end wall 22a' (Fig. 3D (I)). This outward piston movement releases the single piston 22' from the pulp, i.e. prevents it from pressing hard against the pulp cake, which allows for wash liquid to enter a space between the end wall 22a' and the perforated surface 12a. After providing wash liquid to the pulp chamber 12 of zone Z2 in this way, the single piston 22' is moved towards the perforated surface 12a of the rotor 10 (Fig. 3D (H)). Thus, displacement is, in this single piston embodiment, accomplished by moving the single piston 22' in a radially inward direction with respect to the pulp transport rotor 10. As the single piston 22' slides within the pulp chamber 12, dirty liquid in the pulp is replaced by the cleaner washing liquid and filtrate is led away from the pulp chamber through the perforated surface 12a.

The wash liquid addition may, as illustrated, occur via the single piston 22', in which case said end wall 22a' is perforated so as to allow wash liquid to flow from an internal wash liquid compartment 26' of the single piston 22' and into the pulp chamber 12. Alternatively, the wash liquid can be added to the pulp chamber from outside the piston, i.e. from a line, or other suitable means in the washing arrangement 100, which is external to the single piston 22'. With external wash liquid addition, the end wall 22a' of the single piston 22' with its pressing surface need not be perforated.

As illustrated by Fig. 3E, the displacement may with advantage be followed by a further consistency increase by pressing the pulp to a desirable discharge consistency. This is, in this single piston embodiment, accomplished by moving the single piston 22' towards the pulp in a radial direction with respect to the pulp transport rotor 10. As the single piston 22' travels further within the pulp chamber 12, its end wall 22a' compresses the pulp in the pulp chamber

12, causing more liquid to flow through the perforated surface 12a.

The washing at the displacement and dewatering zone Z2 may with advantage be followed by pulp removal as described above with reference to Fig. 2F and/or rotor cleaning as described above with reference to Fig. 2G.

Example: single piston with pump wash liquid feed

Figs. 4A to 4E illustrate various operational stages of another example embodiment of the invention using a single piston device. A double apostrophe (") is used to indicate particular elements, which are more or less different in design and/or function as compared to the elements of the previous embodiments. The ranges for pulp consistency (input consistency, consistency after increase, discharge consistency) and/or pressure can for example be as described above in connection with Fig. 2. The example piston device 20" of Fig. 4 is a single piston device comprising a single piston 22" arranged to cooperate with the pulp chamber 12. It may, for example, be arranged to travel in the pulp chamber 12, at least for a part of its radial movement. The single piston 22" can then with advantage be of a corresponding shape and slightly smaller than the pulp chamber 12 so as to slide within the pulp chamber. The single piston device 20" is externally pressurized, for example by comprising or being connected to a pump 40 or the like, as illustrated in Fig. 4.

In the pulp input zone Zl, cellulose pulp 14 is fed to the pulp chamber 12 (Fig. 4A). After rotating one step, the pulp chamber arrives at the displacement and dewatering zone Z2 (Fig. 4B), where the actual washing takes place. Firstly, the consistency of the pulp 14 is increased by piston movement. This is, in this single piston embodiment, accomplished by moving the single piston 22" in a radially inward direction with respect to the pulp transport rotor 10, see Fig. 4C. The single piston 22" travels within the pulp chamber 12, its end wall 22a" being pressed towards the pulp in the pulp chamber 12. Filtrate flows through the perforated surface

12a to be collected by the inner filtrate collection structure 16.

Referring to Fig. 4D, displacement washing is performed of the pulp at its increased consistency. This is, in this single piston embodiment, accomplished by means of a wash liquid pump 40 or the like. The pump is arranged to, when being activated, pump wash liquid through the perforated end wall 22a" of the single piston 22" and further through the pulp in the pulp chamber 12. Dirty liquid in the pulp is replaced by the cleaner washing liquid and filtrate is led away from the pulp chamber through the perforated surface 12a. There may be embodiments where the single piston 22" can move, or slide, within the pulp chamber 12, in a radially inward direction with respect to the pulp transport rotor 10 also when the pump 40 is active.

In order to achieve efficient wash liquid distribution and displacement washing, the wash liquid addition with the pump 40 preferably occurs via the single piston 22", the end wall 22a" of which is then perforated so as to allow wash liquid to flow from a wash liquid compartment

26" inside the single piston 22" and into the pulp chamber 12.

As illustrated by Fig. 4E, the displacement may with advantage be followed by a further consistency increase by pressing the pulp to a desirable discharge consistency. This is, in this single piston embodiment, accomplished by moving the single piston 22" in a radially inward direction with respect to the pulp transport rotor 10. As the single piston 22" travels further within the pulp chamber 12, its end wall 22a" compresses the pulp in the pulp chamber 12, causing more liquid to flow through the perforated surface 12a. The washing at the displacement and dewatering zone Z2 may with advantage be followed by pulp removal as described above with reference to Fig. 2F and/or rotor cleaning as described above with reference to Fig. 2G. Fig. 5 is a schematic flow chart of a method, in accordance with an exemplifying embodiment of the present invention, for washing cellulose pulp in a washing arrangement of the above- described type with rotor and pulp chambers. The pulp transport rotor is rotated stepwise or intermittently such that the perforated surface of the respective pulp chambers is moved between different pulp handling zones (step Sl). In step S2, cellulose pulp is fed to one of the pulp chambers. Thereafter, consistency increase and displacement follows, advantageously due to pressing and displacement by piston movement in a radial direction with respect to the pulp transport rotor. More specifically, firstly the consistency of the pulp is increased in step S3 during movement of a piston device arranged at one of said pulp handling zones radially towards the pulp. This generally implies that the pulp is pressed with the piston device. The consistency of the pulp is typically increased more than one % unit, but it could be increased by far more than that depending on the application. Displacement washing of the pulp with increased consistency thereafter takes place in step S4 using the piston device. The displacement is generally effected by means of the piston device. Step S4 may for example comprise addition of wash liquid via, i.e. from inside, the piston device. Alternatively, wash liquid can be provided externally, i.e. from outside. Finally, the consistency of the pulp may be increased further by moving the piston device further towards the pulp (step S5). This generally involves pressing the pulp further with the piston device in order to obtain a desired discharge consistency. Step S5 is optional, as indicated by the dashed box. In step S6, the pulp is removed from the pulp chamber.

As described above, step S2, steps S3, S4, S5, and step S6, respectively, may with advantage be performed at different pulp handling zones in the washing arrangement, in which case the rotation step will be repeated before step S3 and before step S6. Moreover, a person skilled in the art realizes that the displacement step and the subsequent pressing may not always be truly isolated from each other. This means that the displacement step S4 may include some amount of pressing and/or the subsequent pressing S5 may include some displacement.

In an example embodiment using a double piston device, the step(s) S3, S5 of moving the piston device 20 and increase the consistency of the pulp involve moving the outer piston 22 towards the pulp, and the step S4 of displacement washing involves moving the inner piston 24 towards the pulp. In an example embodiment using a single piston device, the step(s) S3, S5 of moving the piston device 20' and increase the consistency of the pulp involve moving the single piston 22' towards the pulp 14, and the step S4 of displacement washing involves backing the single piston 22' for wash liquid to enter the pulp chamber 12 and thereafter moving the single piston 22' towards the pulp again.

In another example embodiment using a single piston device, the step(s) S3, S5 of moving the piston device 20" and increase the consistency of the pulp involve moving the single piston 22" towards the pulp 14, and the step S4 of displacement washing involves activating an external pressure device, such as a pump 40, to press wash liquid through the pulp 14 in the pulp chamber 12.

It is to be understood that the number of pulp chambers and/or the number of pulp handling zones may be varied within the scope of the invention. Accordingly, fewer than four as well as more than four pulp chambers and/or pulp handling zones are possible. However, it is suitable to have at least four pulp chambers in order to obtain an efficient and smooth-operated washing apparatus. The number of pulp handling zones may with advantage correspond to the number of pulp chambers. There may also be embodiments where the washing arrangement comprises two (or more) piston devices, each arranged at a respective associated pulp handling zone.

Furthermore, a person skilled in the art realizes that the invention is not limited to the rotation direction and orientation shown in the drawings. Washing arrangement embodiments with pulp transport rotors arranged to rotate counter-clockwise are of course also possible. So are embodiments that are turned in the sense that the pulp input zone Zl is not at the top. The washing arrangement is typically pressurized and there may be applications where it is suitable with vertically oriented washing arrangements. It would also be possible to use a piston device in accordance with the present invention on a displacement washer with a compartmentalized drum. The piston device could for instance be mounted on existing multistage drum washers or filters as an upgrade feature. The drum should then be adapted so as to rotate stepwise and this intermittent motion should be synchronized with the piston movement. Alternatively, the piston device could be arranged to travel along with the drum for a part of a revolution, in which case the drum can rotate continuously but the piston device need to be arranged for both rotational and radial movement with respect to the drum.

Although the invention has been described with reference to specific illustrated embodiments, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a man skilled in the art, and the scope of the invention is only limited by the appended claims.

Claims

1. An arrangement (100) for washing cellulose pulp comprising a pulp transport rotor (10) and a number of pulp chambers (12), each partly defined by a perforated surface (12a) of the pulp transport rotor, whereby filtrate is allowed to pass through the perforated surface when there is pulp in the pulp chamber,

the pulp transport rotor being arranged to rotate stepwise so as to transfer the perforated surface of the respective pulp chambers to a subsequent one of a number of pulp handling zones (Zl, Z2, Z3, Z4) in the washing arrangement, and

the washing arrangement further comprising

a piston device (20; 20'; 20") arranged at one of the pulp handling zones (Z2), said piston device being arranged for piston movement (M) in a radial direction with respect to the pulp transport rotor so as to, in operation and upon movement towards the pulp, increase the consistency of pulp in the pulp chamber currently located in that pulp handling zone, and

means for displacement washing, at the pulp handling zone (Z2) associated with the piston device, of the pulp with increased consistency using the piston device.

2. The arrangement of claim 1, wherein the means for displacement washing is adapted for displacement washing at a pressure of 5-20OkPa on a pulp having a consistency of 10-30%.

3. The arrangement of claim 1 or 2, wherein the means for displacement washing in turn comprises means for wash liquid addition, via the piston device (20; 20'; 20"), to the pulp chamber (12) currently located in the pulp handling zone (Z2) associated with the piston device.

4. The arrangement of any of previous claims, wherein the means for displacement washing comprises means for piston movement (M) of the piston device (20; 20'; 20").

5. The arrangement of any of previous claims, wherein the piston device (20) is a double piston device comprising an outer piston (22) and an inner piston (24), the inner piston being arranged inside the outer piston and being movable in relation to the outer piston.

6. The arrangement of claim 5, wherein the piston device (20) comprises a wash liquid compartment (26) defined between an end wall (22a) of the outer piston (22) facing the perforated surface (12a), and a corresponding end wall (24a) of the inner piston (24), the end wall (22a) of the outer piston (22) being perforated so as to allow wash liquid into the pulp chamber (12).

7. The arrangement of any of claims 1-4, wherein the piston device (20'; 20") is a single piston device comprising a single piston (22'; 22") with an end wall (22a'; 22a") facing the perforated surface (12a).

8. The arrangement of claim 7, wherein the single piston (22") is connected to an external pressure device (40) arranged to press wash liquid through the pulp in the pulp chamber (12).

9. The arrangement of claim 7 or 8, wherein the single piston (22'; 22") comprises a wash liquid compartment (26'; 26") and the end wall (22a'; 22a") is perforated so as to allow wash liquid from the wash liquid compartment into the pulp chamber (12).

10. The arrangement of any of previous claims, wherein the pulp transport rotor (10) and the piston device (20; 20'; 20") are enclosed by a housing forming a controllable pressurized environment.

11. The arrangement of any of previous claims, comprising means (30) for cleaning the perforated surface (12a) after pulp removal, said means (30) being arranged to use filtrate passing through the perforated surface (12a) as cleaning liquid.

12. The arrangement of any of previous claims, comprising at least four pulp chambers (12).

13. The arrangement of claim 12, comprising four pulp chambers (12) and being arranged to rotate in steps of about 90 degrees.

14. The arrangement of claim 13, wherein the pulp handling zones (Zl, Z2, Z3, Z4) include at least a pulp input zone (Zl), a displacement and dewatering zone (Z2), and a pulp removal o zone (Z3), the piston device (20; 20'; 20") being arranged at the displacement and dewatering zone (Z2).

15. A method for washing cellulose pulp in a washing arrangement (100) comprising a pulp transport rotor (10) and a number of pulp chambers (12), each partly defined by a perforated surface (12a) of the pulp transport rotor, whereby filtrate is allowed to pass through the perforated surface when there is pulp in the pulp chamber, comprising the steps of:

rotating the pulp transport rotor stepwise such that the perforated surface of the respective pulp chambers is moved between different pulp handling zones (Zl, Z2, Z3, Z4) in the washing arrangement;

feeding pulp (14) to one of the pulp chambers;

moving a piston device (20; 20'; 20") arranged at one of said pulp handling zones towards the pulp in a radial direction with respect to the pulp transport rotor, whereby the consistency of the pulp is increased;

displacement washing of the pulp with increased consistency using the piston device; and removing the washed pulp from the pulp chamber.

16. The method of claim 15, wherein the step of moving the piston device (20; 20'; 20") involves increasing the pulp consistency to 10-30% before the displacement washing.

17. The method of claim 15 or 16, wherein the step of displacement washing is performed at 5- 200 kPa pressure.

18. The method of any of claims 15-17, comprising the additional step of

moving the piston device (20; 20'; 20") further towards the pulp (14) in a radial direction with respect to the pulp transport rotor (10), after the displacement washing, whereby the consistency of the pulp is further increased.

19. The method of any of claims 15-18, wherein the piston device is a double piston device (20) comprising an outer piston (22) and an inner piston (24), the inner piston being arranged inside the outer piston and being movable in relation to the outer piston, and

the step(s) of moving the piston device (20) whereby the consistency of the pulp is increased involve moving the outer piston (22) towards the pulp (14), and the step of displacement washing involves moving the inner piston (24) towards the pulp.

20. The method of claim 19, wherein the step of displacement washing comprises adding wash liquid from a wash liquid compartment (26) in the piston device (20), whereby wash liquid flows through a perforated end wall (22a) of the outer piston (22) and to the pulp chamber

(12).

21. The method of any of claims 15-18, wherein the piston device is a single piston device (20') comprising a single piston (22') with an end wall (22a') facing the perforated surface (12a) and the step(s) of moving the piston device (20) whereby the consistency of the pulp is increased involve moving the single piston (22') towards the pulp (14), and

the step of displacement washing involves backing the single piston (22') for wash liquid to enter the pulp chamber (12) and thereafter moving the single piston (22') towards the pulp again.

22. The method of any of claims 15-18, wherein the piston device is a single piston device (20") comprising a single piston (22") with an end wall (22a") facing the perforated surface (12a) and

the step(s) of moving the piston device (20) whereby the consistency of the pulp is increased involve moving the single piston (22") towards the pulp (14), and

the step of displacement washing involves activating an external pressure device (40) to press wash liquid through the pulp in the pulp chamber.

23. The method of any of claims 21 and 22, wherein the step of displacement washing comprises adding wash liquid from a wash liquid compartment (26'; 26") in the piston device

(20'; 20"), whereby wash liquid flows through a perforated end wall (22a'; 22a") of the single piston (22'; 22") and to the pulp chamber (12).

24. The method of any of claims 15-23, further comprising the step of cleaning the perforated surface (12a), after the removing step, using filtrate from the washing arrangement (100).

25. The method of any of claims 15-24, wherein the rotating step involves rotating the respective pulp chambers (12) stepwise at least to a pulp input zone (Zl), a displacement and dewatering zone (Z2), and a pulp removal zone (Z3) in the washing arrangement (100), the piston device (20; 20'; 20") being arranged at the displacement and dewatering zone (Z2).

26. A piston device (20; 20'; 20") for use in an arrangement (100) for washing cellulose pulp, which arrangement comprises a pulp transport rotor (10) and a number of pulp chambers (12), each partly defined by a perforated surface (12a) of the pulp transport rotor, whereby filtrate is allowed to pass through the perforated surface when there is pulp in the pulp chamber, the pulp transport rotor being arranged to rotate stepwise so as to transfer the perforated surface of the respective pulp chambers to a subsequent one of a number of pulp handling zones (Zl, Z2, Z3, Z4) in the washing arrangement,

the piston device (20; 20'; 20") being arranged for piston movement (M) in a radial direction with respect to the pulp transport rotor so as to, in operation and upon movement towards the pulp (14), increase the consistency of pulp in the pulp chamber currently located in the pulp handling zone (Z2) associated with the piston device, and being adapted for displacement washing, at the pulp handling zone associated with the piston device, of the pulp with increased consistency.

27. The piston device (20) of claim 26, being a double piston device (20) comprising an outer piston (22) and an inner piston (24), the inner piston being arranged inside the outer piston and being movable in relation to the outer piston.

28. The piston device (20) of claim 27, comprising a wash liquid compartment (26) defined between an end wall (22a) of the outer piston (22) facing the perforated surface (12a), and a corresponding end wall (24a) of the inner piston (24), the end wall (22a) of the outer piston (22) being perforated so as to allow wash liquid into the pulp chamber (12).

29. The piston device (20'; 20") of claim 26, being a single piston device comprising a single piston (22'; 22") with an end wall (22a'; 22a") facing the perforated surface (12a).

30. The piston device (20") of claim 29, wherein the single piston (22") is connected to an external pressure device (40) arranged to press wash liquid through the pulp in the pulp chamber (12).

31. The piston device (20'; 20") of claim 29 or 30, wherein the single piston (22'; 22") comprises a wash liquid compartment (26'; 26") and the end wall (22a'; 22a") is perforated so as to allow wash liquid from a wash liquid compartment (26'; 26") into the pulp chamber (12).