EP2079872A1

EP2079872A1 - Press washer comprising separation of filtrates

Info

Publication number: EP2079872A1
Application number: EP07835351A
Authority: EP
Inventors: Rickard Andersson
Original assignee: Metso Paper Oy
Current assignee: Metso Paper Oy
Priority date: 2006-10-27
Filing date: 2007-10-23
Publication date: 2009-07-22
Also published as: WO2008051157A1; CA2667242A1; SE530282C2; CN101529016A; EP2079872A4; CN101529016B; SE0602268L

Abstract

The invention relates to a washing arrangement (100) comprising two co-operating cylindrical press rolls (102), at least one having a perforated outer surface (104). There is a guide surface (106) at a distance from the perforated outer surface partially enclosing the press roll in the circumferential direction, and a pulp passage (108) is provided between the perforated outer surface and the guide surface. During operation, pulp fed into the pulp passage is guided in direction of rotation and pressed between the press rolls. Filtrate passes through the perforated surface and into the press roll. It is proposed to provide the washing arrangement with means for separate collection of a filtrate flow (128) from a first portion of the pulp passage. Such filtrate contains comparatively high amounts of fiber and can by means of the invention be handled separately from the main filtrate flow (130), reducing the negative effects associated with fibers in the filtrate.

Description

Press washer comprising separation of filtrates

TECHNICAL FIELD

The present invention relates to washing of cellulose pulp and in particular to press washing with two co-operating cylindrical press rolls,

BACKGROUND

Pulp washing is a key operation in the chemical pulping line. There are many different types of washers available, some of which are based on press washing and comprise means for pressing the pulp such that fluid is removed. After pressing, the pulp is normally diluted to a desired thickness.

A well-known type of press washer has two co-operating cylindrical press rolls, arranged with their rotation center in the same horizontal plane. The outer surface of each press roll is perforated and, during operation, cellulose pulp is input to a restricted space between the perforated roll surface and a restriction member, such as a trough, whereby a pulp web is formed on the perforated roll surface. The press rolls are arranged to rotate in opposite directions so as to transport the respective pulp webs in the direction of rotation to be pressed in a so-called pinch or nip where the distance between the press rolls is smallest.

The fluid removed from the pulp (i.e. the filtrate) passes through the perforated roll surface in a radially inward direction and can for example be transported to the ends of the respective press roll by means of axial filtrate channels. There is normally a filtrate tank arranged in connection with the press washer to collect all filtrate resulting from the washing in the press.

There is often a supply of washing liquid to the press washer and since the washing liquid displaces fluid in the pulp, the washing principle will in such a case be a combination of dewatering, displacement and pressing.

A press washer of the described general type is disclosed in US Patent 3,980,518, for example. A press washer outputs considerable amounts of filtrate and in line with the general aim for closed cellulose pulp treatment systems, the filtrate is normally to as great extent as possible returned to previous process stages for dilution. However, the handling of the filtrate can be rather problematic, in particular since the filtrate often contains high amounts of fiber. Recirculation of filtrate to previus process stages may lead to problems with clogging and blocking of certain parts of the equipment. External equipment, such as fiber filters, is sometimes required to recover the fibers if the filtrate is to be purged to external treatment or to a recipient.

Accordingly, there is a need for appropriate mechanisms reducing the problems associated with filtrate produced by press washers.

SUMMARY

A general object of the invention is to provide an improved arrangement for washing cellulose pulp. A specific object is to achieve improved filtrate handling in a press washer with cylindrical press rolls. Another object is to reduce the fiber content in the output flow of filtrate from the press washer. Still other objects are to enable press washers associated with higher capacity and/or less complicated fiber recovery mechanisms.

These objects are achieved in accordance with the attached claims.

Briefly, the present invention is based on the recognition that a majority of all fibers that end up in the filtrate come from a quite small flow and from an area relatively close to point where incoming pulp meets a clean press roll surface. It is proposed to collect that filtrate separately, and thereby lower the fiber level in the main filtrate flow. In this way, the negative effects associated with fibers in the filtrate can be avoided or reduced.

In accordance with the invention is thus provided an arrangement for washing cellulose pulp comprising two press rolls, arranged to rotate in opposite directions during operation and at least one having a perforated outer surface; a guide surface, arranged at a distance from the perforated outer surface of the at least one press roll so as to partially enclose the press roll in the circumferential direction, a pulp passage being provided substantially between the perforated outer surface and the guide surface, and, during operation, pulp fed into the pulp passage is transported in direction of rotation and pressed in a pinch between the press rolls at the end of the pulp passage; and a new kind of filtrate collection means, arranged to collect filtrate passing through the perforated outer surface and into the press roll. The proposed means for filtrate collection comprises a first device for filtrate collection arranged to receive a main filtrate flow; and a second device for filtrate collection arranged to receive filtrate from a first portion of the pulp passage, whereby filtrate associated with the first portion of the pulp passage can be handled separately from the main filtrate flow. Typically, the filtrate associated with the first portion of the pulp passage is a high fiber content filtrate and the second device for filtrate collection enables this filtrate to be led to a process stage suitable for high fiber content.

In one embodiment, the washing arrangement further comprises means for separate output, from the washing arrangement, of filtrate collected by the second filtrate collection device (or by more than one second filtrate collection device), the separate output means being connectable for transport to suitable external process stages. Such separate output means could be implemented as one filtrate outlet or, alternatively, as two or more separate filtrate outlets.

Preferably, the first portion of the pulp passage extends from the beginning of the pulp passage and has a length, in the circumferential direction, of up to 40 % of the total length of the pulp passage.

The proposed washing arrangement leads to a number of advantages, including:

• Reduced fiber content in the filtrate

• Possibility to reduce the size of equipment recovering fibers from filtrates • Possibility to increase the open area of the press rolls (giving higher capacity) without suffering from increased fiber losses

• Possibility to increase the diameter of the holes in the press rolls (giving higher capacity and less expensive manufacturing) without suffering from increased fiber losses

According to another aspect of the invention, a system for treatment of cellulose pulp is provided. BREIF 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 transverse cross-section of an exemplifying washing arrangement in which the present invention may be used;

Fig. 2 is a perspective view illustrating the principle of operation of the two co-operating press rolls of the washing arrangement in Fig. 1 ;

Fig. 3 is a schematic cross-sectional end view of a conventional washing arrangement with filtrate collection means;

Fig. 4 is a schematic cross- sectional end view of a washing arrangement with filtrate collection means according to an exemplifying embodiment of the present invention;

Fig. 5 is a schematic side view of a washing arrangement with filtrate collection means according to an exemplifying embodiment of the present invention;

Fig. 6 illustrates a washing arrangement with filtrate collection means according to another exemplifying embodiment of the present invention;

Fig. 7 illustrates a washing arrangement with filtrate collection means according to still another exemplifying embodiment of the present invention;

Fig. 8 is a schematic block diagram of a system for treatment of cellulose pulp according to an exemplifying embodiment of the present invention; and

Fig. 9 is a schematic block diagram of a system for treatment of cellulose pulp according to another exemplifying embodiment of the present invention. DETAILED DESCRIPTION

In the drawings, similar or corresponding elements will be denoted by the same reference numbers.

Fig. 1 illustrates an exemplifying washing arrangement in which the invention may be used. The washing arrangement 100 is of the general type described in the background section and comprises two co-operating cylindrical press rolls/drums 102 inside a casing 103. The two press rolls 102 are arranged to rotate in opposite directions during operation (as indicated by the arrows) and each has a perforated outer surface 104, such as a surface of perforated metal sheet.

The washing arrangement 100 further presents guide surfaces 106, arranged at a distance from the perforated outer surface 104 of the respective press roll 102 so as to partially enclose the press roll in the circumferential direction, whereby a pulp passage 108 is defined between the perforated outer surface 104 and the guide surface 106.

During operation, pulp fed from pulp inlet 109 into the pulp passage 108 is guided by the guide surface 106 in direction of rotation and pressed in so-called pinch or nip 110 between the press rolls 102. In the illustrated example, pulp is input at the upper portion of the respective press roll 102 to enter the pulp passage 108, where it forms a pulp web 112 on the perforated roll surface 104. The press rolls 102 rotate in opposite directions so as to transport the respective pulp web

112 in the direction of rotation to be pressed in the pinch 110 where the distance between the press rolls 102 is smallest. The illustrated press rolls 102 comprise axial filtrate channels 114 which receive the filtrate that passes through the perforated roll surface 104. Washing liquid is in this example supplied to the pulp web at two different points (lines 116) per press roll 102. Pulp is output by means of a screw arrangement 118 which transfers the pulp to a subsequent process stage (not shown), such as a standpipe or another unit where the pulp may be diluted and processed.

Fig. 2 illustrates the principle of operation of the two co-operating press rolls 102 of a washing arrangement, such as the one in Fig. 1 for example. The perforated outer surface 104 of the rolls is partially cut off in order to illustrate the movement of the filtrate. (The actual perforating with holes/openings in the perforated outer surface 104 has been left out in the simplified view of Fig. 2.) Filtrate is transported through the perforated outer surface 104 and radially inwards into the axial filtrate channels 114 of the press roll 102, and thereafter, as illustrated by arrows, the filtrate moves towards the ends 120 of the press rolls 102, where it is output from the press rolls 102. The directions of rotation of the press rolls 102 are also indicated in Fig. 2. The rotation centers are normally arranged in the same horizontal plane and it may often be preferred that one rotation center is fixed, whereas the other rotation center is allowed to slide somewhat.

Measurements have been performed on the filtrate produced by a washing arrangement based on the described principle of operation. In Table 1 , the filtrate content (mg/liter) at different positions A₅ B, .., E in the circumferential direction is presented for two trials. Positions A, B,

.., E corresponds to about 20, 75, 130, 200 and 250 degrees from the pulp inlets, in direction of rotation, for a press washer with the pulp inlets close to the uppermost part of the press rolls. In Trial 1, the average fiber in the filtrate was 496 mg/liter, the feed consistency 5.2%, and the feed flow 200 m³/h. In Trial 2, the average fiber in the filtrate was 131 mg/liter, the feed consistency 6.0%, and the feed flow 120 m³/h.

Table 1

It was found that a filtrate flow from the first portion of the pulp passage, typically a rather small filtrate flow, held an unexpectedly high amount of fiber. Based on this understanding, the present invention suggests an arrangement which is adapted for collecting this high fiber content filtrate separately.

The principle of the invention will now be explained by reference to Fig. 3 and 4. Fig. 3 illustrates a conventional washing arrangement 100 with filtrate collection means. Filtrate from all axial filtrate channels 114 flows down into one and the same collecting vessel 122 as the filtrate reaches the end of the press roll 102. The filtrate is generally led from the collecting vessel 122 to an external filtrate tank (not shown) as the filtrate in the collection vessel 122 reaches a predetermined level. Fig. 4 illustrates a washing arrangement 100 with filtrate collection means according to an exemplifying embodiment of the invention. Here, filtrate from the first portion of the pulp passage 108 is collected separately. There is a first device 124 for filtrate collection, arranged to receive the main filtrate flow, but also a second device 126 for filtrate collection arranged to receive filtrate from a first portion of the pulp passage 108. The second device 126 for filtrate collection will in a washing arrangement 100 like the one in Fig. 4 be adapted for collecting filtrate transported to at least one end of the press roll by a subset of the axial filtrate channels 114. There will be at least two filtrate output flows 128, 130 from the washing arrangement, one for the main filtrate flow and one for the separately collected high fiber content filtrate. Thus, there is a separate output of filtrate with a comparatively high fiber content from the washing arrangement, hi the example of Fig. 4, two output flows 128 with high fiber content filtrate are disclosed, one from each trough 126. The devices 124, 126 for filtrate collection in accordance with the invention enable filtrate associated with the first portion of the pulp passage 108 to be handled separately from the main filtrate flow 130.

In accordance with the present invention filtrate from a first and a second part of the pulp passage, as seen in the circumferential direction, are collected separately. This implies that there will be separate filtrate flows, which originate from the same pulp passage and the "same" pulp. The filtrate separation by means of the present invention is hence a filtrate separation in the circumferential direction.

In the circumferential direction, the pulp passage 108 extends from the position or area where pulp is introduced onto the outer surface 104 of the press roll 102 and to the pinch 110 between the press rolls. The first portion of the pulp passage is the portion of the pulp passage 108 furthest away from the pinch 110 between the press rolls 102. In other words, "first" refers to the pulp passage portion where pulp first meets a substantially clean metal sheet surface of the press roll 102. For a washing arrangement 100 with a pulp inlet 109 arranged at the upper half of the at least one press roll (with respect to the rotation center) in direct connection with the pulp passage 108, such as in Fig. 4, the first portion of the pulp passage 108 is positioned near (in the vicinity of) the pulp inlet 109.

Furthermore, expressions like main filtrate portion, main filtrate flow, etc are for the purpose of this disclosure referring to the filtrate not collected separately by the second filtrate collection device 126, i.e. the remaining filtrate from the pulp passage as the filtrate from the first pulp passage portion has been removed. This filtrate will in general correspond to the filtrate flow that in prior art systems is transferred to the filtrate tank and handled as one volume irrespective of origin or content. The main filtrate flow 130 is normally considerably larger than the flow 128 of filtrate from the first pulp passage portion. The relationship between separately collected flow

128 and main flow 130 can for example be in the order of 1 :4 or 1 :5, but both smaller and larger differences between the flows are possible.

The first portion of the pulp passage extends from the beginning of the pulp passage as seen in direction of rotation of the press roll, i.e. from a point in the area where pulp is introduced onto the perforated outer surface of the press roll. The first portion of the pulp passage typically has a length, in the circumferential direction, of up to 40 % of the total length of the pulp passage 108.

In other words, the second filtrate collection device 126 is typically arranged to receive filtrate from a pulp passage portion extending from a position close to the point were the pulp first gets in contact with the clean surface of the press roll 102 to a position at x % of the pulp passage 108 in the circumferential direction, 0 < x ≤ 40. In a preferred embodiment the length of the first portion of the pulp passage is up to 30 % of the total length of the pulp passage.

For a washing arrangement 100 with a pulp inlet 109 at the beginning of the pulp passage 108 (typically at the upper part of the press roll 102), the first portion of the pulp passage 108 may for example cover a sector corresponding to up to x % of an angle defined by the lines intersecting the rotation center and the pinch 110 and the pulp inlet 109, respectively, 0 < x < 40 (preferably 0 < x < 30).

Fig. 5 shows a side view of a washing arrangement 100 with filtrate collection means according to an exemplifying embodiment of the invention, for example corresponding to Fig. 4. The perforated surface 104 (perforating not shown) has been partly removed for illustrative purposes. This view shows that the principle of the invention may be implemented by a primary trough 124 arranged to receive the main filtrate flow and secondary troughs 126 arranged to receive filtrate from the first portion of the pulp passage 108 before it falls into the primary trough 124. The secondary troughs 126 are thus arranged in the vicinity of the first portion of the pulp passage 108, whereas the primary trough 124 catches filtrate from the remaining portion of the pulp passage 108. Suitably, for press rolls 102 where filtrate is output at both ends 120, there is one pair of secondary troughs 126 per roll 102. However, embodiments where all filtrate is led to one and the same end 120, for example, are also possible and then one secondary trough 126 is obviously sufficient. Moreover, in the example of Fig. 5 the primary trough 124 extends along the entire length of the roll(s) 102 without interruptions. The skilled person recognizes that also variants with two shorter troughs arranged at the respective ends of the roll are possible. Also, it should be emphasized that implementations which use other kinds of filtrate collection structures instead of troughs, shoes, baffle walls and the like also lie within the scope of the invention. The filtrate collection device can for example comprise sealed valves arranged at one or both ends of the press rolls. Irrespective of which filtrate collection device is used, it will normally be advisable to arrange sealing means between the actual filtrate collection structure and the area where filtrate is output at the end(s) of the press roll.

In the above-described washing arrangements (Fig. 4 and 5), both press rolls 102 have perforated outer surfaces 104 and respective pulp passages 108 and the second device 126 for filtrate collection is arranged to collect filtrate from the first portion of each of the two pulp passages. Fig. 6 illustrates a washing arrangement 100 with filtrate collection means according to another exemplifying embodiment of the invention. In this case, one of the press rolls 102-1 has an perforated outer surface 104, e.g. a perforated metal sheet, whereas the other press roll

102-2 has a continuous non-perforated surface. Pulp is input to a pulp passage 108 associated with the perforated roll 102-1 and pressed between the perforated and the non-perforated roll 102-1, 102-2 at the pinch 110. A device 126 for separate collection of the filtrate from the first position of the pulp passage 108 is arranged close to the end of the perforated roll 102-1.

Fig. 6 also illustrates the fact that the press rolls 102 of a washing arrangement 100 according to the present invention need not be of the same size. Rolls of different diameters and/or lengths may be used, although equally large rolls are normally used.

The washing arrangement 100 of Fig. 6 further has a pulp passage 108 of different length as compared to the one in Fig. 4. The present invention is applicable e.g. to pulp passages extending to the top of the roll, to pulp passages enclosing about 90 degrees less of the roll, and to pulp passage lengths therebetween. Fig. 7 illustrates a washing arrangement 100 with filtrate collection means according to still another exemplifying embodiment of the invention. Here, the pulp inlet 109 is arranged at the bottom of the press washer 100, in the area beneath the pinch 110 formed by the co-operating press rolls 102. The pulp thus enters the pulp passage 108 in a different manner and the guide surface 106 has a different appearance than in the previous embodiments. However, the pulp passage 108 is still a space where pulp, during operation, is transported in the direction of rotation of the press roll 102 substantially between the perforated outer roll surface 104 and the guide surface 106 and further to the pinch 110. The pulp passage 108 extends from where the pulp meets a substantially clean press roll surface to the pinch 110. The device 126 for collecting filtrate with high fiber content is arranged so as to receive filtrate from a first portion of the pulp passage ("first" referring to the first pulp passage portion that meets a free roll surface). A washing arrangement 100 like the one in Fig. 7 typically has no addition of washing liquid. In other words, the press 100 may operate entirely based on dewatering and pressing processes.

The embodiments in Fig. 4-7 are simplified, schematic illustrations. For instance, the pulp outlet is not illustrated. The pulp may be output from the washing arrangement e.g. by means of a screw arrangement, such as in Fig. 1, but other solutions are also possible.

Fig. 8 illustrates a system 1000 for treatment of cellulose pulp according to an exemplifying embodiment of the invention, which includes washing arrangements 100-1, 100-2, stand pipes 200-1, 200-2, a filtrate tank 300, and a number of pumping devices 210, 310. The washer 100- 2 is arranged to output, at one hand, a main filtrate flow transported by line/pipe 315 and, at the other hand, a filtrate flow separately collected from a first portion of the pulp passage and transported by represented by line/pipe 305. The main filtrate flow 315 is returned to washer 100-1 for use as wash liquor, whereas the separated filtrate flow 305 is handled differently. Preferably, the system 1000 is provided with means for recirculating that filtrate such that a major portion thereof is led to a previous process stage in the system capable of handling high fiber contents, said previous process stage being separate from the process stage(s) (the washer

100-1 in Fig. 8) which receives a major portion of the main filtrate flow 315. In the illustrated example, the previous process stage includes a standpipe 200-1 and the means for recirculating is arranged to transport filtrate collected by the second device for filtrate collection to the standpipe 200-1 as dilution liquid.

By separating high fiber content filtrate and leading it to the standpipe 200-1, the fibers will to a large extent be returned into the main fiber flow. Moreover, the main filtrate flow 315 used as wash liquor in the illustrated example can have a comparatively low fiber content. This is advantageous since problems with clogging and blocking can be avoided in the parts of the washer 100-1 receiving the wash liquor.

As illustrated in Fig. 8, the means for recirculating the separately collected filtrate can for example be based on a pump 310-1 (e.g. a dilution pump) arranged in the filtrate tank 300. The filtrate tank 300 is in such a case preferably arranged to receive substantially all filtrate collected in the washer 100-2 and provided with a pipe 305 for the filtrate collected by the second device for filtrate collection. The pipe 305 is arranged with its output end in the vicinity of a feeding region to a pump 310-1 of the filtrate tank 300, whereby said pump 310-1 recirculates mainly high fiber content filtrate to a predetermined process stage.

Embodiments leading the high fiber content filtrate to a pump 310-1 in the filtrate tank are easy to implement. In general, an existing system will already have a dilution pump in association with which appropriate standard components (pipes, connectors, etc) are arranged so as to output the high content filtrate flow in the feeding region of the pump. However, even though there are advantages associated with the described arrangement with filtrate tank 300 and pump 310-1, it should be understood that also other arrangements for transporting the collected high fiber content filtrate are possible. For example, the system may be provided with a separate tank and pump for the "new" high fiber content filtrate flow.

Fig. 9 illustrates a system 1000 for treatment of cellulose pulp according to another exemplifying embodiment of the invention. The system is provided with means 400 for recovering fiber from the filtrate collected by the second device for filtrate collection. The filtrate flow of line/connection 305 is lead to the fiber recovery unit 400, directly or via the filtrate tank

300 as in Fig. 9, and after the processing at the recovery unit 400, fiber and washing liquor are output as separate flows. Still referring to Fig. 9, the example arrangement with filtrate tank 300 and pump 310-1 is the same as the one described in connection with Fig. 8. Also this system could of course comprise other arrangements for transporting the collected high fiber content filtrate to the predetermined process stage, such as a separate tank and pump for the separately collected high fiber content filtrate.

Although the invention has be 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. Thus, the scope of the invention is only limited by the appended claims.

Claims

1. An arrangement (100) for washing cellulose pulp comprising: two press rolls (102), arranged to rotate in opposite directions during operation and at least one having a perforated outer surface ( 104); a guide surface (106), arranged at a distance from the perforated outer surface of the at least one press roll so as to partially enclose the press roll in the circumferential direction, a pulp passage (108) being provided substantially between the perforated outer surface and the guide surface, and, during operation, pulp fed into the pulp passage is transported in direction of rotation and pressed in a pinch (110) between the press rolls at the end of the pulp passage; and means for filtrate collection, arranged to collect filtrate passing through the perforated outer surface and into the press roll, characterized in that the means for filtrate collection in turn comprises: a first device (124) for filtrate collection arranged to receive a main filtrate flow; and a second device (126) for filtrate collection arranged to receive filtrate from a first portion of the pulp passage, whereby filtrate associated with the first portion of the pulp passage can be handled separately from the main filtrate flow, and where the first portion of the pulp passage (108) extends from the beginning of the pulp passage and, in the circumferential direction, has a length of up to 40 % of the total length of the pulp passage (108).

2. The washing arrangement of claim 1, characterized in that the filtrate associated with the first portion of the pulp passage (108) is a high fiber content filtrate and in that the second device (126) for filtrate collection enables this filtrate to be led to a process stage suitable for high fiber content.

3. The washing arrangement of any of claim 1 or 2, characterized by means for separate output, from the washing arrangement, of filtrate collected by the second filtrate collection device, the separate output means being connectable for transport to suitable external process stages.

4. The washing arrangement of any of previous claims, characterized in that the first portion of the pulp passage (108) extends from the beginning of the pulp passage and, in the circumferential direction, has a length of up to 30 % of the total length of the pulp passage (108).

5. The washing arrangement of any of previous claims, characterized in that the at least one press roll (102) has axial filtrate channels (114) arranged to transport the filtrate passing through the perforated outer surface (104) to at least one end (120) of the press roll and that the second device (126) for filtrate collection is adapted for collecting filtrate transported to the at least one end by a subset of the axial filtrate channels.

6. The washing arrangement of any of previous claims, characterized by a pulp inlet (109) arranged at the upper half of the at least one press roll (102) with respect to the rotation center, whereby the pulp passage (108), in the circumferential direction, extends from a point near the pulp inlet to a point near the pinch (110) between the press rolls.

7. The washing arrangement of any of previous claims, characterized in that both press rolls (102) have perforated outer surfaces (104) and respective pulp passages (108) and in that the second device (126) for filtrate collection is arranged to collect filtrate from the first portion of each of the two pulp passages.

8. The washing arrangement of any of previous claims, characterized in that the first filtrate collection device (124) comprises a primary trough arranged to receive the main filtrate flow (130) and the second filtrate collection device (126) comprises at least one secondary trough arranged to receive filtrate from the first portion of the pulp passage before it falls into the primary trough.

9. A system (1000) for treatment of cellulose pulp comprising the washing arrangement (100) of any of previous claims.

10. The system of claim 9, characterized by means for recirculating the filtrate collected by the second device (126) for filtrate collection such that a major portion thereof is led to a previous process stage in the system capable of handling high fiber contents, said previous process stage being separate from the process stage(s) which receives a major portion of the main filtrate flow (130).

11. The system of claim 10, characterized in that the previous process stage includes a standpipe (200) and the means for recirculating is arranged to transport filtrate collected by the second device (126) for filtrate collection to the standpipe as dilution liquid.

12. The system of claim 9, characterized by means (400) for recovering fiber from the filtrate collected by the second device (126) for filtrate collection.

13. The system of any of claims 9-12, characterized by further comprising a filtrate tank (300) arranged to receive substantially all filtrate collected in the washing arrangement (100-2) and provided with a pipe (305) for the filtrate collected by the second device (126) for filtrate collection, said pipe being arranged with its output end in the vicinity of a feeding region to a pump (310-1) of the filtrate tank, whereby said pump recirculates mainly high fiber content filtrate to a predetermined process stage (200-1 ; 400).