Field of the invention

The 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 wood chips, or other fibrous raw material, into pulp for use e.g. in papermaking accomplished by cooking the chips in a cooking liquor aiming to liberate the fibers from the lignin matrix 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 to benefit 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 names are applicable for spent liquors from sulfite processes. This is done in order to minimize the water content of the liquor entering the fumace/recovery boiler. It is clear that the higher the organic content of the spent liquor is, the higher is the heat value of it and the more economically interesting it is to bum the liquor.

To provide an overview of a typical modem fiberline for the manufacture of bleached chemical pulp, figure 1 illustrates such a fiberline, more precisely for bleached kraft 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 to be used, these would most likely be combined with a separate washer to reach the same performance as the continuous digester, why the systems are still comparable with regards to waste water treatment.

The fiberline is in figure 1 is shown in two rows, the upper row illustrating the so-called closed part of the fiberline, being the brownstock and oxygen delignification areas.“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 oxygen delignification, D to chlorine dioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Highlighted are fresh water intake positions (grey thick arrows) and filtrate discharge positions (black thick arrows) in a fiberline.

In chemical pulping the wood chips are first subjected to steaming,

impregnation and cooking. In the lower part of a continuous digester (the dominant technology for chemical pulping) the pulp is washed and in the blowline the washing continues, in this particular case using a pressure diffuser washer, before the pulp enters a blowtank/storage tower. Following screening the pulp is again washed, here on a wash press and then subjected to oxygen delignification in two subsequent reactors. After venting 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 wash press before the bleaching. This position also marks the end of the closed loop of the fiberline, meaning that the clean wash water applied on this last wash press produces a dirty filtrate, which is used as wash liquor to the wash press next before in line etc, so that the wash liquor is used counter-currently. The filtrate from the first washer, in this case the washer in the lower part of the digester, is taken care of. It goes first to evaporation and then to incineration in the fumace/recovery boiler. The high organic content of the stream is then utilized for energy recovery, whereas the inorganic components are reduced such that the cooking chemicals sodium hydrosulfide and sodium hydroxide can be formed again after dissolution and recaustization.

In a typical modem fiberline (such as exemplified in figure 1) a compromise has been made between a number of different conflicting desires, including the wish to use as little water as possible to:

- Get the dissolved organic substance as highly concentrated as possible for most efficient energy recovery, implying that it is first and foremost the early washers that are of interest to collect filtrate from - but running the wash liquor counter-currently means that the concentrations build up.

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

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

- Mechanically treat the pulp as little as possible in order to maintain as much as possible of the inherent strength of the pulp fibers, implying the 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 fiberline, speaking in favor of using as short sequence as possible by means e.g. of using clean 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 several reasons. The dirt concentration in the form of dissolved organics is way lower in this part of the plant meaning that the cost for energy recovery is very high. Moreover, in bleach 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 be rich in chloride ions. Would such a filtrate be burnt in a recovery boiler it would cause severe corrosion and stickies formation, which obviously is not desired.

In the bleaching, washing is also important in order to minimize the carry-over of dissolved organic substance into the bleaching. Doing so, the COD to effluent in the open bleach plant is kept low as well as the discharge of AOX. About 80 % of the AOX from the bleaching is normally formed in the first chlorine dioxide stage, the DO-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 entering into a chlorine dioxide stage together with the pulp suspension consumes about 0.2-0.4 kg chlorine dioxide/ADt pulp (in this patent the definitions ADt=Air Dried Ton of pulp and BDt=Bone Dried Ton of pulp are used to describe pulp that is 90 % dry (ADt) and 100 % dry (BDt) respectively). This argument is also valid as argument for the importance of washing between different stages in the bleaching just as the argument that washing here is important also as a means of transferring heat between stages and to adjust other process conditions, e.g. pH.

In the bleaching of chemical pulp different bleaching chemicals are applied to the pulp in a number of subsequent bleaching stages, each of which is usually followed by a washer to wash out the dissolved substance and the degradation products of the bleaching chemicals. Some bleaching chemicals (e.g. chlorine dioxide, ozone and peracetic acid) are used in acidic conditions, whereas others (e.g. hydrogen peroxide and oxygen) are used in alkaline conditions. This means that throughout the passage of the 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 in the chlorine dioxide stage (usually this is the first bleaching stage and chlorine dioxide is usually also used in the third stage and possibly in yet another stage) filtrates from the washers in the bleaching could not be used as wash liquor in the closed part of the fiberline as it would then end up in the recovery boiler and cause severe corrosion there. For this reason, both one acidic and one alkaline filtrate from the bleaching always go to the sewer.

The current setup is a compromise between a number of different conflicting desires, 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 low viscosity for the water and thereby a more efficient washing, implying the counter-current use of the wash liquor is a good idea.

- Boost bleaching efficiency by making productive use of residual hydrogen peroxide (valid for hydrogen peroxide stages), residual chlorine dioxide (valid for chlorine dioxide stages) or chloride ions formed in the chlorine 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 clean water used as wash liquor on each washer

- Minimize undesired side reactions of the bleaching chemicals with dissolved dirt instead of with the pulp to be bleached

- Mechanically treat the pulp as little as possible in order to maintain as much as possible of the inherent strength of the pulp fibers, implying the 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 fiberline, speaking in favor of using as short sequence as possible by means e.g. of using clean water as wash liquor on all the washers

Not mentioned, but obvious, is that the acidic and alkaline wash liquor flows need to be kept apart. The costs for changing pH of a certain flow by adding sulfuric acid 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 is of 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 low to do so. To a certain extent, washers are usually possible to run at a higher load, but always at the expense of a decreased washing efficiency. Eventually the matter becomes more of managing to push the pulp through the washer considering only the hydraulic capacity of the washer and completely neglecting the washing efficiency. At an early stage bleaching chemical consumptions begin to increase as a result of less efficient washing, but also pulp strength and other quality parameters soon will be negatively affected. And at some point, further along that path the pulp simply becomes too dirty for its aimed utilization. Accordingly, investments in enhanced washing are on the agenda 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 or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a method 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 purifying said wash filtrate comprising contaminants originating 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 floe formed in the flocculating 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.

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 for receiving wash liquor for use in washing paper pulp, a wash liquor outlet for discharging wash filtrate comprising contaminants originating from the pulp and pulping process, wherein the system further comprises for the washer; a purifier, connected to the wash liquor outlet, for purifying the wash filtrate comprising contaminants originating from the pulp and pulping process to purified wash liquor and separating the contaminates originating from the pulp and pulping process as floe, a re- circulation conduit, connected to the purifier and the wash liquor inlet, for re-circulating purified wash liquor to the pulp washer liquor inlet, a floe outlet, connected to the purifier, for removal of the separated flocculated contaminants originating from the pulp and pulping process, and optionally a pulp dilution loop, connected to the re-circulation conduit, 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 is capable of will be apparent and elucidated from the following description of

embodiments of the present invention, reference being made to the accompanying drawings, 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 (grey thick arrow) and filtrate discharge positions (black thick arrow) in a fiberline. Clean water (either fresh water or condensate or a mixture) is used as wash liquor in three positions indicated by grey thick arrows. Two thick black arrows indicate two flows of dirty 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, being the brownstock and oxygen delignification areas.“Dig” stands for digester and“scr” for screening. Figures relate to the amount of wash liquor prevailing in different positions during continuous operation (in m ³ /BDt pulp) and with assumptions of a dilution factor of 1.0 nfVBDt, 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 discharge consistency of 11 % and the same goes, theoretically, for the HiHeat-washer inside the digester. The thick grey arrow indicates the clean water used for washing. This clean water is usually made up of condensate. The thick black arrow from the wash in Dig is the flow of black liquor going to evaporation and incineration,

Fig. 3 illustrated wash liquor flows in the open part of the fiberline, being the bleaching area. O relates to oxygen delignification, D to chlorine dioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Figures relate to the amount of wash liquor prevailing in different positions during continuous operation (in nfVBDt pulp) and with assumptions of a dilution factor of 1.0 nfVBDt, a consistency in the stages of 11 % and a discharge consistency of 32 % from the washpresses. The grey thick arrows indicate the clean water used for washing. The black thick arrows indicate acidic (from Do) and alkaline (from (OP)) filtrates going to the sewer.

Fig. 4 illustrates wash liquor flows in the closed part of the fiberline, being the brownstock and oxygen delignification areas.“Dig” stands for digester and“scr” for screening. Figures relate to the amount of wash liquor prevailing in different positions during continuous operation (in m ³ /BDt pulp) and with assumptions of a dilution factor of 1.0 nfVBDt, 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 discharge consistency of 11 % and the same goes, theoretically, for the HiHeat-wash inside the digester. The grey thick arrow indicates the clean water used for washing. This clean water is usually made up of condensate. The black thick arrow from the wash in Dig is the stream of black liquor going to evaporation and incineration. Purifiers are labelled AxoPur™, and are can be seen coupled to the outlet of the scr-, and OO-washers. The thick horizontal arrows from the purifiers is the floe, here is going to incineration together with the black liquor.

Fig. 5 illustrates wash liquor flows in the open part of the fiberline, being the bleaching area. O relates to oxygen delignification, D to chlorine dioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxide and oxygen. Figures relate to the amount of wash liquor prevailing in different positions during continuous operation (in nfVBDt pulp) and with assumptions of a dilution factor of 1.0 nfVBDt, a consistency in the stages of 11 % and a discharge consistency of 32 % from the washpresses. The thick grey arrows indicate the clean water used for washing. The thick black arrows indicate acidic (from Do) and alkaline (from (OP)) filtrates going to the sewer. Purifiers are labelled Axopur™, and are can be seen coupled to the outlet of each of the washers, where the thick horizontal arrows from the purifiers is the separated floe.

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 floe separator, and

Fig. 7 illustrates two subsequent washers, here exemplified by two wash presses after an oxygen delignification stage. Part of the wash filtrate flow is shown for and a purifier connected to the second (labelled AxoPur). In the example a stage consistency of 11 %, an outlet consistency from the wash press of 32 % and a dilution factor of 1.0 nfVBDt are employed. In this case that means a purified wash filtrate of 9.1 m ³ /BDt. 6.0 m ³ /BDt of these are typically used for dilution of the pulp suspension into the stage before the washer (flow“A”), whereas 3.1 m ³ /BDt are traditionally used as wash liquor on the previous (or even earlier) washer (flow“B”). The current invention makes use of the purified filtrate in flow“A” in the conventional way (although with purified 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 invention applicable to the washing of paper pulp. In very general terms, washing is a unit operation 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 treatment whereby the dissolved dirt could be separated from the water.

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

When upgrading an overloaded fiberline, the installation of an additional washer is usually considered. This means a costly and time-consuming investment for the mill as manufacturing times are long, but also space within the mill has to be found/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 the pulp strength negatively. This option is thus not very attractive, although usually the one chosen.

Another option available is to use fresh, clean water as wash liquor early in the fiberline (one of the first washers after the digester and in any case before the first bleach stage) where the pulp is particularly dirty, making the pulp suspension become cleaner than if using the (dirty) water from the downstream cleaner. However, the water consumption of the mill also increases, which in most countries today would mean a problem as water resources begin to be scarce and as environmental permits usually would 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 more fresh water also means an increased amount of water in this flow, thus decreasing its heat value. If, instead, some of the wash filtrate is led to the sewer, the result will be an increased load on the waste water treatment facility, neither of which is desired. So in case more fresh water should be needed, it must be found internally in the mill. In such a situation closest at hand would be considering the opportunity to take water from the 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 a treatment pond, the residence time may be a matter of weeks, in which case the temperature of the wash water after having passed the treatment plant will be ambient or low - probably in the range 5-20 °C, which will mean that the washer will not perform very well due to the high viscosity of the water at such temperatures. In theory, it is of course possible to use a heat exchanger between the wash liquor going to the water treatment plant and the clean water coming back from there. However, due to the high content of dissolved solids and fibres in the wash filtrate, the risk for plugging and scaling of the heat exchanger is evident. It seems that whatever choice is made, there is a negative coming along. And, as practice shows, the usual answer to the challenge is accepting the negatives of the investment in an additional washer in the fiberline.

The invention relates to the filtrate coming from a pulp washer and the rapid inline cleaning of this filtrate in such a way that the organic matter could be separated as a high density floe while maintaining most of the heat so that the cleaned filtrate (i.e purified wash liquor) could be used as wash liquor on the same washer.

The recirculation of the cleaned filtrate thereby decreases the carry-over of dirt in the pulp suspension from the washer at which it is used, to the subsequent treatment stage both due to the utilisation of a cleaner wash liquor and due to a higher washing temperature decreasing the viscosity of the wash liquor and thereby enhancing the washing efficiency. To illustrate this, Figs. 2 to 5 are showing the flow of the wash liquor in a fiberline such as the example fiberline of Fig. 1. Figs. 2 and 3 show a conventional mode of wash liquor utilisation, and Figs. 4 and 5 illustrates an example of wash liquor utilisation when cleaning and re-circulation is used in accordance with the invention. Figs. 4 to 5 show a system where the method of the invention is used on each of the washers 1 from the washer 1 after screening 31 to the washer 1 after chlorine dioxide bleaching 37. It may however be desirable to only implement this for one or a few 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 and a wash filtrate comprising contaminants originating from the pulp and pulping process. The method further comprises purifying said wash filtrate comprising contaminants originating 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 floe formed in the flocculating 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 1 to be used as wash liquor, calculated from the total mass of said wash filtrate comprising contaminants originating from the pulp and pulping process.

A high level of re-circulation is often favorable. As such, 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-%, 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 said wash filtrate comprising contaminants originating from the pulp and pulping process.

To simplify examples, the pulp washers in Figures 2 to 5 are being illustrated by wash presses, since they are a water effective solution, however, the washer may be of other suitable types. The optimum pulp consistency is about 11 %, so since the wash presses press the pulp to a consistence of 32 %, 6 m ³ is used to dilute the pulp to a suitable consistency of 11%, as shown in Figures 2 to 5. In one embodiment, the washer 1 is selected from the group consisting a wash press, pressure diffuser, atmospheric diffuser, 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 fiberline, it is illustrated how in a closed loop system with a counter- flow, the wash liquor not used for dilution of the pulp (referred to as 3.1 m ³ flow) will be transferred to the previous washer, counter to the pulp direction. One effect of this is that the wash liquor will build up more and more dirt. However, since wash liquor will travel from one washer to another, its properties will also differ to some degree, such as in temperature or pH, from the properties of the existing liquor in the washer 1. Such variability of the counter-flow wash liquor may have an impact on the wash result, and can in worst case lead to scaling, which in the long run will lead to cleaning procedures which may possibly interfere with or even interrupt the pulp production process on the fiberline. This can be a continuous flow, to make sure that temperature is maintained at a steady level. In one embodiment, the flocculation, the purification of said filtrate is carried out using a continuous flow to provide a continuous re-circulation of purified wash liquor to the washer.

Another effect of this is that a closed loop reverse flow of waste water can only be fed to pulp washers 1 using similar conditions. This can be seen in Figure 3, where the Do 34, Di 36, and D ₂ -stages 37 (chlorine dioxide bleaching stages) are operating under acidic conditions, while the (OP) stage 35 (pressurized extraction stage, fortified with 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, or else serious problems such as rapid heat generation and scaling, will occur. The result of this is that in the open part of the fiberline, one acidic waste water flow and one alkaline waste water flow (each corresponding to the 3.1 m ³ water) are obtained, as seen in Figure 3.

By re-circulating the cleaned filtrate as wash liquor at the same pulp washer, a closed loop washing system may be formed around this particular washer, one common limitation can be removed. Normally, in the bleach plant the pulp suspension shifts between 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 and one alkaline filtrate going to the sewer). However, by re-circulating the cleaned filtrate as wash liquor at the same pulp washer, this limitation can be overcome.

In one embodiment, a system for washing pulp in a pulping process comprises at least one washer 1. The washer 1 comprises a wash liquor inlet 2 for receiving wash liquor for use in washing paper pulp, a wash liquor outlet 3 for discharging wash filtrate comprising contaminants originating from the pulp and pulping process. The system further comprises for the washer 1 : a purifier 20, connected to the wash liquor outlet 3, for purifying the wash filtrate comprising contaminants originating from the pulp and pulping process to purified wash liquor and separating the contaminates originating from the pulp and pulping process as floe; a re-circulation conduit 13, connected to the purifier 20 and the wash liquor inlet 2, for re-circulating purified wash liquor to the pulp washer liquor inlet 2; a floe outlet 22, connected to the purifier 20, for removal of the separated flocculated contaminants originating from the pulp and pulping 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. In one further embodiment, the inline purifier comprises a wash liquor inlet 21, a floe outlet 22, and purified water outlet 23. This is visualized in Figure 6A.

As can be seen in Figures 4 to 6, by having an individual inline purifier for the wash liquor of the washer 1 , this flow is reused for the same washer 1 , resulting in a wash liquor of similar properties constantly being fed to the washer. The advantages of this 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 to the same washer after inline purification and thus avoiding the problems of having to change the properties of the wash liquor. In the example fiberline of Figure 3, this represents a substantial waste water flow of 6.2 m ³ per ton of pulp.

However, there might be situations where only a part of the purified wash liquor is re-circulated. Remaining purified wash liquor may for instance be led to evaporation and incineration in the recovery boiler, fed to another kind of drier or furnace for disposal, or fed to a second washer in a manner similar to a conventional counter-current wash liquor utilization. If so, the re-circulation could also be defined as a 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 fed to a second washer in a counter-current flow. Here, a stage consistency of 11 %, an outlet consistency from the wash press of 32 % and a dilution factor of 1.0 nfVBDt are employed. In this case that means a wash filtrate of 9.1 nfVBDt. 6.0 nfVBDt of these are typically (although in unpurified form) used for dilution of the pulp suspension into the stage before the washer (flow“A”). The remaining 3.1 m ³ /BDt would typically be used as wash liquor on the previous (or even earlier) washer (flow“B”). The current invention makes use of the filtrate in flow“A” in the conventional way (although in purified form), but replaces the flow“B” by the flow“C” to a high degree, so that at least 25 % of the combined flows“B” and“C” goes the“C” route. Preferably, an even higher flow, such as at least 35%, 45%, 55%, 65%, 75%, 85%, or 95% of the combined flows of“B” and“C” goes the“C” route. As illustrated by figures 4, 5, even 100% of the combined flows“B” and“C” my go the“C” route. For other kinds of washers than wash presses, flow“A” is typically much smaller as the discharge consistency is in the range from 10-18 %, rather than around 30 %. Accordingly, the combined flows of“B” and“C” is also significantly higher.

In one embodiment, said purified wash liquor is further used for a second application, 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- 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-%, or even 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 further comprises a conduit to a second application (14) for use of the purified wash liquor at the second application, which is connected to the purifier (20) and to an inlet for the second application.

Although Figure 5 does not indicate any filtrate reaching the sewer at all, bleed-outs will be required at certain times. But still the water consumption of the kind described 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 liquor or purified water, and an inlet for adding clean water or wash liquor. This enables exchange or dilution of the wash liquor in the fluid conduit. This may be advisable if the wash liquor contains ions that are not removed together with the floe, to prevent a concentration build-up of such ions. In one embodiment, the re-circulation conduit 13 comprises an outlet for bleeding out wash liquor or purified 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 separation unit. In one embodiment, the inline purifier 20 is an electrocoagulation unit 24, potentially complemented by a separation unit 25, such as an electro flotation unit, for separation of the floe.

In Figures. 4, 5 and 7, the purifier 20 is exemplified with an electrocoagulation unit of the AxoPur™ brand including a suitable floe separation device and installations are exemplified at three different positions. The AxoPur™ electrocoagulation unit is an electrocoagulation unit 24 designed to provide a particularly uniform and thereby efficient 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 first patent relating to the field was filed by Albert E. Dietrich, US823671 A. Little utilised until the second half of the 20th century the technology has become more popular in the 21 st 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 method for water, electrocoagulation could be considered. Electrocoagulation is a rapid way of separating suspended and dissolved solids from a liquor flow thus producing a high density floe and a“clean” water. Separation of suspended material, transition metal ions, organic matter like COD, phosphate etc. is usually almost complete. Very small organic molecules e.g. methanol and alkali metal ions e.g. sodium ion and halide ions e.g. chloride ion usually substantially follow the“clean” water. The electrocoagulation process makes use of sacrificing electrodes producing coagulation aids in the form of charged 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 of cellulose semi-finished goods production as a way to reduce the load to the recipient.

In patent EP2486187 electrocoagulation is applied on a fibre and particle containing suspension in a paper mill, focusing on the production of board. It also discusses the recirculation of the purified water back to the process. However, the patent does not address the in-line purification of a con-currently flow of wash liquor in a fiberline.

WO 2014/072586 relates to a method for treating liquid flows at a chemical pulp mill and comprises conveying at least a portion of white waters from a pulp drying machine to an oxygen delignification unit.

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

WO 00/43589 Al relates to a method for treatment of pulp, where part of the liquor to be fed to a washer is fractionated into two liquor fractions, a cleaner liquor fraction and a fouler liquor fraction. The concept centers on using both these liquor fractions, the cleaner liquor fraction in the flow direction of the fiber suspension and the fouler liquor fraction counter-currently relative to the flow direction of the fiber suspension.

JP2004218150A relates to a manufacturing method of bleached pulp where the filtrate from two pulp washers are intermixed, the COD in the intermixed filtrate is oxidized/degraded, and the mixed filtrate is fed back to one washer. Instead of separating the COD from the washing liquor, the concept centers on oxidizing COD in the intermixed washing liquor. The theory being that the oxidized COD does not need to be re-oxidized when used as washing liquor, whereby the usage of bleaching chemicals in the washing step may be reduced. CN1110452C relates to a process for recovering and treating waste water in paper-making featuring the use of a filtering device where it is said that all of the useful substances are physically and chemically recovered and fully used and the treated water takes part in closed cycle for reuse. However, it seems to pool filtered waste water for further 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 a cleaning problem and not an inline solution for purification in such a way that also the heat of the wash liquor is preserved. However, as shown in the invention,

electrocoagulation has turned out surprisingly suitable for use in purification of wash filtrate from washers 1 in the pulping process of the invention.

In one embodiment, the purifier 20 comprises an electrocoagulation unit 24 for flocculating contaminants in said wash liquor comprising contaminants from said pulping process. In one embodiment, 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, manganese, titanium, zirconium and alloys including one or many of these metals. In one embodiment, 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, the electrocoagulation unit 24 comprises a non- wearing electrode of the same material as the wearing electrode.

The acquired floe may be separated using a floe separator 25, as can be seen in figure 6B, together with the electrocoagulation unit 24. Thus, in one embodiment, the purifier 20 further comprises a floe separator 25 to separate the floe from the purified water. Such a floe separator 25 is not intended to be particularly limited. For example, the floe separator 25 may include a flotation device, a sedimentation tank, a filter, and a microfiltration tube, centrifuge. Several other floe separators 25 are may also be used, such as flotation unit equipped with a scraper, a discharge pipe fitted with a screw conveyor, a belt filter centrifuge depth filter electrostatic precipitator evaporator filter press fractionating column leachate mixer-settler protein skimmer rotary vacuum-drum filter scrubber spinning cone still sublimation apparatus vacuum ceramic filter. In one embodiment, the floe separator 25 is selected from the group consisting of a flotation device, sedimentation tank, filter, microfiltration tube, and centrifuge. According to the invention the filtrate from a washer 1 equipped with rapid inline cleaning reuses the cleaned filtrate as wash liquor on the same washer 1. This means that the wash liquor is“clean”, thus having a very low content of suspended matter and dissolved organic material. This fact, together with the fact that the temperature of the wash liquor in the closed system created this way will be higher than in today’s counter-current mode of washing, such that the carry-over of organics from one 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 120 min, preferably 60 min, most preferably 30 min, in order to maintain a high temperature and low viscosity of the wash liquor.

In one embodiment, the pulping process is a kraft pulping process. In one further embodiment, the kraft pulping process comprises a cooking step of the pulp fibre source. A washing step after screening 31 the pulp for removing any pulp fibre bundles that have failed to separate. Optionally, a washing step after an optional oxygen delignification 32 for removing residual lignin left in pulp after cooking using oxygen and alkali conditions. Optionally, a washing step after an optional post oxygen washing 33, to further remove oxidised lignin from the pulp. An acidic chlorine dioxide bleaching stage 34, 36, 37 to bleach the pulp. At least one washing step after an acidic chlorine 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 one alkaline extraction/bleaching stage 35. In one further embodiment, at least one washer 1 is located 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 at the washing step after the alkaline bleaching stage 35.

Thus, in one embodiment, a pulping process may comprise at least one, such as at least two, such as at least three, such as at least four, such as at least five, such as at least six, such as at least seven, such as at least eight, such as least nine, such at least ten, such at least eleven, such at least twelve washers 1 , each washer 1 having a purifier 20 for purifying the wash filtrate comprising contaminants originating from the pulp and pulping process to purified wash liquor and re-circulating at least part of the filtrate as wash liquor to the same washer 1.

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 delignification areas.“Dig” stands for digester and“scr” for screening. Also, the so-called open part of the fiberline, being the bleaching area. O relates to oxygen delignification, D to chlorine dioxide bleaching and (OP) to pressurized extraction fortified with hydrogen peroxide and oxygen.

In the design of a new fiberline, it would become possible to reduce the number of washers in the closed area from todays 5/4 to at least 4/3, potentially even 3/2. This means dramatic savings in investment costs for the washer as such, but also for reduced needs for pumps, piping - and even the building. At the same time the lower number of washers then utilised is also a good way to maintain more of the inherent strength of the pulp fibre, in the end enabling a lower amount of fibres in a certain paper product. Also, for some paper qualities, notably for use as insulation material in electric cables, a particularly low conductivity of the paper is desired. Pulp for such papers with a particularly low metal content could be achieved by means of a particularly efficient washing using closed loop filtrate purification as in the invention.

The high density floe made up of the organic substance separated in the inline purifier 20 when this is equal to an electrocoagulation unit 24 is brought to and blended with the filtrate from the first washer 1 (typically the HiHeat washer in the digester), thus not more than marginally increasing the amount of water going to evaporation and incineration, but also marginally increasing the amount of organic matter taken care of this way due to a de-creased carry-over to the bleaching.

According to the invention the filtrate from a washer 1 equipped with rapid inline cleaning reuses the cleaned filtrate as wash liquor on the same washer 1. This means that the wash liquor is“clean”, thus having a very low content of suspended matter and dissolved organic material.

In the closed loop process, a substantial part of the sodium ions (in the closed loop) will also be separated as a high density floe, since the will be sorbed in the organic matter.

For the open loop area the utilisation of the invention is illustrated in Figure 5, where the cleaning device is exemplified with an (AxoPur™) electrocoagulation unit and installations are exemplified at four different positions. Chloride ions are to a substantial degree left in the cleaned filtrate from D-stages. The use of this liquor for dilution of the pulp suspension incoming to the bleaching stage is positive as the chloride ions could react with chlorate ion deadload to regenerate chlorine dioxide in situ at the same time as the environmentally harmful chlorate is removed. Thus, in one embodiment, purified wash liquor containing chloride ions is used for diluting the pulp to a desired dilution factor before it is washed in the washer 1 , whereby the chloride ions react with chlorate whereby chlorate ion deadload is regenerated to chlorine dioxide and environmentally harmful chlorate is removed.

As mentioned above the commonly used electrode materials in

electrocoagulation are aluminium and iron. In the electrocoagulation process the electrodes are consumed such that a small amount of these metals is dissolved into the treated water flow and thus transferred onwards in the process. However, in the recovery 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 process running as intended. Thus, the use of aluminium electrodes in the traditionally closed loop 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 electrodes, whereby the concentration of non-process elements, such as aluminium compounds, will not increase to any substantial degree in the recovery cycle.

In the traditionally open loop of the fiberline, the situation is different. In stages using hydrogen peroxide as a bleaching agent the presence of iron ions is potentially detrimental since they could catalyse hydrogen peroxide decomposition into harmful hydroxyl radicals causing the pulp strength to deteriorate at the same time as the bleaching efficiency is reduced. The same goes for manganese ions. Since the high density floe produced in this part of the fiberline is not burnt in the recovery boiler, aluminium electrodes here are on the other hand not a problem. Magnesium is also a good option here, whereas thus iron and manganese electrodes should preferably be avoided, since iron and/or manganese ions and hydrogen peroxide are capable of oxidizing a wide range of substrates and causing biological damage through the Fenton reaction, a complex reaction capable of generating both hydroxyl radicals and higher oxidation states of the iron/manganese. Thus, in one embodiment, for a washer located at or downstream of the bleaching stages 34, 35, the purifier 20 comprises an electrocoagulation unit 24 comprising aluminium or magnesium electrodes, whereby potential hydrogen peroxide decomposition is avoided.

The fact that the purified wash liquor is“clean”, together with the fact that the temperature of the wash liquor in the system of the invention will be higher than in today’s counter-current mode of washing, leads to that the carry-over of organics from one stage to the next decreases significantly. Together with the chloride ion effect described above, a new bleach plant for the manufacture of bleached kraft pulp should be very possible to shorten from currently typical 4 stages to 3 or 2 wash stages. This corresponds also to an equivalent reduction in the number of washers in the open area. This means dramatic savings in investment costs for the washer 1 as such, but also for reduced needs for pumps, piping - and even the building. At the same time the lower number of washers then utilised is also a good way to maintain more of the inherent strength of the pulp fibre, in the end enabling a lower amount of fibres in a certain paper product.

The high density floe made up of the organic substance from washers 1 located at or downstream of the bleaching stages 34, 35 (i.e. in the open loop), when the purifier 20 comprises an electrocoagulation unit 24, will be chloride containing and as such should not be burnt in the recovery boiler, but should to be burnt in some other controlled manner. However, the fact that this chloride containing matter does not reach the sewer but instead may be collected as compact floe means a significantly decreased demand for secondary water treatments e.g. in an aerated lagoon. It also means that the water consumption of the fiberline is reduced. In one embodiment, separated floes from wash filtrate from washers 1 located upstream of the bleaching stages 34, 35 are collected and sent to thickening/evaporation and incineration for heat and chemicals’ recovery. Separated floes from wash filtrate containing chloride ions are recovered separately, and not burnt in the recovery boiler. There is of course also always the opportunity, in both cases, to separate the floe and use it for other purposes e.g. as a fertilizer or as a resource for chemical production.

The invention should primarily be considered in conjunction with the manufacture of chemical kraft pulp. In one embodiment, the pulp is chemical pulp, preferably kraft pulp. Some aspects of the invention relates specifically to the chlorine dioxide bleaching stage(s) of kraft pulp. However, it will be appreciated that the invention is not limited to this application but may be applied to the manufacture of chemical sulfite pulp, neutral sulfite semi-chemical pulp (NSSC), chemimechanical and/or mechanical pulps e.g. CTMP, TMP, PGW or SGW - and also de-ink pulp made from waste paper. Thus, in one embodiment, the pulp is paper pulp, fluff pulp, dissolving pulp or derivative pulp.

Example

In a trial to evaluate the invention calculations were carried out to simulate the effect of inline wash liquor purification units. The fiberline in Figure 1 was used as a basis 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 is likely that guarantee figures for such a fiberline would have been washing efficiency values for the wash presses of 0.98 (as Fnio - Ragnar et al. 2006), discharge consistency of 32 % and a dilution factor of 2.0 nfVBDt. However, in practice a mill is usually not possible to run like that, not even directly after startup. More realistic values include washing efficiencies of around 0.93-0.95 and towards the end of the bleaching rather in the range 0.70-0.75 and discharge consistencies from 26-30 %. In this example this mode of operation is called reference. Most mills become overloaded over time to some degree. Running this fiberline at its maximum hydraulic capacity would probably end up with washing efficiencies of 0.60 and discharge consistencies of 23 % at a dilution factor of -2.0 nfVBDt. This is referred to as the overload case. Installation of AxoPur™ inline cleaning devices for all seven positions indicated in Figures 4 and 5 leads to the AxoPur™ overload scenario, whereas AxoPur™ reference is the same AxoPur™ 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 are made on incoming COD of 800 kg/BDt to the wash press after screening. Observe that the 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 formation (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 five different scenarios for the operation of the fiberline in Figure 1 with an additional wash press inbetween the last two chlorine dioxide bleaching towers.

Point of arantee Reference Overload AxoPur™ AxoPur™ measure Overload reference

After Scr 229 281 438 139 155

1 after O 65 88 318 117 49

2 after O 56 106 166 63 41 After DO 40 60 89 40 42 After (OP) 31 54 52 30 14 After D 1 4 11 42 16 6 After D2 2 5 15 7 4

The comparison shows several interesting things. The AxoPur™ reference scenario compared to the Reference shows significant and important improvements in several positions in spite of 0 used fresh water and a completely closed bleach plant in this case. Notably differences occur e.g. 2 after O 42 kg/BDt instead of 106 kg/BDt and after Scr 155 kg/BDt instead of 281 kg/BDt. Position 2 after O is equivalent with ingoing to the DO-stage. In the literature, 1 kg of COD has been reported to consume between 0.7 (Pettersson et al. 2002) and 1.0 kg a. Cl/BDt [kilogram of active chlorine per 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 these two cases could be estimated to an additional chlorine dioxide requirement of around 20 kg Cl02/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 oxidized COD into an oxygen delignification stage leading to a reduced delignification degree and a reduced selectivity of the process.

Comparing the Overload scenario with AxoPur™ overload shows that the former would come at very high operational costs for chemicals etc, whereas the AxoPur™ overload scenario in almost all positions mean better performance that the Reference case. Another way of interpreting these results is thus to conclude that instead of putting in new additional washers in the overloaded line, installation of a number of AxoPur™ units could fix the overload situation.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims, e.g. different than those described above.

In the claims, the term“comprises/comprising” does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit.

Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, 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 provided merely 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, Reliability and 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 of Carbon Dioxide on the Efficiency of Various Brown Stock Washers, Tappi Pulping

Conference, Vol. 3: 1225-1234.

3. Pettersson, E. A. K., Ragnar, M. and Dahllof, H. (2002): A Compact Approach to Washing in the Bleaching, 35th annual meeting of Associagao Brasileira Tecnica de Celulose e Papel (ABTCP), Sao Paulo, Brazil

4. Ragnar, M.; Boosting Oxygen Delignification by means of Maximising

Availability of Dissolved Oxygen, 94th PAPTAC conference 2008, B347-B355