WO1995031600A1 - Method for improving capacity when filtering green liquor - Google Patents

Method for improving capacity when filtering green liquor Download PDF

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Publication number
WO1995031600A1
WO1995031600A1 PCT/SE1995/000461 SE9500461W WO9531600A1 WO 1995031600 A1 WO1995031600 A1 WO 1995031600A1 SE 9500461 W SE9500461 W SE 9500461W WO 9531600 A1 WO9531600 A1 WO 9531600A1
Authority
WO
WIPO (PCT)
Prior art keywords
aluminium
green liquor
liquor
magnesium
green
Prior art date
Application number
PCT/SE1995/000461
Other languages
French (fr)
Inventor
Martin Wimby
Original Assignee
Kvaerner Pulping Technologies Ab
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 Kvaerner Pulping Technologies Ab filed Critical Kvaerner Pulping Technologies Ab
Priority to AU25411/95A priority Critical patent/AU2541195A/en
Priority to JP7529551A priority patent/JPH10500180A/en
Publication of WO1995031600A1 publication Critical patent/WO1995031600A1/en
Priority to FI964547A priority patent/FI964547A0/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
    • D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P40/00—Technologies relating to the processing of minerals
    • Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills

Definitions

  • the present invention relates to a method for improving the filterability of the green liquor in the kraft pulp process.
  • cooking chemicals are to a large extent recovered in the following manner.
  • This is transferred to a soda smelt dissolver in which the smelt is dissolved in weak liquor which is obtained by washing lime sludge in the causticizing plant.
  • the liquor which is thereby obtained is termed green liquor, which is fed to a green liquor clarifier in which solid particles are removed by sedimentation or filtration.
  • the clarified green liquor is fed to a lime slaker, where burnt lime (calcium oxide) is added.
  • lime milk which is obtained is transferred to the causticizing plant, where the content of sodium carbonate in the green liquor reacts with the slaked lime, resulting in the formation of a solution of sodium hydroxide and a precipitate of calcium carbonate, the so-called lime sludge.
  • lime sludge is separated off, by filtration, from the white liquor which is obtained in the said reaction (causticization) .
  • the lime sludge is washed with water in order to recover soluble alkali metal compounds, and the weak liquor thereby obtained as washing solution is fed to the soda smelt dissolver.
  • the lime sludge is burnt, in a rotary kiln, to burnt lime which is returned to the lime slaker.
  • burnt lime or lime sludge must be drawn off from the system, the quantity of lime or lime sludge having to be removed from the system depending in part on the efficacy of the green liquor clarification.
  • the smelt which is formed in the recovery boiler contains sodium compounds and sulphur compounds which are necessary for the process, it also contains small quantities of inorganic, difficultly soluble compounds which derive from the wood or the burnt lime and have to be removed from the process.
  • the smelt contains a certain quantity of carbon particles. All these compounds are present in the green liquor as particles which are more or less fine, and the intention is that these particles should be removed by sedimentation in the green liquor clarifier.
  • the present invention is based on the insight that it is necessary to decrease the precipitation of magnesium in the green liquor as pure magnesium hydroxide, which is difficult to filter off, and this is achieved, according to the present invention, by adding aluminium to the process and precipitating out magnesium and aluminium in the green liquor together as hydrotalcite, which is a magnesium/aluminium precipi- tate which can be filtered relatively easily.
  • hydrotalcite is a magnesium/aluminium precipi- tate which can be filtered relatively easily.
  • Magnesium and aluminium are precipitated out together as hydrotalcite in a ratio of approximately 4 g of Mg per 1 g of Al.
  • Aluminium is, of course, regarded as a trouble- some impurity in all recovery plants within the kraft pulp industry since aluminium gives rise to incrustations in the evaporation section of the recovery process.
  • the content of aluminium has been reduced, owing to the magnesium supplied to the oxygen delignification stage, to such an extent that aluminium has ceased to give rise to incrustations. It has to be regarded as being parti ⁇ cularly surprising that these mills have, instead, been affected by aluminium shortage.
  • US 4.218.284 relates to a method for inhibiting the formation of incrustations in pulp manufacture by means of adding compounds of multivalent metals, which compounds are able to form complexes with incrustation- forming anions, as a result of which the incrustation- forming anions are maintained in the form of a liquor- soluble complex.
  • the multivalent metal in this context can be aluminium.
  • this patent specification does not touch upon the problem of the filterability of the green liquor at all; instead, the process exploits the ability of the aluminium to form liquor-soluble complexes of incrustation-forming anions, and not its ability to form a readily filterable precipitate with magnesium in the form of hydrotalcite.
  • the object of US 4.049.489 is to decrease the quantity of sulphur and chlorine in the cooking liquor, which quantities can reach levels which are undesirable owing to environmental restrictions.
  • certain metals for example aluminium
  • these metals are therefore burnt together with the spent liquor, and, under these circumstances, these metals form compounds with the sulphur and the chlorine which can be readily separated from the chemicals which participate in the normal process cycle.
  • This patent specification does not deal with the problem of the filterability of the green liquor either.
  • JP 60.045.692 relates to the purification of green liquor by adding aluminium or magnesium.
  • the object of the invention according to this patent publication is to precipitate out silicon dioxide and subsequently to separate the latter from the green liquor by adding a magnesium or aluminium salt.
  • the said patent publication does not touch upon the joint precipitation of magnesium and aluminium.
  • the SCAN-FORSK report which is referred to above proposes adding magnesium as an agent for decreasing aluminium in green liquor.
  • the purpose of this addition is to decrease aluminium incrustations in association with evaporation.
  • that which the present invention proposes is the exact opposite, i.e. adding aluminium in order to decrease the precipitation of magnesium as pure magnesium hydroxide.
  • the present invention relates to a method for improving the filterability of the green liquor in the kraft pulp process, which method is distinguished by the fact that aluminium in the form of elemental aluminium or an aluminium- containing chemical compound is added to the process in a quantity such that the major part of the magnesium content of the green liquor is precipitated out together with aluminium in the form of hydrotalcite, and that an optimal content of aluminium is obtained in the clarified green liquor.
  • the aluminium is added upstream of the recovery boiler, thereby rendering- it possible to choose at will the form in which the aluminium is to be added.
  • the aluminium can also, for example, be added to the green liquor upstream of the clarification stage or directly into the recovery boiler.
  • the quantity of aluminium which is added is such that a content of 5-50 mg/1, preferably 5-15 mg/1, is obtained in the clarified green liquor.
  • Mill III produces approximately 40 tonne/h, denoting that at least 2 kg of aluminium have to be added per hour.

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  • Paper (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

Abstract

The invention relates to a method for improving the filterability of the green liquor in the kraft pulp process by means of adding aluminium in the form of elemental aluminium or an aluminium-containing chemical compound to the process in a quantity such that the major part of the magnesium content of the green liquor is precipitated out together with aluminium in the form of hydrotalcite.

Description

Method for improving capacity when filtering green liquor
The present invention relates to a method for improving the filterability of the green liquor in the kraft pulp process.
When producing pulp in accordance with the kraft process by cooking wood, for example, cooking chemicals are to a large extent recovered in the following manner.
The spent liquor which is obtained when cooking wood, and which is also termed black liquor, is evaporated to form a thick waste liquor which is combusted in a recovery boiler, resulting in the formation of a smelt. This is transferred to a soda smelt dissolver in which the smelt is dissolved in weak liquor which is obtained by washing lime sludge in the causticizing plant. The liquor which is thereby obtained is termed green liquor, which is fed to a green liquor clarifier in which solid particles are removed by sedimentation or filtration. The clarified green liquor is fed to a lime slaker, where burnt lime (calcium oxide) is added. This results in a slaking of the lime, and the mixture of slaked lime and green liquor, so-called "lime milk", which is obtained is transferred to the causticizing plant, where the content of sodium carbonate in the green liquor reacts with the slaked lime, resulting in the formation of a solution of sodium hydroxide and a precipitate of calcium carbonate, the so-called lime sludge. This latter is separated off, by filtration, from the white liquor which is obtained in the said reaction (causticization) . The lime sludge is washed with water in order to recover soluble alkali metal compounds, and the weak liquor thereby obtained as washing solution is fed to the soda smelt dissolver. The lime sludge is burnt, in a rotary kiln, to burnt lime which is returned to the lime slaker. In order to prevent inert compounds accumulating in the white liquor and in the lime cycle, burnt lime or lime sludge must be drawn off from the system, the quantity of lime or lime sludge having to be removed from the system depending in part on the efficacy of the green liquor clarification. While the smelt which is formed in the recovery boiler contains sodium compounds and sulphur compounds which are necessary for the process, it also contains small quantities of inorganic, difficultly soluble compounds which derive from the wood or the burnt lime and have to be removed from the process. In addition, the smelt contains a certain quantity of carbon particles. All these compounds are present in the green liquor as particles which are more or less fine, and the intention is that these particles should be removed by sedimentation in the green liquor clarifier.
Attempts have therefore been made to improve the clarification (i.e. sedimentation) and filtration properties in the green liquor clarifier in a variety of ways, including supplying dry, burnt unslaked lime to the unclarified green liquor (SE 456254, the so- called KORSNAS method) . It has subsequently been found that a better result is obtained by adding slaked lime to unclarified green liquor (SE 467765) .
It has also been found that finely divided precipitates of certain metal hydroxides, for example magnesium hydroxide, in the green liquor cause it to sediment slowly. A high content of magnesium in the green liquor sludge can result in poor sedimentation of the green liquor sludge and consequently in a high risk of the green liquor sludge being conveyed onwards in the process to the causticizing stage. If the magnesium content is very high in the lime cycle, the filter¬ ability of the lime sludge can be seriously impaired (Per Ulmgren, "SCAN-FORSK REPORT", 535, June 1989, pp. 21-23) .
The present invention is based on the insight that it is necessary to decrease the precipitation of magnesium in the green liquor as pure magnesium hydroxide, which is difficult to filter off, and this is achieved, according to the present invention, by adding aluminium to the process and precipitating out magnesium and aluminium in the green liquor together as hydrotalcite, which is a magnesium/aluminium precipi- tate which can be filtered relatively easily. Magnesium and aluminium are precipitated out together as hydrotalcite in a ratio of approximately 4 g of Mg per 1 g of Al.
Aluminium is, of course, regarded as a trouble- some impurity in all recovery plants within the kraft pulp industry since aluminium gives rise to incrustations in the evaporation section of the recovery process. However, in mills which have introduced oxygen delignification, the content of aluminium has been reduced, owing to the magnesium supplied to the oxygen delignification stage, to such an extent that aluminium has ceased to give rise to incrustations. It has to be regarded as being parti¬ cularly surprising that these mills have, instead, been affected by aluminium shortage.
In the abovementioned "SCAN-FORSK" report, it is stated, on p. 28, that the intake of aluminium into the kraft pulp process should be minimized, and a number of methods are proposed for decreasing the aluminium intake.
US 4.218.284 relates to a method for inhibiting the formation of incrustations in pulp manufacture by means of adding compounds of multivalent metals, which compounds are able to form complexes with incrustation- forming anions, as a result of which the incrustation- forming anions are maintained in the form of a liquor- soluble complex. According to Patent Claim 2, the multivalent metal in this context can be aluminium. However, this patent specification does not touch upon the problem of the filterability of the green liquor at all; instead, the process exploits the ability of the aluminium to form liquor-soluble complexes of incrustation-forming anions, and not its ability to form a readily filterable precipitate with magnesium in the form of hydrotalcite.
The object of US 4.049.489 is to decrease the quantity of sulphur and chlorine in the cooking liquor, which quantities can reach levels which are undesirable owing to environmental restrictions. According to this patent specification, certain metals, for example aluminium, are therefore burnt together with the spent liquor, and, under these circumstances, these metals form compounds with the sulphur and the chlorine which can be readily separated from the chemicals which participate in the normal process cycle. This patent specification does not deal with the problem of the filterability of the green liquor either. JP 60.045.692 relates to the purification of green liquor by adding aluminium or magnesium. The object of the invention according to this patent publication is to precipitate out silicon dioxide and subsequently to separate the latter from the green liquor by adding a magnesium or aluminium salt. However, the said patent publication does not touch upon the joint precipitation of magnesium and aluminium.
The SCAN-FORSK report which is referred to above proposes adding magnesium as an agent for decreasing aluminium in green liquor. The purpose of this addition is to decrease aluminium incrustations in association with evaporation. However, that which the present invention proposes is the exact opposite, i.e. adding aluminium in order to decrease the precipitation of magnesium as pure magnesium hydroxide.
More specifically, the present invention relates to a method for improving the filterability of the green liquor in the kraft pulp process, which method is distinguished by the fact that aluminium in the form of elemental aluminium or an aluminium- containing chemical compound is added to the process in a quantity such that the major part of the magnesium content of the green liquor is precipitated out together with aluminium in the form of hydrotalcite, and that an optimal content of aluminium is obtained in the clarified green liquor.
Preferably, the aluminium is added upstream of the recovery boiler, thereby rendering- it possible to choose at will the form in which the aluminium is to be added. The aluminium can also, for example, be added to the green liquor upstream of the clarification stage or directly into the recovery boiler.
The quantity of aluminium which is added is such that a content of 5-50 mg/1, preferably 5-15 mg/1, is obtained in the clarified green liquor.
We have carried out a detailed study in three different installations (mill I, mill II and mill III) of the variation in the contents of magnesium and aluminium with the filter capacity when filtering green liquor sludge, and the results are shown in the following table.
Table: Capacity in m /h and contents in mg/1
Mill Mg in Mg in Al in Al in Mg/Al in Filter Sludge unfil- fil¬ unfil- fil¬ unfil- capa¬ content tered tered tered tered tered city green green green green green liquor liquor liquor liquor liquor
Mill 90.4 0 10.4 2 8.7 1 880
III
Mill I 108 0 23.1 9.6 4.7 2.3 1520
Mill 125 0 16.2 2.3 7.8 1.9 approx.
III, 211 5000
Mill II 47 0 37 26 1.3 1.9 900
1) Korsnas method
On the basis of the abovementioned KORSNAS method (SE 456.254) , which involves improving the filtration properties by admixing a relatively small quantity of lime in the green liquor, it can be assumed that the filter capacity should increase with the sludge content. Calcification experiments were carried out in mill III, and it was possible to increase the filter capacity in this way. The calcification experiments are shown as "mill III, 2" in the above table.
It is reasonably clear from the table that the KORSNAS method has a positive effect on the filter capacity. However, the capacity reached in mill III was not as high as that in mill II despite the fact that the green liquor was calcified. We are of the view that the explanation for this can be found in the fact that the aluminium content in the clarified green liquor is very low in the case of mill III, signifying that part of the magnesium in the green liquor sludge is present in the form of magnesium hydroxide.
As can be seen in the table, almost all the aluminium in mill III is used for precipitating magnesium. The probable explanation for this is that magnesium hydroxide which is difficult to filter off is present in the green liquor sludge of this mill. As is evident from the table, the green liquors in mill I and mill II are substantially easier to filter than is the liquor in mill III and, since there is aluminium left in the filtered green liquor in mill I and mill II, the probable explanation is that, in these cases, the magnesium in the green liquor sludge is present in the form of hydrotalcite.
Approximately 50 g of aluminium per tonne of pulp would therefore be required in the green liquor of the mill III installation in order to bind magnesium in the green liquor. Mill III produces approximately 40 tonne/h, denoting that at least 2 kg of aluminium have to be added per hour.

Claims

PATENT CLAIMS
1. Method for improving the filterability of the green liquor sludge in the kraft pulp process, c h a r a c t e r i z e d i n that aluminium in the form of elemental aluminium or an aluminium-containing chemical compound is added to the process in a quantity such that the major part of the magnesium content of the green liquor is precipitated out together with aluminium in the form of hydrotalcite, and in that an optimal content of aluminium is obtained in the clarified green liquor.
2. Method according to Claim 1, c h a r a c t e r i z e d i n that the aluminium is added to the black liquor upstream of the recovery boiler, to the green liquor upstream of the clarification stage, or directly into the recovery boiler.
3. Method according to Claim 1 or 2, c h a r a c t e r i z e d i n that the quantity of aluminium added is such that an aluminium content of 5- 50 mg/1, preferably 5-15 mg/1, is obtained in the clarified green liquor.
PCT/SE1995/000461 1994-05-13 1995-04-26 Method for improving capacity when filtering green liquor WO1995031600A1 (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
AU25411/95A AU2541195A (en) 1994-05-13 1995-04-26 Method for improving capacity when filtering green liquor
JP7529551A JPH10500180A (en) 1994-05-13 1995-04-26 How to improve the ability when filtering green liquor
FI964547A FI964547A0 (en) 1994-05-13 1996-11-13 Method for increasing the capacity in green liquor filtration

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9401663-1 1994-05-13
SE9401663A SE501877C2 (en) 1994-05-13 1994-05-13 Procedure for precipitating magnesium with aluminum during green clearing

Publications (1)

Publication Number Publication Date
WO1995031600A1 true WO1995031600A1 (en) 1995-11-23

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JP (1) JPH10500180A (en)
AU (1) AU2541195A (en)
CA (1) CA2188941A1 (en)
FI (1) FI964547A0 (en)
SE (1) SE501877C2 (en)
WO (1) WO1995031600A1 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000042251A1 (en) * 1999-01-12 2000-07-20 Andritz-Ahlstrom Oy Method of reducing the silicon content of green liquor
WO2021232133A1 (en) * 2020-05-22 2021-11-25 Suzano S.A. Methods of treating a kraft process recovery cicle to reduce metal levels at the kraft process

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988006203A1 (en) * 1987-02-12 1988-08-25 Korsnäs, Aktiebolag Method for green liquor cleaning in sulphate pulp mills
WO1994012723A1 (en) * 1992-12-03 1994-06-09 Mo Och Domsjö Aktiebolag A method of producing green liquor in chemical recovery in sulfate and sulfite pulp mills

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988006203A1 (en) * 1987-02-12 1988-08-25 Korsnäs, Aktiebolag Method for green liquor cleaning in sulphate pulp mills
WO1994012723A1 (en) * 1992-12-03 1994-06-09 Mo Och Domsjö Aktiebolag A method of producing green liquor in chemical recovery in sulfate and sulfite pulp mills

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
DIALOG INFORMATION SERVICES, File 351, World Patent Index 81-95, Dialog Accession No. 004269873, WPI Accession No. 85-096751/16, ISHIKAWAJIMA-HARIMA JUKO: "Black or Green Liquor Purification by Adding Aluminium or Magnesium Salt to Precipitate Ionic Silica"; & JP,A,60 045 692, (12-03-85), 8516 (Basic). *
PER ULMGREN, "Processfraemmande Grundaemnen i Kemikalieatervinningen", Juni 1989; SCAN FORSK - RAPPORT 535, STFI, pages 21-23, 26-28, 45-47. *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000042251A1 (en) * 1999-01-12 2000-07-20 Andritz-Ahlstrom Oy Method of reducing the silicon content of green liquor
WO2021232133A1 (en) * 2020-05-22 2021-11-25 Suzano S.A. Methods of treating a kraft process recovery cicle to reduce metal levels at the kraft process
CN115667622A (en) * 2020-05-22 2023-01-31 苏扎诺公司 Method of treating kraft process recovery cycle to reduce metal levels in kraft process

Also Published As

Publication number Publication date
SE9401663L (en) 1995-06-12
AU2541195A (en) 1995-12-05
FI964547A (en) 1996-11-13
JPH10500180A (en) 1998-01-06
SE9401663D0 (en) 1994-05-13
CA2188941A1 (en) 1995-11-23
FI964547A0 (en) 1996-11-13
SE501877C2 (en) 1995-06-12

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