Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents
  • D21C9/04—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents in diffusers ; Washing of pulp of fluid consistency without substantially thickening
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
  • D21C11/0028—Effluents derived from the washing or bleaching plants
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents

Definitions

• 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.
• 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.
• 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.
• 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.
• Compromising means that none of the desires above is reached.
• 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.
• 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
• others e.g. hydrogen peroxide and oxygen
• 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.
• the current setup is a compromise between a number of different conflicting desires, including the wish to use as little water as possible to:
• 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
• 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
• 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 chlorine dioxide bleaching
• 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 AxoPurTM, 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 chlorine dioxide bleaching
• 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 AxopurTM, 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).
• AxoPur a stage consistency of 11 %
• an outlet consistency from the wash press of 32 % and a dilution factor of 1.0 nfVBDt are employed.
• 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.
• washing is a unit operation aiming to separate the pulp from dissolved dirt by means of using water.
• the resulting dirty water could itself be subject to a separation treatment whereby the dissolved dirt could be separated from the water.
• 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.
• 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.
• 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
• Figs. 4 and 5 illustrates an example of wash liquor utilisation when cleaning and re-circulation is used in accordance with the invention.
• the pulp in a method for a for washing pulp in a pulping process, 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the inline purifier comprises a wash liquor inlet 21, a floe outlet 22, and purified water outlet 23. This is visualized in Figure 6A.
• 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.
• a stage consistency of 11 % an outlet consistency from the wash press of 32 % and a dilution factor of 1.0 nfVBDt are employed.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the re-circulation conduit 13 comprises an outlet for bleeding out wash liquor or purified water.
• 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.
• 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.
• the purifier 20 is exemplified with an electrocoagulation unit of the AxoPurTM brand including a suitable floe separation device and installations are exemplified at three different positions.
• the AxoPurTM 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.
• 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.
• 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.
• 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.
• electrocoagulation has turned out surprisingly suitable for use in purification of wash filtrate from washers 1 in the pulping process of the invention.
• the purifier 20 comprises an electrocoagulation unit 24 for flocculating contaminants in said wash liquor comprising contaminants from said pulping process.
• 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.
• 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 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.
• the purifier 20 further comprises a floe separator 25 to separate the floe from the purified water.
• a floe separator 25 is not intended to be particularly limited.
• the floe separator 25 may include a flotation device, a sedimentation tank, a filter, and a microfiltration tube, centrifuge.
• 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.
• the floe separator 25 is selected from the group consisting of a flotation device, sedimentation tank, filter, microfiltration tube, and centrifuge.
• 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.
• 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.
• the pulping process is a kraft pulping process.
• 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.
• 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.
• An alkaline extraction/bleaching stage 35 to extract degraded structures and further bleach the pulp.
• 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.
• 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.
• 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.
• 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.
• the first washer 1 typically the HiHeat washer in the digester
• the filtrate from a washer 1 equipped with rapid inline cleaning reuses the cleaned filtrate as wash liquor on the same washer 1.
• the wash liquor is“clean”, thus having a very low content of suspended matter and dissolved organic material.
• 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.
• 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.
• the purifier 20 comprises an electrocoagulation unit 24 comprising aluminium or magnesium electrodes, whereby potential hydrogen peroxide decomposition is avoided.
• 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.
• 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.
• 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.
• 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.
• the pulp is paper pulp, fluff pulp, dissolving pulp or derivative pulp.
• 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 %.
• 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 AxoPurTM inline 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 installation.
• the comparison shows several interesting things.
• the AxoPurTM 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.
• 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).

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