System for producing high-purity sodium chloride from brine and production method thereof

Abstract

The invention provides a system for producing high-purity sodium chloride from brine and a production method thereof, belongs to the technical field of sodium chloride production, and solves the problems of low product quality and large fluctuation influence of a main production process of high-purity sodium chloride at present. The method comprises a reaction barrel, an ultrafiltration process, a first-stage nanofiltration process, a second-stage nanofiltration process and an MVR (mechanical vapor recompression) evaporation salt production process which are connected in sequence, the ultrafiltration process is sequentially connected with a salty mud washing process and a salty mud drying process; the primary nanofiltration process is also sequentially connected with a precooling process, a freezing nitre extraction process, a nitre melting process and an MVR nitre preparation process; according to the invention, ultrafiltration, first-stage nanofiltration, second-stage nanofiltration, an MVR salt production process and an MVR saltpeter production process are adopted for production. According to the method disclosed by the invention, the processes of pretreatment primary refining, ultrafiltration and nanofiltration secondary refining, evaporative crystallization deep refining and the like are utilized, so that the high-purity sodium chloride can be produced by utilizing the high-mirabilite brine, the production cost of the high-purity sodium chloride is greatly reduced, and the energy consumption is reduced.

Description

Technical Field

[0001] This invention belongs to the field of sodium chloride production technology, and relates to new energy, power battery, salt production technology, and nitrate production technology. Specifically, it relates to a system and method for producing high-purity sodium chloride from brine. Background Technology

[0002] High-purity sodium chloride is the main raw material for the production of metallic sodium, which is a crucial raw material for the cathode material and electrolyte of sodium-ion batteries. The demand for high-purity salts in new energy batteries is surging, and companies developing high-purity sodium chloride can achieve gross profit margins exceeding 40%. Sodium chloride with a purity of 99.9%, sulfate content <0.01%, and calcium and magnesium ion content of 0% is currently the main production process for high-purity sodium chloride: multiple recrystallization and chemical treatment. Multiple recrystallization involves evaporating and crystallizing the brine, then dissolving it again, and evaporating and crystallizing it again. This process suffers from high energy consumption, low product quality, and significant susceptibility to fluctuations in feed composition. Chemical treatment involves adding soda ash and caustic soda to the brine to remove calcium and magnesium ions, then adding barium chloride to react with sulfate ions to form barium sulfate precipitate, removing sulfate from the brine, followed by evaporation and concentration to produce salt. This process uses highly toxic barium chloride, generates barium slag, causing environmental pollution, poses health risks during the addition of barium chloride, and is costly. With the gradual maturation of membrane filtration technology, ultrafiltration membranes are used to filter suspended solids in brine with SS < 1 ppm, and nanofiltration membranes are used to selectively retain divalent ions, achieving green and pollution-free sulfate removal. Combined with processes such as freeze nitrification, MVR salt production, and MVR nitrification, high-purity sodium chloride can be produced. Summary of the Invention

[0003] The purpose of this invention is to provide a system and method for producing high-purity sodium chloride from brine, in order to solve the problems of low product quality and large fluctuations in the main production processes of high-purity sodium chloride.

[0004] The technical solution of the present invention is: a system for producing high-purity sodium chloride from brine, comprising a reaction tank, an ultrafiltration process, a primary nanofiltration process, a secondary nanofiltration process, and an MVR evaporation salt production process connected in sequence; the ultrafiltration process is connected in sequence to a salt mud washing process and a salt mud drying process; the primary nanofiltration process is also connected in sequence to a precooling process, a freeze-drying nitrate extraction process, a nitrification process, and an MVR nitrate production process.

[0005] As a further improvement of the present invention, the MVR evaporation salt production process includes a salt production condensate preheater, an MVR salt production evaporator, a wet salt washing system and a salt slurry dehydration and drying system connected in sequence. The MVR salt production evaporator is also connected in sequence to a secondary steam washing tower and an MVR salt production compressor.

[0006] As a further improvement of the present invention, the MVR nitrification process includes a nitrification condensate preheater, an MVR nitrification evaporator, a nitrification slurry dewatering system and a wet sodium sulfate drying system connected in sequence. The MVR nitrification evaporator is also connected in sequence to a secondary steam scrubbing tower and an MVR nitrification compressor.

[0007] As a further improvement of the present invention, the ultrafiltration process includes a pH adjustment device, an activated carbon filter, a security filter, and a high-pressure pump.

[0008] A method for producing high-purity sodium chloride from brine includes the following steps: S1: Place the brine raw material liquid containing high concentration of sodium sulfate into the reaction tank, add sodium hydroxide and sodium carbonate to the reaction tank to precipitate calcium and magnesium ions in the brine, and then pump the brine in the reaction tank into the ultrafiltration process. S2: The concentrated component of the brine after ultrafiltration is salt mud. The salt mud enters the salt mud washing process and the salt mud drying process in sequence. The dried salt mud is sold as building material. The condensate after washing in the salt mud washing process is returned to the mine for brine extraction and reuse. S3: The dilute phase of the brine after ultrafiltration and the ultrafiltration permeate enter the primary nanofiltration process for primary sodium sulfate removal. The permeate from the primary nanofiltration process enters the secondary nanofiltration process for secondary sodium sulfate removal. The concentrate from the secondary nanofiltration process is returned to the primary nanofiltration process for reuse. The permeate from the secondary nanofiltration process enters the MVR evaporation salt production process for tertiary sodium sulfate removal. The salt slurry generated in the MVR evaporation salt production process is washed by the permeate from the secondary nanofiltration process to remove the small amount of sodium sulfate entrained in the salt slurry again. After washing, the salt slurry enters the salt slurry dehydration and drying system. The dried product reaches the standard of high-purity sodium chloride. S4: The concentrate from the first-stage nanofiltration process enters the pre-cooling process and the cryogenic nitration process in sequence. The cryogenic nitration process yields sodium sulfate solid and cryogenic clear liquid. The cryogenic clear liquid returns to the first-stage nanofiltration process after passing through the nanofiltration concentrate in the pre-cooling process. The sodium sulfate solid enters the nitration process. The sodium sulfate solution obtained from the nitration process enters the MVR nitration process, and finally, dry sodium sulfate product is obtained.

[0009] As a further improvement of the present invention, in step S1, the ultrafiltration membrane in the ultrafiltration process adopts a pore size of 1-6 micrometers, and the brine after ultrafiltration has SS≤1ppm. The permeate in the ultrafiltration process is adjusted to pH value between 5 and 9 by a pH adjustment device, and then enters the activated carbon filter in the ultrafiltration process. After passing through the activated carbon filter, it enters the security filter. The brine containing sodium chloride and sodium sulfate after being filtered by the security filter is transported to the first-stage nanofiltration process by a high-pressure pump.

[0010] As a further improvement of the present invention, in step S3, both the primary nanofiltration process and the secondary nanofiltration process use nanofiltration membranes specifically designed for high-salt brine, with a sodium chloride concentration of 290-310 g / L. The side with a high sodium sulfate content is the concentrate from the primary nanofiltration process, which has a sodium sulfate content of 45-65 g / L. The side with a low sodium sulfate content is the permeate from the primary nanofiltration process, which has a sodium sulfate content of 1-5 g / L. The permeate from the secondary nanofiltration process has a sodium sulfate content of 0.1-0.5 g / L.

[0011] As a further improvement of the present invention, in step S3, the permeate from the secondary nanofiltration process is preheated by the salt condensate preheater and then enters the MVR salt evaporator. The evaporated secondary steam is heated and pressurized by the MVR salt compressor driven by green electricity, and the steam temperature is increased by 19-21°C compared with the secondary steam. The secondary steam from the MVR salt compressor is then returned to the MVR salt evaporator for reuse. The high-temperature condensate generated by the heat exchange of the MVR salt evaporator is used to preheat the feed of the salt condensate preheater. The MVR salt evaporator obtains a salt slurry containing sodium chloride through evaporation. After the salt slurry is washed and floated by the permeate from the secondary nanofiltration process, the small-particle-size powdered salt and impurities are returned to the MVR salt evaporator. The large-particle-size high-purity salt slurry is washed by the wet salt washing system to obtain wet sodium chloride solid. The wet sodium chloride solid is then dried by the salt slurry dehydration and drying system to obtain high-purity sodium chloride product.

[0012] As a further improvement of the present invention, in step S4, the concentrate from the first-stage nanofiltration process is pre-cooled sequentially by a plate precooler and a tubular precooler in the precooling process to obtain a low-temperature high-nitrate, high-salt feed solution. The high-nitrate, high-salt feed solution then enters the cryogenic crystallizer in the cryogenic extraction process. The cryogenic crystallizer includes a first-stage direct-cooling tubular cryogenic crystallizer and a second-stage direct-cooling tubular cryogenic crystallizer. By increasing the circulation volume of the cold source in the shell of the cryogenic heat exchanger and the heat exchange temperature difference, the temperature of the cold source in the shell of the tube heat exchanger is controlled between 2°C and -7°C, freezing the raw material to -4°C to 5°C, i.e., the heat transfer temperature difference is controlled within 3°C. At the same time, the supersaturation of the material in the heat exchanger is controlled to be <1 kg / m³. 3 The residence time of the material in the cryogenic crystallizer is controlled to be greater than 3 hours. The first-stage direct-cooling tube cryogenic crystallizer and the second-stage direct-cooling tube cryogenic crystallizer produce a slurry of Glauber's salt and a first-stage low-nitrate, high-salt solution containing 1-20 g / L sodium sulfate. The Glauber's salt slurry is dehydrated by cryogenic centrifugation to obtain Glauber's salt solid. The Glauber's salt solid enters the nitration process. The low-nitrate, high-salt solution from the first-stage direct-cooling tube cryogenic crystallizer and the second-stage direct-cooling tube cryogenic crystallizer enters the precooling process to precool the concentrated water in the first-stage nanofiltration process.

[0013] As a further improvement of the present invention, in step S4, the sodium sulfate solid is heated to above 32.38°C by introducing a heat source in the nitration process, and the temperature is controlled between 35-90°C to obtain a solution or slurry containing sodium sulfate. The mother liquor of the solution or slurry containing sodium sulfate is then centrifuged and dehydrated to obtain wet sodium sulfate solid and then enters the MVR nitration process.

[0014] The beneficial effects of this invention are as follows: To address the technical problems of high energy consumption, low product quality, and significant susceptibility to fluctuations in feed composition in existing processes, which do not meet the national "dual carbon" target, this invention provides a system and method for producing high-purity sodium chloride from brine. Utilizing an ultrafiltration + two-stage nanofiltration + MVR evaporation + freeze-drying process, high-purity sodium chloride is produced, along with sodium sulfate or mirabilite. Green electricity is used as the primary energy source, enabling comprehensive resource utilization, improving resource efficiency, and reducing energy consumption.

[0015] The present invention also has the following beneficial effects: 1. This invention recovers sodium chloride and sodium sulfate from raw materials to produce high-quality products, wherein the sodium chloride purity is ≥99.9%, meeting the requirements for high-purity sodium chloride, and the sodium sulfate purity is ≥99.1%, exceeding the requirement of ≥99% purity for Class I Grade 1 anhydrous sodium sulfate in GB / T 6009, thus improving product quality and making it suitable for sale.

[0016] 2. This invention adopts an ultrafiltration + primary nanofiltration + secondary nanofiltration + MVR salt production process and an MVR nitrate production process. In particular, the unique design of the ultrafiltration + primary nanofiltration + secondary nanofiltration process ensures stable product quality and greatly reduces energy consumption. The overall energy consumption level is about 34 kgce / ton of salt, which is significantly lower than the existing process of 75-110 kgce / ton of salt and nitrate, thus reducing production costs.

[0017] 3. This invention uses green electricity as the main energy source, requiring only a small amount of steam for starting up and drying products, thus significantly reducing carbon emissions. It is green and environmentally friendly, and suitable for industrial production. Attached Figure Description

[0018] Figure 1 This is a system structure diagram of the present invention.

[0019] In the diagram: 1-Reaction tank; 2-Ultrafiltration process; 3-Salt mud washing process; 4-Salt mud drying process; 5-First-stage nanofiltration process; 6-Second-stage nanofiltration process; 7-Salt condensate preheater; 8-MVR salt evaporator; 9-Secondary steam scrubbing tower; 10-MVR salt compressor; 11-Wet salt washing system; 12-Salt slurry dehydration and drying system; 13-Precooling process; 14-Freezing nitrate extraction process; 15-Nitrogenation process; 16-Nitrogen condensate preheater; 17-MVR nitrate evaporator; 18-MVR nitrate compressor; 19-Nitrogen slurry dehydration system; 20-Wet sodium sulfate drying system. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0022] Example 1 A system for producing high-purity sodium chloride from brine includes a reaction tank 1, an ultrafiltration process 2, a primary nanofiltration process 5, a secondary nanofiltration process 6, and an MVR evaporation salt-making process connected in sequence. The ultrafiltration process 2 is connected in sequence to a salt mud washing process 3 and a salt mud drying process 4. The primary nanofiltration process 5 is also connected in sequence to a precooling process 13, a freeze-drying nitrate extraction process 14, a nitrification process 15, and an MVR nitrate production process. The MVR evaporation salt-making process includes a salt-making condensate preheater 7, an MVR salt-making evaporator 8, a wet salt washing system 11, and a salt slurry dehydration and drying system 12 connected in sequence. The MVR salt-making evaporator 8 is also connected in sequence to a secondary steam scrubbing tower 9 and an MVR salt-making compressor 10. The MVR nitrate production process includes a nitrate-making condensate preheater 16, an MVR nitrate-making evaporator 17, a nitrate slurry dehydration system 19, and a wet sodium sulfate drying system 20 connected in sequence. The MVR nitrate-making evaporator 17 is also connected in sequence to a secondary steam scrubbing tower 9 and an MVR nitrate-making compressor 10. The ultrafiltration process 2 includes a pH adjustment device, an activated carbon filter, a security filter, and a high-pressure pump.

[0023] The specific processing steps are as follows: The brine containing sodium chloride and sodium sulfate has the following composition: NaCl: 295g / L, Na2SO4: 29g / L. The brine raw material with a high concentration of sodium sulfate enters reaction tank 1. Sodium hydroxide and sodium carbonate are added to reaction tank 1 to precipitate calcium and magnesium ions in the brine. Then, the brine in reaction tank 1 is pumped into ultrafiltration step 2. The brine and calcium and magnesium salt mud in reaction tank 1 do not require the addition of flocculants and directly enter the ultrafiltration membrane. Through the diameter analysis of calcium and magnesium salt mud, the ultrafiltration membrane adopts a pore size of 1-6 microns. If the pore size is too large, the product quality will not meet the standards; if the pore size is too small, the ultrafiltration membrane will easily clog. To ensure the filtration effect, the SS of the brine after ultrafiltration is ≤1ppm, which ensures the operation of subsequent processes and product quality.

[0024] The concentrated product from ultrafiltration step 2 is called salt mud. The salt mud enters the salt mud washing step 3, and after being washed with evaporative condensate, it enters the salt mud drying step 4. The dried salt mud is sold as building material. The condensate after washing is returned to the mine for brine extraction. The salt mud is directly washed with the evaporative condensate generated by this system, without the need to introduce external industrial water, thus saving water. Moreover, the evaporative condensate after washing is returned to the mine for brine extraction and recycling.

[0025] The dilute phase of ultrafiltration step 2 is the ultrafiltration permeate. The permeate from ultrafiltration step 2 has its pH value adjusted to between 5 and 9 by a pH adjustment device. It then enters an activated carbon filter and then a security filter. To prevent fine particles from entering the membrane system and clogging the membrane elements, the feed liquid containing sodium chloride and sodium sulfate after filtration by the security filter is transported by a high-pressure pump to the first-stage nanofiltration step 5 for primary sodium sulfate removal.

[0026] The permeate from the first-stage nanofiltration process 5 enters the second-stage nanofiltration process 6 for secondary sodium sulfate removal. The concentrate from the second-stage nanofiltration process 6 is returned to the first-stage nanofiltration process 5. The permeate from the second-stage nanofiltration process 6 enters the MVR evaporation salt production process for tertiary sodium sulfate removal. The salt slurry generated in the MVR evaporation salt production process is washed by the permeate from the second-stage nanofiltration process 6 to remove the small amount of sodium sulfate carried by the salt slurry again. After washing, the salt slurry enters the dehydration and drying process. The dried product meets the high-purity sodium chloride standard.

[0027] In the first-stage nanofiltration process 5, the pressure is controlled at 2-3.5 MPa. The first-stage nanofiltration process 5 uses a special nanofiltration membrane for brine that is resistant to high salt (sodium chloride concentration 290-310 g / L, close to saturation). The side with high sodium sulfate content is the first-stage nanofiltration concentrate, with the sodium sulfate content controlled at 45-65 g / L and the NaCl content at 290 g / L. If the sodium sulfate content is too high, it will easily crystallize and clog the membrane; if it is too low, the nanofiltration yield will be low and the investment will be too high. The side with low sodium sulfate content is the first-stage nanofiltration permeate, with the sodium sulfate content controlled at 1-5 g / L and the NaCl content at 300 g / L. The concentrate from the first-stage nanofiltration process 5 enters the freezing process, and the permeate from the first-stage nanofiltration process 5 enters the second-stage nanofiltration process 6.

[0028] The secondary nanofiltration process 6 also uses a special nanofiltration membrane for high-salt brine. The concentrate from the secondary nanofiltration process 6 enters the primary nanofiltration process 5. The sodium sulfate content on the secondary nanofiltration concentrate side is 50g / L. The sodium sulfate content in the permeate from the secondary nanofiltration process 6 is controlled between 0.1-0.5g / L. If it is too high, the quality of subsequent products will easily fail to meet the standards; if it is too low, the investment will be too large. The permeate from the secondary nanofiltration process 6 enters the salt production process.

[0029] The permeate from the secondary nanofiltration process 6 is preheated by the salt condensate preheater 7 and then enters the MVR salt evaporator 8. The evaporated secondary steam is heated and pressurized by the green electric power-driven MVR salt compressor 10, and the steam temperature is increased by 19-21°C compared with the secondary steam. It then returns to the MVR salt evaporator 8. The high-temperature condensate generated by the heat exchange in the MVR salt evaporator 8 is used to preheat the feed in the salt condensate preheater 7. The MVR salt evaporator 8 obtains a salt slurry containing sodium chloride through evaporation. After the salt slurry is washed and floated by the permeate from the secondary nanofiltration process 6, the small-particle-size powdered salt and impurities are returned to the MVR salt evaporator 8. The large-particle-size high-purity salt is washed by the wet salt washing system 11 to obtain wet sodium chloride solid. The wet sodium chloride solid is then dried by the salt slurry dehydration and drying system 12 to obtain high-purity sodium chloride product.

[0030] The concentrate from the first-stage nanofiltration process 5 is first precooled sequentially through a plate precooler and a tubular precooler in the precooling process 13 to obtain a low-temperature, high-nitrate, high-salt feed solution. This high-nitrate, high-salt feed solution then enters the cryogenic crystallizer in the cryogenic extraction process 14. The cryogenic crystallizer includes a first-stage direct-cooling tubular cryogenic crystallizer and a second-stage direct-cooling tubular cryogenic crystallizer. By increasing the circulation rate of the cold source within the cryogenic exchanger shell and the temperature difference during heat exchange, the temperature of the cold source within the tube exchanger shell is controlled between 2°C and -7°C, freezing the raw material to -4°C to 5°C. This means the heat transfer temperature difference is controlled within 3°C to avoid excessive heat transfer temperature differences that could cause scaling in the exchanger. Simultaneously, the supersaturation of the material within the exchanger is controlled to be <1 kg / m³. 3To avoid producing excessively fine sodium sulfate and scaling in the cooler, the residence time of the material in the freeze crystallizer is controlled to be greater than 3 hours to increase the particle size of the crystallized sodium sulfate. Fine sodium sulfate is then floated out to participate in crystallization, which facilitates the separation of sodium sulfate from low-nitrate clear liquid and thus reduces the floating nitrate content of the low-nitrate clear liquid.

[0031] The first-stage direct-cooling tube-type cryogenic crystallizer in the cryogenic extraction process 14 produces a slurry of Glauber's salt and a first-stage low-nitrate, high-salt solution containing 1-20 g / L sodium sulfate. The Glauber's salt slurry is then dehydrated by centrifugation in the first stage to obtain solid Glauber's salt, which enters the nitration process 15. The sodium sulfate solution obtained in the nitration process 15 enters the MVR nitration process to obtain sodium sulfate product. The first-stage low-nitrate, high-salt solution enters the second-stage direct-cooling tube-type cryogenic crystallizer. The cryogenic clarified liquid produced in the cryogenic extraction process 14 is pre-cooled by the nanofiltration concentrate in the pre-cooling process 13 and then returned to the first-stage nanofiltration process 5.

[0032] The sodium sulfate solid enters the nitration process 15. In the nitration process 15, the sodium sulfate is heated to above 32.38℃ by introducing a heat source. The temperature is controlled within the range of 35-90℃. If the temperature is too low, the sodium sulfate solid cannot be completely converted into sodium sulfate solution. If the temperature is too high, too much energy is wasted and additional steam needs to be added. The resulting solution or slurry contains sodium sulfate. The solution or slurry containing sodium sulfate is centrifuged and dehydrated to obtain wet sodium sulfate solid. The centrifuged mother liquor then enters the MVR nitration process.

[0033] The solution from the nitration process 15 is first preheated by the condensate preheater 16 and then enters the MVR nitration evaporator 17 for evaporation. The evaporated secondary steam is heated and pressurized by the secondary steam scrubbing tower 9 and the MVR nitration compressor 10 which is powered by green electricity, and then enters the MVR nitration evaporator 17. The evaporation process yields a slurry containing sodium sulfate. The sodium sulfate slurry enters the sodium sulfate nitration slurry dewatering system 19, and the solid is dried by the wet sodium sulfate drying system 20 to obtain the sodium sulfate product.

[0034] Comparative Example 1 A system for producing high-purity sodium chloride from brine includes a flocculation and sedimentation process, a two-stage nanofiltration process, a freeze-drying nitrate extraction process, an MVR salt production process, and an MVR nitrate production process. The brine containing sodium chloride and sodium sulfate has the following composition: NaCl: 295 g / L, Na2SO4: 29 g / L. The specific process is as follows: S1. Add brine, soda ash, and caustic soda to multiple reaction tanks; S2. Add flocculant to the reaction tank; S3, calcium, and magnesium salt mud settled naturally in the reaction tank. S4. The clear liquid in the upper part of the reaction tank enters the activated carbon adsorption tower; S5. Then it enters the security filter to prevent fine particles from entering the membrane system and clogging the membrane elements; S6. The water is then pumped to the first-stage nanofiltration process by a high-pressure pump. The pressure of the first-stage nanofiltration is controlled at 2-3.5 MPa. The sodium sulfate content on the first-stage nanofiltration permeate side is 2 g / L and the NaCl content is 300 g / L. The sodium sulfate content on the first-stage nanofiltration concentrate side is 56 g / L and the NaCl content is 290 g / L. The first-stage nanofiltration concentrate enters the freeze-drying and nitrification process, and the first-stage nanofiltration permeate enters the second-stage nanofiltration process. S7, the sodium sulfate content on the concentrate side of the secondary nanofiltration is 50 g / L, and the sodium sulfate content on the product water side of the secondary nanofiltration is 0.5 g / L, which enters the MVR salt production process; S8. The secondary nanofiltration permeate, after being preheated by the condensate from the MVR salt production process, enters the MVR salt evaporator. The evaporated secondary steam is washed in a scrubbing tower and then heated and pressurized by a steam compressor to a temperature 19°C higher than the secondary steam. It then enters the heating chamber of the MVR salt evaporator to exchange heat with the circulating material. The slurry containing sodium chloride obtained by evaporation in the MVR salt production process is washed and floated. The high-purity salt product with large particle size is discharged into the centrifugal dehydration process. After centrifugal dehydration, wet sodium chloride solid is obtained. The solid is then dried to obtain high-purity sodium chloride product.

[0035] S9. The first-stage nanofiltration concentrate first passes through a precooler to exchange cooling with the low-nitrate, high-salt feed solution from the cryogenic crystallizer before entering the cryogenic crystallizer to exchange cooling with the cold source from the refrigeration ice machine. S10. The slurry of Glauber's salt obtained from the freeze crystallizer and the low-nitrate, high-salt solution containing 28 g / L sodium sulfate are fed to the pre-cooling feed and then returned to the first-stage nanofiltration process. The Glauber's salt slurry is centrifuged and dehydrated to obtain Glauber's salt solid, which then enters the nitration process.

[0036] S11. The sodium sulfate solid from the freezing process enters the nitrification tank of the nitrification process. A heat source is introduced into the nitrification tank to heat the sodium sulfate, resulting in a slurry containing sodium sulfate. The slurry then enters the MVR nitrification process. S12. The MVR nitration process includes the following steps: The solution from the nitration process is first preheated by the condensate from the MVR nitration process, and then enters the MVR nitration evaporator. The evaporated secondary steam is heated and pressurized by a steam compressor and then enters the MVR nitration evaporator. Through evaporation, a slurry containing sodium sulfate is obtained. The sodium sulfate slurry then enters the sodium sulfate dehydration and drying process. The solid is dried to obtain the sodium sulfate product.

[0037] Comparative Example 2 A system for producing high-purity sodium chloride from brine includes an ultrafiltration process, a primary nanofiltration process, a freeze-drying nitrate extraction process, an MVR salt production process, and an MVR nitrate production process. The brine containing sodium chloride and sodium sulfate has the following composition: NaCl: 295 g / L, Na2SO4: 29 g / L. The specific process is as follows: S1. Add brine, soda ash, and caustic soda to the reaction tank; S2, Inject into the ultrafiltration process; S3. The ultrafiltration permeate is then adjusted to pH 7 by a pH adjustment device. S4, and then it enters the activated carbon adsorption tower; S5. Then it enters the security filter to prevent fine particles from entering the membrane system and clogging the membrane elements; S6. The solution is then pumped to the nanofiltration process, where the pressure is controlled at 2-3.5 MPa. The sodium sulfate content in the nanofiltration permeate is 2 g / L, and the NaCl content is 300 g / L. The sodium sulfate content in the nanofiltration concentrate is 56 g / L, and the NaCl content is 290 g / L. The nanofiltration concentrate enters the cryogenic nitrification process, while the nanofiltration permeate enters the MVR salt production process. S7. Nanofiltration permeate, after being preheated by the condensate from the MVR salt production process, enters the MVR salt evaporator. The evaporated secondary steam is washed in a scrubbing tower and then heated and pressurized by a steam compressor before entering the heating chamber of the MVR salt evaporator to exchange heat with the circulating material. The slurry containing sodium chloride obtained by evaporation in the MVR salt production process is washed and floated. The high-purity salt product with large particle size is discharged into the centrifugal dehydration process. After centrifugal dehydration, wet sodium chloride solid is obtained. The solid is dried to obtain sodium chloride product.

[0038] S8. The first-stage nanofiltration concentrate first passes through a precooler and exchanges cold with the low-nitrate, high-salt feed liquid from the cryogenic crystallizer to cool down before entering the cryogenic crystallizer to exchange cold with the cold source from the refrigeration ice machine. S9. The slurry of Glauber's salt obtained from the freeze crystallizer and the low-nitrate, high-salt solution containing 26 g / L sodium sulfate are fed to the pre-cooling feed and then returned to the first-stage nanofiltration process. The Glauber's salt slurry is centrifuged and dehydrated to obtain Glauber's salt solid, which then enters the nitration process.

[0039] S10. The nitration process includes the following steps: the sodium sulfate solid from the freezing process enters the nitration tank of the nitration process, and a heat source is introduced into the nitration tank to heat the sodium sulfate to obtain a slurry containing sodium sulfate. The slurry enters the MVR nitration process. S12. The MVR nitration process includes the following steps: The solution from the nitration process is first preheated by the condensate from the MVR nitration process and then enters the MVR nitration evaporator. The evaporated secondary steam is heated and pressurized by a steam compressor and then enters the MVR nitration evaporator. Through evaporation, a slurry containing sodium sulfate is obtained. The sodium sulfate slurry then enters the sodium sulfate dehydration and drying process. The solid is dried to obtain the sodium sulfate product.

[0040] Comparative Example 3 The thermal salting-out process for co-producing sodium chloride and nitrate uses raw materials of NaCl: 295 g / L, Na2SO4: 29 g / L, and a temperature of 28℃. The evaporation salt production system employs a six-effect evaporation process. The raw materials are preheated with condensate and secondary steam before entering the first effect of a five-effect evaporator. The process then proceeds to the second, third, fourth, fifth, and sixth effects. The salt slurry discharged from the sixth effect is washed, cooled, and re-dissolved with sodium sulfate. After dehydration, wet salt and salt precipitation mother liquor are obtained. The wet salt is dissolved again with condensate and then enters a secondary evaporation crystallization unit. The salt slurry discharged from the secondary evaporation crystallization unit is washed, dehydrated, and separated to obtain wet salt. The wet salt is dried to produce sodium chloride. A certain amount of powdered salt is added to the salt precipitation mother liquor, which is then preheated, kept at a constant temperature, and allowed to settle. Dehydration produces crude nitrate, and the nitrate mother liquor is returned to the evaporation salt precipitation unit.

[0041] Comparative Example 4 The mother liquor circulating evaporation method for co-production of salt and nitrate uses raw materials with NaCl: 295 g / L and Na2SO4: 29 g / L at a temperature of 28℃. The evaporation salt production system employs a five-effect process, while the evaporation nitrate production system utilizes TVR evaporation combined with a two-stage flash evaporation process. The raw materials are preheated with condensate and secondary steam before entering the first effect of the five-effect evaporator. The process then proceeds to the second, third, fourth, and fifth effects. The salt slurry discharged from each effect is washed and cooled before dehydration and separation to obtain wet salt and precipitated mother liquor. The wet salt is dissolved a second time with condensate and then enters the second-stage evaporation crystallization unit. The salt slurry discharged from the second-stage evaporation crystallization unit is washed and dehydrated to obtain wet salt. The wet salt is then dried to produce sodium chloride product. The mother liquor from the salt precipitation process is preheated by two stages of flash steam in the nitration system before entering the nitration evaporator. A portion of the secondary steam from the evaporator is drawn back to the nitration heating chamber by a steam jet pump to heat the materials, while the remaining secondary steam enters the flash heating chamber. The mother liquor from the salt precipitation process is then cooled by two stages of flash steam before entering the five-effect evaporator for salt production. The mother liquor is then returned to the evaporator for heating and reuse, thus separating NaCl and Na₂SO₄.

[0042] The test results of Example 1 and Comparative Examples 1-4 are shown in Table 1 below:

[0043] The product quality inspection report for Example 1 is shown in Table 2:

[0044] As shown in Tables 1 and 2, the product quality of Example 1, which is the main process promoted in this patent, meets the requirements for high-purity sodium chloride, and the quality is stable with a relatively long operating cycle. Comparative Example 1 omits the ultrafiltration step; although the quality remains relatively stable, the nanofiltration process is prone to clogging, resulting in a short operating cycle. Comparative Example 2 uses a single-stage nanofiltration process; although the operating cycle is long, the quality fluctuates due to operational factors and influent water quality. Comparative Example 3 uses salt precipitation for salt and nitrate separation; the salt produced by the first-stage evaporation and crystallization fluctuates, causing fluctuations in the final product. Comparative Example 4 has stable product quality and a relatively long operating cycle, but its overall energy consumption is high.

[0045] Meanwhile, the energy structure of the process in Example 1 uses green electricity as the main energy source, and the carbon emissions of Example 1 are significantly reduced, making it environmentally friendly and suitable for industrial production. Although the energy structures of the processes in Comparative Examples 1 and 2 also use green electricity as the main energy source, they suffer from short operating cycles and some fluctuations in quality. The energy structures of the processes in Comparative Examples 3 and 4 use steam generated by boilers as the main energy source, which is not suitable for the requirements and trends of low-carbon production.

Claims

1. A system for producing high-purity sodium chloride from brine, characterized in that: It includes a reaction tank (1), an ultrafiltration process (2), a primary nanofiltration process (5), a secondary nanofiltration process (6), and an MVR evaporation salt production process connected in sequence; the ultrafiltration process (2) is connected in sequence to a salt mud washing process (3) and a salt mud drying process (4); the primary nanofiltration process (5) is also connected in sequence to a precooling process (13), a freeze-drying nitrate extraction process (14), a nitrate conversion process (15), and an MVR nitrate production process.

2. A system for producing high-purity sodium chloride from brine according to claim 1, characterized in that: The MVR evaporation salt production process includes a salt production condensate preheater (7), an MVR salt production evaporator (8), a wet salt washing system (11), and a salt slurry dehydration and drying system (12) connected in sequence. The MVR salt production evaporator (8) is also connected in sequence to a secondary steam washing tower (9) and an MVR salt production compressor (10).

3. A system for producing high-purity sodium chloride from brine according to claim 1 or 2, characterized in that: The MVR nitrification process includes a nitrification condensate preheater (16), an MVR nitrification evaporator (17), a nitrification slurry dewatering system (19), and a wet sodium sulfate drying system (20) connected in sequence. The MVR nitrification evaporator (17) is also connected in sequence to a secondary steam scrubbing tower (9) and an MVR nitrification compressor (18).

4. A system for producing high-purity sodium chloride from brine according to claim 3, characterized in that: The ultrafiltration process (2) includes a pH adjustment device, an activated carbon filter, a security filter and a high-pressure pump.

5. A method for producing high-purity sodium chloride from brine according to claim 4, characterized in that: Includes the following steps: S1: Place the brine raw material liquid containing high concentration of sodium sulfate into the reaction tank (1), add sodium hydroxide and sodium carbonate to the reaction tank (1) to precipitate calcium and magnesium ions in the brine, and then pump the brine in the reaction tank (1) into the ultrafiltration process (2). S2: The concentrated brine after ultrafiltration is salt mud. The salt mud enters the salt mud washing process (3) and the salt mud drying process (4) in sequence. The dried salt mud is sold as building material. The condensate after washing in the salt mud washing process (3) is returned to the mine for brine extraction and reuse. S3: The dilute phase of the brine after ultrafiltration and the ultrafiltration permeate enter the first-stage nanofiltration process (5) for primary sodium sulfate removal. The permeate of the first-stage nanofiltration process (5) enters the second-stage nanofiltration process (6) for secondary sodium sulfate removal. The concentrate of the second-stage nanofiltration process (6) is returned to the first-stage nanofiltration process (5) for reuse. The permeate of the second-stage nanofiltration process (6) enters the MVR evaporation salt production process for tertiary sodium sulfate removal. The salt slurry generated by the MVR evaporation salt production process is washed by the permeate of the second-stage nanofiltration process (6) to remove the small amount of sodium sulfate carried by the salt slurry again. After washing, the salt slurry enters the salt slurry dehydration and drying system (12). The dried product reaches the standard of high-purity sodium chloride. The concentrated water from the first-stage nanofiltration process (5) enters the precooling process (13) and the cryogenic extraction process (14) in sequence. The cryogenic extraction process (14) yields sodium sulfate solid and cryogenic clear liquid. The cryogenic clear liquid returns to the first-stage nanofiltration process (5) after passing through the nanofiltration concentrated water in the precooling process (13). The sodium sulfate solid enters the nitration process (15). The sodium sulfate solution obtained from the nitration process (15) enters the MVR nitration process, and finally, dry sodium sulfate product is obtained.

6. A method for producing high-purity sodium chloride from brine according to claim 5, characterized in that: In step S1, the ultrafiltration membrane in the ultrafiltration process (2) has a pore size of 1-6 micrometers. The brine after ultrafiltration has SS≤1ppm. The permeate in the ultrafiltration process (2) is adjusted to pH between 5 and 9 by a pH adjustment device, and then enters the activated carbon filter in the ultrafiltration process (2). After passing through the activated carbon filter, it enters the security filter. The brine containing sodium chloride and sodium sulfate after being filtered by the security filter is transported to the first-stage nanofiltration process (5) by a high-pressure pump.

7. A method for producing high-purity sodium chloride from brine according to claim 5, characterized in that: In step S3, both the primary nanofiltration process (5) and the secondary nanofiltration process (6) use nanofiltration membranes that are resistant to high-salt brine. The sodium chloride concentration is 290-310 g / L. The side with a high sodium sulfate content is the concentrate of the primary nanofiltration process (5), and the sodium sulfate content of the concentrate of the primary nanofiltration process (5) is 45-65 g / L. The side with a low sodium sulfate content is the permeate of the primary nanofiltration process (5), and the sodium sulfate content of the permeate of the primary nanofiltration process (5) is 1-5 g / L. The sodium sulfate content of the permeate of the secondary nanofiltration process (6) is 0.1-0.5 g / L.

8. A method for producing high-purity sodium chloride from brine according to claim 5, characterized in that: In step S3, the permeate from the secondary nanofiltration process (6) is preheated by the salt condensate preheater (7) and then enters the MVR salt evaporator (8). The evaporated secondary steam is heated and pressurized by the green electric power driven MVR salt compressor (10), and the steam temperature is 19-21°C higher than that of the secondary steam. The secondary steam from the MVR salt compressor (10) is then returned to the MVR salt evaporator (8) for reuse. The high-temperature condensate generated by the heat exchange of the MVR salt evaporator (8) is used to preheat the feed of the salt condensate preheater (7). The MVR salt evaporator (8) obtains a salt slurry containing sodium chloride through evaporation. After the salt slurry is washed and floated by the permeate from the secondary nanofiltration process (6), the small-particle-size powdered salt and impurities are returned to the MVR salt evaporator (8). The large-particle-size high-purity salt slurry is processed by the wet salt washing system (11) to obtain wet sodium chloride solid. The wet sodium chloride solid is processed by the salt slurry dehydration and drying system (12) to obtain high-purity sodium chloride product.

9. A method for producing high-purity sodium chloride from brine according to claim 5, characterized in that: In step S4, the concentrate from the first-stage nanofiltration process (5) is pre-cooled sequentially by the plate precooler and the tubular precooler in the precooling process (13) to obtain a low-temperature high-nitrate and high-salt feed solution. The high-nitrate and high-salt feed solution then enters the cryogenic crystallizer in the cryogenic extraction process (14). The cryogenic crystallizer includes a first-stage direct-cooling tubular cryogenic crystallizer and a second-stage direct-cooling tubular cryogenic crystallizer. By increasing the circulation volume of the cold source in the shell of the cryogenic heat exchanger and the heat exchange temperature difference, the temperature of the cold source in the shell of the tube heat exchanger is controlled between 2°C and -7°C, and the raw material is frozen to -4°C to 5°C. That is, the heat transfer temperature difference is controlled within 3°C, while the supersaturation of the material in the heat exchanger is controlled to be <1kg / m³. 3 The residence time of the material in the freeze crystallizer is controlled to be >3 hours; the first-stage direct-cooling tube freeze crystallizer and the second-stage direct-cooling tube freeze crystallizer obtain the slurry of Glauber's salt and the first-stage low-nitrate high-salt liquid containing 1-20g / L sodium sulfate. The Glauber's salt slurry is dehydrated by freezing and centrifugation to obtain Glauber's salt solid. The Glauber's salt solid enters the nitration process (15). The low-nitrate high-salt liquid from the first-stage direct-cooling tube freeze crystallizer and the second-stage direct-cooling tube freeze crystallizer enters the precooling process (13) to precool the concentrated water in the first-stage nanofiltration process (5).

10. A method for producing high-purity sodium chloride from brine according to claim 5, characterized in that: In step S4, the sodium sulfate solid is heated to above 32.38°C by introducing a heat source in the nitration process (15), and the temperature is controlled between 35-90°C to obtain a solution or slurry containing sodium sulfate. The solution or slurry containing sodium sulfate is centrifuged and dehydrated to obtain wet sodium sulfate solid. The centrifuged mother liquor is then introduced into the MVR nitration process.

Images