SE1751468A1 - Method and a system for washing paper pulp - Google Patents

Abstract

The invention pertains to a method and system for washing pulp in a pulping process, wherein the pulp is washed in a pulp washer using aqueous wash liquor to obtain washed pulp and a wash filtrate comprising contaminants originating from the pulp and pulping process, wherein the method further comprises the steps of purifying said wash filtrate comprising contaminants from the pulp to provide purified wash liquor by flocculating said wash filtrate comprising contaminants originating from the pulp and pulping process; and separating the contaminants, as floe formed in the flocculation of said wash filtrate comprising contaminants originating from the pulp and pulping process, from the wash filtrate to provide purified wash liquor, the method further comprising re-circulating at least 25 mass-% of said purified wash liquor to the washer to be used as wash liquor, calculated from the total mass of said wash filtrate comprising contaminants originating from the pulp and pulping process.

Description

171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 1 METHOD AND A SYSTEM FOR WASHING PAPER PULP Field of the inventionThe present invention relates to a method and a system for washing paper pulp.More specifically, the invention relates to ways to enhance the washing efficiency of a pulp washer by means of utilizing cleaner wash liquor, through inline water cleaning.

Background of the invention Chemical pulping methods produce high-quality papers by converting woodchips, or other fibrous raw material, into pulp for use e. g. in papermaking accomplishedby cooking the chips in a cooking liquor aiming to liberate the fibers from the ligninmatrix of the wood.

In chemical pulping, after cooking and as long as the liquor loop is closed(usually until the last washer before the bleaching) washing is important in order tobenefit from the high-energy content of the dissolved organic substance and to regain the spent cooking chemicals for economical as well as environmental reasons. The energy recovery is achieved by means of evaporation of the dirty water, the spent liquor.

In the case of Kraft pulping, the spent liquor is called black liquor, whereas other namesare applicable for spent liquors from sulphite processes. This is done in order tominimize the water content of the liquor entering the fumace/recovery boiler. It is clearthat the higher the organic content of the spent liquor is, the higher is the heat value of itand the more economically interesting it is to bum the liquor.

To provide an overview of a typical modem fiberline for the manufacture ofbleached chemical pulp, figure 1 illustrates such a fiberline, more precisely for bleachedkraft pulp. The system uses a continuous digester with an integrated HiHeat washer,which is used in the majority of the fiberlines today. If instead batch digesters were tobe used, these would most likely be combined with a separate washer to reach the sameperformance as the continuous digester, why the systems are still comparable withregards to waste water treatment.

The fiberline is in figure 1 is shown in two rows, the upper row illustrating theso-called closed part of the fiberline, being the brownstock and oxygen delignificationareas. “Dig” stands for digester and “scr” for screening. The lower row shows the so-called open part of the fiberline, being the bleaching area. O relates to oxygendelignification, D to chlorine dioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Highlighted are fresh water intake 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 2 positions (grey thick arrows) and filtrate discharge positions (black thick arrows) in afiberline.

In chemical pulping the wood chips are first subj ected to steaming,impregnation and cooking. In the lower part of a continuous digester (the dominanttechnology for chemical pulping) the pulp is washed and in the blowline the washingcontinues, in this particular case using a pressure diffuser washer, before the pulp entersa blowtank/ storage tower. Following screening the pulp is again washed, here on a washpress and then subj ected to oxygen delignification in two subsequent reactors. AfterVenting off excess gas the pulp is again washed and then taken to another storage tower,also working as a diffusion washing unit before being washed a last time on a washpress before the bleaching. This position also marks the end of the closed loop of thefiberline, meaning that the clean wash water applied on this last wash press produces adirty filtrate, which is used as wash liquor to the wash press next before in line etc, sothat the wash liquor is used counter-currently. The filtrate from the first washer, in thiscase the washer in the lower part of the digester, is taken care of. It goes first toevaporation and then to incineration in the furnace/recovery boiler. The high organiccontent of the stream is then utilized for energy recovery, whereas the inorganiccomponents are reduced such that the cooking chemicals sodium hydrosulfide andsodium hydroxide can be formed again after dissolution and recaustization.

In a typical modern fiberline (such as exemplified in figure l) a compromisehas been made between a number of different conflicting desires, including the wish touse as little water as possible to: - Get the dissolved organic substance as highly concentrated aspossible for most efficient energy recovery, implying that it is first andforemost the early washers that are of interest to collect filtrate from - butrunning the wash liquor counter-currently means that the concentrations buildup.

- Conserve the heat of the wash liquor in order to ensure a lowViscosity for the water and thereby a more efficient washing, implying thecounter-current use of the was liquor is a good idea.

- Only use clean water as wash liquor to get a clean pulp as quick aspossible, implying the use of clean water used as wash liquor on each washer.

- Mechanically treat the pulp as little as possible in order to maintainas much as possible of the inherent strength of the pulp fibers, implying the 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 3 passage of as few as possible pumps, mixers, washers etc. speaking in favor of clean water as wash liquor.

- Minimize capital as well as operating costs for the f1berline,speaking in favor of using as short sequence as possible by means e.g. of usingclean water as wash liquor on all the washers.

Compromising means that none of the desires above is reached.

Contrary to the closed loop (brown side), washing in the subsequent bleaching(so-called open loop) is not a matter of regaining chemicals. This is true for severalreasons. The dirt concentration in the form of dissolVed organics is way lower in thispart of the plant meaning that the cost for energy recovery is very high. Moreover, inbleach plants using chlorine dioxide (or any other chlorine containing bleaching agent),which is Valid for almost all bleach plants in the world, the filtrates produced will berich in chloride ions. Would such a filtrate be bumt in a recovery boiler it would causesevere corrosion and stickies formation, which obviously is not desired.

In the bleaching, washing is also important in order to minimize the carry-overof dissolved organic substance into the bleaching. Doing so, the COD to effluent in theopen bleach plant is kept low as well as the discharge of AOX. About 80 % of the AOXfrom the bleaching is norrnally formed in the first chlorine dioxide stage, the D0-stage,and partly in a reaction between dissolved organic matter and the bleaching chemical.Washing before the bleaching is also important since each kilogram of COD enteringinto a chlorine dioxide stage together with the pulp suspension consumes about 0.2-0.4kg chlorine dioxide/ADt pulp (in this patent the definitions ADt=Air Dried Ton of pulpand BDt=Bone Dried Ton of pulp are used to describe pulp that is 90 % dry (ADt) andl00 % dry (BDt) respectiVely). This argument is also Valid as argument for theimportance of washing between different stages in the bleaching just as the argumentthat washing here is important also as a means of transferring heat between stages andto adjust other process conditions, e. g. pH.

In the bleaching of chemical pulp different bleaching chemicals are applied tothe pulp in a number of subsequent bleaching stages, each of which is usually followedby a washer to wash out the dissolved substance and the degradation products of thebleaching chemicals. Some bleaching chemicals (e. g. chlorine dioxide, ozone andperacetic acid) are used in acidic conditions, whereas others (e.g. hydrogen peroxideand oxygen) are used in alkaline conditions. This means that throughout the passage ofthe bleach plant the pulp suspension shifts between alkaline and acidic conditions, alkaline conditions being the starting point. Due to the fact that chloride ions are formed 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 4 in the chlorine dioxide stage (usually this is the first bleaching stage and chlorinedioxide is usually also used in the third stage and possibly in yet another stage) filtratesfrom the washers in the bleaching could not be used as wash liquor in the closed part ofthe fiberline as it would then end up in the recovery boiler and cause severe corrosionthere. For this reason both one acidic and one alkaline filtrate from the bleaching alwaysgo to the sewer.

The current setup is a compromise between a number of different conflictingdesires, including the wish to use as little water as possible to:- Minimize the filtrate Volumes going to the sewer.- Conserve the heat of the wash liquor in order to ensure a lowViscosity for the water and thereby a more efficient washing, implying thecounter-current use of the wash liquor is a good idea.- Boost bleaching efficiency by making productive use of residualhydrogen peroxide (valid for hydrogen peroxide stages), residual chlorinedioxide (valid for chlorine dioxide stages) or chloride ions formed in thechlorine dioxide stage (Valid for chlorine dioxide stages)- Only use clean water as wash liquor- Get a clean pulp as quick as possible, implying the use of cleanwater used as wash liquor on each washer- Minimize undesired side reactions of the bleaching chemicals withdissolved dirt instead of with the pulp to be bleached- Mechanically treat the pulp as little as possible in order to maintainas much as possible of the inherent strength of the pulp fibers, implying thepassage of as few as possible pumps, mixers, washers etc. speaking in favor ofclean water as wash liquor- Minimize capital as well as operating costs for the f1berline,speaking in favor of using as short sequence as possible by means e.g. of usingclean water as wash liquor on all the washersNot mentioned, but obvious, is that the acidic and alkaline wash liquor flowsneed to be kept apart. The costs for changing pH of a certain flow by adding sulfuricacid or sodium hydroxide would be too high and also risk extensive scaling.

Again, compromising means that none of the desires above is reached.

When a mill is designed and built from the beginning an appropriate washing isof course an integral part of the mill. However, oVer time there are many good economical motives to try to increase the production as marginal costs are usually Very 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc low to do so. To a certain extent Washers are usually possible to run at a higher load, butalways at the expense of a decreased Washing efficiency. Eventually the matter becomesmore of managing to push the pulp through the Washer considering only the hydrauliccapacity of the Washer and completely neglecting the Washing efficiency. At an earlystage bleaching chemical consumptions begin to increase as a result of less efficientWashing, but also pulp strength and other quality parameters soon Will be negativelyaffected. And at some point further along that path the pulp simply becomes too dirtyfor its aimed utilization. Accordingly, investments in enhanced Washing are on theagenda in most pulp mills.

As such, solutions are sought after Which could increase the pulp Washing efficiency Without increasing Water usage or loWering the pulp quality.

Summary of the invention The present invention preferably seeks to mitigate, alleviate or eliminate one ormore of the above-identified deficiencies in the art and disadvantages singly or in anycombination and solves at least the above mentioned problems by providing a methodfor Washing pulp in a pulping process, Wherein the pulp is Washed in a pulp Washerusing aqueous Wash liquor to obtain Washed pulp and a Wash filtrate comprisingcontaminants originating from the pulp and pulping process, Wherein the method furthercomprises purifying said Wash f1ltrate comprising contaminants originating from thepulp and pulping process to provide purified Wash liquor by; flocculating said Washfiltrate comprising contaminants originating from the pulp and pulping process; andseparating the contaminants, as floc formed in the flocculating of said Wash filtratecomprising contaminants originating from the pulp and pulping process, from the Washfiltrate to provide purified Wash liquor, the method further comprising re-circulating atleast 25 mass-% of said purified Wash liquor to the Washer to be used as Wash liquor,calculated from the total mass of said Wash f1ltrate comprising contaminants originatingfrom the pulp and pulping process.

Furthermore is provided a system for Washing pulp in a pulping process,comprising at least one Washer, Wherein the Washer comprises; a Wash liquor inlet forreceiving Wash liquor for use in Washing paper pulp, a Wash liquor outlet fordischarging Wash f1ltrate comprising contaminants originating from the pulp andpulping process, Wherein the system further comprises for the Washer; a purifier,connected to the Wash liquor outlet, for purifying the Wash f1ltrate comprising contaminants originating from the pulp and pulping process to purified Wash liquor and 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 6 separating the contaminates originating from the pulp and pulping process as floc, a re-circulation conduit, connected to the purifier and the wash liquor inlet, for re-circulatingpurified wash liquor to the pulp washer liquor inlet, a floc outlet, connected to thepurifier, for removal of the separated flocculated contaminants originating from the pulpand pulping process, and optionally a pulp dilution loop, connected to the re-circulationconduit, for transferring purified wash liquor to the pulp for diluting the pulp to a desired dilution factor before it is washed in the washer.

Brief description of the drawings These and other aspects, features and advantages of which the invention iscapable of will be apparent and elucidated from the following description ofembodiments of the present invention, reference being made to the accompanyingdrawings, in which; Fig. 1 shows a typical fiberline for manufacture of bleached chemical pulp,more precisely bleached kraft pulp. Highlighted are fresh water intake positions (greythick arrow) and filtrate discharge positions (black thick arrow) in a fiberline. Cleanwater (either fresh water or condensate or a mixture) is used as wash liquor in threepositions indicated by grey thick arrows. Two thick black arrows indicate two flows ofdirty water leaving the fiberline - one having an acidic pH and the other an alkaline pH, Fig. 2 illustrates the wash liquor flows in the closed part of the fiberline, beingthe brownstock and oxygen delignification areas. “Dig” stands for digester and “scr” forscreening. Figures relate to the amount of wash liquor prevailing in different positionsduring continuous operation (in m3/BDt pulp) and with assumptions of a dilution factorof 1.0 m3/BDt, a consistency in the stages of 11 % and a discharge consistency of 32 %from the washpresses. The pressure diffuser has an inlet as well as dischargeconsistency of 11 % and the same goes, theoretically, for the HiHeat-washer inside thedigester. The thick grey arrow indicates the clean water used for washing. This cleanwater is usually made up of condensate. The thick black arrow from the wash in Dig isthe flow of black liquor going to evaporation and incineration, Fig. 3 illustrated wash liquor flows in the open part of the fiberline, being thebleaching area. O relates to oxygen delignification, D to chlorine dioxide bleaching and(OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Figuresrelate to the amount of wash liquor prevailing in different positions during continuousoperation (in m3/BDt pulp) and with assumptions of a dilution factor of 1.0 m3/BDt, a consistency in the stages of 11 % and a discharge consistency of 32 % from the 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 7 washpresses. The grey thick arrows indicate the clean Water used for washing. Theblack thick arrows indicate acidic (from Do) and alkaline (from (OP)) f1ltrates going tothe sewer.

Fig. 4 illustrates wash liquor flows in the closed part of the fiberline, being thebrownstock and oxygen delignification areas. “Dig” stands for digester and “scr” forscreening. Figures relate to the amount of wash liquor prevailing in different positionsduring continuous operation (in m3/BDt pulp) and with assumptions of a dilution factorof 1.0 m3/BDt, a consistency in the stages of ll % and a discharge consistency of 32 %from the washpresses. The pressure diffuser has an inlet as well as dischargeconsistency of 11 % and the same goes, theoretically, for the HiHeat-wash inside thedigester. The grey thick arrow indicates the clean water used for washing. This cleanwater is usually made up of condensate. The black thick arrow from the wash in Dig isthe stream of black liquor going to evaporation and incineration. Purifiers are labelledAxoPurTM, and are can be seen coupled to the outlet of the scr-, and OO-washers. Thethick horizontal arrows from the purifiers is the floc, here is going to incinerationtogether with the black liquor.

Fig. 5 illustrates wash liquor flows in the open part of the fiberline, being thebleaching area. O relates to oxygen delignification, D to chlorine dioxide bleaching and(OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Figuresrelate to the amount of wash liquor preVailing in different positions during continuousoperation (in m3/BDt pulp) and with assumptions of a dilution factor of 1.0 m3/BDt, aconsistency in the stages of 11 % and a discharge consistency of 32 % from thewashpresses. The thick grey arrows indicate the clean water used for washing. The thickblack arrows indicate acidic (from Do) and alkaline (from (OP)) filtrates going to thesewer. Purifiers are labelled AxopurTM, and are can be seen coupled to the outlet of eachof the washers, where the thick horizontal arrows from the purif1ers is the separatedfloc.

Fig. 6 illustrates a purifier, in (A) illustrating its inlet, outlet and flock outlet,and in (B) it is illustrated comprising an electrocoagulation unit and a floc separator,and Fig. 7 illustrates two subsequent washers, here exemplified by two washpresses after an oxygen delignification stage. Part of the wash f1ltrate flow is shown forand a purif1er connected to the second (labelled AxoPur). In the example a stageconsistency of ll %, an outlet consistency from the wash press of 32 % and a dilution factor of 1.0 m3/BDt are employed. In this case that means a purif1ed wash f1ltrate of 9.1 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 8 m3/BDt. 6.0 m3/BDt of these are typically used for dilution of the pulp suspension intothe stage before the washer (flow “A”), whereas 3.1 m3/BDt are traditionally used aswash liquor on the previous (or even earlier) washer (flow “B”). The current inventionmakes use of the purified filtrate in flow “A” in the conventional way (although withpurified filtrate), but replaces the flow “B” by the flow “C” to a high degree.

Description of embodiments The following description focuses on an embodiment of the present inventionapplicable to the washing of paper pulp. In very general terrns, washing is a unitoperation aiming to separate the pulp from dissolved dirt by means of using water.However, the resulting dirty water could itself be subject to a separation treatmentwhereby the dissolved dirt could be separated from the water.

When designing a fiberline for the manufacture of unbleached or bleachedchemical pulp, a compromise has got to be found between the conflicting desires to onthe one hand minimising the water consumption and on the other utilising as clean washliquor as possible. This balance typically means that at least 4, but usually 5 washers areused in the closed loop before the bleaching including the HiHeat washing inside thedigester bottom. For the bleaching, a 4-stage bleach plant is typical, including 4 washersproducing one alkaline and one acidic filtrate going to the sewer thus calling for asignificant need for fresh water for the washing.

When upgrading an overloaded fiberline, the installation of an additionalwasher is usually considered. This means a costly and time-consuming investment forthe mill as manufacturing times are long, but also space within the mill has to befound/ created for the new washer to be placed and concrete works arranged. In addition,every additional machine in direct contact with the pulp in the fiberline will affect thepulp strength negatively. This option is thus not very attractive, although usually the onechosen.

Another option available is to use fresh, clean water as wash liquor early in thefiberline (one of the first washers after the digester and in any case before the firstbleach stage) where the pulp is particularly dirty, making the pulp suspension becomecleaner than if using the (dirty) water from the downstream cleaner. However, the waterconsumption of the mill also increases, which in most countries today would mean aproblem as water resources begin to be scarce and as environmental perrnits usuallywould not allow an increased specific water consumption. If all washer filtrates are led to the evaporation and incineration to recover heat and cooking chemicals, the use of 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 9 more fresh Water also means an increased amount of water in this flow, thus decreasingits heat value. If, instead, some of the wash filtrate is led to the sewer, the result will bean increased load on the waste water treatment facility, neither of which is desired. Soin case more fresh water should be needed, it must be found intemally in the mill. Insuch a situation closest at hand would be considering the opportunity to take water fromthe waste water treatment plant and return back to use as the clean wash water.However, since the treatment plant often includes water treatment or storage in atreatment pond, the residence time may be a matter of weeks, in which case thetemperature of the wash water after having passed the treatment plant will be ambient orlow - probably in the range 5-20 °C, which will mean that the washer will not performvery well due to the high viscosity of the water at such temperatures. In theory, it is ofcourse possible to use a heat exchanger between the wash liquor going to the watertreatment plant and the clean water coming back from there. However, due to the highcontent of dissolved solids and fibres in the wash filtrate, the risk for plugging andscaling of the heat exchanger is evident. It seems that whatever choice is made, there isa negative coming along. And, as practice shows, the usual answer to the challenge isaccepting the negatives of the investment in an additional washer in the f1berline.

The invention relates to the f1ltrate coming from a pulp washer and the rapidinline cleaning of this f1ltrate in such a way that the organic matter could be separated asa high density floc while maintaining most of the heat so that the cleaned f1ltrate (i.epurified wash liquor) could be used as wash liquor on the same washer.

The recirculation of the cleaned filtrate thereby decreases the carry-over of dirtin the pulp suspension from the washer at which it is used, to the subsequent treatmentstage both due to the utilisation of a cleaner wash liquor and due to a higher washingtemperature decreasing the viscosity of the wash liquor and thereby enhancing thewashing efficiency. To illustrate this, Figs. 2 to 5 are showing the flow of the washliquor in a fiberline such as the example f1berline of Fig. l. Figs. 2 and 3 show aconventional mode of wash liquor utilisation, and Figs. 4 and 5 illustrates an example ofwash liquor utilisation when cleaning and re-circulation is used in accordance with theinvention. Figs. 4 to 5 show a system where the method of the invention is used on eachof the washers l from the washer l after screening 3l to the washer l after chlorinedioxide bleaching 37. It may however be desirable to only implement this for one or afew of the washers 1, depending on the need for the specific fiberline.

In one embodiment, in a method for a for washing pulp in a pulping process, the pulp is washed in a pulp washer 1 using aqueous wash liquor to obtain washed pulp 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc and a Wash f1ltrate comprising contaminants originating from the pulp and pulpingprocess. The method further comprises purifying said Wash f1ltrate comprisingcontaminants originating from the pulp and pulping process to provide purified Washliquor by; flocculating said Wash f1ltrate comprising contaminants originating from thepulp and pulping process, and separating the contaminants, as floc forrned in theflocculating of said Wash f1ltrate comprising contaminants originating from the pulp andpulping process, from the Wash f1ltrate to provide purified Wash liquor. The methodfurther comprising re-circulating at least 25 mass-% of said purified Wash liquor to theWasher 1 to be used as Wash liquor, calculated from the total mass of said Wash filtratecomprising contaminants originating from the pulp and pulping process.

A high level of re-circulation is often favorable. As such, in one furtherembodiment, at least 35 mass-%, such as at least 45 mass-%, such as at least 55 mass-%, such as at least 65 mass-%, such as at least 75 mass-%, such as at least 85 mass-%,such as at least 95 mass-%, or even 100 mass-% of said purified Wash liquor is re-circulated to the Washer to be used as Wash liquor, calculated from the total mass of saidWash f1ltrate comprising contaminants originating from the pulp and pulping process.

To simplify examples, the pulp Washers in Figures 2 to 5 are being illustratedby Wash presses, since they are a Water effective solution, however, the Washer may beof other suitable types. The optimum pulp consistency is about 11 %, so since the Washpresses press the pulp to a consistence of 32 %, 6 m3 is used to dilute the pulp to asuitable consistency of 11%, as shown in Figures 2 to 5. In one embodiment, the Washer1 is selected from the group consisting a Wash press, pressure diffuser, atrnosphericdiffuser, filter Washer, Compaction Baffle-Washer (CB-Washer), Drum Displacer-Washer(DD-Washer), dewatering press, and belt Washer.

In Figure 2, relating to the closed part of the f1berline, it is illustrated hoW in aclosed loop system With a counter-flow, the Wash liquor not used for dilution of the pulp(referred to as 3.1 m3 flow) Will be transferred to the previous Washer, counter to thepulp direction. One effect of this is that the Wash liquor Will build up more and moredirt. HoWever, since Wash liquor Will travel from one Washer to another, its propertiesWill also differ to some degree, such as in temperature or pH, from the properties of theexisting liquor in the Washer 1. Such variability of the counter-flow Wash liquor mayhave an impact on the Wash result, and can in Worst case lead to scaling, Which in thelong run Will lead to cleaning procedures Which may possibly interfere With or eveninterrupt the pulp production process on the fiberline. This can be a continuous flow, to make sure that temperature is maintained at a steady level. 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 11 In one embodiment, the flocculation, the purif1cation of said f1ltrate is carriedout using a continuous flow to provide a continuous re-circulation of purified washliquor to the washer.

Another effect of this is that a closed loop reverse flow of waste water can onlybe fed to pulp washers l using similar conditions. This can be seen in Figure 3, wherethe Do 34, D1 36, and Dg-stages 37 (chlorine dioxide bleaching stages) are operatingunder acidic conditions, while the (OP) stage 35 (pressurized extraction stage, fortifiedwith hydrogen peroxide) operates under alkaline conditions. As such, the water from(OP) 35 cannot be fed back to the Do-stage 34 without first modifying its properties, orelse serious problems such as rapid heat generation and scaling, will occur. The result ofthis is that in the open part of the f1berline, one acidic waste water flow and one alkalinewaste water flow (each corresponding to the 3.l m3 water) are obtained, as seen inFigure 3.

By re-circulating the cleaned filtrate as wash liquor at the same pulp washer, aclosed loop washing system may be formed around this particular washer, one commonlimitation can be removed. Norrnally, in the bleach plant the pulp suspension shiftsbetween alkaline and acidic conditions throughout the passage of the subsequent stages.This limits how filtrates from the washers can be used (and resulting in one acidic andone alkaline filtrate going to the sewer). However, by re-circulating the cleaned filtrateas wash liquor at the same pulp washer, this limitation can be overcome.

In one embodiment, a system for washing pulp in a pulping process comprisesat least one washer l. The washer l comprises a wash liquor inlet 2 for receiving washliquor for use in washing paper pulp, a wash liquor outlet 3 for discharging wash filtratecomprising contaminants originating from the pulp and pulping process. The systemfurther comprises for the washer l: a purifier 20, connected to the wash liquor outlet 3,for purifying the wash f1ltrate comprising contaminants originating from the pulp andpulping process to purif1ed wash liquor and separating the contaminates originatingfrom the pulp and pulping process as floc; a re-circulation conduit l3, connected to thepurif1er 20 and the wash liquor inlet 2, for re-circulating purif1ed wash liquor to the pulpwasher liquor inlet 2; a floc outlet 22, connected to the purif1er 20, for removal of theseparated flocculated contaminants originating from the pulp and pulping process; andoptionally a pulp dilution loop ll, connected to the re-circulation conduit l3, fortransferring purif1ed wash liquor to the pulp for diluting the pulp to a desired dilution factor before it is washed in the washer l. In one further embodiment, the inline purif1er 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 12 comprises a wash liquor inlet 21, a floc outlet 22, and purified Water outlet 23. This isvisualized in Figure 6A.

As can be seen in Figures 4 to 6, by having an individual inline purifier for thewash liquor of the washer 1, this flow is reused for the same washer 1, resulting in awash liquor of similar properties constantly being fed to the washer. The advantages ofthis is shown even better in Figure 5, where the acidic (from DO 34) and alkaline (from(OP) 35) filtrates going to the sewer are completely avoided, by re-feeding them back tothe same washer after inline purif1cation and thus avoiding the problems of having tochange the properties of the wash liquor. In the example f1berline of Figure 3, thisrepresents a substantial waste water flow of 6.2 m3 per ton of pulp.

However, there might be situations where only a part of the purif1ed washliquor is re-circulated. Remaining purif1ed wash liquor may for instance be led toevaporation and incineration in the recovery boiler, fed to another kind of drier orfumace for disposal, or fed to a second washer in a manner similar to a conventionalcounter-current wash liquor utilization. If so, the re-circulation could also be defined asa ratio of the re-circulation flow to the counter-current flow of wash liquor.

Figure 7 shows such an example, where part of the purified wash liquor is fedto a second washer in a counter-current flow. Here, a stage consistency of 11 %, anoutlet consistency from the wash press of 32 % and a dilution factor of 1.0 m3/BDt areemployed. In this case that means a wash f1ltrate of 9.1 m3/BDt. 6.0 m3/BDt of these aretypically (although in unpurif1ed form) used for dilution of the pulp suspension into thestage before the washer (flow “A”). The remaining 3.1 m3/BDt would typically be usedas wash liquor on the previous (or even earlier) washer (flow “B”). The currentinvention makes use of the f1ltrate in flow “A” in the conventional way (although inpurified form), but replaces the flow “B” by the flow “C” to a high degree, so that atleast 25 % of the combined flows “B” and “C” goes the “C” route. Preferably, an evenhigher flow, such as at least 35%, 45%, 55%, 65%, 75%, 85%, or 95% ofthe combinedflows of “B” and “C” goes the “C” route. As illustrated by figures 4, 5, even 100% ofthe combined flows “B” and “C” my go the “C” route. For other kinds of washers thanwash presses, flow “A” is typically much smaller as the discharge consistency is in therange from 10-18 %, rather than around 30 %. Accordingly, the combined flows of “B”and “C” is also signif1cantly higher.

In one embodiment, said purified wash liquor is further used for a secondapplication, such as a counter-current flow to a second washer, wherein at least 25 mass-% of the flow is re-circulated, calculated from the combined flow for re- 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 13 circulation and for the second application. In one further embodiment, at least 35 mass-%, such as at least 45 mass-%, such as at least 55 mass-%, such as at least 65 mass-%,such as at least 75 mass-%, such as at least 85 mass-%, such as at least 95 mass-%, oreven 100 mass-% of the flow is re-circulated, calculated from the combined flow for re-circulation and for the second application. In one embodiment, the system furthercomprises a conduit to a second application (14) for use of the purif1ed wash liquor atthe second application, which is connected to the purif1er (20) and to an inlet for thesecond application.

Although Figure 5 does not indicate any f1ltrate reaching the sewer at all,bleed-outs will be required at certain times. But still the water consumption of the kinddescribed here will be significantly more water and energy efficient than the prior art.The re-circulation conduit 13 may thus comprise an outlet for bleeding out wash liquoror purif1ed water, and an inlet for adding clean water or wash liquor. This enablesexchange or dilution of the wash liqour in the fluid conduit. This may be advisable if thewash liquor contains ions that are not removed together with the floc, to prevent aconcentration build-up of such ions. In one embodiment, the re-circulation conduit 13comprises an outlet for bleeding out wash liquor or purif1ed water. In one embodiment,re-circulation conduit 13s comprises, an inlet for adding clean water or wash liquor.

The invention may utilize any suitable electrocoagulation unit or separationunit. In one embodiment, the inline purif1er 20 is an electrocoagulation unit 24,potentially complemented by a separation unit 25, such as an electroflotation unit, forseparation of the floc.

In Figures. 4, 5 and 7, the purif1er 20 is exemplified with an electrocoagulationunit of the AxoPurTM brand including a suitable floc separation device and installationsare exemplified at three different positions. The AxoPurTM electrocoagulation unit is anelectrocoagulation unit 24 designed to provide a particularly uniform and therebyefficient electrochemical dosage of coagulating cations like Fe(II), Al(III) and Mg(II)thanks to its coaxial design and automated control program.

The principle of electrocoagulation has been known since 1906 when the firstpatent relating to the field was filed by Albert E. Dietrich, US823671 A. Little utiliseduntil the second half of the 20th century the technology has become more popular in the21st century. One reason for this development is the prices of electricity coming down,being the main consumable of the process. For a smaller footprint purification methodfor water, electrocoagulation could be considered. Electrocoagulation is a rapid way of separating suspended and dissolved solids from a liquor flow thus producing a high 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 14 density floc and a “clean” water. Separation of suspended material, transition metalions, organic matter like COD, phosphate etc. is usually almost complete. Very smallorganic molecules e.g. methanol and alkali metal ions e. g. sodium ion and halide ionse. g. chloride ion usually substantially follow the “clean” water. The electrocoagulationprocess makes use of sacrificing electrodes producing coagulation aids in the form ofcharged metal ions. Commonly used electrode materials include aluminium and iron,but also other metals are sometimes used.

In patent RU2002124194 electrocoagulation is utilised to purify sewage ofcellulose semi-finished goods production as a way to reduce the load to the recipient.

In patent EP2486l87 electrocoagulation is applied on a fibre and particlecontaining suspension in a paper mill, focusing on the production of board. It alsodiscusses the recirculation of the purified water back to the process. However, the patentdoes not address the in-line purification of a con-currently flow of wash liquor in af1berline.

WO 2014/072586 relates to a method for treating liquid flows at a chemicalpulp mill and comprises conVeying at least a portion of white waters from a pulp dryingmachine to an oxygen delignif1cation unit.

WO 2008/ 152186 relates to a method for treating liquid flows at a chemicalpulp mill with an effluent purification plant for treating bleaching plant effluents andother effluents generated at the mill where at least a portion of the effluents is retumedafter the purification to the pulp production line as source of process water.

CN1110452C relates to a process for recovering and treating waste water inpaper-making featuring the use of a f1ltering device where it is said that all of the usefulsubstances are physically and chemically recovered and fully used and the treated watertakes part in closed cycle for reuse. However, it seems to pool filtered waste water forfurther filtering and possibly storage before any suggested re-use of the waste water.

As such, from the prior art usage, it seems to be an end-of-pipe solution to acleaning problem and not an inline solution for purification in such a way that also theheat of the wash liquor is preserved. However, as shown in the invention,electrocoagulation has tumed out surprisingly suitable for use in purification of washfiltrate from washers l in the pulping process of the inVention.

In one embodiment, the purif1er 20 comprises an electrocoagulation unit 24 forflocculating contaminants in said wash liquor comprising contaminants from saidpulping process. In one embodiment, the electrocoagulation unit 24 comprises at least one wearing electrode, the wearing electrode comprising a metal selected from the 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc group consisting of iron, aluminium, magnesium, zinc, molybdenum, manganese,titanium, zirconium and alloys including one or many of these metals. In oneembodiment, the electrocoagulation unit 24 comprises a non-wearing electrode, the non-wearing electrode comprising a material selected from the group consisting of iron,aluminium, magnesium, stainless steel, MMO (Mixed Metal Oxides), platinum,graphite, titanium, and a boron-doped metal.

The electrodes may be of the same materials. Thus, in one embodiment, theelectrocoagulation unit 24 comprises a non-wearing electrode of the same material asthe wearing electrode.

The acquired floc may be separated using a floc separator 25, as can be seen infigure 6B, together with the electrocoagulation unit 24. Thus, in one embodiment, thepurifier 20 further comprises a floc separator 25 to separate the floc from the purifiedwater. Such a floc separator 25 is not intended to be particularly limited. For example,the floc separator 25 may include a flotation device, a sedimentation tank, a filter, and amicrofiltration tube, centrifuge. Several other floc separators 25 are may also be used,such as flotation unit equipped with a scraper, a discharge pipe fitted with a screwconveyor, a belt filter centrifuge depth filter electrostatic precipitator evaporator filterpress fractionating column leachate mixer-settler protein skimmer rotary vacuum-drumfilter scrubber spinning cone still sublimation apparatus vacuum ceramic filter. In oneembodiment, the floc separator 25 is selected from the group consisting of a flotationdevice, sedimentation tank, filter, microfiltration tube, and centrifuge.

According to the invention the filtrate from a washer l equipped with rapidinline cleaning reuses the cleaned filtrate as wash liquor on the same washer l. Thismeans that the wash liquor is “clean”, thus having a very low content of suspendedmatter and dissolved organic material. This fact, together with the fact that thetemperature of the wash liquor in the closed system created this way will be higher thanin today°s counter-current mode of washing, such that the carry-over of organics fromone stage to the next decreases heavily.

In one embodiment, the purification of said wash filtrate and the subsequent re-circulation of said purified wash liquor in total take at most 240 min, preferably l20min, preferably 60 min, most preferably 30 min, in order to maintain a high temperatureand low viscosity of the wash liquor.

In one embodiment, the pulping process is a kraft pulping process. In onefurther embodiment, the kraft pulping process comprises a cooking step of the pulp fibre source. A washing step after screening 3l the pulp for removing any pulp fibre 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 16 bundles that have failed to separate. Optionally, a washing step after an optional oxygendelignif1cation 32 for removing residual lignin left in pulp after Cooking using oxygenand alkali conditions. Optionally, a washing step after an optional post oxygen washing33, to further remove oxidised lignin from the pulp. An acidic chlorine dioxidebleaching stage 34, 36, 37 to bleach the pulp. At least one washing step after an acidicchlorine dioxide bleaching stage 34, 36, 37. An alkaline extraction/bleaching stage 35,to extract degraded structures and further bleach the pulp. A washing step after onealkaline extraction/bleaching stage 35. In one further embodiment, at least one washer lis located at the washing step after screening 3 l , and/or at the washing step after theoptional oxygen delignif1cation 32, and/or at the washing step after the optional postoxygen washing 33, and/or at the washing step after the acidic chlorine dioxidebleaching 34, 36, 37, and/or at the washing step after the alkaline bleaching stage 35.

Thus, in one embodiment, a pulping process may comprise at least one, such asat least two, such as at least three, such as at least four, such as at least five, such as atleast six, such as at least seven, such as at least eight, such as least nine, such at leastten, such at least eleven, such at least twelve washers 1, each washer 1 having a purifier20 for purifying the wash f1ltrate comprising contaminants originating from the pulpand pulping process to purified wash liquor and re-circulating at least part of the f1ltrateas wash liquor to the same washer l.

As described above, the example fiberline of figures 1 to 5 comprises the so-called closed part of the fiberline, being the brownstock and oxygen delignificationareas. “Dig” stands for digester and “scr” for screening. Also, the so-called open part ofthe f1berline, being the bleaching area. O relates to oxygen delignification, D to chlorinedioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxideand oxygen.

In the design of a new fiberline, it would become possible to reduce thenumber of washers in the closed area from todays 5/4 to at least 4/3, potentially even3/2. This means dramatic savings in investment costs for the washer as such, but alsofor reduced needs for pumps, piping - and even the building. At the same time the lowernumber of washers then utilised is also a good way to maintain more of the inherentstrength of the pulp f1bre, in the end enabling a lower amount of f1bres in a certain paperproduct. Also, for some paper qualities, notably for use as insulation material in electriccables, a particularly low conductivity of the paper is desired. Pulp for such papers witha particularly low metal content could be achieved by means of a particularly efficient washing using closed loop f1ltrate purif1cation as in the invention. 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 17 The high density floc made up of the organic Substance separated in the inlinepurif1er 20 When this is equal to an electrocoagulation unit 24 is brought to and blendedWith the filtrate from the first Washer l (typically the HiHeat Washer in the digester),thus not more than marginally increasing the amount of Water going to evaporation andincineration, but also marginally increasing the amount of organic matter taken care ofthis Way due to a de-creased carry-over to the bleaching.

According to the invention the filtrate from a Washer l equipped With rapidinline cleaning reuses the cleaned filtrate as Wash liquor on the same Washer l. Thismeans that the Wash liquor is “c1ean”, thus having a very low content of suspendedmatter and dissolved organic material.

In the closed loop process, a substantial part of the sodium ions (in the closedloop) Will also be separated as a high density floc, since the Will be sorbed in theorganic matter.

For the open loop area the utilisation of the invention is illustrated in Figure 5,Where the cleaning device is exemplified With an (AxoPurTM) electrocoagulation unitand installations are exemplified at four different positions. Chloride ions are to asubstantial degree left in the cleaned filtrate from D-stages. The use of this liquor fordilution of the pulp suspension incoming to the bleaching stage is positive as thechloride ions could react With chlorate ion deadload to regenerate chlorine dioxide insítu at the same time as the environmentally harrnful chlorate is removed. Thus, in oneembodiment, purified Wash liquor containing chloride ions is used for diluting the pulpto a desired dilution factor before it is Washed in the Washer 1, Whereby the chlorideions react With chlorate Whereby chlorate ion deadload is regenerated to chlorinedioxide and environmentally harrnful chlorate is removed.

As mentioned above the commonly used electrode materials inelectrocoagulation are aluminium and iron. In the electrocoagulation process theelectrodes are consumed such that a small amount of these metals is dissolved into thetreated Water flow and thus transferred onWards in the process. HoWever, in therecovery process of kraft pulping spent cooking liquors aluminium is considered a non-process element, Which needs to be purged at certain times to keep the cooking processrunning as intended. Thus, the use of aluminium electrodes in the traditionally closedloop area should be avoided. Instead, iron, magnesium and zinc are good options here.Thus, in one embodiment, for a Washer located upstream of the bleaching stages 34, 35, the purifier 20 comprises an electrocoagulation unit 24 comprising iron or magnesium 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 18 electrodes, whereby the concentration of non-process elements, such as aluminiumcompounds, will not increase to any substantial degree in the recovery cycle.

In the traditionally open loop of the f1berline, the situation is different. Instages using hydrogen peroxide as a bleaching agent the presence of iron ions ispotentially detrimental since they could catalyse hydrogen peroxide decomposition intoharrnful hydroxyl radicals causing the pulp strength to deteriorate at the same time asthe bleaching efficiency is reduced. The same goes for manganese ions. Since the highdensity floc produced in this part of the f1berline is not bumt in the recovery boiler,aluminium electrodes here are on the other hand not a problem. Magnesium is also agood option here, whereas thus iron and manganese electrodes should preferably beavoided, since iron and/or manganese ions and hydrogen peroxide are capable ofoxidizing a wide range of substrates and causing biological damage through the Fentonreaction, a complex reaction capable of generating both hydroxyl radicals and higheroxidation states of the iron/manganese. Thus, in one embodiment, for a washer locatedat or downstream of the bleaching stages 34, 35, the purif1er 20 comprises anelectrocoagulation unit 24 comprising aluminium or magnesium electrodes, wherebypotential hydrogen peroxide decomposition is avoided.

The fact that the purif1ed wash liquor is “clean”, together with the fact that thetemperature of the wash liquor in the system of the invention will be higher than intoday°s counter-current mode of washing, leads to that the carry-over of organics fromone stage to the next decreases signif1cantly. Together with the chloride ion effectdescribed above, a new bleach plant for the manufacture of bleached kraft pulp shouldbe very possible to shorten from currently typical 4 stages to 3 or 2 wash stages. Thiscorresponds also to an equivalent reduction in the number of washers in the open area.This means dramatic savings in investment costs for the washer l as such, but also forreduced needs for pumps, piping - and even the building. At the same time the lowernumber of washers then utilised is also a good way to maintain more of the inherentstrength of the pulp f1bre, in the end enabling a lower amount of f1bres in a certain paperproduct.

The high density floc made up of the organic substance from washers llocated at or downstream of the bleaching stages 34, 35 (i.e. in the open loop), when thepurif1er 20 comprises an electrocoagulation unit 24, will be chloride containing and assuch should not be bumt in the recovery boiler, but should to be bumt in some othercontrolled manner. However, the fact that this chloride containing matter does not reach the sewer but instead may be collected as compact floc means a significantly decreased 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 19 demand for secondary Water treatments e. g. in an aerated lagoon. It also means that theWater consumption of the fiberline is reduced. In one embodiment, separated flocs fromWash filtrate from Washers l located upstream of the bleaching stages 34, 35 arecollected and sent to thickening/evaporation and incineration for heat and chemicals”recovery. Separated flocs from Wash filtrate containing chloride ions are recoveredseparately, and not bumt in the recovery boiler. There is of course also always theopportunity, in both cases, to separate the floc and use it for other purposes e.g. as afertilizer or as a resource for chemical production.

The invention should primarily be considered in conjunction With themanufacture of chemical kraft pulp. In one embodiment, the pulp is chemical pulp,preferably kraft pulp. Some aspects of the invention relates specifically to the chlorinedioxide bleaching stage(s) of kraft pulp. HoWever, it Will be appreciated that theinvention is not limited to this application but may be applied to the manufacture ofchemical sulphite pulp, neutral sulphite semi-chemical pulp (NSSC), chemimechanicaland/or mechanical pulps e. g. CTMP, TMP, PGW or SGW - and also de-ink pulp madefrom Waste paper. Thus, in one embodiment, the pulp is paper pulp, fluff pulp, dissolving pulp or derivative pulp.

ExampleIn a trial to evaluate the invention calculations Were carried out to simulate the effect of inline Wash liquor purif1cation units. The f1berline in Figure l Was used as abasis for the calculations With one exception being the introduction of a Wash press in-betWeen the last tWo chlorine dioxide reactors as this is the typical setup for a mill. It islikely that guarantee figures for such a fiberline Would have been Washing efficiencyvalues for the Wash presses of 0.98 (as Ynio - Ragnar et al. 2006), discharge consistencyof 32 % and a dilution factor of 2.0 m3/BDt. HoWever, in practice a mill is usually notpossible to run like that, not even directly after startup. More realistic values includeWashing efficiencies of around 0.93-0.95 and towards the end of the bleaching rather inthe range 0.70-0.75 and discharge consistencies from 26-30 %. In this example thismode of operation is called reference. Most mills become overloaded over time to somedegree. Running this f1berline at its maximum hydraulic capacity Would probably endup With Washing efficiencies of 0.60 and discharge consistencies of 23 % at a dilutionfactor of -2.0 m3/BDt. This is referred to as the overload case. Installation of AxoPurTMinline cleaning devices for all seven positions indicated in Figures 4 and 5 leads to the AxoPurTM overload scenario, Whereas AxoPurTM reference is the same AxoPurTM 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc installation. Indicative COD values of the pulp suspension after dilution down to 10 %consistency to the following treatment stage are given in Table 1. Anticipations aremade on incoming COD of 800 kg/BDt to the wash press after screening. Observe thatthe below values are not carry-over at high consistency, but de facto COD contents at 10% consistency after dilution. In the calculation figures for COD forrnation (or liberation from the lumen of the fibres by means of diffusion) are also included.

Table 1. Indicative COD-values (kg/BDt) in calculations simulating fivedifferent scenarios for the operation of the f1berline in Figure 1 with an additional wash press inbetween the last two chlorine dioxide bleaching towers.

Point of Guarantee Reference Overload AxoPurTM AxoPurTMmeasure Overload referenceAfter Scr 229 281 438 139 155 1 after O 65 88 318 117 49 2 after O 56 106 166 63 41After DO 40 60 89 40 42After (OP) 31 54 52 30 14After D1 4 11 42 16 6After D2 2 5 15 7 4 The comparison shows several interesting things. The AxoPurTM referencescenario compared to the Reference shows significant and important improvements inseveral positions in spite of 0 used fresh water and a completely closed bleach plant inthis case. Notably differences occur e.g. 2 after O 42 kg/BDt instead of 106 kg/BDt andafter Scr 155 kg/BDt instead of 281 kg/BDt. Position 2 after O is equivalent withingoing to the D0-stage. In the literature, 1 kg of COD has been reported to consumebetween 0.7 (Pettersson et al. 2002) and 1.0 kg a. Cl/BDt [kilogram of active chlorineper bone dried ton of pulp, where 1.0 kg chlorine dioxide is equal to 2.63 kg a. Cl](Girard et al. 1999). Thus the difference in chlorine dioxide consumption between thesetwo cases could be estimated to an additional chlorine dioxide requirement of around 20kg ClO2/BDt corresponding to an additional cost of some 20 USD/BDt. Similarly,Ragnar has reported negative effects of in particular carry-over but also of oxidizedCOD into an oxygen delignification stage leading to a reduced delignification degreeand a reduced selectivity of the process.

Comparing the Overload scenario with AXoPurTM overload shows that theformer would come at very high operational costs for chemicals etc, whereas the AXoPurTM overload scenario in almost all positions mean better performance that the 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 21 Reference case. Another Way of interpreting these results is thus to conclude that insteadof putting in new additional Washers in the overloaded line, installation of a number of AxoPurTM units could fix the overload situation.

Although the present invention has been described above With reference tospecific embodiments, it is not intended to be limited to the specific forrn set forthherein. Rather, the invention is limited only by the accompanying claims and, otherembodiments than the specific above are equally possible Within the scope of theseappended claims, e. g. different than those described above.

In the claims, the terrn “comprises/comprising” does not exclude the presenceof other elements or steps. Furthermore, although individually listed, a plurality ofmeans, elements or method steps may be implemented by e. g. a single unit.Additionally, although individual features may be included in different claims, thesemay possibly advantageously be combined, and the inclusion in different claims doesnot imply that a combination of features is not feasible and/or advantageous. Inaddition, singular references do not exclude a plurality. The terms “a”, “an”, “first”,“second” etc do not preclude a plurality. Reference signs in the claims are providedmerely as a clarifying example and shall not be construed as limiting the scope of the claims in any Way.

References 1. Ragnar, M.; Backa, S.; Wilgotson, F.; On the Comparability, Reliabilityand Repeatability of Washing Efficiency Measures in a Fiberline for Chemical Pulp,Workshop on Chemical Pulping Processes 2006, 58-64 2. Girard, R., Ho, C., Gutherie, P. and Campbell, P. (1999): The Effects ofCarbon Dioxide on the Efficiency of Various Brown Stock Washers, Tappi PulpingConference, Vol. 3: 1225-1234. 3. Pettersson, E. A. K., Ragnar, M. and Dahllöf, H. (2002): A CompactApproach to Washing in the Bleaching, 35th annual meeting of Associaçäo BrasileiraTecnica de Celulose e Papel (ABTCP), Sao Paulo, Brazil 4. Ragnar, M.; Boosting Oxygen Delignification by means of MaximisingAvailability of Dissolved Oxygen, 94th PAPTAC conference 2008, B347-B355

Claims (20)

1. Method for Washing pulp in a pulping process, Wherein the pulp is Washedin a pulp Washer (1) using aqueous Wash liquor to obtain Washed pulp and a Washfiltrate comprising contaminants originating from the pulp and pulping process, Whereinthe method further comprises purifying said Wash filtrate comprising contaminantsoriginating from the pulp and pulping process to provide purified Wash liquor by; - flocculating said Wash filtrate comprising contaminants originating from the pulp and pulping process; and - separating the contaminants, as floc formed in the flocculating of said Wash filtrate comprising contaminants originating from the pulp and pulpingprocess, from the Wash filtrate to provide purified Wash liquor, the method further comprising re-circulating at least 25 mass-% of saidpurified Wash liquor to the Washer (1) to be used as Wash liquor, calculated from thetotal mass of said Wash filtrate comprising contaminants originating from the pulp and pulping process.

2. Method according to claim 1, Wherein the purification of said Washfiltrate, and the subsequent re-circulation of said purified Wash liquor in total take atmost 240 min, preferably 120 min, preferably 60 min, most preferably 30 min, in order to maintain a high temperature and low viscosity of the Wash liquor.

3. Method according to claim 1 or 2, Wherein at least at least 35 mass-%,such as at least 45 mass-%, such as at least 55 mass-%, such as at least 65 mass-%, suchas at least 75 mass-%, such as at least 85 mass-%, such as at least 95 mass-%, or even100 mass-% of said purified Wash liquor is re-circulated to the Washer to be used asWash liquor, calculated from the total mass of said Wash filtrate comprising contaminants originating from the pulp and pulping process.

4. Method according to any of claims 1 or 3, Wherein the purification of saidfiltrate is carried out using a continuous flow to provide a continuous re-circulation of purified Wash liquor to the Washer.

5. Method according to any of claims 1 to 4, Wherein the method further comprises using said purified Wash liquor for a second application, such as a counter- 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 23 current flow to a second Washer, Wherein at least 25 mass-% of the flow is re-circulated, calculated from the combined flow for re-circulation and for the second application.

6. The method according to claim 5, Wherein at least 35 mass-%, such as atleast 45 mass-%, such as at least 55 mass-%, such as at least 65 mass-%, such as at least75 mass-%, such as at least 85 mass-%, such as at least 95 mass-%, or even 100 mass-%of the flow is re-circulated, calculated from the combined flow for re-circulation and for the second application.

7. A method according to any of the previous claims, Wherein the pulping process is a kraft pulping process.

8. A method according to claim 7, Wherein the kraft pulping processcomprises; a cooking step of the pulp fibre source, a Washing step after screening (31) the pulp for removing any pulp fibrebundles that have failed to separate, optionally a Washing step after an optional oxygen delignif1cation (32) forremoving residual lignin left in pulp after cooking using oxygen and alkali conditions, optionally a Washing step after an optional post oxygen Washing (33) to furtherremove oxidised lignin from the pulp, an acidic chlorine dioxide bleaching stage (34, 36, 37) to bleach the pulp, at least one Washing step after acidic chlorine dioxide bleaching stage (34, 36,37) an alkaline extraction/bleaching stage (35) to to extract degraded structures andfurther bleach the pulp, and a Washing step after one alkaline extraction/bleaching stage (35).

9. A method according to any of the proceeding claims, Wherein the pulp is paper pulp, fluff pulp, dissolving pulp, or derivative pulp.

10. A system for Washing pulp in a pulping process according to any one ofclaims 1 to 9, comprising at least one Washer (1), Wherein the Washer (1) comprises; 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 24 a Wash liquor inlet (2) for receiving Wash liquor for use in Washing paperpu1p, a Wash liquor Outlet (3) for discharging Wash filtrate comprisingcontaminants originating from the pulp and pulping process, Wherein the system further comprises for the Washer (1); a purifier (20), connected to the Wash liquor outlet (3), for purifying theWash f1ltrate comprising contaminants originating from the pulp andpulping process to purified Wash liquor and separating thecontaminates originating from the pulp and pulping process as floc, a re-circulation conduit (13), connected to the purifier (20) and the Washliquor inlet (2), for re-circulating purif1ed Wash liquor to the pulpWasher liquor inlet (2), a floc outlet (22), connected to the purif1er (20), for removal of theseparated flocculated contaminants originating from the pulp andpulping process, and optionally a pulp dilution loop (11), connected to the re-circulation conduit(13), for transferring purified Wash liquor to the pulp for diluting the pulp to a desired dilution factor before it is Washed in the Washer (1).

11. The system according to claim 10, Wherein the purifier (20) comprises, aWash liquor inlet (21), a floc outlet (22), and a purified Water outlet (23).

12. The system according to claim 11 or 12, Wherein the Washer (1) is selectedfrom the group consisting a Wash press, pressure diffuser, atmospheric diffuser, filterWasher, Compaction Baffle-Washer, Drum Displacer-Washer, deWatering press, and belt Washer.

13. The system according to any one of the claims 10 to 12, Wherein thepurifier (20) comprises an electrocoagulation unit (24) for coagulating and flocculating contaminants in said Wash liquor comprising contaminants from said pulping process.

14. The system according to claim 13, Wherein the electrocoagulation unit(24) comprises at least one wearing electrode, the wearing electrode comprising a metal selected from the group consisting of iron, aluminium, magnesium, zinc, molybdenum, 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc manganese, titanium, zirconium or alloys consisting of comprising at last one of these metals.

15. The system according to claim 13 or 14, Wherein the electrocoagulationunit (24) comprises a non-wearing electrode, the non-wearing electrode comprising amaterial selected from the group consisting of iron, aluminium, magnesium, stainlesssteel, MMO (Mixed Metal Oxides), platinum, graphite, titanium, and a boron-doped metal.

16. The system according to any of claims 10 to 15, Wherein the purif1er (20) further comprises a floc separator (25) to separate the floc from the purif1ed Water.

17. The system according to claim 16, Wherein the floc separator (25) isselected from the group consisting of a flotation device, sedimentation tank, filter,microfiltration tube, and centrifuge.

18. The system according to any of claims 10 to 17, Wherein the systemfurther comprises a conduit to a second application (14) for use of the purified Washliquor at the second application, Which is connected to the purif1er (20) and to an inlet atthe second application.

19. The system according to any of claims 10 to 18, Wherein the pulp ischemical pulp, preferably kraft pulp.

20. The system according to any of claims 10 to 19, Wherein the pulpingprocess comprises; a Washing step after screening (31) the pulp for remoVing any pulp fibrebundles that have failed to separate, and optionally a Washing step after an optional oxygen delignif1cation (32) forremoVing residual lignin left in pulp after cooking using oxygen and alkali conditions, optionally a Washing step after an optional post oxygen Washing (33) to furtherremove oxidised lignin from the pulp, an acidic chlorine dioxide bleaching stage (34, 36, 37) to bleach the pulp, at least one Washing step after acidic chlorine dioxide bleaching stage (34, 36,37) 171128 I:\Patrawin\TEMP\AJ\~P130960011*Tvättpatent massa s.2017112912470426304.doc 26 an alkaline extraction/bleaching stage (35) to to extract degraded structures andfurther bleach the pulp, and a Washing step after one alkaline extraction/bleaching stage (35), Wherein in the system at least one Washer is located; 5 at the Washing step after screening (31), and/or at the Washing step after the optional oxygen delignification (32), and/or at the Washing step after the optional post oxygen Washing (33), and/or at the Washing step after the acidic chlorine dioxide bleaching (34, 36, 37),and/or 10 at the Washing step after the alkaline bleaching stage (35).