WO2008063123A1

WO2008063123A1 - Leaching of chloride and potassium from electric filter ashes

Info

Publication number: WO2008063123A1
Application number: PCT/SE2007/050856
Authority: WO
Inventors: Patrik LÖWNERTZ
Original assignee: Metso Power Ab
Priority date: 2006-11-22
Filing date: 2007-11-16
Publication date: 2008-05-29
Also published as: SE530373C2; SE530373C3; SE0602475L

Abstract

The invention concerns an arrangement and a method for the leaching of electrical filter ash that contains potassium and chlorides. The electrical filter ash undergoes mixing in the invention with leaching fluid in a first recirculation zone. The mixture of electrical filter ash and leaching fluid is subsequently led to a second recirculation zone. The mixture in this zone is placed into a flow that is toroidal relative to the horizontal plane. The fluid is forced to recirculate in the toroidal flow downwards at its outer edges and upwards at its centre, whereby crystal growth of sludge particles is promoted in this toroidal flow. Fluid that is enriched in potassium and chlorides is withdrawn from the second recirculation zone from the outer edge of the enforced toroidal flow, while slurry particles sink down into a third sedimentation zone. Condensate is added in the sedimentation zone such that this is expelled through the slurry particles. Leaching fluid is withdrawn from the sedimentation zone, which fluid is added at the first zone in order to be mixed with the electrical filter ash.

Description

Leaching of chloride and potassium from electric filter ashes

TECHNICAL ARFA The invention concerns an arrangement and a method for leaching electrical filter ash that contains potassium and chlorides according to the preamble of to claim 1 and 7.

THE PRIOR ART Sodium and sulphur-based chemicals are used and recycled during the sulphate pulp process for the release of wood substance. The chemicals are used in what is essentially a closed cycle. Contaminants in the form of potassium and chlorides in this cycle of chemicals contribute to scaling in the recovery boiler of the sulphate pulp process, i.e. the soda boiler, and they contribute to corrosion of the hot surfaces of the boiler. This is a problem that becomes more acute since the steam temperature of the boiler is high in order to obtain a high yield of electrical power in the subsequent steam turbine.

Potassium and chlorides are added primarily with the wood raw material, and to a certain extent with make-up chemicals and water. Chlorides can also be added to the closed cycle of chemicals when recycling alkaline bleaching filtrate from the bleaching plant of the sulphate pulp mill.

Even low levels of potassium and chlorides, which are foreign substances for the pulping process, supplied in the input flows to the process contribute to high levels in the cycle of chemicals when the degree of closure is high.

The electrical filter ash that is separated from the flue gases of the soda boiler contains principally Na2SO4, with a lesser amount of sodium carbonate Na₂CO₃, but due to the relatively high volatility of potassium and chlorides under the conditions that are prevalent in the soda boiler, the electrical filter ash contains also the highest concentrations of potassium and chlorides within the pulp mill.

RECORD COPY - TRANSLATION (Rule 12.4) One way of reducing the levels of potassium and chlorides in the liquor stock of the pulp mill is to separate out a subfraction of the electrical filter ash from the cycle of chemicals of the mill. This separation normally takes place through the ash being dissolved in water and subsequently led to the water purification plant of the mill. These types of separation, however, entail also significant loss of sodium sulphate, which has a value, and they entail heavy metals that are present in the electrical filter ash placing a load on the waste water system.

The loss of sodium sulphate that takes place during separation of the electrical filter ash from the cycle of chemicals must be compensated by the addition of make-up chemicals. A number of processes of different types are available for the recovery of Na2SO4 from the separated electrical filter ash through cleaning the electrical filter ash from, primarily, potassium and chlorides. The solutions that are suggested often contain filters or substages in which dissolved or leached electrical filter ash is cooled, either in its entirety or in the form of subfractions. Other processes for the recovery of Na2SO4 are also available in which the electrical filter ash is first fully dissolved in order subsequently to be recrystallised by vaporisation in a crystalliser with the subsequent separation of the solid and fluid phases.

Filtering technology

SE 504374 C2 reveals a method in which the electrical filter ash is processed in a pressurised filter, in which not only filtration but also wash of the filter cake take place, and any leaching that takes place does so in one and the same vessel. This process gives a good result as long as the filter can be kept free from clogging. Methods for cleaning the filter are required, in which the cleaning takes place either during operation or with temporary interruptions of the filtering process. US 3,833,462 reveals a further method in which leaching takes place at pH 3-6 and at 40-80⁰C, followed by filtration in a filter.

B. Blackwell and A. Hitzroth present experiences from applied leaching processes in the Tappi Proceedings, Chemical Recovery Conference 1992 in Seattle, pages 329-350: "Recycle of Bleach Plant Extraction Stage Effluent to the Kraft Liquour Cycle". Leaching of the electrical filter ash in water was applied in one system at a concentration of dry matter of around 15% by weight and at pH 5. It was pointed out that this low pH constituted a significant problem with corrosion. The leached fluid was filtered in a filter (a belt filter), and this filter came to experience significant problems with clogging, due to small particles of organic material that clogged the filter cloth. One measure suggested was to use an even larger filter, which was expected to increase the period of operation. The filter must, however, sooner or later be cleaned. It was stated that just over 89% of the sodium chloride in the electrical filter ash could be leached from the electrical filter ash.

Cooling technology SE 515987 reveals an alternative method that includes cooling. The leaching in this case takes place at pH 10 and a temperature higher than 20 °C, typically at 30 ⁰C, after which the total quantity of slurried material is subject to a cooling process to a temperature lower than 20 ⁰C. This process includes a requirement for cooling and the accompanying consumption of energy.

Also WO9819003 reveals a further method for the leaching of electrical filter ash in which the electrical filter ash first undergoes a leaching process at a temperature higher than 32 ⁰C, preferably in the vicinity of the boiling point of the aqueous solution, and in which a first substance that crystallises out after the separation is returned to black liquor.

The leachate that is obtained from the process subsequently undergoes also cooling to approximately 10-15 ⁰C, after which also a second substance that has crystallised out is returned to the black liquor. This system provides effective leaching, but it has a significant requirement for energy for the cooling, even though the cooling is required only for the smaller quantities that the leachate constitutes. Separation by centrifugation

SE 517587 C2 reveals a method for cleaning electrical filter ash, primarily from chlorides, through leaching and the subsequent separation of the substance that crystallises out, which substance is returned to the process for the formation of black liquor.

Good separation of the solid phase in the form of crystalline Na2SO4 is obtained through a repeated mild agitation of the slurried electrical filter ash in a leaching tank with a total retention time of between 1 and 5 hours, immediately followed by centrifugation of the leached electrical filter ash. The solid phase is added to the black liquor before the final evaporation stage, and before the recovery process in the soda boiler, or before both of these processes. More than 70% of the chloride content of the electrical filter ash and more than 70% of its potassium content are efficiently leached out, and an improved use of the process chemicals is obtained, in which the electrical filter ash is formed into a slurry and treated in a leaching tank. The electrical filter ash undergoes at least two stirring operations in the leaching tank, in at least two zones of agitation. The electrical filter ash is subsequently fed to a separation stage in which the leached electrical filter ash undergoes a separation process of the crystallised substance through centrifugal separation. Dry material and a fraction of the fluid are obtained in this process. The dry material is returned to the liquor cycle or to further leaching stages with the subsequent separation of the solid phase from the liquid phase, and where the liquid fraction is returned to parts of the leaching tank, or away from the process.

The disadvantage of this method is that the stage of centrifugal separation is rather expensive.

THE AIM OF THE INVENTION A first aim of the invention is to offer an arrangement and a method without the disadvantages and problems that the prior art technology demonstrates. A second aim is to obtain a continuous process that does not comprise filtration stages, which filter stages entail problems with clogging and the requirement that the filter medium be regenerated.

A third aim is to achieve a continuous process that is cheaper and that does not have the disadvantages that the invention revealed by SE 517587 C2 has.

The aims described above are achieved with an arrangement and a method according to the characterising parts of claims 1 and 7.

DESCRIPTION OF DRAWINGS

Figure 1 shows a drawing of a preferred embodiment of the arrangement for leaching electrical filter ash according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The concepts "first compartment", "second compartment" and "third compartment" will be used in the following detailed description.

The term "first compartment" is here used to denote a mixing chamber with a first recirculation zone, where the electrical filter ash is mixed under vigorous agitation with leaching fluid, to form a mixture of electrical filter ash and leaching fluid.

The term "second compartment" is here used to denote a crystal growth chamber with a second recirculation zone, in which crystal growth of the mixture of electrical filter ash and leaching fluid is promoted, and where fluid enriched in potassium and chlorides is withdrawn.

The term "third compartment" is here used to denote a sedimentation chamber with a sedimentation zone, at which leached electrical filter ash is withdrawn.

Figure 1 shows a preferred embodiment of an arrangement for leaching electrical filter ash that contains potassium and chlorides. The arrangement in Figure 1 comprises a vessel (2) with a first compartment (V1) situated uppermost in the vessel (2). The first compartment (V1) is contained by stationary limiting walls. The stationary limiting walls are constituted by cylindrical limiting walls arranged vertically in the vessel (2), and a bottom (105). The first compartment (V1) contains a first fluid phase in essentially the complete volume of the first compartment, the part of the first compartment that is not fluid phase is constituted by a gas phase, which is located above the fluid surface (104) of the fluid phase. Furthermore, a first motor-driven stirrer (101) is arranged in the first compartment (V1). The first stirrer (101) is fixed arranged on a shaft (103) that is caused to rotate by means of a motor (M 1).

Electrical filter ash (11) that contains potassium and chlorides is fed from a soda boiler (not shown in the drawing) into the first compartment (V1) through an inlet arrangement (12) that functions as an airlock. The electrical filter ash (11) is mixed in the first compartment (V1) with leaching fluid under vigorous agitation from the first stirrer (101). The small particles of ash, which are rich in chlorides and potassium, will grow through recrystallisation, in association with the mixing of the electrical filter ash with the leaching fluid. The electrical filter ash is subsequently led through a first outlet (102) arranged at the bottom of the first compartment into a second compartment (V2), arranged vertically under the first compartment (V1) in the vessel (10). The cross-sectional area (a1) of the first outlet is smaller than the cross-sectional area (A1) of the first compartment. A ventilating fan (14) is arranged in the upper part of the first compartment (V1) with the aim of preventing condensed moisture from entering the inlet of the electrical filter ash into the first compartment.

The second compartment (V2) is arranged vertically under the first compartment (V1) and it is arranged such that it can retain the mixture and provide the conditions required to promote crystal growth. The second compartment (V2) is contained by stationary limiting walls. The stationary limiting walls are constituted by cylindrical limiting walls arranged vertically in the vessel (2), and a bottom (205). The second compartment (V2) contains a second fluid phase essentially throughout the complete second compartment (V2). Furthermore, a second motor-driven stirrer (201) is arranged in the second compartment. The second stirrer (201) is fixed arranged on a shaft (103) that is caused to rotate by means of a motor (M 1). The second stirrer is arranged inside a cylinder (204), which is open at both of its ends.

The mixture of electrical filter ash and leaching fluid undergoes a second stirring operation by the second stirrer (201) in the second compartment. The second stirrer (201) presses the mixture up through the cylinder (204), such that a flow that is toroidal relative to the horizontal plane is formed around the cylinder (204), where the toroid has its centre at the centre of the cylinder and where the outer edges of the toroid are located outside the jacket cover of the cylinder. The mixture of electrical filter ash and leaching fluid is thus forced to recirculate in the toroidal flow downwards at its outer edges and upwards at its centre, whereby crystal growth of sludge particles is promoted in this toroidal flow.

A second outlet (202) is arranged at the bottom (205) of the second compartment (V2), which outlet opens out into a third compartment (V3). The cross-sectional area (a2) of the second outlet (202) is smaller than the cross-sectional area (A2) of the second compartment (V2). The bottom (205) of the second compartment, at which the second outlet (202) opens, has the shape of a cone in such a manner that the outlet (202) opens out into the tip of the cone.

Withdrawal channels (203) are arranged in the limiting walls of the second compartment. Clear and purified fluid rich in potassium and chlorides is withdrawn from the withdrawal channels (203) from the outer edge of the toroidal flow. A box with an opening that faces downwards is arranged around the withdrawal channels (203) in such a manner that the volume under the withdrawal channels is not stirred, and thus crystal particles in this volume sink, and the withdrawal channels in this manner withdraw only fluid from the compartment V2.

The slurry particles that have grown through recrystallisation in the toroidal flow tend to sink downwards into the second outlet and into the third compartment (V3), while smaller particles tend to be retained by the flow in the compartment V2 for further crystal growth.

The third compartment (V3) is arranged vertically under the second compartment (V2). The third compartment is contained by stationary limiting walls. The stationary limiting walls are constituted by cylindrical limiting walls arranged vertically in the vessel (2), and a bottom (308). The third compartment (V3) contains a third fluid phase essentially throughout the complete third compartment. The bottom (308) of the third compartment has the shape of a cone vertically downwards, with a third outlet (302) arranged at the point of the cone-shaped bottom. A slowly moving motor-driven rake (301) acts at the cone- shaped bottom of the third compartment, the task of this rake being to facilitate thickening and output of the crystallised slurry. The motor-driven rake (301) is fixed arranged on a shaft that is caused to rotate by means of a motor (M2).

The cross-sectional area (a3) of the third outlet is smaller than 20% of the cross- sectional area of the third compartment (V3). The outlet is equipped with an outlet means (303) for sluiced output of leached slurry particles from the third compartment (V3). The slurry particles are subsequently mixed with black liquor and returned to the evaporation plant (not shown in the drawing).

Supply means (304) are arranged in the limiting walls of the third compartment for the supply of condensate (307) to the third compartment (V3) in such a manner that leaching fluid is expelled from the slurry that is output from the third compartment. The condensate maintains a temperature below 70 ⁰C, preferably below 65 ⁰C.

A circulatory line (305) comprising a pump (306) is arranged between the first compartment (V1) and the third compartment (V3). The suction inlet of the circulatory line is arranged in the upper part of the stationary limiting wall of the third compartment (V3), and the outlet of the circulatory line is arranged in the limiting wall of the first compartment. Leaching fluid from the third compartment is fed in the circulatory line from the third compartment to the first compartment, in order there to be mixed with the electrical filter ash.

The invention is not limited to the embodiments shown above: several variants are possible within the scope of the attached patent claims.

One such variant is one in which the three compartments do not need to be arranged in one common vessel. Nor is it necessary that the compartments be arranged vertically.

It is also possible that a further compartment for recrystallisation is inserted between the first and second compartments described here.

It is not necessary, either, that the first stirrer (101) and the second stirrer (201) be driven by a common motor and be located on the same shaft. It is an advantage that the first and second stirrers rotate at different rates of revolution, and in that case they will then be driven by separate drive shafts.

Claims

1. An arrangement (1) for the leaching of electrical filter ash that contains potassium and chlorides, characterised in that the arrangement comprises:

- a first compartment (V1) contained by stationary limiting walls, containing a first fluid phase in essentially the complete volume of this first compartment and a first motor-driven stirrer (101) arranged in the compartment, and to which first compartment electrical filter ash (11) that contains potassium and chlorides is fed through an inlet arrangement (12),

- a first outlet (102) arranged at the bottom (105) of the first compartment and that opens out into a second compartment, where the cross-sectional area (a1) of the first outlet is less than the cross-sectional area (A1) of the first compartment, - a second compartment (V2) contained by stationary limiting wall containing a second fluid phase in essentially the complete volume of this second compartment and a second motor-driven stirrer (201) arranged in the second compartment, and in that at least one withdrawal channel (203) is arranged in the limiting walls of the second compartment, - a second outlet (202) arranged at the bottom (205) of the second compartment and that opens out into a third compartment, where the cross-sectional area (a2) of the second outlet is less than the cross-sectional area (A2) of the second compartment,

- a third compartment (V3) contained within stationary limiting walls and containing a third fluid phase in this third compartment, and a motor-driven rake

(301),

- a third outlet (302) from the third compartment arranged at the bottom of the third compartment, where the cross-sectional area (a3) of the third outlet is less than 20% of the cross-sectional area (A3) of the third compartment, and which outlet is equipped with an outlet means (303) for the sluiced output of leached slurry particles,

- addition means (304) arranged in the stationary limiting walls of the third compartment for the addition of condensate to the third compartment, - a circulatory line (402) with pump (401) arranged between the first and the third compartments, with the suction inlet of the circulatory line arranged in an upper part of the stationary limiting wall of the third compartment and the outlet of the circulatory line arranged in the limiting wall of the first compartment,

- whereby electrical filter ash is mixed in the first compartment during stirring and the addition of fluid withdrawn from the third compartment, crystal growth takes place in the second compartment during withdrawal of leaching fluid that contains chlorides and potassium from the withdrawal channel (203), and that condensate is added into the third compartment during the expulsion of leaching fluid from the crystal slurry in the lower part of the third compartment.

2. The arrangement according to claim 1, characterised in that the first, second and third compartments are arranged vertically.

3. The arrangement according to either claim 1 or 2, characterised in that the first, second and third compartments are arranged in a common vessel.

4. The arrangement according to any one of claims 1-3, characterised in that the bottom of the third compartment has the form of a cone.

5. The arrangement according to any one of claims 1-4, characterised in that the bottom of the second compartment has the form of a cone.

6. The arrangement according to any one of claims 1-5, characterised in that a ventilation means (14) is arranged in the upper part of the first compartment (V1) for the removal of moisture.

7. A method for the leaching of electrical filter ash that contains potassium and chlorides, characterised in that electrical filter ash is mixed during vigorous agitation in a first recirculation zone with leaching fluid, followed by the electrical filter ash that has been mixed with leaching fluid being led to a second recirculation zone, in which the mixture of electrical filter ash and leaching fluid is placed in a flow that is toroidal relative to the horizontal plane, where the fluid is caused to recirculate downwards at the outer edges and upwards in the centre of the toroidal flow, whereby crystal growth of slurry particles in this toroidal flow is promoted, and where fluid that is enriched in potassium and chlorides from leaching is withdrawn from the outer edge of the forced toroidal flow while slurry particles sink down into a third sedimentation zone, and in that condensate is added in the sedimentation zone such that this is expelled through the slurry particles, and in that leaching fluid is withdrawn from the sedimentation zone, which fluid is added at the first zone in order to be mixed with the electrical filter ash.