Note: Within nine months of the publication of the mention of the grant of the European patent in the European PatentBulletin, any person may give notice to the European Patent Office of opposition to that patent, in accordance with theImplementing Regulations. Notice of opposition shall not be deemed to have been filed until the opposition fee has beenpaid. (Art. 99(1) European Patent Convention).

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(12) EUROPEAN PATENT SPECIFICATION

(45) Date of publication and mention of the grant of the patent: 02.03.2016 Bulletin 2016/09

(21) Application number: 11874958.9

(22) Date of filing: 04.11.2011

(51) Int Cl.:D21C 11/04 (2006.01) D21C 11/00 (2006.01)

(86) International application number: PCT/SE2011/051317

(87) International publication number: WO 2013/066225 (10.05.2013 Gazette 2013/19)

(54) METHOD FOR THE CAUSTICIZING PROCESS FOR PRODUCING WHITE LIQUOR

VERFAHREN FÜR EINEN KAUSTIFIZIERUNGSPROZESS ZUR HERSTELLUNG VON WEISSLAUGE

PROCÉDÉ POUR LE TRAITEMENT DE CAUSTIFICATION DESTINÉ À LA PRODUCTION DE LIQUEUR BLANCHE

(84) Designated Contracting States: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(43) Date of publication of application: 10.09.2014 Bulletin 2014/37

(73) Proprietor: Valmet AB851 94 Sundsvall (SE)

(72) Inventors: • HOGEBRANDT, Eva

SE-653 46 Karlstad (SE)• LABIGALINI BARANOVSKY, Alessandra

CEP 82.530-090 Curitiba - PR (BR)

(74) Representative: Becker Kurig StrausPatentanwälte Bavariastrasse 780336 München (DE)

(56) References cited: WO-A1-97/22752 WO-A1-2005/116329US-A- 4 668 342 US-A- 4 929 355US-A- 5 145 556 US-A- 5 628 875US-A- 5 788 813

• STRID, K. ET AL.: ’A New Filter System for Filtration of Green and White Liquor’ 1992 PULPING CONFERENCE, TAPPI PROCEEDINGS 1992, pages 1 - 5, XP008173548

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Description

FIELD OF THE INVENTION

[0001] The present invention relates to a method forpreparation of white liquor in a chemical recovery processof the kraft process. It affects the total system lay out ofthe causticizing process between input of raw green liq-uor and final production of a clear white liquor.

BACKGROUND OF THE INVENTION

[0002] The causticizing process has conventionallyused a lot of different process steps for;

• reception of the green liquor;• separation of dregs from green liquor;• washing and drying dregs obtained from the previous

separation step;• mixing of clear green liquor and burnt lime in order

to slake lime and start the causticizing reaction;• tanks for completion of the causticizing reaction;• separation of lime mud from white liquor;• lime mud washing and drying.

[0003] A typical conventional causticizing process isshown in figure 1. The raw green liquor RGL is first re-ceived in an equalizing tank EQT and from there pumpedto a first green liquor separation process, here shown asa green liquor pressurized disc filter GLF. The green liq-uor filter separates dregs from the raw green liquor andproduces clear green liquor which is sent to a green liquorstorage tank GLT. The clear green liquor is then sent,most often via a green liquor cooler GLC, to the slakerSL where burnt lime is mixed into the green liquor. Thecooler is needed to reduce temperature ahead of the slak-er to keep the slurry in the slaker under boiling point asthe reactions occurring in and after the slaker are exo-thermic. Grits, i.e. unreacted fractions of the burnt lime,are also separated out from the slaker. After mixing inthe slaker, the slurry is sent to a series of causticizingvessels CT1-CT2-CT3, often named the causticizingtrain, wherein the chemical causticizing reactions arecompleted. Once these causticizing reactions are com-pleted, the slurry is pumped to a white liquor separationprocess, here shown as white liquor pressurized disc fil-ter WLF. The white liquor filter separates lime mud fromthe caustiziced liquor and produces clear white liquor,which is sent to a white liquor storage tank WLT. Theclear white liquor is then sent directly to be used in thekraft cooking or bleaching line, or alternatively via apolysulfide modification process to said kraft cooking.The lime mud, which still may have a residual content ofalkali, is sent to a lime mud washing and drying stage,here shown as a lime mud pressurized disc filter LMF.[0004] Once the lime mud is washed and dried it maybe passed to the lime kiln in order to convert it to burntlime to be used in the slaker again.

[0005] In these conventional causticizing processes asshown in figure 1, a specific start up procedure for thegreen liquor separation process has been used. Duringstart up, the green liquor filter has initially been filled withcausticizised liquor from the causticizing train CT1-CT2-CT3 in order to build up a precoat of lime mud on thesurface of the filter cloth. The reason for this formationof lime mud precoat is that this precoat exhibit a far betterseparation efficiency than the cloth itself and has a betterfilterability than would a precoat formed by dregs fromgreen liquor. The filterability improves by a factor of 6 ifa precoat is formed by lime mud instead of green liquormud (dregs). However, this short establishment of theprecoat using causticizised liquor from the causticizingtrain CT1-CT2-CT3 has never been used for longer pe-riods than about 5% of the total cycle time of the greenliquor filter, and as soon as this precoat has been formed,the major part of the operating time for the green liquorfilter has been used for green liquor filtering, and the mainpart of the white liquor produced, typically more than 90%of the total amount, is obtained from the dedicated whiteliquor filter.[0006] However, usage of pressurized disc filters, onefor white liquor filtration and one for green liquor filtration,are expensive as the costs for these filters are high. Fil-tering techniques are often better as cleaner product liq-uors could be obtained with small amounts of suspendedsolids in the product liquors, typically with content lessthan 20 ppm, as compared with typical green liquor hav-ing more than 1500 ppm. Another advantage is that dregsor lime mud separated from these filters could be ob-tained at very high dryness in the range 40-60% and60-75% respectively. Alternative techniques has there-fore been considered, and usage of conventional settlingtanks for green liquor has once again been consideredsimply due to less investment costs, even though theamount of suspended solids often are much higher, typ-ically four times more.Another problem with these conventional processes isthat so many different and dedicated separation appara-tuses are needed, requiring a lot of free building area.This will be problematic when trying to increase capacityof the causticizing plant, as most often no available roomis at hand for additional apparatuses increasing the ca-pacity.[0007] In WO 2005/116329 is disclosed a filter wherethe cleaning nozzles for the filter discs are supplied withthe very filtrate obtained from the filter. This solution im-plemented in order to reduce dilution of the filtrate withwater. The preferred embodiment shown use a dedicatedsingle stage pressurized white liquor filter.[0008] In US 4668342 is disclosed an alternative filter-ing technique for raw white liquor, where the raw whiteliquor is first subjected to a centrifuge, separating mostof the lime mud particles, and finally using a white liquorclarifier. Here the green liquor from the dissolving tankis separately handled first in a separate clarifier andthereafter in a separate green liquor filter.

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[0009] In US 5145556 is disclosed another set up witha dedicated white liquor filter and also a dedicated greenliquor clarifier and dregs filter. US 5145556 per se is allabout an integration of the green liquor clarifier and thecaustizising vessel in one and the same vessel.[0010] In US 5628875 is disclosed yet another set upwith dedicated process equipment for green liquor han-dling, and dedicated process equipment for white liquorhandling. US 5628875 per se is all about improvementof the green liquor filtration where calcium is added in anamount that corresponds to at least % of the content ofmagnesium.[0011] In WO 97/22752 is disclosed a similar set upwith dedicated process equipment for green liquor han-dling, and dedicated process equipment for white liquorhandling.[0012] US 51455562 is trying to integrate dual func-tionality in the recovery operation where the green liquorclarifier and the caustizising vessel is integrated in oneand the same vessel.

SUMMARY OF THE INVENTION

[0013] The present invention is provided by appendedclaim 1. Beneficial embodiments are provided in the de-pendent claims. The invention is based upon the surpris-ing finding that using a common separation process ap-paratus for white and green liquor separation will main-tain a very efficient green liquor separation process asof reduced content of suspended solids, low residual al-kali in dregs separated as well as high dryness in dregs.There is thus no need for a multitude of dedicated sep-aration processes for white and green liquor.[0014] The present invention also shows a method forsimplification of the recausticizing process using far lessseparation apparatuses and thus may provide a solutionfor increasing capacity in any given available area nothaving the possibility of increasing the building area ofthe causticizing plant.[0015] Another objective is to reduce the risk for downtime. Normally the MTBF (mean time between failures)for the causticizing process will increase as the numbersof apparatuses needed in sequence in the process floware decreased.[0016] The invention will enable replacement of twoseparate and dedicated separation processes for white-and green liquor separation with only one separationprocess used for both the entire white- and green liquorseparation. The new separation apparatus will have aslightly larger footprint area than one of the previouslyused separation apparatuses, but require far less footprint area than the two previous separation apparatusesput together. Even though buffer tanks preceding thecommon separation apparatus will increase in size,would the net foot print area be reduced in the system.[0017] The method according to the invention is intend-ed for preparation of white liquor in a chemical recoveryprocess of the kraft process. Here the raw green liquor

is first fed to a green liquor separation process whereindregs are separated out and clear green liquor is ob-tained. Thereafter burnt lime is added to the clear greenliquor in a slaker, followed by a causticizing train with anumber of causticizing vessels wherein the causticizingprocess is finished producing causticized liquor. There-after the causticized liquor is sent to a white liquor sep-aration process wherein lime mud is separated out anda clear white liquor is obtained to be used as cookingliquor in the kraft process either in form of the clear whiteliquor or as modified by polysulfide modification in apolysulfide process. The separated lime mud is sent toa lime mud washing and drying process before feedingthe washed lime mud to a lime kiln. In this type of processthe method is characterized in that the green liquor sep-aration process and the white liquor separation processtakes place in the same common filter apparatus with nodedicated green liquor separation apparatus nor anydedicated white liquor separation apparatus, and wherethe white liquor separation process and the green liquorseparation process are conducted in sequence in thesame filter apparatus and where the white liquor sepa-ration process has a part of the cycle time in the range20-50% of the total cycle time in the same filter apparatus.[0018] In order to maintain the flexibility of the processthe method is further characterized in that an equalizingbuffer tank is preceding the green liquor separation proc-ess and where the equalizing buffer tank has a storagecapacity holding raw green liquor for at least 5 hours insaid equalizing buffer tank, and where a last buffer tankin the causticizing train has a storage capacity holding acausticized liquor for at least 2 hours in said last buffertank in the causticizing train. With this embodiment couldthe causticizing process be maintained even in case ofany interruption in the dissolving tank (where green liquoris formed) or any interruption in the causticizing reactionprocess following the slaker operation.[0019] In order to further improve the flexibility of theprocess, the method is further characterized in that theequalizing buffer tank is filled with raw green liquor whileemptying the buffer tank in the causticizing train whenperforming the white liquor separation in the commonfilter apparatus, and thereafter emptying the equalizingbuffer tank of raw green liquor while filling the buffer tankin the causticizing train when performing the green liquorseparation in the common filter apparatus. By this alter-nating filling and emptying the buffer tanks the separationprocess can be in continuous operation producing thenecessary volumes of both separated green and whiteliquors.[0020] In order to use the buffer tanks as much as pos-sible the method is further characterized in that the levelof liquors in the buffer tanks are controlled within 20-95%of the total retention capacity during white and green liq-uor separation. A certain minimum content of liquor isneeded to maintain a stabilizing volume in the equalizingtank as well as a minimum level for agitation in the buffertank, and filling of buffer tanks should not reach a full

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100% filling degree which may risk overflow of liquorsand special handling actions for such overflow.[0021] In order to improve formation of an optimal limemud precoat with a minimum of residual dregs content,which content of dregs may reduce filterability, is themethod further characterized in that the green liquor sep-aration process in said common filter apparatus is endedby a complete emptying of raw green liquor and additionof an intensified wash out process using a volume ofwashing liquid of at least 5% of the liquor volume held inthe common filter apparatus, said washing liquid not con-taining any dregs or lime mud particles, said intensifiedwash out process also entailing intense agitation in theliquid volume held in the common filter apparatus. In thiscontext it would be beneficial for the volume of washingliquid used during the intensified wash out process toexceed 3 m3 in most typical processes having a capacityof over 5300 m3 green liquor per day and over 5000 m3

white liquor per day. The wash liquid should be clean insuch aspects that any content of dregs are less than1/100 of the content in the green liquor to be filtered.[0022] According to one further aspect of the inventivemethod is also a cake of precoat maintained on the filtersurface during the intensified wash out process. Thewash out process ending each cycle after green liquorseparation is intended to flush out the vat of the separat-ing apparatus with the objective to flush out any dregsaccumulated in the vat, while maintaining the precoat sothat the following white liquor separation process couldstart immediately after termination of the wash out proc-ess.[0023] According to yet a further embodiment of theinventive method is also a total removal of the precoaton the common filter apparatus including a filter clothwash activated after two or more green liquor separationcycles and wherein a total new precoat is established insubsequent white liquor separation process in said com-mon filter apparatus. In some cases could as many asup to 3-4 green liquor separation cycles be performed insequence, interrupted by white liquor separation cyclesin between, before a total removal of the precoat is acti-vated. The number of green liquor cycles possible is de-pendent on the current status of the green liquor or thecausticized white liquor as of impurities and is very muchspecific for each mill and current type of kraft pulpingoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]

FIG. 1 is a schematic representation of a conven-tional causticizing process;

FIG. 2 is a schematic representation of the causti-cizing process according to the invention;

FIG. 3 is showing the liquor flows during the white

liquor cycle according to the invention;

FIG. 4; is showing the liquor flows during the greenliquor cycle according to the invention;

FIG. 5; is showing a typical sequence with white- andgreen liquor cycles according to the invention;

FIG. 6; is showing the usage in buffer tanks duringgreen and white liquor cycles according to the inven-tion;

FIG. 7; is showing precoat removal on filter surfacesof the common filter apparatus;

FIG 8; is showing a typical disc filter apparatus pref-erably used for the common filter apparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The inventive method is described in connec-tion with a system set up as shown in Figure 2. In hereis one single common filter apparatus GLF/WLF used forthe green and white liquor cycles.[0026] The raw green liquor RGL is first received in anequalizing tank EQT and from there pumped to the greenliquor separation process when the feed valve for greenliquor FVGL is open and the feed valve for white liquorFVWL is closed (black valves indicate closed status). Theseparation process is here shown implemented in a pres-surized disc filter GLF/WLF. The common filter apparatusGLF/WLF now operating as a green liquor filter separatesout dregs from the raw green liquor and produces cleargreen liquor sent to a green liquor storage tank GLT whenthe output valve for green liquor OVGL is open and theoutput valve for white liquor OVWL is closed. The cleargreen liquor is then sent, most often via a green liquorcooler GLC, to the slaker SL where burnt lime is mixedinto the green liquor. The cooler is needed to reduce tem-perature to well below boiling point as the reactions oc-curring in and after the slaker are exothermic. Grits, i.e.unreacted components from the burnt lime, are also sep-arated out from the slaker. After mixing in the slaker themixture is sent to a series of causticizing vessels CT1-CT2-CT3, often named the causticizing train, whereinthe chemical causticizing reactions are completed. Asthe feed valve for white liquor FVWL is closed the vesselsCT1-CT2-CT3, preferably only the last vessel CT3, areused as storage vessels for the causticizised liquor whenthe common filter apparatus GLF/WLF is used as a greenliquor filter during the green liquor cycle.[0027] When the storage vessel CT3 is reaching theupper storage capacity limit, the common filter is switch-ing to white liquor filtration. During the white liquor filtra-tion the feed valve for green liquor FVGL is closed andthe feed valve for white liquor FVWL is opened, while theoutput valve for green liquor OVGL is closed and the out-put valve for white liquor OVWL is opened. During the

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white liquor cycle the liquid is pumped from storage ves-sel CT3 to a white liquor separation process in the com-mon filter apparatus GLF/WLF, here shown as a whiteliquor pressurized disc filter. During the white liquor cyclethe filter separates out lime mud from the caustizicedliquor and produces clear white liquor sent to a whiteliquor storage tank WLT. The clear white liquor is thensent directly to be used in the kraft cooking or bleachingline, or alternatively via a polysulfide modification processto said kraft cooking. The lime mud, which still may havea residual content of alkali, is sent to a lime mud washingand drying stage, here shown as a lime mud pressurizeddisc filter LMF. Once the lime mud is washed and driedit may be passed to the lime kiln in order to convert it toburnt lime to be used in the slaker again.[0028] In figure 3 only the flows during the white liquorcycle are shown when operating the common filter ap-paratus GLF/WLF. This cycle is preferably initiated dur-ing 1.5-2 hours, during which the equalizing tank EQTfor receiving raw green liquor RGL is only used as buff-ering tank, i.e. with no outflow of any raw green liquor.As no filtered green liquor is produced, the green liquortank GLT is in an emptying process, feeding clear greenliquor to the slaker and onwards via the causticizing trainCT1-CT2-CT3 to the common filter apparatus GLF/WLF.The resulting filtered white liquor is fed from the commonfilter apparatus GLF/WLF to the white liquor tank WLT.[0029] In figure 4 only the flows during the green liquorcycle are shown when operating the common filter ap-paratus GLF/WLF. This cycle is preferably initiated dur-ing 2.5-3 hours, during which the causticizing train CT1-CT2-CT3 for receiving causticizised liquor is only usedas buffering tank, i.e. with no outflow of any causticizisedliquor. As no filtered white liquor is produced, the whiteliquor tank WLT is in an emptying process, feeding clearwhite to the cooking or bleaching process in the kraftpulping process. Raw green liquor RGL is fed from theequalizing tank EQT to the common filter apparatusGLF/WLF. The resulting filtered green liquor is fed fromthe common filter apparatus GLF/WLF to the green liquortank GLT.[0030] In figure 5 are shown a number of white andgreen liquor cycles in sequence operated according tothe inventive method. Typically within a 10 hour total cy-cle there are preferably a first white liquor cycle during1.8 hours followed by a first green liquor cycle during 2.8hours, and repeated with a subsequent second white liq-uor cycle during 1.8 hours followed by a second greenliquor cycle during 2.8 hours. After the white liquor cyclesthere are preferably only an emptying of the commonfilter apparatus GLF/WLF from causticizised white liquorduring the time interval A. But after the green liquor cyclesthere are preferably not only an emptying of the commonfilter apparatus GLF/WLF from raw green liquor duringthe time interval B, but also an improved addition of anintensified wash out process using a volume of washingliquid of at least 5% of the liquor volume held in the vatof the common filter apparatus during filtering. As indi-

cated before, the washing liquid should not contain anylarger amounts of dregs, as the objective is to flush outany dregs that may have settled into the vat of the filterapparatus, whose presence may have a negative impactduring the start of the white liquor cycle and formation ofa precoat with only lime mud on the filter cloth of thefiltering apparatus. If any dregs are still kept in the com-mon filtering apparatus when filling it up with causticizedliquor, these dregs residuals may be suspended in thecausticized liquor and then remain in the precoat formed,thus reducing the filtering capacity. In order to flush outany dregs should preferably also said intensified washout process be complemented by intense agitation in theliquid volume held in the common filter apparatus. Thiscould be implemented by any intense recirculation insidethe vat of the common filtering apparatus or adding thewashing liquid trough so called mammoth pumps locatedin the bottom area of the vat. The mammoth pumps areduring filtering operations fed with pressurized air in orderto prevent settling in the vat, and looks like an educatornozzle that is driven by the air flow and which induce asuction effect around the nozzles at the bottom wall ofthe vat.[0031] As indicated in figure 5 is also a total renewalof the precoat including a thorough cloth wash imple-mented after a last green liquor cycle, here indicated asa 30 minutes cloth wash.[0032] In figure 6 are shown how the equalizing tankEQT and the last tank CT3 in the causticizing train CT1-CT2-CT3 are used as buffer tanks during the white liquorcycle (left hand part of figure) and the green liquor cycle/right hand side of figure). During the white liquor cyclethe liquid level in the equalizing tank EQT is rising froma level of 20% and up to about 95%, while the liquid levelin CT3 is dropping from a level of 95% and down to about20%. In the subsequent green liquor cycle the oppositeeffect occurs, i.e. the liquid level in the equalizing tankEQT is dropping from a level of 95% and down to about20%, while the liquid level in CT3 is rising from a level of20% and up to about 95%.[0033] In figure 7 is shown a filter disc section used ina disc filter apparatus as shown in figure 8. Knives locatedon each side of the rotating disc, are scraping off an outerlayer of the precoat. In figure 7 is shown the principleconstitution of the precoat after a green liquor cycle,where an outermost layer of dregs has been caught ontop of the lime mud base precoat. The knives advance alittle bit into the lime mud base precoat and create a cleanlime mud surface for the following white liquor cycle. Dur-ing the white liquor cycle the knives are retracted allowingthe lime mud base precoat to build up again in thickness.[0034] In a preferred mode of operation, the knives arelocated about 12 mm from the filter cloth during start ofWL filtration and is retracted to position about 22 mmwhen a precoat of lime mud is built up on the filter cloth.At the end of the WL filtration period a lime mud precoatwith a thickness of 22 mm is thus established. When GLfiltration is started, the knives are successively moved

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towards the filter cloth and when reaching a distance of12 mm the GL filtration stops. WL filtration starts by mov-ing the knives to a distance of 10 mm in order to exposea fresh lime mud precoating and rebuilding a new limemud precoat with 22 mm thickness.[0035] In a test of the inventive method using a cyclesequence as shown in figure 5, the total cycle time wasabout 619 minutes (the "10 h" in figure). In this total cyclethe WL filtration was about 230 minutes, i.e. 37 % of thetotal cycle, and the GL filtration about 330 minutes, i.e.53 % of the total cycle. The rest of the total cycle, about10%, is non productive time (A, B and 30 min cloth washin figure 5). In the test a common filter apparatus wasused with a pressurized disc filter, see figure 8, having atotal filter area of 280 m2 and a vat holding some 55 m3

liquor to be filtered, producing 5 100 m3 WL/day and 5350 m3 GL/day.

Claims

1. Method for preparation of white liquor in a chemicalrecovery process of the kraft process, wherein rawgreen liquor is first fed to a green liquor separationprocess wherein dregs are separated out and a cleargreen liquor is obtained, thereafter adding burnt limeto the clear green liquor in a slaker, followed by acausticizing train (CT1-CT2-CT3) with a number ofcausticizing vessels wherein the causticizing proc-ess is finished producing a causticized liquor, there-after sending the causticized liquor to a white liquorseparation process wherein lime mud is separatedout and a clear white liquor is obtained to be usedas cooking liquor in the kraft process either in formof the clear white liquor or as modified by polysulfidemodification in a polysulfide process, and whereinthe separated lime mud is sent to a lime mud washingand drying process before feeding the washed anddried lime mud to a lime kiln characterized in thatthe green liquor separation process and the whiteliquor separation process takes place in the samecommon filter apparatus (GLF/WLF) with no dedi-cated green liquor separation apparatus nor anydedicated white liquor separation apparatus, andwhere the white liquor separation process and thegreen liquor separation process are conducted in se-quence in the same filter apparatus (GLF/WLF) andwhere the white liquor separation process has a partof the cycle time in the range 20-50% of the totalcycle time in the same filter apparatus (GLF/WLF).

2. Method according to claim 1, wherein an equalizingbuffer tank is preceding the green liquor separationprocess and where the equalizing buffer tank has astorage capacity holding raw green liquor (RGL) forat least 5 hours in said equalizing buffer tank, andwhere a last buffer tank in the causticizing train (CT1-CT2-CT3) has a storage capacity holding the caus-

ticized liquor for at least 2 hours in said last buffertank in the causticizing train (CT1-CT2-CT3).

3. Method according to claim 2, wherein the equalizingbuffer tank is filled with raw green liquor (RGL) whileemptying the buffer tank in the causticizing train(CT1-CT2-CT3) when performing the white liquorseparation in the common filter apparatus(GLF/WLF), and thereafter emptying the equalizingbuffer tank of raw green liquor (RGL) while filling thebuffer tank in the causticizing train (CT1-CT2-CT3)when performing the green liquor separation in thecommon filter apparatus (GLF/WLF).

4. Method according to claim 3, wherein the level ofliquors in the buffer tanks are controlled within20-95% of the total retention capacity during whiteand green liquor separation.

5. Method according to claim 1, wherein said the greenliquor separation process in said common filter ap-paratus (GLF/WLF) is ended by a complete emptyingof raw green liquor (RGL) and addition of an inten-sified wash out process using a volume of washingliquid at least 5% to that of the liquor volume held inthe common filter apparatus (GLF/WLF) during fil-tering, said washing liquid not containing any dregs,said intensified wash out process also complement-ed by intense agitation in the liquid volume held inthe common filter apparatus (GLF/WLF).

6. Method according to claim 5, wherein the volume ofwashing liquid used during the intensified wash outprocess exceeds 3 m3.

7. Method according to claim 6, wherein a cake of pre-coat is maintained on the filter surface during theintensified wash out process.

8. Method according to any of preceding claims, where-in a total removal of the precoat on the common filterapparatus (GLF/WLF) including a filter cloth wash isactivated after 2 or more green liquor separation cy-cles and wherein a total new precoat is establishedin subsequent white liquor separation process in saidcommon filter apparatus (GLF/WLF).

Patentansprüche

1. Verfahren zur Zubereitung von Weißlauge in einemchemischen Rückgewinnungsprozess des Kraftpro-zesses, wobei unverarbeitete Grünlauge zunächstin einen Grünlaugentrennprozess eingeleitet wird,wobei Bodensatz abgetrennt und eine klare Grün-lauge erhalten wird, anschließend Hinzufügen vongebranntem Kalk zu der klaren Grünlauge in einerLöschanlage, gefolgt von einem Kaustifizierungszug

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(CT1-CT2-CT3) mit einer Anzahl von Kaustifizie-rungsbehältern, wobei der Kaustifizierungsprozessbeendet wird, wodurch eine kaustifizierte Lauge pro-duziert wird, anschließend Senden der kaustifizier-ten Lauge zu einem Weißlaugentrennprozess, wo-bei Kalkschlamm abgetrennt und eine klareWeißlauge erhalten wird, um als Kochlauge in demKraftprozess verwendet zu werden, entweder inForm der klaren Weißlauge oder modifiziert durcheine Polysulfidmodifikation in einem Polysulfidpro-zess, und wobei der getrennte Kalkschlamm in einenKalkschlammwasch- und Trocknungsprozess ge-schickt wird vor dem Einleiten des gewaschenen undgetrockneten Kalkschlamms in einen Kalkbrenno-fen, gekennzeichnet dadurch, dass der Grünlau-gentrennprozess und der Weißlaugentrennprozessin der gleichen gemeinsamen Filtervorrichtung(GLF/WLF) stattfinden, ohne eine zugeordneteGrunlaugentrennvorrichtung oder irgendeiner zuge-ordneten Weißlaugentrennvorrichtung, und wobeider Weißlaugentrennprozess und der Grünlaugen-trennprozess nacheinander in der gleichen Filtervor-richtung (GLF/WLF) durchgeführt werden und wobeider Weißlaugentrennprozess einen Teil der Zyklus-zeit in dem Bereich von 20-50% der Gesamtzyklus-zeit der gleichen Filtervorrichtung (GLF/WLF) auf-weist.

2. Verfahren gemäß Anspruch 1, wobei ein ausglei-chender Puffertank dem Grünlaugentrennprozessvorausgeht und wobei der ausgleichende Puffertankeine Speicherkapazität aufweist, die unverarbeiteteGrünlauge (RGL) für wenigstens fünf Stunden indem ausgleichenden Puffertank hält, und wobei einletzter Puffertank in dem Kaustifizierungszug (CT1-CT2-CT3) eine Speicherkapazität aufweist, die diekaustifizierte Lauge für wenigstens zwei Stunden indem letzten Puffertank in dem Kaustifizierungszug(CT1-CT2-CT3) hält.

3. Verfahren gemäß Anspruch 2, wobei der ausglei-chende Puffertank während des Leerens des Puf-fertanks in dem Kaustifizierungszug (CT1-CT2-CT3)mit unverarbeiteter Grünlauge (RGL) gefüllt wird,wenn die Weißlaugentrennung in der gemeinsamenFiltervorrichtung (GLF/WLF) durchgeführt wird, undanschließend Leeren der Grünlauge (RGL) aus demausgleichenden Puffertank während der Puffertankin dem Kaustifizierungszug (CT1-CT2-CT3) gefülltwird, wenn die Grünlaugentrennung in der gemein-samen Filtervorrichtung (GLF/WLF) durchgeführtwird.

4. Verfahren gemäß Anspruch 3, wobei das Niveau vonLaugen in den Puffertanks während der Weiß- undGrünlaugentrennung innerhalb von 20-95% der ge-samten Speicherkapazität geregelt wird.

5. Verfahren gemäß Anspruch 1, wobei der Grünlau-gentrennungsprozess in der gemeinsamen Filter-vorrichtung (GLF/WLF) durch ein vollständiges Lee-ren der unverarbeiteten Grünlauge (RGL) und Hin-zufügen eines intensivierten Auswaschprozessesunter Verwendung eines Volumens einer Wasch-flüssigkeit von wenigstens 5% des Laugenvolu-mens, das während des Filterns in der gemeinsamenFiltervorrichtung (GLF/WLF) gehalten wird, beendetwird, wobei die Waschflüssigkeit keinen Bodensatzenthält, wobei der intensivierte Auswaschprozessauch von intensiver Bewegung in dem Flüssigkeits-volumen begleitet wird, das in der gemeinsamen Fil-tervorrichtung (GLF/WLF) gehalten wird.

6. Verfahren gemäß Anspruch 5, wobei das Volumender Waschflüssigkeit, die während des intensiviertenAuswaschprozesses verwendet wird, 3m3 über-steigt.

7. Verfahren gemäß Anspruch 6, wobei ein Filterrück-stand eines Precoats während des intensiviertenAuswaschprozesses auf der Filteroberfläche auf-rechterhalten wird.

8. Verfahren gemäß irgendeinem der vorstehendenAnsprüche, wobei ein vollständiges Entfernen desPrecoats an der gemeinsamen Filtervorrichtung(GLF/WLF) einschließlich einer Filtergewebewä-sche nach zwei oder mehr Grünlaugentrennungszy-klen aktiviert wird, und wobei eine ganz neues Pre-coat in der gemeinsamen Filtervorrichtung(GLF/WLF) in einem anschließenden Weißlaugen-trennprozess aufgebaut wird.

Revendications

1. Procédé de préparation d’une liqueur blanche dansun procédé de récupération chimique du procédékraft, dans lequel une liqueur verte brute est alimen-tée en premier dans un procédé de séparation deliqueur verte dans lequel les dépôts sont séparés etune liqueur verte limpide est obtenue, puis de lachaux vive est ajoutée à la liqueur verte limpide dansun extincteur, suivie par un train de caustification(CT1-CT2-CT3) avec un certain nombre de réci-pients de caustification dans lesquels le procédé decaustification est fini produisant une liqueur causti-fiée, et ensuite la liqueur caustifiée est envoyée versun procédé de séparation de liqueur blanche danslequel la boue de chaux est séparée et une liqueurblanche limpide est obtenue qui est utilisée commeliqueur de cuisson dans le procédé kraft sous la for-me de liqueur blanche limpide ou modifiée par unemodification par polysulfure dans un procédé à basede polysulfure, et dans lequel la boue de chaux sé-parée est envoyée vers un procédé de lavage et de

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séchage de boue de chaux avant d’envoyer la bouede chaux lavée et séchée vers un four à chaux, ca-ractérisé en ce que le procédé de séparation deliqueur verte et le procédé de séparation de liqueurblanche se produisent dans le même appareil de fil-tration classique (GLF/WLF) sans appareil de sépa-ration de liqueur verte dédié ni appareil de séparationde liqueur blanche dédié, et dans lequel le procédéde séparation de liqueur blanche et le procédé deséparation de liqueur verte sont réalisés séquentiel-lement dans le même appareil de filtration(GLF/WLF) et dans lequel le procédé de séparationde liqueur blanche a une partie du temps de cycledans la plage de 20 à 50 % du temps de cycle totaldans le même appareil de filtration (GLF/WLF).

2. Procédé selon la revendication 1, dans lequel un ré-servoir de tampon d’équilibrage précède le procédéde séparation de liqueur verte et dans lequel le ré-servoir de tampon d’équilibrage a une capacité destockage maintenant la liqueur verte brute (LVB)pendant au moins 5 heures dans ledit réservoir detampon d’équilibrage, et dans lequel un dernier ré-servoir de tampon dans le train de caustification(CT1-CT2-CT3) a une capacité de stockage main-tenant la liqueur caustifiée pendant au moins 2 heu-res dans ledit dernier réservoir de tampon dans letrain de caustification (CT1-CT2-CT3).

3. Procédé selon la revendication 2, dans lequel le ré-servoir de tampon d’équilibrage est chargé avec laliqueur verte brute (LVB) tout en vidant le réservoirde tampon dans le train de caustification (CT1-CT2-CT3) lors de la séparation de la liqueur blanche dansl’appareil de filtration classique (GLF/WLF) et ensui-te le réservoir de tampon d’équilibrage est déchargéde la liqueur verte brute (LVB) tout en remplissantle réservoir de tampon dans le train de caustification(CT1-CT2-CT3) lors de la séparation de la liqueurverte dans l’appareil de filtration classique(GLF/WLF).

4. Procédé selon la revendication 3, dans lequel lestaux de liqueurs dans les réservoirs de tampon sontrégulés à hauteur de 20 à 95 % de la capacité derétention totale pendant la séparation de la liqueurverte et de la liqueur blanche.

5. Procédé selon la revendication 1, dans lequel leditprocédé de séparation de liqueur verte dans leditappareil de filtration classique (GLF/WLF) se termi-ne par une vidange totale de la liqueur verte brute(LVB) et l’ajout d’un procédé de lavage intensifié enutilisant un volume de liquide de lavage d’au moins5 % du volume de liqueur contenu dans l’appareil defiltration classique (GLF/WLF) pendant la filtration,ledit liquide de lavage ne contenant pas de dépôts,ledit procédé de lavage intensifié étant également

complété par une agitation intense dans le volumede liquide contenu dans l’appareil de filtration clas-sique (GLF/WLF).

6. Procédé selon la revendication 5, dans lequel le vo-lume du liquide de lavage utilisé pendant le procédéde lavage intensifié est supérieur à 3 m3.

7. Procédé selon la revendication 6, dans lequel un gâ-teau de prérevêtement est maintenu sur la surfacedu filtre pendant le procédé de lavage intensifié.

8. Procédé selon l’une quelconque des revendicationsprécédentes, dans lequel une élimination totale duprérevêtement sur l’appareil de filtration classique(GLF/WLF) incluant un lavage de la toile filtrante estactivée pendant 2 cycles de séparation de liqueurverte ou plus et dans lequel un tout nouveau prére-vêtement est créé dans un procédé de séparationde liqueur blanche ultérieur dans ledit appareil defiltration classique (GLF/WLF).

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REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the Europeanpatent document. Even though great care has been taken in compiling the references, errors or omissions cannot beexcluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description

• WO 2005116329 A [0007]• US 4668342 A [0008]• US 5145556 A [0009]

• US 5628875 A [0010]• WO 9722752 A [0011]• US 51455562 B [0012]