Method and device for treating sulfur dioxide flue gas and crystallizing sodium sulfite

Abstract

This invention specifically relates to a method and apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite. Low-concentration sulfur dioxide flue gas generated during smelting production is purified through a two-stage wet scrubbing process, then absorbed in an absorption tower. The resulting tail gas meets emission standards. The absorbed liquid, sodium bisulfite, is neutralized with alkali and impurities are removed, followed by evaporation, concentration, and centrifugal separation. The aim is to use the collected filtrate from the underground tank to feed into a double-effect evaporator, thereby disrupting the existing solid-liquid ratio distribution within each effect, controlling the number of seed crystals in the first effect, increasing the number of seed crystals in the second effect evaporator, accelerating crystal growth, reducing oxidation during evaporation, increasing the crystal growth rate, and improving the centrifugal separation effect by adding sodium chloride as an impurity.

Description

Technical Field

[0001] This invention belongs to the field of chemical environmental protection, specifically relating to a method and apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite. Background Technology

[0002] During the smelting of sulfide ores, a large amount of low-concentration sulfur dioxide flue gas is generated. Due to its low sulfur concentration, this flue gas cannot be used for acid production. However, it cannot be directly discharged into the air without treatment. Currently, the common method for treating this low-concentration sulfur dioxide flue gas is to use caustic soda to absorb the low-concentration sulfur dioxide flue gas from smelting, so that it meets emission standards. Sodium sulfite is then produced as a byproduct after absorption. This solves the sulfur dioxide pollution problem and also produces sodium sulfite, which has certain economic benefits.

[0003] However, in the existing sodium sulfite crystallization process, solid-liquid separation is achieved through a single evaporation crystallization process. When using relatively pure diaphragm caustic soda to absorb low-concentration sulfur dioxide flue gas from smelting, and then evaporating the sodium sulfite solution through an evaporator, the efficiency of sodium sulfite evaporation crystallization is not high, resulting in fewer seed crystals, poor crystallization effect, and inability to effectively achieve the purpose of solid-liquid separation. Summary of the Invention

[0004] To address the issues of sulfur dioxide absorption and treatment, as well as sodium sulfite crystallization, this invention provides a method and apparatus for sulfur dioxide flue gas treatment and sodium sulfite crystallization. The method involves treating low-concentration sulfur dioxide flue gas generated during smelting processes through a two-stage wet scrubbing purification process. The resulting flue gas is then absorbed in an absorption tower, and the treated exhaust gas meets emission standards. The resulting absorbent, sodium bisulfite, is evaporated, concentrated, and centrifuged. The collected filtrate from an underground tank is then fed into a double-effect evaporator. This process disrupts the existing solid-liquid ratio distribution within each effect, controls the number of seed crystals in the first effect, increases the number of seed crystals in the second effect evaporator, accelerates crystal growth, reduces oxidation during evaporation, and improves the crystal growth rate. The addition of sodium chloride impurities further enhances the centrifugal separation effect.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is: a method for treating sulfur dioxide flue gas and crystallizing sodium sulfite, characterized by comprising the following steps:

[0006] S1: Purify and absorb low-concentration sulfur dioxide flue gas, and discharge the absorbed gas in compliance with emission standards. The generated sodium bisulfite absorbent is then neutralized.

[0007] S2: The neutralized sodium bisulfite solution is concentrated by circulating it through a two-stage evaporator, so that the solid-liquid ratio in the concentrated solution reaches 3:2.

[0008] S3: Solid-liquid separation is performed on the concentrated solution. The separated crystalline sodium sulfite material enters the drying process for further processing. The separated filtrate is stored and then enters the secondary evaporator for further evaporation and concentration.

[0009] Preferably, in step S1, the sulfur dioxide flue gas is absorbed using liquid alkali. When the sodium chloride content in the added liquid alkali is as low as 1 g / L, sodium chloride is mixed into the liquid alkali to form a liquid alkali-sodium chloride solution, so that the concentration of sodium chloride in the liquid alkali-sodium chloride solution is in the range of 70-80 g / L, thereby increasing the crystallization rate of sodium sulfite in the secondary evaporator.

[0010] Preferably, the secondary evaporator includes a first-effect evaporator and a second-effect evaporator, which are connected in series. The sodium bisulfite solution entering the first-effect evaporator circulates within the first-effect evaporator and is processed within it. When the first-effect evaporator reaches a set position, it enters the second-effect evaporator.

[0011] Preferably, in step S2, the solution concentrated by evaporation in the secondary evaporator passes through a flash tank and then enters a secondary hydrocyclone separator for solid-liquid separation.

[0012] Preferably, the filtrate after solid-liquid separation in step S3 is directly fed into the first-effect evaporator for further evaporation and concentration; or, sodium chloride is added before it is fed into the second-effect evaporator to adjust the number of seed crystals in the second-effect evaporator.

[0013] An apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite includes a sulfur dioxide absorption tower and a sodium sulfite staged crystallization evaporator. The liquid outlet pipe of the sulfur dioxide absorption tower is connected to the sodium sulfite staged crystallization evaporator. The sulfur dioxide absorption tower is connected to a first regulating tank, which is used to add liquid alkali-sodium chloride solution into the absorption tower. The sodium sulfite staged crystallization evaporator is connected to a solid-liquid separator. The liquid outlet of the solid-liquid separator is connected to a second regulating tank, which is also connected to the sodium sulfite staged crystallization evaporator. The solid outlet of the solid-liquid separator is connected to a drying process.

[0014] Preferably, the sodium sulfite graded crystallization evaporator includes a first-effect evaporator and a second-effect evaporator, which are connected in series, and the outlet end of the second-effect evaporator is connected to a solid-liquid separator.

[0015] Preferably, the solid-liquid separator includes a hydrocyclone separator and a centrifuge. The hydrocyclone separator includes a primary hydrocyclone separator and a secondary hydrocyclone separator. A flash tank is connected between the primary hydrocyclone separator and the secondary hydrocyclone separator. The outlet end of the secondary hydrocyclone separator is connected to the centrifuge.

[0016] Preferably, the absorption tower is connected to the liquid alkali storage tank; the first regulating tank includes a first underground stirring tank, which is connected to the liquid alkali storage tank and the first sodium chloride storage tank respectively, and the first underground stirring tank is connected to the absorption tower through a first outlet valve.

[0017] Preferably, the second regulating tank includes a second underground stirring tank, which is connected to a second sodium chloride storage tank; the outlet of the second underground stirring tank is connected to the inlet of a first-effect evaporator and / or a second-effect evaporator.

[0018] The present invention has the following beneficial effects: The method and apparatus of the present invention are used to treat low-concentration sulfur dioxide flue gas generated during the smelting production process. However, when the acid production system malfunctions, it can also be temporarily used to treat high-concentration sulfur dioxide flue gas from smelting, ensuring the normal operation of the smelting system. The treatment apparatus of the present invention uses a two-stage evaporator, and sodium chloride is added to the liquid alkali to increase the crystallization rate of sodium sulfite and the crystal growth rate, thereby ensuring the effect of subsequent centrifugal separation. The present invention uses a two-stage hydrocyclone separator for solid-liquid separation, and a flash tank is added between the two-stage hydrocyclone separators to further improve the evaporation and crystallization effect, ensuring that the solid-liquid ratio reaches 4:3. The filtrate is stored again and can be directly fed into the first-effect evaporator to continue participating in the evaporation and crystallization process, or it can be fed into the second-effect evaporator after adding sodium chloride to increase the number of seed crystals in the second-effect evaporator, accelerate crystal growth, control the number of seed crystals in the entire system, increase the crystal growth rate, and improve the centrifugal separation effect. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the present invention.

[0020] In the diagram: 1-Smelting flue gas; 2-Purification process; 3-Absorption tower; 4-Neutralization process; 5-First underground mixing tank; 6-First stirring paddle; 7-First submersible pump; 8-First sodium chloride storage tank; 9-Liquid alkali pump; 10-Liquid alkali storage tank; 11-First valve; 12-Second valve; 13-Third valve; 14-Fourth valve; 15-Neutralization liquid storage tank; 16-Neutralization liquid pump; 17-Fifth valve; 18-Sixth valve; 19-First-effect evaporator; 20-First-effect forced circulation pump ; 21-First-stage feed pump; 22-Seventh valve; 23-Eighth valve; 24-Second-stage feed pump; 25-Second-stage forced circulation pump; 26-Second-stage evaporator; 27-First-stage hydrocyclone separator; 28-Flash tank; 29-Flash pump; 30-Second-stage hydrocyclone separator; 31-High-level tank; 32-High-level tank discharge valve; 33-Centrifuge; 34-Drying process; 35-Second underground mixing tank; 36-Second stirring paddle; 37-Second submersible pump; 38-Second sodium chloride storage tank. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0022] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0023] like Figure 1 As shown, a method for treating sulfur dioxide flue gas and crystallizing sodium sulfite includes the following steps:

[0024] S1: Purify and absorb low-concentration sulfur dioxide flue gas. The absorbed gas meets emission standards, and the generated sodium bisulfite absorbent is neutralized. Open the absorption tower 3 and the third valve 13, close the fourth valve 14, the first valve 11, and the second valve 12, start the liquid alkali pump 9, and pump the liquid alkali in the liquid alkali storage tank 10 into the absorption tower 3. After reaching the set liquid level, turn off the liquid alkali pump 9 to stop adding alkali, and close the third valve 13.

[0025] The low-concentration sulfur dioxide smelting flue gas 1 generated during the smelting process enters the purification process 2. After two-stage wet scrubbing, cooling, dust removal and electrostatic precipitator demisting in the purification process 2, the purified sulfur dioxide flue gas enters the absorption tower 3. In the absorption tower 3, the sulfur dioxide flue gas reacts with caustic soda in the absorption tower 3. The tail gas after absorption meets the emission standards, and the generated sodium bisulfite solution enters the neutralization process 4.

[0026] Open the fourth valve 14, close the third valve 13, the second valve 12, and the first valve 11 of the absorption tower 3, turn on the liquid alkali pump 9, add caustic soda, and the absorbent sodium bisulfite solution undergoes a neutralization reaction with the caustic soda. After impurity removal treatment, the neutralized sodium bisulfite solution is sent to the neutralized solution storage tank 15 for later use.

[0027] Sulfur dioxide from the flue gas is absorbed using liquid alkali. When the sodium chloride content in the liquid alkali storage tank 10 drops to 1 g / L, sodium chloride is mixed into the liquid alkali to form a liquid alkali-sodium chloride solution. This ensures that the sodium chloride concentration in the liquid alkali-sodium chloride solution is in the range of 70-80 g / L, thereby increasing the crystallization rate of sodium sulfite in the secondary evaporator. First valve 11 is opened, third valve 13 and fourth valve 14 are closed, and liquid alkali pump 9 is started to pump the liquid alkali from the liquid alkali storage tank 10 into the first underground stirring tank 5. Once the set liquid level is reached, liquid alkali pump 9 is turned off to stop alkali addition, and first valve 11 is closed. Sodium chloride is added from the first sodium chloride storage tank 8 to the first underground mixing tank 5. Under the action of stirring, the sodium chloride in the first underground mixing tank 5 rotates and tumbles. After the sodium chloride dissolves, the third valve 13 is closed, the second valve 12 is opened, and the first submersible pump 7 is started to pump the liquid alkali containing sodium chloride from the first underground mixing tank 5 into the absorption tower 3. After the set liquid level is reached, the addition of alkali is stopped, and the second valve 12 is closed.

[0028] Using relatively pure caustic soda with low sodium chloride content is not conducive to crystal growth. By artificially adding industrial sodium chloride salt or other media, the impurity content in the solution can be appropriately increased to improve the crystal growth rate and enhance the centrifugal separation effect.

[0029] S2: The neutralized sodium bisulfite solution is concentrated by circulating it through a two-stage evaporator, resulting in a solid-liquid ratio of 3:2 in the concentrated solution.

[0030] The secondary evaporator includes a first-effect evaporator 19 and a second-effect evaporator 26, which are connected in series. The sodium bisulfite solution entering the first-effect evaporator 19 circulates and is processed within the first-effect evaporator 19. When the first-effect evaporator reaches the set position (lower edge of the second sight glass), it then enters the second-effect evaporator 26.

[0031] When starting the evaporation process, open the fifth valve 17, close the sixth valve 18, start the neutralization liquid pump 16, and pump the neutralization liquid from the neutralization liquid storage tank 15 into the first-effect evaporator 19. When the liquid level in the first-effect evaporator 19 reaches the set position, start the first-effect forced circulation pump 20 to evaporate and concentrate the neutralization liquid sodium sulfite. Open the seventh valve 22, close the eighth valve 23, start the first-effect feed pump 21 to feed the second-effect evaporator 26. When the liquid level in the second-effect evaporator reaches the set position, start the second-effect forced circulation pump 25 to continue evaporating and concentrating the neutralization liquid sodium sulfite.

[0032] The solution, after being concentrated by evaporation in the secondary evaporator, enters the secondary hydrocyclone separator after passing through the flash tank 28 for further solid-liquid separation. When the liquid level in the second-effect evaporator 26 stabilizes and reaches the set concentration (set solid-liquid ratio of 3:2), the second-effect feed pump 24 is started to feed the primary hydrocyclone separator 27. The concentrated liquid enters the flash tank 28. When the liquid level in the flash tank 28 reaches the set position, the flash pump 29 is started, and the material enters the secondary hydrocyclone separator 30. The concentrated liquid then enters the high-level tank 31. When the liquid level in the high-level tank 31 reaches the set position, the centrifuge 33 is started, and the discharge valve 32 of the high-level tank is opened, allowing the concentrated liquid to enter the centrifuge 33 for solid-liquid separation. The separated crystalline sodium sulfite material enters the drying process 34 for further processing.

[0033] S3: Solid-liquid separation is performed on the concentrated solution. The separated crystalline sodium sulfite material enters the drying process for further processing. The separated filtrate is stored and then enters the secondary evaporator for further evaporation and concentration.

[0034] The filtrate after solid-liquid separation is directly fed into the first-effect evaporator for further evaporation and concentration; or, sodium chloride is added before it enters the second-effect evaporator to regulate the number of seed crystals. In other words, the filtrate after solid-liquid separation is stored and then used directly in the first-effect evaporator for further evaporation and concentration; or, when the sodium chloride content is low, sodium chloride is added before it enters the second-effect evaporator to disrupt the existing solid-liquid ratio distribution within each effect, control the number of seed crystals in the first effect, increase the number of seed crystals in the second-effect evaporator (26), accelerate crystal growth, reduce oxidation during evaporation, increase the crystal growth rate, and improve the centrifugal separation effect after the addition of sodium chloride as an impurity.

[0035] Based on the above method, the present invention also provides an apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite, comprising a sulfur dioxide absorption tower 3 and a sodium sulfite graded crystallization evaporator. The liquid outlet pipe of the sulfur dioxide absorption tower 3 is connected to the sodium sulfite graded crystallization evaporator. The sulfur dioxide absorption tower 3 is connected to a first regulating tank, which is used to add liquid alkali-sodium chloride solution into the absorption tower 3. The sodium sulfite graded crystallization evaporator is connected to a solid-liquid separator 33. The liquid outlet of the solid-liquid separator is connected to a second regulating tank, which is connected to the sodium sulfite graded crystallization evaporator. The solid outlet of the solid-liquid separator is connected to a drying process.

[0036] The sodium sulfite graded crystallization evaporator includes a first-effect evaporator 19 and a second-effect evaporator 26, which are connected in series. The outlet end of the second-effect evaporator 26 is connected to a solid-liquid separator.

[0037] The solid-liquid separator includes a hydrocyclone separator and a centrifuge 33. The hydrocyclone separator includes a primary hydrocyclone separator 27 and a secondary hydrocyclone separator 30. A flash tank 28 is connected between the primary hydrocyclone separator 27 and the secondary hydrocyclone separator 30. The outlet end of the secondary hydrocyclone separator 30 is connected to the centrifuge 33.

[0038] The absorption tower 3 is connected to the liquid alkali storage tank 10; the first regulating tank includes a first underground stirring tank 5, which is connected to both the liquid alkali storage tank 10 and the first sodium chloride storage tank 8. The first underground stirring tank 5 is connected to the absorption tower 3 through a first outlet valve 12. A first stirring paddle 6 is installed in the first underground stirring tank 5. The liquid alkali-sodium chloride is stirred evenly by the first stirring paddle 6 and then pumped into the absorption tower 3, thereby controlling the number of seed crystals in the entire system.

[0039] The second regulating tank includes a second underground stirring tank 35, which is connected to a second sodium chloride storage tank 38. The outlet of the second underground stirring tank 35 is connected to the inlet of a first-effect evaporator 19 and / or a second-effect evaporator 26. Before feeding into the centrifuge 33, the second stirring paddle 36 is turned on, and the filtrate from the centrifuge 33 enters the second underground stirring tank 35. Under the action of stirring, the filtrate in the second underground stirring tank 35 rotates and tumbles, preventing the filtrate from settling, crystallizing, and clogging the inlet of the second submersible pump 37. When the liquid level in the second underground stirring tank 35 reaches the set position, the sixth valve 18 is opened, the fifth valve 17 is closed, and the second submersible pump 37 is started. The filtrate enters the first-effect evaporator 19 for further evaporation and concentration. Alternatively, when the liquid alkali in the liquid alkali storage tank 10 is relatively pure and the sodium chloride content is low, sodium chloride is added from the second sodium chloride storage tank 38 to the second underground stirring tank 35. Under the action of stirring, the sodium chloride in the second underground stirring tank 35 rotates and tumbles. After the sodium chloride dissolves, the filtrate of sodium chloride added to the second underground stirring tank 35 is pumped into the double-effect evaporator 26. This breaks the existing solid-liquid ratio distribution within each effect, controls the number of seed crystals in the first effect, increases the number of seed crystals in the second-effect evaporator 26, accelerates crystal growth, reduces oxidation during the evaporation process, increases the crystal growth rate, and improves the centrifugal separation effect after the addition of sodium chloride impurities.

[0040] Anhydrous solid sodium sulfite is produced using an evaporation crystallization process. When the liquid alkali in the liquid alkali storage tank 10 is relatively pure and the sodium chloride content is low, the evaporation system operates poorly, resulting in poor evaporation crystallization, inadequate centrifuge separation, and a high solid content in the mother liquor (solid-liquid ratio reaching 1:3). This leads to a high solid-liquid ratio in the first-effect evaporator and a low solid-liquid ratio in the second-effect evaporator, resulting in a chaotic distribution of crystal seeds within the evaporators and hindering crystal growth. Furthermore, the low impurity content in the raw materials is detrimental to evaporation crystallization. The filtrate from the centrifuge enters the second underground stirring tank 35. After adding sodium chloride, the filtrate is reused in the second-effect evaporator. Appropriately increasing the impurity content in the solution can improve the crystal growth rate, disrupt the existing solid-liquid ratio distribution within each effect (first-effect and second-effect evaporators), control the number of crystal seeds in the first-effect evaporator, accelerate crystal growth, and reduce oxidation during the evaporation process. Increasing the number of crystal seeds in the second-effect evaporator improves the crystal growth rate and enhances the centrifugal separation effect.

[0041] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, substitutions, or variations made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for treating sulfur dioxide flue gas and crystallizing sodium sulfite, characterized in that, Includes the following steps: S1: Purify and absorb low-concentration sulfur dioxide flue gas. The absorbed gas meets emission standards. The generated sodium bisulfite absorbent is neutralized and purified. Liquid alkali is used to absorb and treat sulfur dioxide flue gas. When the sodium chloride content in the added liquid alkali is as low as 1g / L, sodium chloride is mixed into the liquid alkali to form a liquid alkali-sodium chloride solution. This ensures that the concentration of sodium chloride in the liquid alkali-sodium chloride solution is 70-80g / L, thereby increasing the crystallization rate of sodium sulfite in the secondary evaporator. S2: The neutralized sodium sulfite solution is concentrated by circulating evaporation through first-effect and second-effect evaporators, so that the solid-liquid ratio in the concentrated solution reaches 3:

2. S3: Solid-liquid separation is performed on the concentrated solution. The separated crystalline sodium sulfite material enters the drying process for further processing. The separated filtrate is stored and then enters a first-effect or second-effect evaporator for further evaporation and concentration. The filtrate after solid-liquid separation is directly fed into the first-effect evaporator for further evaporation and concentration; or, sodium chloride is added before it is fed into the second-effect evaporator to adjust the number of seed crystals in the second-effect evaporator.

2. The method for treating sulfur dioxide flue gas and crystallizing sodium sulfite according to claim 1, characterized in that: The first-effect evaporator and the second-effect evaporator are connected in series. The sodium sulfite solution entering the first-effect evaporator circulates and is processed in the first-effect evaporator. When the first-effect evaporator reaches the set position, it enters the second-effect evaporator.

3. The method for treating sulfur dioxide flue gas and crystallizing sodium sulfite according to claim 2, characterized in that: The solution concentrated by evaporation in step S2 passes through a flash tank and then enters a secondary hydrocyclone separator for solid-liquid separation.

4. An apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite, characterized in that: The system includes a sulfur dioxide absorption tower (3) and a sodium sulfite staged crystallization evaporator. The liquid outlet pipe of the sulfur dioxide absorption tower (3) is connected to the sodium sulfite staged crystallization evaporator. The sulfur dioxide absorption tower (3) is connected to a first regulating tank, which is used to add liquid alkali-sodium chloride solution into the absorption tower (3). When the sodium chloride content in the added liquid alkali is as low as 1 g / L, sodium chloride is mixed into the liquid alkali to form a liquid alkali-sodium chloride solution. The concentration of sodium chloride in the liquid alkali-sodium chloride solution is 70-80 g / L. The sodium sulfite staged crystallization evaporator... The evaporator is connected to the solid-liquid separator. The liquid outlet of the solid-liquid separator is connected to the second regulating tank. The second regulating tank is connected to the sodium sulfite graded crystallization evaporator. The solid outlet of the solid-liquid separator is connected to the drying process. The sodium sulfite graded crystallization evaporator includes a first-effect evaporator (19) and a second-effect evaporator (26). The first-effect evaporator (19) and the second-effect evaporator (26) are connected in series. The outlet end of the second-effect evaporator (26) is connected to the solid-liquid separator. The first-effect evaporator (19) is connected to a first-effect forced circulation pump (20).

5. The apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite according to claim 4, characterized in that: The solid-liquid separator includes a hydrocyclone separator and a centrifuge (33). The hydrocyclone separator includes a primary hydrocyclone separator (27) and a secondary hydrocyclone separator (30). A flash tank (28) is connected between the primary hydrocyclone separator (27) and the secondary hydrocyclone separator (30). The outlet end of the secondary hydrocyclone separator (30) is connected to the centrifuge (33).

6. The apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite according to claim 5, characterized in that: The absorption tower (3) is connected to the liquid alkali storage tank (10); the first regulating tank includes a first underground stirring tank (5), which is connected to the liquid alkali storage tank (10) and the first sodium chloride storage tank (8) respectively, and the first underground stirring tank (5) is connected to the absorption tower (3) through the first outlet valve (12).

7. The apparatus for treating sulfur dioxide flue gas and crystallizing sodium sulfite according to claim 6, characterized in that: The second regulating tank includes a second underground stirring tank (35), which is connected to a second sodium chloride storage tank (38); the outlet of the second underground stirring tank (35) is connected to the inlet of a first-effect evaporator (19) and / or a second-effect evaporator (26).

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