WO2017089656A1 - A method for treating recovery boiler ash - Google Patents

A method for treating recovery boiler ash Download PDF

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Publication number
WO2017089656A1
WO2017089656A1 PCT/FI2016/050830 FI2016050830W WO2017089656A1 WO 2017089656 A1 WO2017089656 A1 WO 2017089656A1 FI 2016050830 W FI2016050830 W FI 2016050830W WO 2017089656 A1 WO2017089656 A1 WO 2017089656A1
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Prior art keywords
ash
slurry
tall oil
leaching
brine
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PCT/FI2016/050830
Other languages
French (fr)
Inventor
Ulf Alterby
Anders LOVELL
Original Assignee
Andritz Oy
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Publication date
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Publication of WO2017089656A1 publication Critical patent/WO2017089656A1/en

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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
    • D21C11/12—Combustion of pulp liquors
    • 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
    • D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
    • D21C11/063—Treatment of gas streams comprising solid matter, e.g. the ashes resulting from the combustion of black liquor
    • D21C11/066—Separation of solid compounds from these gases; further treatment of recovered products
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
    • C01D3/00—Halides of sodium, potassium or alkali metals in general
    • C01D3/04—Chlorides
    • C01D3/06—Preparation by working up brines; seawater or spent lyes
    • C—CHEMISTRY; METALLURGY
    • C11—ANIMAL OR VEGETABLE OILS, FATS, FATTY SUBSTANCES OR WAXES; FATTY ACIDS THEREFROM; DETERGENTS; CANDLES
    • C11B—PRODUCING, e.g. BY PRESSING RAW MATERIALS OR BY EXTRACTION FROM WASTE MATERIALS, REFINING OR PRESERVING FATS, FATTY SUBSTANCES, e.g. LANOLIN, FATTY OILS OR WAXES; ESSENTIAL OILS; PERFUMES
    • C11B13/00—Recovery of fats, fatty oils or fatty acids from waste materials
    • C11B13/005—Recovery of fats, fatty oils or fatty acids from waste materials of residues of the fabrication of wood-cellulose (in particular tall-oil)

Definitions

  • the present method relates to removing chloride and potassium in the leaching of ash originating from the recovery boiler of a pulp mill.
  • the objective of the ash- leaching process is to purge chlorine and potassium, two of the so-called non- process elements, from the chemical recovery cycle of the pulp mill.
  • the chemicals of a pulping process are recovered from spent liquor, e.g., black liquor in kraft pulping.
  • the black liquor is evaporated and then fired in a recovery boiler.
  • Flue gases from the recovery boiler contain inorganic dry solids particles, which are separated in an electrostatic precipitator (ESP).
  • the main components in the ash are sodium sulfate and sodium carbonate.
  • Wood contains small amounts of potassium (K) and chlorine (CI). These elements remain in the black liquor during cooking. They may enter black liquor also via make-up chemicals, or via internal connections inside the mill.
  • K potassium
  • CI chlorine
  • These elements remain in the black liquor during cooking. They may enter black liquor also via make-up chemicals, or via internal connections inside the mill.
  • the ash amount is typically 6 - 12 % of the dry solids fired in the recovery boiler, equal to about 80 - 200 kg/ADT.
  • the ash is returned back to the evaporator or to the firing liquor to recover valuable chemicals.
  • Chloride and potassium are enriched in ESP ash and therefore chloride and potassium are favorably removed from the ash.
  • Leaching of recovery-boiler ash e.g. ash separated by an electrostatic precipitator
  • these elements would, in general, accumulate to levels which would lead to unacceptable extents of fouling and corrosion in the recovery boiler of the pulp mill.
  • the carbonate content in the ash is 10 %
  • about 80 kg of H 2 S0 4 per ton of ash is typically added into the leaching tank in order to decrease the carbonate concentration in the solution sufficiently.
  • a low suction in the top of the leaching tank will draw carbon dioxide out from the solution and drive the reaction 1 ) towards the right.
  • the conventional ash-leaching process is described in more detail in, for example, the article: Honkanen, R. & Kaila, J., Experiences in Various Chloride Removal Technologies, Proc. 2010 International Chemical Recovery Conference, Williamsburg, USA, Tappi Press, Vol 2, pp. 259-267.
  • An example of a variation on this process is a two-stage leaching process disclosed in the patent application document WO201 1/002354A1 .
  • the use of H 2 S0 4 as an additive in the leaching process is becoming more and more problematic.
  • a drawback is that the total sulfur amount in the recovery cycle gets higher.
  • the use of this fresh H 2 S0 4 can be decreased by using CI0 2 plant spent acid for this acidification, as described e.g. in US Patent 3833462.
  • a drawback of using this spent acid is that it contains chloride which will be added to the leaching stage.
  • An object of the present invention is to provide an ash leaching method in which the use of sulfuric acid is decreased or eliminated. Another object is to improve a chemical recovery process of a pulp mill.
  • the present invention relates to a method for removing chloride and potassium from ash of a chemical recovery boiler at a pulp mill having a tall oil plant, wherein the ash is slurried in an aqueous solution in at least one leaching stage and the pH of the slurry is adjusted, and the slurry is led from the at least one leaching stage to a separation stage.
  • a solid phase containing sodium compounds is separated from a liquid phase containing chloride and potassium, and the solid phase is fed to the recovery cycle.
  • the aqueous solution comprises tall oil brine obtained from the tall oil plant and the pH of the slurry is adjusted by adding tall oil brine or tall oil brine in combination with acid.
  • the tall oil brine (tall oil spent acid) is formed at a tall oil plant of the pulp mill.
  • the major chemical by-product derived from the kraft pulp industry is crude tall oil (CTO).
  • CTO crude tall oil
  • the primary source is pine.
  • Sodium soaps are formed in the kraft pulping, and these dissolve in the pulping liquor, but float to the surface upon
  • soap skimmings are separated from the black liquor, and further treated with acid to form CTO.
  • the standard process for converting soap to CTO is the addition of sulfuric acid to soap in a batch reactor or continuous reactor system. Many pulp mills have a chlorine dioxide plant, and thus sulfuric acid (spent acid) is available as by product. Acidulation of tall oil soap to a pH of about 3 or lower, yields a CTO stream and a spent acid stream. During this reaction three phases are formed: crude tall oil, a mixed phase containing fiber and lignin residuals and an aqueous sulfate brine solution (spent acid). The crude tall oil is separated and sent out as a product.
  • the other phases, the brine solution and the mixed phase can be combined as one phase or separated and, as well known, typically returned to the recovery system: to weak liquor evaporator or to heavy black liquor.
  • the pH of the tall oil brine is increased typically above 10 with an alkaline solution, such as white liquor or NaOH, before the return.
  • the acidic tall oil brine is used in a new way. It is used in ash leaching. It is used to adjust the pH of the ash slurry in the leaching stage. This decreases or even eliminates the use of sulfuric acid in ash leaching.
  • the pH is decreased so that it is typically between 7-1 1 , preferably below 9 and the brine reacts with carbonate in the slurry forming sodium sulfate and carbonate/carbon dioxide according to the reactions described above. This results in less Na 2 C0 3 (s) into the solution phase and in a smaller sodium loss in leaching.
  • the solid phase is separated from the liquid phase after each leaching stage, and the separated solid phase is reslurried in an aqueous solution in a next leaching stage, and the pH is adjusted, if needed.
  • the precipitator ash which is formed at lower as-fired black liquor dry solids (up to 70-75 DS%) mainly consists of sodium sulfate (Na 2 S0 4 ). The higher the dry solids content is, the more the ash contains sodium carbonate (Na 2 C0 3 ). When the black liquor dry solids content is over 80%, the ratio in the precipitator ash is typically 70% Na 2 S0 4 / 30% Na 2 C0 3 minus ratio of other components.
  • tall oil brine in this new position brings many advantages. Its acidity replaces fresh sulfuric acid in the leaching process. In some cases, fresh sulfuric acid in addition to tall oil brine may also be needed to some extent, but in any case the use of the brine will lower the net sulfur addition to the recovery process. In cases where fresh sulfuric acid is not needed at all, it will generate improved efficiency. Also another suitable acid together with tall oil brine can be used, if needed.
  • the invention also decreases the need to re-alkalize the tall oil brine.
  • the aqueous solution used in ash leaching is typically water or condensate.
  • the brine is also an aqueous solution, its use decreases the need of water and condensate.
  • the evaporation load is decreased, because the brine is not fed to the black liquor to be evaporated.
  • the evaporated black liquor is burned in the recovery boiler.
  • the solids from the ash leaching are added to heavy black liquor.
  • the tall oil brine contains sulfate, and thus its use brings a positive sulfate addition (as Na2S04 crystals) to black liquor crystallisation in concentrators at the black liquor evaporation plant of the mill.
  • the brine contains also calcium (as free or solid Ca 2 S0 4 ) and is able to remove some C0 3 as it forms CaC0 3 (s). Some free calcium may also end up in leached liquor (together with CI and K). Thus less free calcium ends up to black liquor, and calcium scaling is decreased at the black liquor evaporation plant.
  • the tall oil brine contains also lignin which together with sodium compound crystals in the solid phase can be separated in a centrifuge or filter and sent to black liquor.
  • Figure 1 shows an apparatus which can be used for applying the new method.
  • Ash which is separated from flue gases of a recovery boiler in an electrostatic precipitator is fed by a screw conveyor 2 through line 3 to a leaching tank 4.
  • the ash is mixed with an aqueous solution containing evaporator condensate, water or other suitable liquids to form a slurry.
  • the aqueous solution is led through line 7 to the leaching tank. Condensate or water is led through line 5 to line 7.
  • the aqueous solution contains also tall oil brine from a tall oil plant, which brine is fed through line 6 to line 7.
  • the ash is alkaline and contains up to 40 wt- % sodium carbonate and rest of the mass is mainly sodium sulfate.
  • chloride and potassium are enriched in the ESP ash and therefore chloride and potassium are removed from the ash.
  • the sum of potassium and chloride content in the ash is about 5 wt-%
  • the ash and the solution are mixed typically in a ratio of 1 .4-1 .7 kg ash/kg condensate or other aqueous solution. Ash is only partially dissolved in the aqueous solution.
  • Chlorine (CI) and potassium (K) are enriched in the solution; i.e., compared to the other elements, their contents are higher in the solution than they are in the ash.
  • a major part of the sodium salts in the original ash material remains as solids in the slurry.
  • the settling velocity of the ash slurry particles becomes very low and the centrifuge solid product remains wet, meaning that chloride and potassium removal rates are decreased.
  • the Na 2 C0 3 crystals are relatively small and light compared to the Na 2 S0 4 crystals. This means that in a centrifuge or filter in a leaching plant the Na 2 C0 3 particles will to a high extent stay in the liquid phase.
  • the pH of the slurry is decreased, typically below 9. According to the new method tall oil brine is added to the slurry through lines 6 and 7.
  • H 2 S0 4 sulfuric acid
  • C0 3 carbonate
  • C0 2 carbon dioxide
  • the tall oil brine contains sulfite, sulfate, calcium and organics, and water (40-90 %).
  • the added tall oil brine decreases the amount of condensate or water needed in leaching.
  • the density in the leaching tank is kept low enough to ensure easy pumping of slurry through line 9 to a separation device 10, such as a decanter centrifuge or filter. Part of the slurry is recycled back to the leaching tank through line 7.
  • the liquid phase containing potassium and chloride and the solid phase containing sodium compounds are separated from each other in the separation device. Organics, such as lignin, contribute to keep sodium compound crystals in the solid phase in the separation device.
  • the liquid phase is led through line 1 1 to a reject tank 12. Part of the solution is usually recycled to the leaching tank through lines 13 and 7 and part is purged through line 14 to the sewer or water treatment.
  • the solution contains also free calcium, which is discharged from the process.
  • the solid phase is fed from the separation device 10 by a screw conveyor 15 through line 16 to a mixing tank 17.
  • the solids are added to heavy black liquor, which is led through line 18.
  • the heavy black liquor having an increased sodium compound content is returned through line 19 to the
  • Gases formed in the leaching tank, separation tank and mixing tank are collected via lines 21 and 22 to a scrubber 20.
  • the pH of the tall oil brine is increased by adding sodium hydroxide or white liquor, and it is mixed to black liquor. This increases the load of the evaporation plant and weakens the steam economy. Free calcium reacts with carbonate in black liquor forming CaC0 3 scaling on heat surfaces, so that the evaporation capacity decreases. Special processes are needed to remove these scales, and evaporators have to be shut down.
  • the sodium, sulfate and carbonate in the brine also increase the inorganic dead load of the evaporation plant, when the brine is mixed to black liquor.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Wood Science & Technology (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Materials Engineering (AREA)
  • Inorganic Chemistry (AREA)
  • Paper (AREA)
  • Processing Of Solid Wastes (AREA)

Abstract

The present invention relates to a method for removing chloride and potassium from ash formed in a recovery boiler at a pulp mill. Evaporated black liquor is fired, wherein the ash is made to a slurry in an aqueous solution in at least one leaching stage and the pH of the slurry is adjusted. The slurry is led from the at least one leaching stage to a separation stage, in which a solid phase containing sodium compounds is separated from a liquid phase containing chloride and potassium, and the solid phase is fed to the recovery cycle. The pH of the slurry is adjusted by adding tall oil brine or tall oil brine in combination with acid.

Description

A METHOD FOR TREATING RECOVERY BOILER ASH
The present method relates to removing chloride and potassium in the leaching of ash originating from the recovery boiler of a pulp mill. The objective of the ash- leaching process is to purge chlorine and potassium, two of the so-called non- process elements, from the chemical recovery cycle of the pulp mill.
In chemical pulp mills, the chemicals of a pulping process are recovered from spent liquor, e.g., black liquor in kraft pulping. The black liquor is evaporated and then fired in a recovery boiler. Flue gases from the recovery boiler contain inorganic dry solids particles, which are separated in an electrostatic precipitator (ESP). The main components in the ash are sodium sulfate and sodium carbonate. Wood contains small amounts of potassium (K) and chlorine (CI). These elements remain in the black liquor during cooking. They may enter black liquor also via make-up chemicals, or via internal connections inside the mill. In the recovery boiler, these elements are enriched into the fly ash and increase the corrosiveness of the flue gas especially in the super heater. The ash amount is typically 6 - 12 % of the dry solids fired in the recovery boiler, equal to about 80 - 200 kg/ADT. The ash is returned back to the evaporator or to the firing liquor to recover valuable chemicals.
Chloride and potassium are enriched in ESP ash and therefore chloride and potassium are favorably removed from the ash. Leaching of recovery-boiler ash (e.g. ash separated by an electrostatic precipitator) is a known method of purging chlorine and potassium from the chemical recovery process of pulp mills employing alkaline pulping processes. Without purging, these elements would, in general, accumulate to levels which would lead to unacceptable extents of fouling and corrosion in the recovery boiler of the pulp mill.
In the conventional ESP ash-leaching process, a limited amount of water is added to the ash so that a slurry is formed. Chlorine (CI) and potassium (K) are enriched in the solution; i.e., compared to the other elements, their contents are higher in the solution than they are in the ash. A major part of the sodium salts in the original ash material remains as solids in the slurry. The solids are separated from the solution using, for example, a decanter centrifuge or a filter, and the solids are returned to the recovery cycle by adding them to the spent pulping liquor (black liquor), which is evaporated. Part of the solution is usually recycled to the leaching tank and part is purged to the sewer or water treatment.
When carbonate (C03) concentration in the ash exceeds about 5 %, the settling velocity of the ash slurry particles becomes very low and the centrifuge solid product remains wet, meaning that chloride and potassium removal rates are decreased. The Na2C03 crystals are relatively small and light compared to the Na2S04 crystals. This means that in a centrifuge or filter in a leaching plant the Na2C03 particles will to a high extent stay in the liquid phase. This results in a sodium loss, as the solution from the centrifuge or filter is purged from the process. Make up sodium is expensive.
To ensure a satisfactory separation of solids from the solution, sulfuric acid (H2S04) can be added in the leaching stage to decreasing the pH from the initial value of 9-13 below 9. Thus part of the solid sodium carbonate is converted to carbonate (C03) or carbon dioxide (C02) according to the reactions:
1 ) Na2C03(s) + H2S04(aq)→ Na2S04 (s)+ C02(g) + H20(aq)
2) Na2C03(s) + ½ H2S04(aq)→ ½ Na2S04 (s)+ Na+ (aq) + C03 2-(aq) + 1 H+ (aq)
3) Na2C03(s) + H2S04(aq)→ Na2S04 (s) + 2C03 2 (aq) + 2 H+ (aq)
For example, if the carbonate content in the ash is 10 %, about 80 kg of H2S04 per ton of ash is typically added into the leaching tank in order to decrease the carbonate concentration in the solution sufficiently. A low suction in the top of the leaching tank will draw carbon dioxide out from the solution and drive the reaction 1 ) towards the right.
The conventional ash-leaching process is described in more detail in, for example, the article: Honkanen, R. & Kaila, J., Experiences in Various Chloride Removal Technologies, Proc. 2010 International Chemical Recovery Conference, Williamsburg, USA, Tappi Press, Vol 2, pp. 259-267. An example of a variation on this process is a two-stage leaching process disclosed in the patent application document WO201 1/002354A1 . The use of H2S04 as an additive in the leaching process is becoming more and more problematic. A drawback is that the total sulfur amount in the recovery cycle gets higher. The use of this fresh H2S04 can be decreased by using CI02 plant spent acid for this acidification, as described e.g. in US Patent 3833462. A drawback of using this spent acid is that it contains chloride which will be added to the leaching stage.
An object of the present invention is to provide an ash leaching method in which the use of sulfuric acid is decreased or eliminated. Another object is to improve a chemical recovery process of a pulp mill.
The present invention relates to a method for removing chloride and potassium from ash of a chemical recovery boiler at a pulp mill having a tall oil plant, wherein the ash is slurried in an aqueous solution in at least one leaching stage and the pH of the slurry is adjusted, and the slurry is led from the at least one leaching stage to a separation stage. A solid phase containing sodium compounds is separated from a liquid phase containing chloride and potassium, and the solid phase is fed to the recovery cycle. The aqueous solution comprises tall oil brine obtained from the tall oil plant and the pH of the slurry is adjusted by adding tall oil brine or tall oil brine in combination with acid. The tall oil brine (tall oil spent acid) is formed at a tall oil plant of the pulp mill. The major chemical by-product derived from the kraft pulp industry is crude tall oil (CTO). The primary source is pine. Sodium soaps are formed in the kraft pulping, and these dissolve in the pulping liquor, but float to the surface upon
concentration of the liquor. These "soap skimmings" are separated from the black liquor, and further treated with acid to form CTO. The standard process for converting soap to CTO is the addition of sulfuric acid to soap in a batch reactor or continuous reactor system. Many pulp mills have a chlorine dioxide plant, and thus sulfuric acid (spent acid) is available as by product. Acidulation of tall oil soap to a pH of about 3 or lower, yields a CTO stream and a spent acid stream. During this reaction three phases are formed: crude tall oil, a mixed phase containing fiber and lignin residuals and an aqueous sulfate brine solution (spent acid). The crude tall oil is separated and sent out as a product. The other phases, the brine solution and the mixed phase can be combined as one phase or separated and, as well known, typically returned to the recovery system: to weak liquor evaporator or to heavy black liquor. The pH of the tall oil brine is increased typically above 10 with an alkaline solution, such as white liquor or NaOH, before the return.
In the new method the acidic tall oil brine is used in a new way. It is used in ash leaching. It is used to adjust the pH of the ash slurry in the leaching stage. This decreases or even eliminates the use of sulfuric acid in ash leaching. The pH is decreased so that it is typically between 7-1 1 , preferably below 9 and the brine reacts with carbonate in the slurry forming sodium sulfate and carbonate/carbon dioxide according to the reactions described above. This results in less Na2C03(s) into the solution phase and in a smaller sodium loss in leaching.
In the new method there may be more than one leaching stage, for instance two leaching stages. The solid phase is separated from the liquid phase after each leaching stage, and the separated solid phase is reslurried in an aqueous solution in a next leaching stage, and the pH is adjusted, if needed. The precipitator ash which is formed at lower as-fired black liquor dry solids (up to 70-75 DS%) mainly consists of sodium sulfate (Na2S04). The higher the dry solids content is, the more the ash contains sodium carbonate (Na2C03). When the black liquor dry solids content is over 80%, the ratio in the precipitator ash is typically 70% Na2S04 / 30% Na2C03 minus ratio of other components. The use of the tall oil brine in this new position brings many advantages. Its acidity replaces fresh sulfuric acid in the leaching process. In some cases, fresh sulfuric acid in addition to tall oil brine may also be needed to some extent, but in any case the use of the brine will lower the net sulfur addition to the recovery process. In cases where fresh sulfuric acid is not needed at all, it will generate improved efficiency. Also another suitable acid together with tall oil brine can be used, if needed.
The invention also decreases the need to re-alkalize the tall oil brine.
The aqueous solution used in ash leaching is typically water or condensate.
Because the brine is also an aqueous solution, its use decreases the need of water and condensate. In addition the evaporation load is decreased, because the brine is not fed to the black liquor to be evaporated. The evaporated black liquor is burned in the recovery boiler.
The solids from the ash leaching are added to heavy black liquor. The tall oil brine contains sulfate, and thus its use brings a positive sulfate addition (as Na2S04 crystals) to black liquor crystallisation in concentrators at the black liquor evaporation plant of the mill. The brine contains also calcium (as free or solid Ca2S04) and is able to remove some C03 as it forms CaC03(s). Some free calcium may also end up in leached liquor (together with CI and K). Thus less free calcium ends up to black liquor, and calcium scaling is decreased at the black liquor evaporation plant. The tall oil brine contains also lignin which together with sodium compound crystals in the solid phase can be separated in a centrifuge or filter and sent to black liquor.
Figure 1 shows an apparatus which can be used for applying the new method.
Ash which is separated from flue gases of a recovery boiler in an electrostatic precipitator is fed by a screw conveyor 2 through line 3 to a leaching tank 4. The ash is mixed with an aqueous solution containing evaporator condensate, water or other suitable liquids to form a slurry. The aqueous solution is led through line 7 to the leaching tank. Condensate or water is led through line 5 to line 7. According to the present invention, the aqueous solution contains also tall oil brine from a tall oil plant, which brine is fed through line 6 to line 7.
The ash is alkaline and contains up to 40 wt- % sodium carbonate and rest of the mass is mainly sodium sulfate. In addition, chloride and potassium are enriched in the ESP ash and therefore chloride and potassium are removed from the ash. The sum of potassium and chloride content in the ash is about 5 wt-% The ash and the solution are mixed typically in a ratio of 1 .4-1 .7 kg ash/kg condensate or other aqueous solution. Ash is only partially dissolved in the aqueous solution. Chlorine (CI) and potassium (K) are enriched in the solution; i.e., compared to the other elements, their contents are higher in the solution than they are in the ash. A major part of the sodium salts in the original ash material remains as solids in the slurry.
When the carbonate (C03) concentration in the ash exceeds about 5 %, the settling velocity of the ash slurry particles becomes very low and the centrifuge solid product remains wet, meaning that chloride and potassium removal rates are decreased. The Na2C03 crystals are relatively small and light compared to the Na2S04 crystals. This means that in a centrifuge or filter in a leaching plant the Na2C03 particles will to a high extent stay in the liquid phase. To ensure a satisfactory separation of solids from the solution, the pH of the slurry is decreased, typically below 9. According to the new method tall oil brine is added to the slurry through lines 6 and 7. If needed, also sulfuric acid (H2S04) may be added to the slurry through line 8. However, the use of tall oil brine decreases substantially the need of fresh H2S04. When the pH decreases, part of the sodium carbonate is converted to carbonate (C03) or carbon dioxide (C02). It is important to have a proper pH control and slurry density control in the leaching tank.
The tall oil brine contains sulfite, sulfate, calcium and organics, and water (40-90 %). The added tall oil brine decreases the amount of condensate or water needed in leaching.
The density in the leaching tank is kept low enough to ensure easy pumping of slurry through line 9 to a separation device 10, such as a decanter centrifuge or filter. Part of the slurry is recycled back to the leaching tank through line 7. The liquid phase containing potassium and chloride and the solid phase containing sodium compounds are separated from each other in the separation device. Organics, such as lignin, contribute to keep sodium compound crystals in the solid phase in the separation device. The liquid phase is led through line 1 1 to a reject tank 12. Part of the solution is usually recycled to the leaching tank through lines 13 and 7 and part is purged through line 14 to the sewer or water treatment. The solution contains also free calcium, which is discharged from the process.
The solid phase is fed from the separation device 10 by a screw conveyor 15 through line 16 to a mixing tank 17. In the mixing tank the solids are added to heavy black liquor, which is led through line 18. The heavy black liquor having an increased sodium compound content is returned through line 19 to the
evaporation plant and further to the recovery boiler.
Gases formed in the leaching tank, separation tank and mixing tank are collected via lines 21 and 22 to a scrubber 20. In known processes the pH of the tall oil brine is increased by adding sodium hydroxide or white liquor, and it is mixed to black liquor. This increases the load of the evaporation plant and weakens the steam economy. Free calcium reacts with carbonate in black liquor forming CaC03 scaling on heat surfaces, so that the evaporation capacity decreases. Special processes are needed to remove these scales, and evaporators have to be shut down. The sodium, sulfate and carbonate in the brine also increase the inorganic dead load of the evaporation plant, when the brine is mixed to black liquor. These drawbacks of the known practice can be avoided in the new method.

Claims

CLAIMS:
1 . A method for removing chloride and potassium from ash formed in a recovery boiler at a pulp mill having a tall oil plant, wherein the ash is made to a slurry in an aqueous solution in at least one leaching stage and the pH of the slurry is adjusted, and the slurry is led from the at least one leaching stage to a separation stage, in which a solid phase containing sodium compounds is separated from a liquid phase containing chloride and potassium, and the solid phase is fed to the recovery cycle of the pulp mill, characterized in that the aqueous solution comprises tall oil brine and the pH of the slurry is adjusted with tall oil brine or tall oil brine in combination with acid.
2. A method according to claim 1 , characterized in that the pH of the tall oil brine is below 7.
3. A method according to claim 1 or 2, characterized in that the pH of the slurry is decreased to 7-1 1 , preferably below 9.
PCT/FI2016/050830 2015-11-27 2016-11-25 A method for treating recovery boiler ash WO2017089656A1 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN109368666A (en) * 2018-12-21 2019-02-22 中冶南方都市环保工程技术股份有限公司 One kind proposing sylvite system

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3833462A (en) 1971-03-15 1974-09-03 Mac Millan Bloedel Ltd Process of removing sodium chloride from kraft pulping process chemical recovery systems
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