System and method for treating delignified cellulose pulp

Abstract

System for treating delignified cellulose pulp comprising: - first and second consecutive washing arrangements (7a, 7b) configured to receive and wash said delignified cellulose pulp; - an acidic bleaching arrangement (13) arranged to receive cellulose pulp from the second washing arrangement (7b); - a post-acidic bleaching washing arrangement (14) connected to the acidic bleaching arrangement to wash cellulose pulp received therefrom; - an alkaline bleaching arrangement (15) arranged downstream of the washing arrangement (14); - a post-alkaline bleaching washing arrangement (16) connected to the alkaline bleaching arrangement to wash cellulose pulp received therefrom; - a filtrate feeding arrangement (19) arranged to recirculate spent washing liquid from the post-alkaline bleaching washing arrangement as dilution liquid between the first and second washing arrangements (7a, 7b) and / or as washing liquid to the first washing arrangement (7a), and - a source (11) of water and / or condensate connected to said second washing arrangement (7b) to provide washing liquid.

Description

TECHNICAL FIELD

[0001] The invention relates to the field of producing cellulose pulp. More specifically, the invention relates to improvements in the post-delignification treatment of the cellulose pulp (washing and bleaching of delignified pulp).BACKGROUND

[0002] Methods for producing cellulose pulp are well known in the art and typically comprises impregnating lignocellulosic biomass material such as wood chips with cooking liquor and cooking the impregnated lignocellulosic biomass material (also referred to as a digestion) to produce cellulose pulp. The cellulose pulp is thereafter washed in a (brown stock) washing arrangement (for example one or more wash press devices). Thereafter, the cellulose pulp is (further) delignified using for example oxygen delignification, and is then subjected to post-delignification washing, usually in at least two consecutive washing stages. After post-delignification washing, the cellulose pulp may be bleached using for example acidic and / or alkaline bleaching stages. The cellulose pulp is also screened using one or more screening arrangements, usually before the brown stock washing arrangement or after delignification (before post-delignification washing).

[0003] Spent chemicals and dissolved substances from cooking and delignification are efficiently handled today by recirculating the spent washing liquid / filtrates upstream in the process and using a recovery boiler. Acidic and alkaline filtrates from the bleaching process are traditionally discharged as effluents. Due to increased environmental concerns, it has been suggested to recirculate the alkaline filtrate as wash liquor in the last post-delignification washing stage to close the alkaline loop. Although this reduces effluent flow by 50-60%, there are several problems associated with such recirculation. For example, the alkaline filtrate increases the pH from the post-delignification washing to the first bleaching stage which, in the case of the first bleaching stage being an acidic bleaching stage, increases the consumption of acid and bleaching chemicals. Another problem is that the extractive and organic substances dissolved in the alkaline bleaching stage are recirculated back to the bleaching process. Thus, the reduction in COD is substantially less than the reduction in effluent flow.SUMMARY

[0004] An object of the invention is to solve or improve on at least some of the problems mentioned above in the background section.

[0005] These and other objects are achieved by the present invention by means of a system and a method according to the independent claims.

[0006] According to a first aspect of the invention, there is provided a system for treating delignified cellulose pulp comprising: a first washing arrangement configured to receive and wash said delignified cellulose pulp; a second washing arrangement connected to the first washing arrangement to wash cellulose pulp received therefrom; an acidic bleaching arrangement arranged to receive cellulose pulp from the second washing arrangement, said acidic bleaching arrangement comprising at least one acidic stage such as at least one acidic bleaching stage; a post-acidic bleaching washing arrangement connected to the acidic bleaching arrangement to wash cellulose pulp received therefrom; an alkaline bleaching arrangement arranged downstream of the post-acidic bleaching washing arrangement to subject the cellulose pulp to alkaline bleaching; a post-alkaline bleaching washing arrangement connected to the alkaline bleaching arrangement to wash cellulose pulp received therefrom; a filtrate feeding arrangement arranged to recirculate spent washing liquid from the post-alkaline bleaching washing arrangement as dilution liquid between the first washing arrangement and the second washing arrangement and / or as washing liquid to the first washing arrangement, and a source of water and / or condensate connected to said second washing arrangement to provide said water and / or condensate as washing liquid.

[0007] In other words, delignified cellulose pulp is supplied to a first washing arrangement. It is understood that delignified cellulose pulp refers to cellulose pulp which has somehow been subjected to delignification, i.e. the delignified pulp can be supplied from a delignification arrangement such as an oxygen delignification or directly from a digester / cooking arrangement. Thereafter, the cellulose pulp is fed to a second washing arrangement. The first and second washing arrangements may together be referred to as a post-delignification or post-oxygen washing arrangement. Thereafter, the washed cellulose pulp is fed to an acidic bleaching arrangement which comprises at least one acidic stage. It is understood that the acidic bleaching arrangement may comprise not only one or more acidic bleaching stages but also one or more acidic stages such as A- or Q-stage(s), where A-stage refers to an acidic stage and Q-stage refers to a chelating stage. Thereafter, the bleached cellulose pulp is fed to a post-acidic bleaching washing arrangement to wash the cellulose pulp. An alkaline bleaching arrangement is arranged downstream of the post-acidic bleaching washing arrangement to subject the cellulose pulp to alkaline bleaching. It is understood that the alkaline bleaching arrangement is arranged downstream of the post-acidic bleaching washing arrangement in the sense that there may be additional intermediate arrangements / devices between the post-acidic bleaching washing arrangement and the alkaline bleaching arrangement, which may include additional alkaline bleaching arrangements. Put differently, the alkaline bleaching arrangement does not necessarily need to form the second bleaching stage, but can be the third, fourth etc. bleaching stage. After the alkaline bleaching arrangement, the cellulose pulp is fed to a post-alkaline bleaching washing arrangement. A filtrate feeding arrangement is arranged to recirculate spent washing liquid (alkaline filtrate) from the post-alkaline bleaching washing arrangement as dilution liquid to the post-delignification washing arrangement upstream of the second washing arrangement, i.e. between the first washing arrangement and the second washing arrangement and / or as washing liquid to the first washing arrangement. Since the alkaline filtrate is not supplied as washing liquid to the second washing arrangement, a source of water and / or condensate connected to said second washing arrangement to provide water and / or condensate as washing liquid. The filtrate feeding arrangement may be arranged to recirculate all of the spent washing liquid (the whole flow) or only a portion thereof (such as 90%, 80, 70, 60, 50, 40, 30, 20, 10%, or any other portion of the flow of spent washing liquid). Recirculating only a portion of the spent washing liquid may, depending on the operating conditions and specific configuration of the system, be advantageous to avoid recirculation of an amount of chlorides from the bleaching which is too great for the recovery boiler / evaporator of the recovery system / evaporation arrangement to handle.

[0008] It is understood that the washing arrangements may each be described as subjecting the cellulose pulp to one or more washing stages, i.e. the washing arrangements may each comprise one or more (consecutive) washing devices such as wash presses.

[0009] The system according to the first aspect of the invention may comprise a delignification arrangement, such as an oxygen delignification arrangement, arranged to receive and delignify cellulose pulp, wherein the first washing arrangement is connected to the delignification arrangement to wash delignified cellulose pulp received therefrom. Such a system may be referred to as a system for treating cellulose pulp.

[0010] According to a second aspect of the invention, there is provided a method for treating delignified cellulose pulp comprising: post-delignification washing of the delignified cellulose pulp in at least one first washing stage and at least one second washing stage being subsequent to the at least one first washing stage; after said post-delignification washing, subjecting the cellulose pulp to acidic bleaching by means of at least one acidic stage such as at least one acidic bleaching stage; after said acidic bleaching, washing the cellulose pulp by means of at least one post-acidic bleaching washing stage; after said washing, subjecting the cellulose pulp to alkaline bleaching by means of at least one alkaline stage; after said alkaline bleaching, washing the cellulose pulp by means of at least one post-alkaline bleaching washing stage, and recirculating spent washing liquid from one or more of said at least one post-alkaline bleaching washing stage to said post-delignification washing upstream of the at least one second washing stage, wherein one or more of said at least one second washing stage uses water and / or condensate as washing liquid.

[0011] The step of recirculating spent washing liquid may comprise recirculating all of the spent washing liquid (the whole flow) or only a portion thereof (such as 90%, 80, 70, 60, 50, 40, 30, 20, 10%, or any other portion of the flow of spent washing liquid).

[0012] The method according to the second aspect of the invention may comprise delignifying, such as oxygen delignifying, cellulose pulp prior to the post-delignification washing step. Such a method may be referred to as a method for treating cellulose pulp.

[0013] The invention is based on the insight to recirculate the alkaline filtrate to the post-delignification washing upstream of the final / last post-delignification washing stage. By recirculating the alkaline filtrate further upstream, it is subjected to at least one washing stage (the final / last post-delignification washing stage) where it is washed with water / condensate prior to the bleaching. Consequently, at least part of the extractive / organic substances dissolved in the alkaline bleaching stage are removed and are recirculated further upstream in the delignification process as part of the spent washing liquid / filtrate. Furthermore, since water or condensate is used as washing liquid in the final post-delignification washing stage rather than alkaline filtrate, there is no increase in pH. The inventors have surprisingly realized that recirculation of the alkaline filtrate to the post-delignification washing upstream of the final / last post-delignification washing stage has additional positive effects: residual alkaline bleaching chemicals (such as hydrogen peroxide) remaining in the alkaline filtrate has a positive effect on brightness of the pulp when added to the cellulose pulp in the post-delignification washing upstream of the final / last washing stage, thereby reducing consumption of bleaching chemicals. This effect is particularly pronounced when the alkaline filtrate is added to, or upstream of, an intermediate storage tower between the first and second washing arrangements. Furthermore, consumption of bleaching chemicals (such as chlorine dioxide) in the acidic bleaching may also be reduced due to less residual alkaline bleaching chemicals such as hydrogen peroxide in the acidic bleaching (such as chlorine dioxide bleaching). It is understood that the above-described additional positive effect is achieved by recirculating the alkaline filtrate specifically to the post-delignification washing (and not further upstream in the process) and specifically to a post-delignification washing stage upstream of the final / last post-delignification washing stage (i.e. not to the final final / last post-delignification washing stage).

[0014] In embodiments of the system, the first washing arrangement comprises at least one first washing device (such as at least one wash press device) and the second washing arrangement comprises at least one second washing device (such as at least one wash press device). In embodiments comprising two or more first / second washing devices, the first / second washing devices may be arranged consecutively.

[0015] In embodiments of the system, the system further comprises an intermediate storage tower for the cellulose pulp between the at least one first washing device and the at least one second washing device. The filtrate feeding arrangement may be arranged to recirculate spent washing liquid from the post-alkaline bleaching washing arrangement according to one or more of the following: as dilution liquid between the at least one first washing device and the storage tower (for example using a dilution screw); as dilution liquid to the intermediate storage tower (for example to a bottom portion thereof where an agitator mixes the cellulose pulp with the filtrate); as dilution liquid to a top screw of a wash press device; as washing liquid to one or more of the at least one first washing device.

[0016] In alternative embodiments of the system, the first and second washing arrangements are respective wash zone portions of a common washing device.

[0017] In embodiments of the system, the filtrate feeding arrangement is connected to dilution means between the first washing arrangement and the second washing arrangement and to a buffer tank, the buffer tank being connected to a wash liquid inlet of the first washing arrangement and / or to said dilution means to provide liquid thereto. The dilution means may for example be a dilution screw or a static dilution device. The alkaline filtrate / spent washing liquid is typically only fed to the buffer tank when the cooking / delignifying is not operating, i.e. when there is no need for dilution liquid between the washing arrangements.

[0018] In embodiments of the method, the post-delignification washing comprises an intermediate storage stage between the at least one first washing stage and at least one second washing stage, and wherein said recirculating comprises recirculating spent washing liquid from said at least one post-alkaline bleaching washing stage as dilution liquid between the at least one first washing stage and the intermediate storage stage and / or to the intermediate storage stage and / or as washing liquid to one or more of the at least one first washing stage. The intermediate storage stage may use a storage tower.

[0019] In embodiments of the method, the recirculating comprises recirculating spent washing liquid to a dilution stage between the at least one first washing stage and the at least one second washing stage and to a buffer tank, and wherein said recirculating comprises feeding liquid from the buffer tank as wash liquid to one or more of the at least one first washing stage and / or to said dilution stage. The dilution stage may use a dilution screw or a static dilution device.

[0020] According to a third aspect of the invention, there is provided a system for producing cellulose pulp from lignocellulosic biomass material such as wood chips, the system comprising: a cooking device (also referred to as a digester) configured to digest lignocellulosic biomass material such as wood chips impregnated with white / black liquor to produce cellulose pulp; a screening arrangement connected to the cooking device to receive cellulose pulp therefrom, wherein the screening arrangement may comprise at least one coarse screen device and at least one fine screen device; a (brown stock) washing arrangement connected to the screening arrangement to receive cellulose pulp therefrom, the washing arrangement comprising at least one consecutively arranged washing device such as wash press devices(s); a delignification arrangement (such as an oxygen delignification arrangement) connected to the washing arrangement to receive cellulose pulp therefrom, the delignification arrangement being configured to delignify the cellulose pulp, and a system for treating delignified cellulose pulp according to the first aspect of the invention or embodiments thereof.

[0021] According to a fourth aspect of the invention, there is provided a method for producing cellulose pulp, the method comprising: cooking (also referred to as digesting) lignocellulosic biomass material such as wood chips impregnated with white / black liquor to produce cellulose pulp; screening the cellulose pulp, wherein the screening may comprise screening in at least one coarse screening stage and at least one fine screening stage; (brown stock) washing of the screened cellulose pulp, the washing comprising at washing in one or more consecutive stages using for example wash press devices(s); delignifying (such as oxygen delignifying) the washed cellulose pulp, and treating the delignified cellulose pulp using the method according to the second aspect of the invention or embodiments thereof.

[0022] The lignocellulosic biomass material may be any type of annual fiber or wood biomass material. Examples of annual fiber biomass material include straw (all types of straw, rice straw, wheat straw etc.), bagasse and EFB (Empty Fruit Brunch). The wood biomass material may comprise softwood, hardwood or a combination thereof. Examples of softwood are spruce and pine. Examples of hardwood are oak and eucalyptus.

[0023] The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments second aspect of the invention and vice versa.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein: fig. 1 shows schematically an embodiment of the system according to the third aspect of the invention (which includes an embodiment of the system according to the first aspect of the invention), and fig. 2 shows schematically another embodiment of the system according to the third aspect of the invention (which includes an embodiment of the system according to the first aspect of the invention), and fig. 3 shows a flowchart illustrating an embodiment of the method according to the second aspect of the invention. DETAILED DESCRIPTION

[0025] Fig. 1 shows a schematic illustration of an embodiment of a system according to the third aspect of the invention.

[0026] The system 1 comprises a cooking device (also referred to as a digester) 2 configured to digest lignocellulosic biomass material such as wood chips to produce cellulose pulp. The cooking device is fed from an impregnation device 9 which impregnates the lignocellulosic biomass material with white / black liquor. The cooking device 2 is connected to a screening arrangement 3 via storage tower 17 in which the pulp is stored at medium consistency (about 12%). The screening arrangement is shown for illustrative purposes as a single screening device. The screening arrangement is connected to a (brown stock) washing arrangement 4 illustrated as a single wash press device (shown as a twin roll press). The washing arrangement is connected to an oxygen delignification arrangement 5 configured to delignify the cellulose pulp after washing. It is understood that the screening arrangement 3, the washing arrangement 4 and the delignification arrangement each normally comprises at least two stages. For instance, the screening arrangement may comprise at least one coarse screen device and at least one fine screen device. A wash liquor buffer tank 6 is arranged adjacent the cooking device and is fluidically connected thereto as well as to the filtrate tank 4' of the washing arrangement 4 to receive filtrate from the washing arrangement for use as washing liquid in the washing section (bottom part) of the cooking device. A post-delignification washing arrangement 7 is connected to the delignification arrangement to wash the cellulose pulp after delignification, the washing arrangement comprising a first washing arrangement in the form of a first wash press 7a and a second washing arrangement in the form of a second wash press 7b with an intermediate storage tower 7c where washed pulp at medium consistency is stored. The filtrate tank 7a' (which receives filtrate from the first wash press 7a) is connected to washing arrangement 4 to provide filtrate as washing liquid thereto. The filtrate tank 7b' (which receives filtrate from the second wash press device 7b) is connected to a liquor buffer tank 8 having a substantially greater volume than the filtrate tank 7b' in order to handle excess filtrate flow from the second wash press device which may be provided during disturbances in the system.

[0027] The first wash press device 7a is provided with washing liquid from buffer tank 8. The second wash press device 7b is provided with water or condensate 11 as washing liquid. A dilution screw 12 is arranged between the first wash press device 7a and the storage tower 7c. Dilution liquid is provided to dilution screw 12 from the filtrate tank 16' (described below) and optionally also from liquor buffer tank 8.

[0028] The second wash press device 7b is connected to a chlorine dioxide bleaching arrangement 13, which in turn is connected to a post-acidic bleaching (third) washing arrangement 14 with associated filtrate tank 14'.

[0029] The third washing arrangement 14 is connected to a hydrogen peroxide (alkaline) bleaching arrangement 15, which in turn is connected to a post-alkaline bleaching (fourth) washing arrangement 16 with associated filtrate tank 16' in which alkaline filtrate is collected.

[0030] The alkaline filtrate is recirculated from filtrate tank 16' as dilution liquid to dilution screw 12. When cooking and delignifying is temporarily not in operation, the alkaline filtrate may alternatively be recirculated to buffer tank 8. In other embodiments, the alkaline filtrate may be provided directly to the intermediate storage tower 7c for mixing with the cellulose pulp.

[0031] The system further comprises an evaporation arrangement 10, also referred to as a recovery system. In the figure, the evaporation arrangement is illustrated as having two evaporation liquor tanks 10a, 10b and one evaporator 10c connected thereto, but may comprise further tanks and evaporators. The first evaporation liquor tank 10a is connected to the cooking device to receive weak black liquor therefrom and is also connected to the wash liquor buffer tank 6 such that also excess wash liquor from the brown stock washing 4 will be sent to evaporation.

[0032] Fig. 1 also illustrates an embodiment of the system according to the first aspect of the invention (all devices in fig. 1 to the right of the oxygen delignification arrangement 5 constitutes an embodiment of the system for treating delignified cellulose pulp).

[0033] Fig. 2 shows schematically another embodiment of the system according to the third aspect of the invention (which includes an embodiment of the system according to the first aspect of the invention). The system comprises a cooking device, an impregnation device, a screening arrangement, a storage tower, a (brown stock) washing arrangement with filtrate tank, an oxygen delignification arrangement and a wash liquor buffer tank as in fig. 1.

[0034] The system in fig. 2 however differs from the system in fig. 1 in that the post-delignification washing arrangement does not comprise an intermediate storage tower and no liquor buffer tank. In the post-delignification washing arrangement 107, filtrate from the filtrate tank 107b' of the second wash press device 107b is recirculated directly to the first wash press device 107a as washing liquid (rather than via the liquor buffer tank as in fig. 1). Furthermore, the cellulose pulp is fed directly from dilution screw 112 to the second wash press device 111 (rather than via the intermediate storage tower as in fig. 1).

[0035] Furthermore, the acidic bleaching arrangement comprises not only a chlorine dioxide bleaching arrangement 113 (D-stage) corresponding to ref. 13 in fig. 1 but also an acidic stage 118 (A-stage) upstream of the D-stage 113. The A-stage is formed by a storage tower similar to the intermediate storage tower 7c in fig. 1 but differs in that acid is added thereto.

[0036] Just like in fig. 1, the chlorine dioxide bleaching arrangement 113 is connected to a post-acidic bleaching (third) washing arrangement 114 with associated filtrate tank 114', and the third washing arrangement 114 is in turn connected to a hydrogen peroxide (alkaline) bleaching arrangement 115 followed by a fourth washing arrangement 116 with associated filtrate tank 116' in which alkaline filtrate is collected. Unlike in fig. 1, alkaline filtrate from 116' is recirculated to the dilution screw 112 only.

[0037] Fig. 2 also illustrates an embodiment of the system according to the first aspect of the invention (all devices in fig. 2 to the right of the oxygen delignification arrangement constitutes an embodiment of the system for treating delignified cellulose pulp).

[0038] The devices and arrangements in the above-described systems are all well known in the art and do not need to be described in further detail here. For example, the system may comprise further devices such as further screening devices, further washing devices and further bleaching devices. Further, it is understood that one or more pump device of known type is / are present to transport the pulp between the shown devices / arrangements.

[0039] Fig. 3 shows a flowchart illustrating an embodiment of the method according to the second aspect of the invention. The method comprises providing delignified pulp (for example from an oxygen delignification stage) and post-delignification washing 200 of the delignified cellulose pulp in a first washing stage 201 and a second washing stage 203 being subsequent to the at least one first washing stage. The post-delignification washing 200 further comprises a dilution stage 202 and an intermediate storage stage 203 between the first washing stage 201 and the second washing stage 204.

[0040] After the post-delignification washing 200, the cellulose pulp is subjected to acid bleaching (for example chlorine dioxide bleaching) 205 and thereafter washed 206. Thereafter, the cellulose pulp is subjected to alkaline bleaching (for example hydrogen peroxide bleaching) 207 and thereafter washed 208. Spent washing liquid (alkaline filtrate) from the (fourth) washing 208 is recirculated to the post-delignification washing 200 upstream of the second washing stage 204, more specifically to the dilution stage 202. In the second washing stage 204 water is used as washing liquid.

[0041] The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. In particular, as explained above, the devices and arrangements shown above may be of other known types.

Claims

1. A system for treating cellulose pulp comprising: - a delignification arrangement (5), such as an oxygen delignification arrangement, arranged to receive cellulose pulp and being configured to delignify the cellulose pulp, - a first washing arrangement (7a; 107a) connected to the delignification arrangement (5) to wash delignified cellulose pulp received therefrom; - a second washing arrangement (7b; 107b) connected to the first washing arrangement to wash cellulose pulp received therefrom; - an acidic bleaching arrangement (13; 113) arranged to receive cellulose pulp from the second washing arrangement (7b; 107b), said acidic bleaching arrangement comprising at least one acidic stage; - a post-acidic bleaching washing arrangement (14; 114) connected to the acidic bleaching arrangement (13; 113) to wash cellulose pulp received therefrom; - an alkaline bleaching arrangement (15; 115) arranged downstream of the post-acidic bleaching washing arrangement (14; 114) to subject the cellulose pulp to alkaline bleaching; - a post-alkaline bleaching washing arrangement (16; 116) connected to the alkaline bleaching arrangement (15; 115) to wash cellulose pulp received therefrom; - a filtrate feeding arrangement (19; 119) arranged to recirculate spent washing liquid from the post-alkaline bleaching washing arrangement (16; 116) as dilution liquid between the first washing arrangement (7a; 107a) and the second washing arrangement (7b; 107b) and / or as washing liquid to the first washing arrangement (7a; 107a), and - a source (11; 111) of water and / or condensate connected to said second washing arrangement (7b; 107b) to provide said water and / or condensate as washing liquid.

2. System according to claim 1, wherein the first washing arrangement (7a; 107a) comprises at least one first washing device and the second washing arrangement comprises at least one second washing device.

3. System according to claim 2, further comprising an intermediate storage tower (7c) for the cellulose pulp between the at least one first washing device (7a) and the at least one second washing device (7b), wherein the filtrate feeding arrangement is arranged to recirculate spent washing liquid from the post-alkaline bleaching washing arrangement (16) as dilution liquid between the at least one first washing device (7a) and the storage tower (7c) and / or to the intermediate storage tower (7c) and / or as washing liquid to one or more of the at least one first washing device (7a).

4. System according to claim 1, wherein the first and second washing arrangements are respective wash zone portions of a common washing device.

5. System according to any of the preceding claims, wherein the filtrate feeding arrangement (19) is connected to dilution means (12) between the first washing arrangement (7a) and the second washing arrangement (7b) and to a buffer tank (8), the buffer tank being connected to a wash liquid inlet of the first washing arrangement (7a) and / or to said dilution means (12) to provide liquid thereto.

6. System for producing cellulose pulp from lignocellulosic biomass material comprising: - a cooking device (2) configured to digest lignocellulosic biomass material impregnated with white or black liquor to produce cellulose pulp; - a screening arrangement (3) connected to the cooking device (2) to receive cellulose pulp therefrom; - a brown stock washing arrangement (4) connected to the screening arrangement (3) to receive cellulose pulp therefrom, the brown stock washing arrangement comprising at least one consecutively arranged washing device, and - a system for treating cellulose pulp according to any of the preceding claims, wherein said delignification arrangement (5) is connected to the brown stock washing arrangement (4) to receive cellulose pulp therefrom.

7. A method for treating delignified cellulose pulp supplied from a delignification arrangement such as an oxygen delignification arrangement, said method comprising: - post-delignification washing (200) of the delignified cellulose pulp in at least one first washing stage (201) and at least one second washing stage (203) being subsequent to the at least one first washing stage; - after said post-delignification washing, subjecting the cellulose pulp to acidic bleaching by means of at least one acidic stage (205); - after said acidic bleaching, washing (206) the cellulose pulp by means of at least one post-acidic bleaching washing stage; - after said washing, subjecting the cellulose pulp to alkaline bleaching by means of at least one alkaline stage (207); - after said alkaline bleaching, washing the cellulose pulp by means of at least one post-alkaline bleaching washing stage (208), and - recirculating spent washing liquid from one or more of said at least one post-alkaline bleaching washing stage (208) to said post-delignification washing (200) upstream of the at least one second washing stage (204), wherein one or more of said at least one second washing stage (204) uses water and / or condensate as washing liquid.

8. Method according to claim 7, wherein said post-delignification washing (200) comprises an intermediate storage stage (203) between the at least one first washing stage (201) and the at least one second washing stage (204), and wherein said recirculating comprises recirculating spent washing liquid from said at least one post-alkaline bleaching washing stage (208) as dilution liquid between the at least one first washing stage (201) and the intermediate storage stage (203) and / or to the intermediate storage stage (203) and / or as washing liquid to one or more of the at least one first washing stage (201).

9. Method according to any claim 7 or 8, wherein said recirculating comprises recirculating spent washing liquid to a dilution stage (202) between the at least one first washing stage (201) and the at least one second washing stage (204) and to a buffer tank, and wherein said recirculating comprises feeding liquid from the buffer tank as wash liquid to one or more of the at least one first washing stage (201) and / or to said dilution stage (202).

10. A method for producing cellulose pulp comprising: - cooking lignocellulosic biomass material impregnated with white or black liquor to produce cellulose pulp; - screening the cellulose pulp; - brown stock washing of the screened cellulose pulp, the washing comprising washing in one or more consecutive stages; - delignifying the washed cellulose pulp; - treating the delignified cellulose pulp using the method according to any of claims 7-9.

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