Sodium sulfate crystallization system and production process

Abstract

The sodium sulfate crystallization system is characterized in that nanofiltration concentrated water enters a sodium sulfate feeding tank, the sodium sulfate feeding tank, a sodium sulfate evaporation crystallizer, a sodium sulfate centrifugal machine and a sodium sulfate mother liquor tank are sequentially connected, mother liquor of the sodium sulfate mother liquor tank respectively enters the sodium sulfate feeding tank and the sodium sulfate evaporation crystallizer, and the sodium sulfate feeding tank, a freezing crystallizer and the sodium sulfate centrifugal machine are sequentially connected; mother liquor of the mirabilite centrifugal machine enters the mirabilite mother liquor tank, the mother liquor of the mirabilite mother liquor tank is discharged, mirabilite of the mirabilite centrifugal machine and nitrate of the nitrate centrifugal machine enter the recrystallization feeding tank, the recrystallization feeding tank, the evaporation crystallizer and the centrifugal machine are sequentially connected, sodium sulfate is produced by the centrifugal machine, the mother liquor discharged by the centrifugal machine enters the mother liquor tank, and the mother liquor tank is connected with the nitrate evaporation crystallizer. The system is applied to industrial wastewater and coal mine water salt zero discharge in coal chemical industry, coking, petrifaction, electric power and other industries, a sodium sulfate product with the purity being 98.5% or above can be obtained, meanwhile, the yield of sodium sulfate is high, and the overall energy consumption of the system is low.

Description

Technical Field

[0001] This invention belongs to the field of sodium sulfate processing technology, and in particular relates to a sodium sulfate crystallization system and production process. Background Technology

[0002] The dissolved solids in the concentrated brine of industrial wastewater from coal chemical, coking, petrochemical, and power industries, as well as coal mine water in zero-salt discharge projects, are mainly sodium chloride and sodium sulfate. The permeate from nanofiltration is mainly sodium chloride, with only a small amount of sodium sulfate. The concentrate from nanofiltration is mainly sodium sulfate, with a small amount of sodium chloride. Therefore, the yield of sodium sulfate in the concentrated brine of zero-salt discharge projects depends on the yield of sodium sulfate in the sodium sulfate crystallization process on the concentrate side of nanofiltration. Currently, the sodium sulfate crystallization process on the concentrate side of nanofiltration in zero-salt discharge projects generally adopts a process of sodium sulfate evaporation crystallization + centrifugal separation + drying to obtain sodium sulfate products; however, it cannot guarantee the production of sodium sulfate products with a purity of over 99%.

[0003] Chinese patent application CN109775939A discloses a zero-discharge and salt-separation crystallization system and method for coal chemical wastewater. The sodium sulfate crystallization device in the method uses an evaporation crystallization + freeze crystallization process to obtain sodium sulfate crystal salt. The purity of the sodium sulfate crystal salt in this patent is guaranteed to be 97%.

[0004] Chinese patent application CN111153540A discloses a salt separation system and process for ensuring the quality of crystallized salt from high-salt wastewater. The sodium sulfate crystallization device in the method uses an evaporation crystallization process to obtain sodium sulfate crystallized salt. The purity of the sodium sulfate crystallized salt in this patent is guaranteed to be 97%. Summary of the Invention

[0005] In view of this, the present invention aims to provide a sodium sulfate crystallization system and production process to solve the problems of low yield and low purity of sodium sulfate crystals in the prior art.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A sodium sulfate crystallization system includes a nitrate feed tank, a nitrate evaporator crystallizer, a nitrate centrifuge, a nitrate mother liquor tank, a sodium sulfate feed tank, a freeze crystallizer, a sodium sulfate centrifuge, a sodium sulfate mother liquor tank, a recrystallization feed tank, an evaporator crystallizer, a centrifuge, and a mother liquor tank; Nanofiltration concentrate enters the nitrate feed tank, and the nitrate feed tank, nitrate evaporator crystallizer, nitrate centrifuge, and nitrate mother liquor tank are connected in sequence. The mother liquor from the nitrate mother liquor tank enters the sodium sulfate feed tank and the nitrate evaporator crystallizer respectively. The Glauber's salt feed tank, the freezing crystallizer, and the Glauber's salt centrifuge are connected in sequence. The mother liquor from the Glauber's salt centrifuge enters the Glauber's salt mother liquor tank, and the mother liquor from the Glauber's salt mother liquor tank is discharged. The sodium sulfate from the sodium sulfate centrifuge and the nitrate from the nitrate centrifuge enter the recrystallization feed tank. The recrystallization feed tank, the evaporator crystallizer, and the centrifuge are connected in sequence. The centrifuge produces sodium sulfate, and the mother liquor discharged from the centrifuge enters the mother liquor tank. The mother liquor tank is connected to the nitrate evaporator crystallizer.

[0007] Furthermore, the outlet of the nitrate feed tank is connected to the first inlet of the nitrate evaporator crystallizer; the outlet of the nitrate evaporator crystallizer is connected to the inlet of the nitrate centrifuge; and the nitrate outlet of the nitrate centrifuge is connected to the inlet of the recrystallization feed tank.

[0008] The mother liquor outlet of the nitric acid centrifuge is connected to the inlet of the nitric acid mother liquor tank; the outlet of the nitric acid mother liquor tank is connected to the second inlet of the nitric acid evaporator crystallizer and the sodium sulfate feed tank. The outlet of the sodium sulfate feed tank is connected to the inlet of the freeze crystallizer; the outlet of the freeze crystallizer is connected to the inlet of the sodium sulfate centrifuge.

[0009] The outlet of the Glauber's salt centrifuge is connected to the inlet of the recrystallization feed tank; the outlet of the Glauber's salt centrifuge is connected to the inlet of the Glauber's salt mother liquor tank; the outlet of the recrystallization feed tank is connected to the inlet of the evaporator crystallizer; and the outlet of the evaporator crystallizer is connected to the inlet of the centrifuge.

[0010] The mother liquor outlet of the centrifuge is connected to the inlet of the mother liquor tank; the solid produced by the centrifuge is sodium sulfate, which enters the drying system; the outlet of the mother liquor tank is connected to the third inlet of the nitrate evaporator crystallizer.

[0011] Furthermore, the nitrate evaporator crystallizer employs one or more of the following: single-effect evaporator crystallization, multi-effect evaporator crystallization, MVR evaporator crystallization, or TVR evaporator crystallization. The nitrate evaporator crystallizer produces a nitrate slurry. The evaporator crystallizer employs one or more of the following: single-effect evaporator crystallization, multi-effect evaporator crystallization, MVR evaporator crystallization, or TVR evaporator crystallization. The evaporator crystallizer produces sodium sulfate crystal slurry.

[0012] Furthermore, the rotation speed of the centrifuge is 2000~3000 r / min; The cooling temperature of the freeze crystallizer is -5~5℃; The rotation speed of the Glauber's salt centrifuge is 2000~3000 r / min; The centrifuge speed is 2000~3000 r / min.

[0013] The above-mentioned process for a sodium sulfate crystallization system includes the following steps: S1: Evaporate and crystallize the nanofiltration concentrate to separate nitrate and nitrate mother liquor. Freeze and crystallize a portion of the nitrate mother liquor to separate mirabilite and mirabilite mother liquor. S2: Dissolve nitrate and mirabilite into a sodium sulfate solution, then evaporate and crystallize the solution and separate the solid and liquid phases to obtain sodium sulfate slurry. Dry the sodium sulfate slurry to obtain sodium sulfate product.

[0014] Furthermore, step S1 also includes the nanofiltration concentrate entering the nitrate feed tank, followed by nitrate evaporation and crystallization. The nitrate evaporation and crystallization produces nitrate slurry, which is then subjected to solid-liquid separation to obtain nitrate and nitrate mother liquor. Part of the nitrate mother liquor is returned to the nitrate evaporation and crystallization tank for further evaporation and crystallization, while the other part is discharged into the sodium sulfate feed tank and enters the cryogenic crystallizer. The cryogenic crystallizer produces sodium sulfate slurry, which is then subjected to solid-liquid separation to obtain sodium sulfate and sodium sulfate mother liquor. Furthermore, step S2 also includes melting nitrate and sodium sulfate in a recrystallization feed tank, evaporating and crystallizing the sodium sulfate solution in an evaporator, producing sodium sulfate slurry, separating the sodium sulfate slurry into solid and liquid, and drying the sodium sulfate obtained from the solid-liquid separation in a drying system to obtain sodium sulfate product. The mother liquor obtained from the solid-liquid separation is discharged to other processing systems through a mother liquor tank.

[0015] Furthermore, the mother liquor is returned to the nitrate evaporation and crystallization system, and after subsequent treatment, it is finally discharged from the system through the mother liquor discharge tank of Glauber's salt.

[0016] Compared with existing technologies, the sodium sulfate crystallization system and production process described in this invention have the following advantages: (1) The sodium sulfate crystallization system and production process described in this invention can reduce the impurity rate while producing high-quality sodium sulfate products, thereby increasing the sales revenue of sodium sulfate and reducing the cost of impurity disposal in the project.

[0017] (2) Most of the nitrate mother liquor is returned to the nitrate evaporator crystallizer for evaporation and crystallization, which further causes the nitrate in it to crystallize and precipitate, so that most of the sodium sulfate in the nanofiltration concentrate is produced in the form of nitrate.

[0018] (3) The sodium chloride content in the feed of the freeze crystallizer is close to saturation, so the freezing point is very low. This allows the freeze crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the freeze crystallization process.

[0019] (4) The nitrate produced by the nitrate evaporator crystallizer and the sodium sulfate produced by the freeze crystallizer are dissolved and then recrystallized in the evaporator crystallizer to obtain sodium sulfate product with high purity. The impurities in the nitrate and sodium sulfate are returned to the nitrate evaporator crystallizer in the form of mother liquor for evaporation and crystallization, so that the nitrate crystals are precipitated out and the impurities are put into the nitrate mother liquor. After subsequent treatment, the mother liquor is finally discharged from the system through the sodium sulfate mother liquor tank 8. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1This is a schematic diagram of a sodium sulfate crystallization system proposed in this invention.

[0021] Explanation of reference numerals in the attached figures 1. Nitrate feed tank; 2. Nitrate evaporator crystallizer; 3. Nitrate centrifuge; 4. Nitrate mother liquor tank; 5. Glauber's salt feed tank; 6. Freezing crystallizer; 7. Glauber's salt centrifuge; 8. Glauber's salt mother liquor tank; 9. Recrystallization feed tank; 10. Evaporator crystallizer; 11. Centrifuge; 12. Mother liquor tank. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0023] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not 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 the invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0025] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0026] Example 1 A sodium sulfate crystallization system includes a nitrate feed tank 1, a nitrate evaporator crystallizer 2, a nitrate centrifuge 3, a nitrate mother liquor tank 4, a sodium sulfate feed tank 5, a freeze crystallizer 6, a sodium sulfate centrifuge 7, a sodium sulfate mother liquor tank 8, a recrystallization feed tank 9, an evaporator crystallizer 10, a centrifuge 11, and a mother liquor tank 12.

[0027] The nanofiltration concentrate from the zero-discharge salt separation project enters the nitrate feed tank 1, and the outlet of the nitrate feed tank 1 is connected to the first inlet of the nitrate evaporator crystallizer 2.

[0028] The outlet of the nitric evaporator crystallizer 2 is connected to the inlet of the nitric centrifuge 3.

[0029] The nitrate outlet and mother liquor outlet of the nitrate centrifuge 3 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the nitrate mother liquor tank 4, respectively.

[0030] The outlet of the nitric acid mother liquor tank 4 is connected to the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5.

[0031] The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6.

[0032] The outlet of the freeze crystallizer 6 is connected to the inlet of the Glauber's salt centrifuge 7.

[0033] The sodium sulfate outlet and mother liquor outlet of the sodium sulfate centrifuge 7 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the sodium sulfate mother liquor tank 8, respectively.

[0034] The outlet of recrystallization feed tank 9 is connected to the inlet of evaporator crystallizer 10.

[0035] The outlet of the evaporator crystallizer 10 is connected to the inlet of the centrifuge 11.

[0036] The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12. The solid produced by the centrifuge enters the drying system as sodium sulfate product.

[0037] The outlet of mother liquor tank 12 is connected to the third inlet of nitric acid evaporator crystallizer 2.

[0038] A sodium sulfate crystallization system includes a nitrate feed tank 1, a nitrate evaporator crystallizer 2, a nitrate centrifuge 3, a nitrate mother liquor tank 4, a sodium sulfate feed tank 5, a freeze crystallizer 6, a sodium sulfate centrifuge 7, a sodium sulfate mother liquor tank 8, a recrystallization feed tank 9, an evaporator crystallizer 10, a centrifuge 11, and a mother liquor tank 12. Nanofiltration concentrate enters the nitrate feed tank 1. The nitrate feed tank 1, nitrate evaporator crystallizer 2, nitrate centrifuge 3, and nitrate mother liquor tank 4 are connected in sequence. The mother liquor from the nitrate mother liquor tank 4 enters the sodium sulfate feed tank 5 and the nitrate evaporator crystallizer 6. The following components are connected in sequence: crystallizer 2; Glauber's salt feed tank 5; freeze crystallizer 6; Glauber's salt centrifuge 7; mother liquor from Glauber's salt centrifuge 7 enters Glauber's salt mother liquor tank 8; mother liquor from Glauber's salt mother liquor tank 8 is discharged; Glauber's salt from Glauber's salt centrifuge 7 and nitrate from nitrate centrifuge 3 enter recrystallization feed tank 9; recrystallization feed tank 9; evaporation crystallizer 10; centrifuge 11 is connected in sequence; centrifuge 11 produces sodium sulfate; mother liquor discharged from centrifuge 11 enters mother liquor tank 12; mother liquor tank 12 is connected to nitrate evaporation crystallizer 2.

[0039] The outlet of the nitric acid feed tank 1 is connected to the first inlet of the nitric acid evaporator crystallizer 2; the outlet of the nitric acid evaporator crystallizer 2 is connected to the inlet of the nitric acid centrifuge 3; the nitric acid outlet of the nitric acid centrifuge 3 is connected to the inlet of the recrystallization feed tank 9. The mother liquor outlet of the nitric acid centrifuge 3 is connected to the inlet of the nitric acid mother liquor tank 4; the outlet of the nitric acid mother liquor tank 4 is connected to both the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5. The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6; the outlet of the freeze crystallizer 6 is connected to the inlet of the sodium sulfate centrifuge 7.

[0040] The outlet of the Glauber's salt centrifuge 7 is connected to the inlet of the recrystallization feed tank 9; the outlet of the mother liquor of the Glauber's salt centrifuge 7 is connected to the inlet of the Glauber's salt mother liquor tank 8; the outlet of the recrystallization feed tank 9 is connected to the inlet of the evaporator crystallizer 10; and the outlet of the evaporator crystallizer 10 is connected to the inlet of the centrifuge 11.

[0041] The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12; the solid produced by centrifuge 11 is sodium sulfate product, which enters the drying system; the outlet of mother liquor tank 12 is connected to the third inlet of nitrate evaporator crystallizer 2.

[0042] Nitrate evaporator crystallizer 2 employs one or more of single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization, or TVR evaporation crystallization, and produces nitrate crystal slurry. Evaporator crystallizer 10 employs one or more of single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization, or TVR evaporation crystallization, and produces sodium sulfate crystal slurry.

[0043] The rotation speed of the nitrate centrifuge 3 is 2000~3000 r / min; the cooling temperature of the freeze crystallizer 6 is -5~5℃; the rotation speed of the Glauber's salt centrifuge 7 is 2000~3000 r / min; and the rotation speed of the centrifuge 11 is 2000~3000 r / min.

[0044] The above-mentioned process for a sodium sulfate crystallization system includes the following steps: S1: Evaporate and crystallize the nanofiltration concentrate to separate nitrate and nitrate mother liquor. Freeze and crystallize a portion of the nitrate mother liquor to separate mirabilite and mirabilite mother liquor. S2: Dissolve nitrate and mirabilite into a sodium sulfate solution, then evaporate and crystallize the solution and separate the solid and liquid phases to obtain sodium sulfate slurry. Dry the sodium sulfate slurry to obtain sodium sulfate product.

[0045] Step S1 also includes nanofiltration concentrate entering the nitrate feed tank 1, followed by nitrate evaporation and crystallization. Nitrate evaporation and crystallization produce nitrate slurry. The nitrate slurry undergoes solid-liquid separation to obtain nitrate and nitrate mother liquor. Part of the nitrate mother liquor is returned to the nitrate evaporation and crystallization tank for further evaporation and crystallization, while the other part is discharged to the sodium sulfate feed tank 5 and enters the cryogenic crystallizer 6. The cryogenic crystallizer 6 produces sodium sulfate slurry, which undergoes solid-liquid separation to obtain sodium sulfate and sodium sulfate mother liquor. Step S2 also includes melting nitrate and sodium sulfate in recrystallization feed tank 9. After melting, the sodium sulfate solution is evaporated and crystallized in an evaporator, producing sodium sulfate slurry. The sodium sulfate slurry is then subjected to solid-liquid separation. The sodium sulfate obtained from the solid-liquid separation is dried in a drying system to obtain the sodium sulfate product. The mother liquor obtained from the solid-liquid separation is discharged to other processing systems through mother liquor tank 12. The mother liquor is returned to the nitrate for evaporation and crystallization, and after further treatment, it is finally discharged from the system through the sodium sulfate mother liquor tank 8.

[0046] Example 2 Example 1 describes a sodium sulfate crystallization process with high yield and product purity, comprising the following steps: 1. Nitrate feed tank, 2. Nitrate evaporator crystallizer, 3. Nitrate centrifuge, 4. Nitrate mother liquor tank, 5. Glauber's salt feed tank, 6. Glauber's salt centrifuge, 7. Glauber's salt mother liquor tank, 8. Recrystallization feed tank, 9. Evaporator crystallizer, 10. Centrifuge, 11. Mother liquor tank, 12.

[0047] The nanofiltration concentrate from the coking wastewater zero-discharge project mainly contains sodium sulfate and a small amount of sodium chloride. The specific parameters are: total dissolved solids: 144690.7 mg / L, sulfate: 67443.7 mg / L, chloride: 24628.9 mg / L, COD: 1858.8 mg / L.

[0048] Nanofiltration concentrate enters nitric acid feed tank 1, and the outlet of nitric acid feed tank 1 is connected to the first inlet of nitric acid evaporator crystallizer 2.

[0049] Nitrate evaporator crystallizer 2 uses multi-effect evaporation crystallization to produce nitrate slurry. The nitrate slurry is connected to the inlet of nitrate centrifuge 3 via the outlet of nitrate evaporator crystallizer 2.

[0050] Nitrate centrifuge 3 performs solid-liquid separation on nit crystal slurry. The nit outlet and mother liquor outlet of nit centrifuge 3 are connected to the inlet of recrystallization feed tank 9 and the inlet of nit mother liquor tank 4, respectively.

[0051] The outlet of the nitric acid mother liquor tank 4 is connected to both the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5. Most of the nitric acid mother liquor is returned to the second inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, further precipitating the nitric acid crystals, thus producing most of the sodium sulfate in the nanofiltration concentrate as nitric acid. A small portion of the nitric acid mother liquor is discharged to the sodium sulfate feed tank 5. The discharge rate is controlled by ensuring that the sodium chloride and sodium sulfate concentrations in the mother liquor reach the co-saturation point at the operating temperature. This ensures that the sodium chloride in the sodium sulfate concentrate reaches the co-saturation point after concentration in the nitric acid evaporator crystallizer 2, but does not precipitate in the nitric acid crystallizer; only sodium sulfate precipitates in the nitric acid evaporator crystallizer 2. The mass flow rate of the mother liquor discharged into the sodium sulfate feed tank 5 is 15.6% of the inlet mass flow rate of the sodium sulfate evaporator crystallizer 2. The specific parameters of the mother liquor are: total dissolved solids: 392481.4 mg / L, sulfate: 37183.1 mg / L, chloride: 180446.5 mg / L, COD: 13618.8 mg / L.

[0052] The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6.

[0053] Because the sodium chloride content in the feed of the cryo-crystallizer is close to saturation, the freezing point is very low, which allows the cryo-crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the cryo-crystallization process. Sodium sulfate precipitates in the cryo-crystallizer 6 in the form of sodium sulfate, producing sodium sulfate crystal slurry. The sodium sulfate crystal slurry is connected to the inlet of the sodium sulfate centrifuge 7 through the outlet of the cryo-crystallizer 6.

[0054] The Glauber's salt centrifuge 7 performs solid-liquid separation on the Glauber's salt slurry. The Glauber's salt outlet and mother liquor outlet of the Glauber's salt centrifuge 7 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the Glauber's salt mother liquor tank 8, respectively. The Glauber's salt mother liquor is discharged to other treatment systems. The mass flow rate of the Glauber's salt mother liquor is 88.9% of the inlet mass flow rate of the freeze crystallizer 6. The specific parameters of the Glauber's salt mother liquor are: total dissolved solids: 349585.6 mg / L, sulfate: 8995.2 mg / L, chloride: 180446.5 mg / L, COD: 13618.8 mg / L.

[0055] The outlet of recrystallization feed tank 9 is connected to the inlet of evaporator crystallizer 10.

[0056] The evaporator crystallizer 10 produces sodium sulfate slurry through multi-effect evaporation crystallization. The sodium sulfate slurry is connected to the inlet of centrifuge 11 via the outlet of the evaporator crystallizer 10.

[0057] Centrifuge 11 performs solid-liquid separation on sodium sulfate crystal slurry. The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12. The solids produced by the centrifuge are discharged as sodium sulfate product to other treatment systems. The mass flow rate of the mother liquor is 7.4% of the inlet mass flow rate of evaporator crystallizer 10. The specific parameters of the mother liquor are: total dissolved solids: 314471.2 mg / L, sulfate: 175649.5 mg / L, chloride: 34105.6 mg / L, COD: 855.4 mg / L.

[0058] The outlet of the mother liquor tank 12 is connected to the third inlet of the nitric acid evaporator crystallizer 2. The mother liquor in the mother liquor tank 12 flows back to the third inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, causing the nitric acid crystals to precipitate out, and the impurities in the crystals to enter the nitric acid mother liquor. After further treatment, the impurities are finally discharged from the system through the mother liquor discharge tank 8.

[0059] Example 3 Example 1 describes a sodium sulfate crystallization process with high yield and product purity, comprising the following steps: 1. Nitrate feed tank, 2. Nitrate evaporator crystallizer, 3. Nitrate centrifuge, 4. Nitrate mother liquor tank, 5. Glauber's salt feed tank, 6. Glauber's salt centrifuge, 7. Glauber's salt mother liquor tank, 8. Recrystallization feed tank, 9. Evaporator crystallizer, 10. Centrifuge, 11. Mother liquor tank, 12.

[0060] The nanofiltration concentrate from the zero-discharge project for wastewater salinization in coal chemical industry mainly contains sodium sulfate and a small amount of sodium chloride. The specific parameters are: total dissolved solids: 154448.0 mg / L, sulfate: 71175.2 mg / L, chloride: 27617.9 mg / L, COD: 2209.1 mg / L.

[0061] Nanofiltration concentrate enters nitric acid feed tank 1, and the outlet of nitric acid feed tank 1 is connected to the first inlet of nitric acid evaporator crystallizer 2.

[0062] Nitrate evaporator crystallizer 2 uses multi-effect evaporation crystallization to produce nitrate slurry. The nitrate slurry is connected to the inlet of nitrate centrifuge 3 via the outlet of nitrate evaporator crystallizer 2.

[0063] Nitrate centrifuge 3 performs solid-liquid separation on nit crystal slurry. The nit outlet and mother liquor outlet of nit centrifuge 3 are connected to the inlet of recrystallization feed tank 9 and the inlet of nit mother liquor tank 4, respectively.

[0064] The outlet of the nitric acid mother liquor tank 4 is connected to both the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5. Most of the nitric acid mother liquor is returned to the second inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, further precipitating the nitric acid crystals, thus producing most of the sodium sulfate in the nanofiltration concentrate as nitric acid. A small portion of the nitric acid mother liquor is discharged to the sodium sulfate feed tank 5. The discharge rate is controlled by ensuring that the sodium chloride and sodium sulfate concentrations in the mother liquor reach the co-saturation point at the operating temperature. This ensures that the sodium chloride in the sodium sulfate concentrate reaches the co-saturation point after concentration in the nitric acid evaporator crystallizer 2, but does not precipitate in the nitric acid crystallizer; only sodium sulfate precipitates in the nitric acid evaporator crystallizer 2. The mass flow rate of the mother liquor discharged into the sodium sulfate feed tank 5 is 22.72% of the inlet mass flow rate of the sodium sulfate evaporator crystallizer 2. The specific parameters of the mother liquor are: total dissolved solids: 348239.8 mg / L, sulfate: 67013.5 mg / L, chloride: 138701.9 mg / L, COD: 11094.4 mg / L.

[0065] The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6.

[0066] Because the sodium chloride content in the feed of the cryo-crystallizer is close to saturation, the freezing point is very low, which allows the cryo-crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the cryo-crystallization process. Sodium sulfate precipitates in the cryo-crystallizer 6 in the form of sodium sulfate, producing sodium sulfate crystal slurry. The sodium sulfate crystal slurry is connected to the inlet of the sodium sulfate centrifuge 7 through the outlet of the cryo-crystallizer 6.

[0067] The Glauber's salt centrifuge 7 performs solid-liquid separation on the Glauber's salt crystal slurry. The Glauber's salt outlet and mother liquor outlet of the Glauber's salt centrifuge 7 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the Glauber's salt mother liquor tank 8, respectively. The Glauber's salt mother liquor is discharged to other treatment systems. The mass flow rate of the Glauber's salt mother liquor is 75.36% of the inlet mass flow rate of the freeze crystallizer 6. The specific parameters of the Glauber's salt mother liquor are: total dissolved solids: 262448.1 mg / L, sulfate: 9107.4 mg / L, chloride: 138701.9 mg / L, COD: 11094.4 mg / L.

[0068] The outlet of recrystallization feed tank 9 is connected to the inlet of evaporator crystallizer 10.

[0069] The evaporator crystallizer 10 produces sodium sulfate crystal slurry using single-effect evaporation crystallization. The sodium sulfate crystal slurry is connected to the inlet of the centrifuge 11 via the outlet of the evaporator crystallizer 10.

[0070] Centrifuge 11 performs solid-liquid separation on sodium sulfate crystal slurry. The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12. The solids produced by the centrifuge are discharged as sodium sulfate product to other treatment systems. The mass flow rate of the mother liquor is 9.7% of the inlet mass flow rate of evaporator crystallizer 10. The specific parameters of the mother liquor are: total dissolved solids: 286951.4 mg / L, sulfate: 174368.7 mg / L, chloride: 19813.9 mg / L, COD: 528.3 mg / L.

[0071] The outlet of the mother liquor tank 12 is connected to the third inlet of the nitric acid evaporator crystallizer 2. The mother liquor in the mother liquor tank 12 flows back to the third inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, causing the nitric acid crystals to precipitate out, and the impurities in the crystals to enter the nitric acid mother liquor. After further treatment, the impurities are finally discharged from the system through the mother liquor discharge tank 8.

[0072] Example 4 Example 1 describes a sodium sulfate crystallization process with high yield and product purity, comprising the following steps: 1. Nitrate feed tank, 2. Nitrate evaporator crystallizer, 3. Nitrate centrifuge, 4. Nitrate mother liquor tank, 5. Glauber's salt feed tank, 6. Glauber's salt centrifuge, 7. Glauber's salt mother liquor tank, 8. Recrystallization feed tank, 9. Evaporator crystallizer, 10. Centrifuge, 11. Mother liquor tank, 12.

[0073] The nanofiltration concentrate from the zero-discharge project for wastewater salinization in coal chemical industry mainly contains sodium sulfate and a small amount of sodium chloride. The specific parameters are: total dissolved solids: 102978.3 mg / L, sulfate: 59775.2 mg / L, chloride: 10013.1 mg / L, COD: 1058 mg / L.

[0074] Nanofiltration concentrate enters nitric acid feed tank 1, and the outlet of nitric acid feed tank 1 is connected to the first inlet of nitric acid evaporator crystallizer 2.

[0075] Nitrate evaporator crystallizer 2 uses TVR evaporation crystallization to produce nitrate slurry. The nitrate slurry is connected to the inlet of nitrate centrifuge 3 via the outlet of nitrate evaporator crystallizer 2.

[0076] Nitrate centrifuge 3 performs solid-liquid separation on nit crystal slurry. The nit outlet and mother liquor outlet of nit centrifuge 3 are connected to the inlet of recrystallization feed tank 9 and the inlet of nit mother liquor tank 4, respectively.

[0077] The outlet of the nitric acid mother liquor tank 4 is connected to both the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5. Most of the nitric acid mother liquor is returned to the second inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, further precipitating the nitric acid crystals, thus producing most of the sodium sulfate in the nanofiltration concentrate as nitric acid. A small portion of the nitric acid mother liquor is discharged to the sodium sulfate feed tank 5. The discharge rate is controlled by ensuring that the sodium chloride and sodium sulfate concentrations in the mother liquor reach the co-saturation point at the operating temperature. This ensures that the sodium chloride in the sodium sulfate concentrate reaches the co-saturation point after concentration in the nitric acid evaporator crystallizer 2, but does not precipitate in the nitric acid crystallizer; only sodium sulfate precipitates in the nitric acid evaporator crystallizer 2. The mass flow rate of the nitrate mother liquor discharged into the Glauber's salt feed tank 5 is 12.19% of the inlet mass flow rate of the nitrate evaporator crystallizer 2. The specific parameters of the nitrate mother liquor are: total dissolved solids: 221298.9 mg / L, sulfate: 38003.9 mg / L, chloride: 93811.8 mg / L, COD: 9911.9 mg / L.

[0078] The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6.

[0079] Because the sodium chloride content in the feed of the cryo-crystallizer is close to saturation, the freezing point is very low, which allows the cryo-crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the cryo-crystallization process. Sodium sulfate precipitates in the cryo-crystallizer 6 in the form of sodium sulfate, producing sodium sulfate crystal slurry. The sodium sulfate crystal slurry is connected to the inlet of the sodium sulfate centrifuge 7 through the outlet of the cryo-crystallizer 6.

[0080] The Glauber's salt centrifuge 7 performs solid-liquid separation on the Glauber's salt slurry. The Glauber's salt outlet and mother liquor outlet of the Glauber's salt centrifuge 7 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the Glauber's salt mother liquor tank 8, respectively. The Glauber's salt mother liquor is discharged to other treatment systems. The mass flow rate of the Glauber's salt mother liquor is 81.95% of the inlet mass flow rate of the freeze crystallizer 6. The specific parameters of the Glauber's salt mother liquor are: total dissolved solids: 181361.4 mg / L, sulfate: 11052 mg / L, chloride: 93811.8 mg / L, COD: 9911.9 mg / L.

[0081] The outlet of recrystallization feed tank 9 is connected to the inlet of evaporator crystallizer 10.

[0082] The evaporator crystallizer 10 produces sodium sulfate crystal slurry using single-effect evaporation crystallization. The sodium sulfate crystal slurry is connected to the inlet of the centrifuge 11 via the outlet of the evaporator crystallizer 10.

[0083] Centrifuge 11 performs solid-liquid separation on sodium sulfate crystal slurry. The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12. The solids produced by the centrifuge are discharged as sodium sulfate product to other treatment systems. The mass flow rate of the mother liquor is 6.35% of the inlet mass flow rate of evaporator crystallizer 10. The specific parameters of the mother liquor are: total dissolved solids: 285855 mg / L, sulfate: 181437.8 mg / L, chloride: 20511.3 mg / L, COD: 722.4 mg / L.

[0084] The outlet of the mother liquor tank 12 is connected to the third inlet of the nitric acid evaporator crystallizer 2. The mother liquor in the mother liquor tank 12 flows back to the third inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, causing the nitric acid crystals to precipitate out, and the impurities in the crystals to enter the nitric acid mother liquor. After further treatment, the impurities are finally discharged from the system through the mother liquor discharge tank 8.

[0085] Example 5 Example 1 describes a sodium sulfate crystallization process with high yield and product purity, comprising the following steps: 1. Nitrate feed tank, 2. Nitrate evaporator crystallizer, 3. Nitrate centrifuge, 4. Nitrate mother liquor tank, 5. Glauber's salt feed tank, 6. Glauber's salt centrifuge, 7. Glauber's salt mother liquor tank, 8. Recrystallization feed tank, 9. Evaporator crystallizer, 10. Centrifuge, 11. Mother liquor tank, 12.

[0086] The nanofiltration concentrate from the coal mine zero-discharge water and salt project mainly contains sodium sulfate and a small amount of sodium chloride. The specific parameters are: total dissolved solids: 119222.6 mg / L, sulfate: 74970.6 mg / L, chloride: 5249.5 mg / L, COD: 514.9 mg / L.

[0087] Nanofiltration concentrate enters nitric acid feed tank 1, and the outlet of nitric acid feed tank 1 is connected to the first inlet of nitric acid evaporator crystallizer 2.

[0088] Nitrate evaporator crystallizer 2 uses MVR evaporation crystallization to produce nitrate slurry. The nitrate slurry is connected to the inlet of nitrate centrifuge 3 via the outlet of nitrate evaporator crystallizer 2.

[0089] Nitrate centrifuge 3 performs solid-liquid separation on nit crystal slurry. The nit outlet and mother liquor outlet of nit centrifuge 3 are connected to the inlet of recrystallization feed tank 9 and the inlet of nit mother liquor tank 4, respectively.

[0090] The outlet of the nitric acid mother liquor tank 4 is connected to both the second inlet of the nitric acid evaporator crystallizer 2 and the sodium sulfate feed tank 5. Most of the nitric acid mother liquor is returned to the second inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, further precipitating the nitric acid crystals, thus producing most of the sodium sulfate in the nanofiltration concentrate as nitric acid. A small portion of the nitric acid mother liquor is discharged to the sodium sulfate feed tank 5. The discharge rate is controlled by ensuring that the sodium chloride and sodium sulfate concentrations in the mother liquor reach the co-saturation point at the operating temperature. This ensures that the sodium chloride in the sodium sulfate concentrate reaches the co-saturation point after concentration in the nitric acid evaporator crystallizer 2, but does not precipitate in the nitric acid crystallizer; only sodium sulfate precipitates in the nitric acid evaporator crystallizer 2. The mass flow rate of the nitrate mother liquor discharged into the Glauber's salt feed tank 5 is 3.32% of the inlet mass flow rate of the nitrate evaporator crystallizer 2. The specific parameters of the nitrate mother liquor are: total dissolved solids: 363968.1 mg / L, sulfate: 37955.4 mg / L, chloride: 180318.5 mg / L, COD: 17657.2 mg / L.

[0091] The outlet of the sodium sulfate feed tank 5 is connected to the inlet of the freeze crystallizer 6.

[0092] Because the sodium chloride content in the feed of the cryo-crystallizer is close to saturation, the freezing point is very low, which allows the cryo-crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the cryo-crystallization process. Sodium sulfate precipitates in the cryo-crystallizer 6 in the form of sodium sulfate, producing sodium sulfate crystal slurry. The sodium sulfate crystal slurry is connected to the inlet of the sodium sulfate centrifuge 7 through the outlet of the cryo-crystallizer 6.

[0093] The Glauber's salt centrifuge 7 performs solid-liquid separation on the Glauber's salt crystal slurry. The Glauber's salt outlet and mother liquor outlet of the Glauber's salt centrifuge 7 are connected to the inlet of the recrystallization feed tank 9 and the inlet of the Glauber's salt mother liquor tank 8, respectively. The Glauber's salt mother liquor is discharged to other treatment systems. The mass flow rate of the Glauber's salt mother liquor is 88.21% of the inlet mass flow rate of the freeze crystallizer 6. The specific parameters of the Glauber's salt mother liquor are: total dissolved solids: 321072.3 mg / L, sulfate: 8867.5 mg / L, chloride: 180318.5 mg / L, COD: 17657.2 mg / L.

[0094] The outlet of recrystallization feed tank 9 is connected to the inlet of evaporator crystallizer 10.

[0095] The evaporator crystallizer 10 uses MVR evaporation crystallization to produce sodium sulfate slurry. The sodium sulfate slurry is connected to the inlet of the centrifuge 11 via the outlet of the evaporator crystallizer 10.

[0096] Centrifuge 11 performs solid-liquid separation on sodium sulfate crystal slurry. The mother liquor outlet of centrifuge 11 is connected to the inlet of mother liquor tank 12. The solids produced by the centrifuge are discharged as sodium sulfate product to other treatment systems. The mass flow rate of the mother liquor is 4.9% of the inlet mass flow rate of evaporator crystallizer 10. The specific parameters of the mother liquor are: total dissolved solids: 338507.9 mg / L, sulfate: 171896.1 mg / L, chloride: 50843.7 mg / L, COD: 1662.3 mg / L.

[0097] The outlet of the mother liquor tank 12 is connected to the third inlet of the nitric acid evaporator crystallizer 2. The mother liquor in the mother liquor tank 12 flows back to the third inlet of the nitric acid evaporator crystallizer 2 for evaporation and crystallization, causing the nitric acid crystals to precipitate out, and the impurities in the crystals to enter the nitric acid mother liquor. After further treatment, the impurities are finally discharged from the system through the mother liquor discharge tank 8.

[0098] Comparison Cases The specific implementation process is the same as in Example 2, except that this process does not include sodium sulfate freezing crystallization or sodium sulfate recrystallization.

[0099] The nanofiltration concentrate from the zero-salt discharge project for coking wastewater mainly contains sodium sulfate and a small amount of sodium chloride. The specific parameters are: total dissolved solids: 119222.6 mg / L, sulfate: 74970.6 mg / L, chloride: 5249.5 mg / L, COD: 514.9 mg / L.

[0100] Table 1. Purity and yield of sodium sulfate products obtained from Examples 2-5 and comparative cases. To achieve sodium sulfate crystallization with low energy consumption, high yield, and high product purity, this invention has comprehensively optimized the zero-emission salt separation process, added key processing stages, and optimized the connections between these stages, thus achieving the intended purpose. Compared to existing technologies, the sodium sulfate crystallization system and process described in this invention, characterized by low energy consumption, high yield, and high product purity, have the following advantages: (1) The sodium sulfate crystallization system and process described in this invention can reduce the impurity rate while producing high-quality sodium sulfate products, thereby increasing the sales revenue of sodium sulfate and reducing the cost of impurity disposal in the project.

[0101] (2) Most of the nitrate mother liquor is returned to the nitrate evaporator crystallizer for evaporation and crystallization, which further causes the nitrate in it to crystallize and precipitate, so that most of the sodium sulfate in the nanofiltration concentrate is produced in the form of nitrate.

[0102] (3) The sodium chloride content in the feed of the freeze crystallizer is close to saturation, so the freezing point is very low. This allows the freeze crystallizer to operate at a very low temperature. The lower the temperature, the lower the solubility of sodium sulfate, and the higher the yield of sodium sulfate during the freeze crystallization process.

[0103] (4) The nitrate produced by the nitrate evaporator crystallizer and the sodium sulfate produced by the freeze crystallizer are dissolved and then recrystallized in the evaporator crystallizer to obtain sodium sulfate product with high purity. The impurities in the nitrate and sodium sulfate are returned to the nitrate evaporator crystallizer in the form of mother liquor for evaporation and crystallization, so that the nitrate crystals are precipitated out and the impurities are put into the nitrate mother liquor. After subsequent treatment, the mother liquor is finally discharged from the system through the sodium sulfate mother liquor tank 8.

[0104] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A sodium sulfate crystallization system, characterized in that: Includes nitrate feed tank, nitrate evaporator crystallizer, nitrate centrifuge, nitrate mother liquor tank, sodium sulfate feed tank, freeze crystallizer, sodium sulfate centrifuge, sodium sulfate mother liquor tank, recrystallization feed tank, evaporator crystallizer, centrifuge, and mother liquor tank; Nanofiltration concentrate enters the nitrate feed tank, and the nitrate feed tank, nitrate evaporator crystallizer, nitrate centrifuge, and nitrate mother liquor tank are connected in sequence. The mother liquor from the nitrate mother liquor tank enters the sodium sulfate feed tank and the nitrate evaporator crystallizer respectively. The Glauber's salt feed tank, the freezing crystallizer, and the Glauber's salt centrifuge are connected in sequence. The mother liquor from the Glauber's salt centrifuge enters the Glauber's salt mother liquor tank, and the mother liquor from the Glauber's salt mother liquor tank is discharged. The sodium sulfate from the sodium sulfate centrifuge and the nitrate from the nitrate centrifuge enter the recrystallization feed tank. The recrystallization feed tank, the evaporator crystallizer, and the centrifuge are connected in sequence. The centrifuge produces sodium sulfate, and the mother liquor discharged from the centrifuge enters the mother liquor tank. The mother liquor tank is connected to the nitrate evaporator crystallizer.

2. The sodium sulfate crystallization system according to claim 1, characterized in that: The solid produced by the centrifuge is sodium sulfate, which then enters the drying system.

3. The sodium sulfate crystallization system according to claim 1, characterized in that: The nitric acid evaporator crystallizer is one or more of the following: single-effect evaporator crystallizer, multi-effect evaporator crystallizer, MVR evaporator crystallizer, or TVR evaporator crystallizer. The evaporator crystallizer produces nitric acid slurry.

4. The sodium sulfate crystallization system according to claim 1, characterized in that: The evaporator crystallizer is one or more of single-effect evaporator crystallizer, multi-effect evaporator crystallizer, MVR evaporator crystallizer or TVR evaporator crystallizer, and the evaporator crystallizer produces sodium sulfate crystal slurry; And / or, the rotation speed of the centrifuge is 2000~3000 r / min; And / or, the cooling temperature of the freeze crystallizer is -5~5℃.

5. A sodium sulfate crystallization system according to claim 1, characterized in that: The rotation speed of the Glauber's salt centrifuge is 2000~3000 r / min.

6. The sodium sulfate crystallization system according to claim 1, characterized in that: The centrifuge speed is 2000~3000 r / min.

7. A process method for a sodium sulfate crystallization system according to any one of claims 1-6, characterized in that: Includes the following steps: S1: Evaporate and crystallize the nanofiltration concentrate to separate nitrate and nitrate mother liquor. Freeze and crystallize a portion of the nitrate mother liquor to separate mirabilite and mirabilite mother liquor. S2: Dissolve nitrate and mirabilite into a sodium sulfate solution, then evaporate and crystallize the solution and separate the solid and liquid phases to obtain sodium sulfate slurry. Dry the sodium sulfate slurry to obtain sodium sulfate product.

8. The process method for a sodium sulfate crystallization system according to claim 7, characterized in that: Step S1 also includes nanofiltration concentrate entering the nitrate feed tank, followed by nitrate evaporation and crystallization. Nitrate evaporation and crystallization produces nitrate slurry. The nitrate slurry is subjected to solid-liquid separation to obtain nitrate and nitrate mother liquor. Part of the nitrate mother liquor is returned to the nitrate evaporation and crystallization tank for further evaporation and crystallization, while the other part is discharged to the sodium sulfate feed tank and enters the freeze crystallizer. The freeze crystallizer produces sodium sulfate slurry, which is then subjected to solid-liquid separation to obtain sodium sulfate and sodium sulfate mother liquor.

9. The process method for a sodium sulfate crystallization system according to claim 7, characterized in that: Step S2 also includes melting nitrate and sodium sulfate in a recrystallization feed tank, followed by evaporation and crystallization of the sodium sulfate solution in an evaporator to produce sodium sulfate slurry. The sodium sulfate slurry is then subjected to solid-liquid separation. The sodium sulfate obtained from the solid-liquid separation is dried in a drying system to obtain the sodium sulfate product. The mother liquor obtained from the solid-liquid separation is discharged to other processing systems through a mother liquor tank.

10. The process method for a sodium sulfate crystallization system according to claim 7, characterized in that: The mother liquor is returned to the nitrate evaporation and crystallization system, and after further treatment, it is finally discharged from the system through the mother liquor discharge tank of Glauber's salt.

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