WO1994001617A1 - Method for filtering a product formed in causticization - Google Patents

Method for filtering a product formed in causticization Download PDF

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Publication number
WO1994001617A1
WO1994001617A1 PCT/FI1993/000287 FI9300287W WO9401617A1 WO 1994001617 A1 WO1994001617 A1 WO 1994001617A1 FI 9300287 W FI9300287 W FI 9300287W WO 9401617 A1 WO9401617 A1 WO 9401617A1
Authority
WO
WIPO (PCT)
Prior art keywords
filtrate
filter
washing
product
liquor
Prior art date
Application number
PCT/FI1993/000287
Other languages
French (fr)
Inventor
Pertti Heitto
Jaakko-Juhani Helsto
Nuutti Vartiainen
Timo Vartiainen
Original Assignee
Larox Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Larox Oy filed Critical Larox Oy
Priority to AU45034/93A priority Critical patent/AU4503493A/en
Priority to JP6502999A priority patent/JPH07508564A/en
Priority to CA002139766A priority patent/CA2139766A1/en
Publication of WO1994001617A1 publication Critical patent/WO1994001617A1/en
Priority to SE9404557A priority patent/SE9404557L/en

Links

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
    • D21C11/00—Regeneration of pulp liquors or effluent waste waters
    • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
    • 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
    • D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
    • D21C11/0078—Treatment of green or white liquors with other means or other compounds than gases, e.g. in order to separate solid compounds such as sodium chloride and carbonate from these liquors; Further treatment of these compounds
    • 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
    • D21C11/04—Regeneration of pulp liquors or effluent waste waters of alkali lye

Definitions

  • the present invention relates to a method according to the preamble of claim 1 for filtering a product formed in causticization.
  • the cooking liquor required in the digestion process of a sulfate cellulose plant is most generally prepared according to the following scheme.
  • the pulp is washed and the cooking liquor with wash waters is recovered (commonly called weak black liquor) .
  • the weak black liquor is dewatered by evaporation and thus obtained heavy black liquor is burned in a soda recovery furnace.
  • the molten smelt formed onto the soda recovery furnace bottom consisting principally of sodium sulfide (Na 2 S) and sodium carbonate (Na 2 C0 3 ) , is taken out in a controlled manner and dis- solved into wash wasters recovered from subsequent stages of the process.
  • the solution thus obtained is called green or fresh liquor, also fresh liquor.
  • the green liquor is clarified and the sludge thus obtained is washed to recover contained chemicals.
  • the spent washing liquid is routed to the weak liquor storage tank for use in the dissolution of the smelt obtained from the furnace output.
  • the clarified green liquor is prepared into white liquor for digestion through a two-stage process .
  • slaking of lime is performed in a unit called lime slaker where lime (CaO) is added to the clarified green liquor, whereby the lime reacts with the water contained in the green liquor thereby forming slaked lime (Ca(OH) 2 ) according to the following reaction formula: CaO + H 2 0 > Ca (OH) 2
  • the causticization reaction starts in the lime slaker and is continued in multiple (2-3 pcs. ) causticization containers wherefrom the reaction products are routed to a sock-type white liquor filter or another type of clarifier.
  • the filtrate from the filter is the cooking liquor (i.e., white liquor) which is routed to a storage tank, and further, to the digester.
  • the obtained white liquor has a carry-over of approx. 25-30 % of the active sodium in the form of Na 2 S which, however, has no detrimental effect on the subsequent reactions.
  • the filter bottom discharge normally obtained as a 30-40 % solids containing sludge is filtered in the lime sludge washing filter whose filtrate is the weak white liquor that is routed via a storage tank to a dissolution tank.
  • the lime sludge forming the bottom discharge is routed via a storage tank to, e.g., a lime sludge filter of the drum filter type where the lime sludge is washed and dried.
  • the filtrate proper and the wash filtrate are used for, e.g., diluting the sludge entering the lime sludge washing filter, while the dried lime sludge is routed to the rotary lime kiln.
  • the lime sludge is incinerated with the help of an auxiliary fuel (oil, gas, etc.) in a rotary lime kiln, whereby calcium oxide and carbon dioxide are formed according to the following formula:
  • the burnt lime (CaO) with makeup lime added to compen ⁇ sate for chemical losses, is used in causticization.
  • the weak white liquor routed to the dissolution tank contains NaOH which is therefrom conveyed to the causticization reaction.
  • the introduction of existing sodium hydroxide into the causticization reaction is detrimental to the reaction-
  • the method according to the present invention aims at overcoming the above-described drawbacks and to simplify the process in a manner which permits omission of a significant portion of equipment needed in concurrent processes. Separation of white liquor, as well as washing and subsequent drying of the lime sludge, take place in a single process stage.
  • the characterizing properties of the method according to the invention are disclosed in the annexed claims.
  • the white liquor and lime sludge received from the causticization tanks are routed to a filter, most advantageously a pressurized filter, wherefrom the white liquor obtained as filtrate is routed to a storage tank and further to the digester.
  • the solids content of the lime sludge cake remaining in the filter is high, which means a low amount of white liquor to be washed away from the cake.
  • the lime sludge cake still remaining in the filter is washed. Owing to the high solids content of the lime sludge cake, a good washing result can be attained using only a small amount of warm water. Thus, the small amount of filtrate from washing, that is, of the weak white liquor can be routed into the storage tank of white liquor already separated in the filtration stage.
  • the lime sludge cake can be further dried by, e.g., blowing compressed air through it. Subsequent to these steps the lime sludge cakes are removed from the filter and conveyed to incineration in the rotary lime kiln.
  • the filtering, washing and drying of the lime sludge can achieve complete elimination of the pre-existing sodium hydroxide, which otherwise would be carried over to the dissolution of the smelt and subsequent detrimental transfer into the causticization process .
  • the method according to the invention achieves a simpler and more effective causticization process over conven ⁇ tional techniques, since unnecessary circulations of liquids are eliminated and the number of remaining cir- culations is thus significantly smaller than those needed in prior art.
  • the above-discussed benefits also offer essential improvements in the form of reduced environmental loading.
  • Smelt received from a soda recovery furnace (1) is routed to a dissolution tank (2) where it is dissolved in water or any suitable recycled process water of the factory. Therefrom the solution is routed via an equalizing tank (3) to a green liquor clarifier (4) .
  • the sludge obtained as the bottom discharge of the clarifier (4) is filtered and washed in a green liquor filter (5) .
  • the liquid filtrate thus obtained can be used from, e.g., a buffer storage (6) for dissolving the smelt, while the solids are removed as waste (7) .
  • the clarified green liquor is routed together with lime (8) obtained from the rotary lime kiln, possibly complemented with makeup lime (9) , into a lime slaking tank (10) .
  • the calcium hydroxide formed in the lime slaking reaction reacts with sodium carbonate to form sodium hydroxide and calcium carbonate, that is, lime sludge.
  • the latter reaction takes place in causticization tanks (11) located after the lime sludge slaker (10) .
  • the calcium carbonate and sodium hydroxide received from the causticization tanks are routed to a filter (12) , where the solids content of the cake formed during filtration is increased by compressing the cake with the help of a pressurized medium acting via a rubber membrane.
  • the fil- trate obtained from the filter is white liquor, which is routed via a white liquor storage tank (13) to the digester.
  • the cake solids con- tent is approx. 75 wt-% at the start of the washing step. Consequently, the washing step can be carried out using a very small amount of wash water when compared to conven ⁇ tional techniques.
  • the amount of wash water required is in the range of 0.8-1 m 3 washing water per ton of dry lime sludge.
  • the solids cake is dried by repeating the compression step via the rubber membrane in the above- described manner, complemented with additional drying by blowing compressed gas (typically compressed air) through the cake.
  • compressed gas typically compressed air
  • the small amount of filtrate obtained from the washing stage and subsequent drying stages, comprised of white liquor, is routed into the white liquor circulation (13) and therefrom further into the digester.
  • the lime sludge cakes are removed from the filter into a storage tank (14) , wherefrom they are further conveyed into a rotary lime kiln (15) .
  • the calcium carbonate is inciner- ated and the calcium oxide thus formed is routed to a lime storage tank (8) for use in the causticization reaction.
  • the above-described method entirely prevents pre-existing sodium hydroxide from reaching the caustici ⁇ zation reaction, thus significantly improving the reaction conditions.
  • a pressurized filter most advantageously a pressurized filter, reduced water consumption is attained and washing can be performed using process waters obtained from, e.g., the evaporation stage.

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  • Paper (AREA)

Abstract

The invention concerns a method for filtering a product formed in a causticization process, in which filtration process the product is subjected to separation of the filtrate from the solids. The product received from a causticization tank is routed to a filter, where the filtrate is separated from the product to be filtered and the solids content of the remaining solids cake is increased by drying the cake, after which the cake is washed with water and dried. The invention is characterized in that the filtrate separated from the product in the filter is combined with the washing filtrate obtained from the washing and drying steps of the filtration process, and the primary liquor thus obtained is routed into a digester for use as cooking liquor.

Description

METHOD FOR FILTERING A PRODUCT FORMED IN CAUSTICIZATION
The present invention relates to a method according to the preamble of claim 1 for filtering a product formed in causticization.
The cooking liquor required in the digestion process of a sulfate cellulose plant is most generally prepared according to the following scheme.
Subsequent to the cooking of the cellulosic raw material with white liquor, the pulp is washed and the cooking liquor with wash waters is recovered (commonly called weak black liquor) . The weak black liquor is dewatered by evaporation and thus obtained heavy black liquor is burned in a soda recovery furnace. The molten smelt formed onto the soda recovery furnace bottom, consisting principally of sodium sulfide (Na2S) and sodium carbonate (Na2C03) , is taken out in a controlled manner and dis- solved into wash wasters recovered from subsequent stages of the process. The solution thus obtained is called green or fresh liquor, also fresh liquor. The green liquor is clarified and the sludge thus obtained is washed to recover contained chemicals. The spent washing liquid is routed to the weak liquor storage tank for use in the dissolution of the smelt obtained from the furnace output.
The clarified green liquor is prepared into white liquor for digestion through a two-stage process .
First, slaking of lime is performed in a unit called lime slaker where lime (CaO) is added to the clarified green liquor, whereby the lime reacts with the water contained in the green liquor thereby forming slaked lime (Ca(OH)2) according to the following reaction formula: CaO + H20 > Ca (OH) 2
In the causticization process the calcium hydroxide (Ca(OH)2) formed as described above is reacted with sodium carbonate (Na2C03) contained in the green liquor, whereby sodium hydroxide (NaOH) and calcium carbonate (CaC03) , also called slake lime, are formed according to the following formula:
Ca(OH)2 + Na2C03 > 2NaOH + CaC03
The causticization reaction starts in the lime slaker and is continued in multiple (2-3 pcs. ) causticization containers wherefrom the reaction products are routed to a sock-type white liquor filter or another type of clarifier. The filtrate from the filter is the cooking liquor (i.e., white liquor) which is routed to a storage tank, and further, to the digester. The obtained white liquor has a carry-over of approx. 25-30 % of the active sodium in the form of Na2S which, however, has no detrimental effect on the subsequent reactions.
The filter bottom discharge normally obtained as a 30-40 % solids containing sludge is filtered in the lime sludge washing filter whose filtrate is the weak white liquor that is routed via a storage tank to a dissolution tank. The lime sludge forming the bottom discharge is routed via a storage tank to, e.g., a lime sludge filter of the drum filter type where the lime sludge is washed and dried. The filtrate proper and the wash filtrate are used for, e.g., diluting the sludge entering the lime sludge washing filter, while the dried lime sludge is routed to the rotary lime kiln.
The lime sludge is incinerated with the help of an auxiliary fuel (oil, gas, etc.) in a rotary lime kiln, whereby calcium oxide and carbon dioxide are formed according to the following formula:
CaC03 > CaO + C02
The burnt lime (CaO) , with makeup lime added to compen¬ sate for chemical losses, is used in causticization.
In the above-described method the weak white liquor routed to the dissolution tank contains NaOH which is therefrom conveyed to the causticization reaction. As is known in the art (cf. e.g. Symposium for Recovery of Pulping Chemicals; K. Kinzner: Untersuchungen zur Kaustizierung von Griinlaugen) , the introduction of existing sodium hydroxide into the causticization reaction is detrimental to the reaction-
It is an object of the present invention to simplify the causticization process in a manner in which the weak cooking liquor, which results from concurrent caustici¬ zation processes and is known to be detrimental to the causticization process, need not be returned to the dis¬ solution tank of the soda recovery furnace circuit for weak alkali recovery, but rather, the alkaline solution resulting from washing with a small amount of warm water in a filter can be combined with the main circulation of the cooking liquor, i.e., the primary liquid containing useful white liquor at a concentration of approx. 90- 110 g/1 (Na20) .
The method according to the present invention aims at overcoming the above-described drawbacks and to simplify the process in a manner which permits omission of a significant portion of equipment needed in concurrent processes. Separation of white liquor, as well as washing and subsequent drying of the lime sludge, take place in a single process stage. The characterizing properties of the method according to the invention are disclosed in the annexed claims.
According to the invention, the white liquor and lime sludge received from the causticization tanks are routed to a filter, most advantageously a pressurized filter, wherefrom the white liquor obtained as filtrate is routed to a storage tank and further to the digester. The solids content of the lime sludge cake remaining in the filter is high, which means a low amount of white liquor to be washed away from the cake.
After filtering is completed, the lime sludge cake still remaining in the filter is washed. Owing to the high solids content of the lime sludge cake, a good washing result can be attained using only a small amount of warm water. Thus, the small amount of filtrate from washing, that is, of the weak white liquor can be routed into the storage tank of white liquor already separated in the filtration stage.
After filtering and washing, the lime sludge cake can be further dried by, e.g., blowing compressed air through it. Subsequent to these steps the lime sludge cakes are removed from the filter and conveyed to incineration in the rotary lime kiln.
By virtue of the above-described method, the filtering, washing and drying of the lime sludge can achieve complete elimination of the pre-existing sodium hydroxide, which otherwise would be carried over to the dissolution of the smelt and subsequent detrimental transfer into the causticization process . The method according to the invention achieves a simpler and more effective causticization process over conven¬ tional techniques, since unnecessary circulations of liquids are eliminated and the number of remaining cir- culations is thus significantly smaller than those needed in prior art. Moreover, the above-discussed benefits also offer essential improvements in the form of reduced environmental loading.
The invention is next examined in greater detail with the help of an exemplifying embodiment by making reference to the attached drawing (Fig. 1) showing the schematic principle of a plant according to the present invention.
Smelt received from a soda recovery furnace (1) is routed to a dissolution tank (2) where it is dissolved in water or any suitable recycled process water of the factory. Therefrom the solution is routed via an equalizing tank (3) to a green liquor clarifier (4) . The sludge obtained as the bottom discharge of the clarifier (4) is filtered and washed in a green liquor filter (5) . The liquid filtrate thus obtained can be used from, e.g., a buffer storage (6) for dissolving the smelt, while the solids are removed as waste (7) .
The clarified green liquor is routed together with lime (8) obtained from the rotary lime kiln, possibly complemented with makeup lime (9) , into a lime slaking tank (10) . The calcium hydroxide formed in the lime slaking reaction reacts with sodium carbonate to form sodium hydroxide and calcium carbonate, that is, lime sludge. The latter reaction takes place in causticization tanks (11) located after the lime sludge slaker (10) .
In the next process steps according to the invention, the calcium carbonate and sodium hydroxide received from the causticization tanks are routed to a filter (12) , where the solids content of the cake formed during filtration is increased by compressing the cake with the help of a pressurized medium acting via a rubber membrane. The fil- trate obtained from the filter is white liquor, which is routed via a white liquor storage tank (13) to the digester.
Subsequent to the compression stage, the cake solids con- tent is approx. 75 wt-% at the start of the washing step. Consequently, the washing step can be carried out using a very small amount of wash water when compared to conven¬ tional techniques. In the method according to the inven¬ tion, the amount of wash water required is in the range of 0.8-1 m3 washing water per ton of dry lime sludge.
After washing, the solids cake is dried by repeating the compression step via the rubber membrane in the above- described manner, complemented with additional drying by blowing compressed gas (typically compressed air) through the cake. The small amount of filtrate obtained from the washing stage and subsequent drying stages, comprised of white liquor, is routed into the white liquor circulation (13) and therefrom further into the digester.
After drying, the lime sludge cakes are removed from the filter into a storage tank (14) , wherefrom they are further conveyed into a rotary lime kiln (15) . In the rotary lime kiln (15) the calcium carbonate is inciner- ated and the calcium oxide thus formed is routed to a lime storage tank (8) for use in the causticization reaction.
In practice, the above-described method entirely prevents pre-existing sodium hydroxide from reaching the caustici¬ zation reaction, thus significantly improving the reaction conditions. Owing to the efficiency of the filter employed, most advantageously a pressurized filter, reduced water consumption is attained and washing can be performed using process waters obtained from, e.g., the evaporation stage.
For those versed in the art it is evident that the diffe¬ rent implementations of the invention are not limited by the exemplifying embodiment described above, but instead, it can be varied within the scope of the annexed claims.

Claims

Claims :
1. A method for filtering a product formed in a causticization process, in which filtration process the product is subjected to separation of the filtrate from the solids in a manner in which the product received from a causticization tank (11) is routed to a filter (12) , where the filtrate is separated from the product to be filtered and the solids content of the remaining solids cake is increased by drying the cake, after which the cake is washed with water and dried, c h a r a c t e r ¬ i z e d in that the filtrate separated in the filter (12) from the product is combined with the washing filtrate obtained from the washing and drying steps of the filtration process.
2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the filtrate removed from the product in the filter (12) is white liquor and the washing filtrate obtained from the washing and drying steps of the filtration process is weak white liquor.
3. A method as defined in claim 1 or 2, c h a r a c ¬ t e r i z e d in that the filtrate proper and washing filtrate are routed either separately or combined into a storage tank (13) and therefrom further into a digester for use as cooking liquor.
. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the amount of wash water used in the filtration process is in the range of 0.8-1 m3 of wash water per ton of dry lime sludge.
5. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that the primary liquor formed from the filtrate proper and the washing filtrate has a concentration in the range of 90-110 g/1 (Na20) .
6. A method as defined in any foregoing claim, c h a r a c t e r i z e d in that said filter (12) is most advantageously a pressurized filter.
PCT/FI1993/000287 1992-07-07 1993-07-06 Method for filtering a product formed in causticization WO1994001617A1 (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
AU45034/93A AU4503493A (en) 1992-07-07 1993-07-06 Method for filtering a product formed in causticization
JP6502999A JPH07508564A (en) 1992-07-07 1993-07-06 Product filtration method using caustic treatment
CA002139766A CA2139766A1 (en) 1992-07-07 1993-07-06 Method for filtering a product formed in causticization
SE9404557A SE9404557L (en) 1992-07-07 1994-12-29 Process for filtering a product formed by causticization

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI923127A FI93752C (en) 1992-07-07 1992-07-07 Method of separating product formed in causticization
FI923127 1992-07-07

Publications (1)

Publication Number Publication Date
WO1994001617A1 true WO1994001617A1 (en) 1994-01-20

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ID=8535586

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/FI1993/000287 WO1994001617A1 (en) 1992-07-07 1993-07-06 Method for filtering a product formed in causticization

Country Status (6)

Country Link
JP (1) JPH07508564A (en)
AU (1) AU4503493A (en)
CA (1) CA2139766A1 (en)
FI (1) FI93752C (en)
SE (1) SE9404557L (en)
WO (1) WO1994001617A1 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997022752A1 (en) * 1995-12-20 1997-06-26 Larox Oy Process for filtering white liquor and a storage container for lime sludge
WO2014131067A1 (en) * 2013-02-26 2014-09-04 Mondi Ag Process for recausticizing green liquor

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3258391A (en) * 1961-08-25 1966-06-28 Eimco Corp Chemical recovery in pulp manufacture
US4695381A (en) * 1985-05-02 1987-09-22 Ab Hedemora Verkstader Filter for continuous filtering of a suspension under pressure
US4929355A (en) * 1985-02-05 1990-05-29 Ab Hedemora Verkstader Method and apparatus for the separation of caustic liquor, lime sludge and sludge in a causticizing process

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3258391A (en) * 1961-08-25 1966-06-28 Eimco Corp Chemical recovery in pulp manufacture
US4929355A (en) * 1985-02-05 1990-05-29 Ab Hedemora Verkstader Method and apparatus for the separation of caustic liquor, lime sludge and sludge in a causticizing process
US4695381A (en) * 1985-05-02 1987-09-22 Ab Hedemora Verkstader Filter for continuous filtering of a suspension under pressure

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997022752A1 (en) * 1995-12-20 1997-06-26 Larox Oy Process for filtering white liquor and a storage container for lime sludge
WO2014131067A1 (en) * 2013-02-26 2014-09-04 Mondi Ag Process for recausticizing green liquor
US10011948B2 (en) 2013-02-26 2018-07-03 Mondi Ag Process for recausticizing green liquor

Also Published As

Publication number Publication date
FI93752B (en) 1995-02-15
CA2139766A1 (en) 1994-01-20
SE9404557D0 (en) 1994-12-29
FI93752C (en) 1995-06-12
AU4503493A (en) 1994-01-31
JPH07508564A (en) 1995-09-21
FI923127A0 (en) 1992-07-07
SE9404557L (en) 1994-12-29
FI923127A (en) 1994-01-08

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