Low-temperature multi-effect evaporation-based impurity salt and salt treatment process method

Abstract

The invention discloses a low-temperature multi-effect evaporation-based impurity salt treatment process method, and relates to the technical field of industrial waste salt treatment in the chemical industry. According to the scheme, firstly, impurity salt is dissolved and then pretreated by a fluorine removal tank and a multi-medium filter, and pretreated saline water is obtained; then, organic matters and dissolved substances are removed through a catalytic oxidation and nanofiltration device, and a monovalent salt solution and a divalent salt solution are separated out; then, the monovalent salt solution is subjected to reverse osmosis concentration and then enters a low-temperature multi-effect evaporation system for sodium chloride crystallization, and the divalent salt solution is subjected to catalytic oxidation and then enters low-temperature multi-effect evaporation for sodium sulfate crystallization. And finally, the mother liquor subjected to low-temperature multi-effect evaporation is refluxed and retreated, and resources are further concentrated. According to the invention, the waste salt treatment efficiency is effectively improved by optimizing the chemical addition of the sedimentation tank. By adopting a low-temperature multi-effect evaporation technology, the energy efficiency is optimized, the energy loss is reduced, the crystallization efficiency is improved, and the resource recovery rate is increased. And a mother liquor backflow retreatment mechanism reduces waste liquid discharge and reduces environmental burden.

Description

Technical Field

[0001] The invention relates to the technical field of industrial waste salt treatment in the chemical industry, and in particular to a salt separation treatment process for miscellaneous salts based on low-temperature multi-effect evaporation. Background Art

[0002] In recent years, with the acceleration of industrialization, the amount of various industrial waste salts generated has increased year by year. How to effectively recycle and treat these waste salts has become a technical problem that needs to be solved urgently. The impurities in waste salts are complex, including fluorides, sulfates, chlorides and other salts. Traditional salt treatment processes usually focus on the removal or separation of certain specific salts, but with increasingly stringent environmental regulations and the increasing demand for resource recycling, how to improve the salt separation efficiency and resource utilization rate of waste salt treatment has become a technical bottleneck that needs to be overcome. Existing waste salt treatment processes mainly include precipitation, membrane separation and crystallization. Among them, low-temperature evaporation technology and membrane separation technology are more common treatment methods, but they still face some technical difficulties in practical applications and fail to meet the needs of high efficiency, low energy consumption and high recovery rate.

[0003] In waste salt treatment, precipitation is widely used to remove harmful components in brine, such as fluoride. The traditional separate design of defluorination sedimentation tanks and high-efficiency sedimentation tanks, although it can effectively remove fluoride and other impurities by adding reagents such as calcium chloride and calcium hydrogen phosphate, still has problems such as low reaction efficiency and unstable precipitation effect. Especially in the treatment of waste salt with high salt concentration, the precipitation reaction rate is slow, and the precipitation effect is greatly affected by the amount of reagents added and the reaction time, resulting in low efficiency of the sedimentation tank. In addition, the existing precipitation process fails to effectively solve the separation and recovery of various salts in complex salt systems, resulting in low resource recovery rate and increased cost of waste salt treatment.

[0004] At the same time, membrane separation technology, especially nanofiltration and reverse osmosis technology, has been widely used in the field of wastewater treatment in recent years. However, these technologies still have many problems in the treatment of waste salt. When treating high-concentration waste salt, membrane pollution and clogging problems are more serious, resulting in a shortened membrane service life and increased treatment costs. Especially in complex salt systems, the membrane has limited separation effect on certain salts and cannot achieve the ideal salt separation effect. Although membrane separation technology can improve separation accuracy, the separation efficiency of the membrane has not been effectively improved in the treatment of high salt concentrations and multi-salt solutions. Therefore, the popularization of membrane technology in large-scale industrial applications still faces challenges such as low energy efficiency, high operating pressure, and frequent maintenance.

[0005] In addition, although evaporation and crystallization technology has been applied in some traditional waste salt treatment processes, the existing evaporation and crystallization technology has significant bottlenecks in energy efficiency. Traditional evaporation technology usually adopts single-effect evaporation or multi-effect evaporation, but due to the insufficient evaporation effect, it leads to high energy consumption and large energy loss in the treatment process. Especially in the treatment of waste salt with high salt concentration, the evaporation rate is low, and there is a large energy loss in the recovery and utilization of steam. The existing technology has failed to effectively improve the thermal efficiency of the evaporation and crystallization process, and cannot improve the salt recovery efficiency while reducing energy consumption, which limits its application effect in waste salt treatment.

[0006] Although the existing waste salt treatment processes have achieved certain results in certain specific occasions, they still have many shortcomings and fail to meet the technical requirements of high efficiency, low energy consumption and high recovery rate. First, although the existing sedimentation tanks and reagent addition processes have certain effects in removing certain impurities, the low reaction efficiency, long reaction time and unstable precipitation effect have greatly restricted the economy and environmental friendliness of the treatment process. Especially in the separation process of complex salts, the existing process has failed to effectively improve the separation efficiency of different salts, resulting in low resource recovery rate and increased cost of waste salt treatment. Secondly, membrane separation technology, especially reverse osmosis technology, faces the problems of membrane pollution and blockage when treating high-concentration waste salts, which not only affects the separation effect, but also greatly increases the replacement frequency and operating cost of the membrane. In addition, when treating complex salt solutions, the existing membrane separation technology often has low separation efficiency for certain salts and cannot meet the needs of efficient separation. Although the introduction of membrane technology has improved the separation accuracy, the application effect in high salt concentration and complex waste salt systems is not ideal, and the ideal salt separation and recovery goals have not been achieved.

[0007] Finally, the existing evaporation crystallization technology has obvious bottlenecks in energy efficiency. The traditional evaporation crystallization process has a large waste in energy efficiency, especially in the multi-effect evaporation process, the evaporation effect is often difficult to fully exert, resulting in excessive energy consumption. In addition, the existing low-temperature evaporation technology still has problems with low evaporation rate and large energy loss when processing complex brine solutions, and fails to improve salt recovery efficiency while reducing energy consumption.

[0008] In summary, although the existing waste salt treatment technology has made certain progress in some areas, it still has major deficiencies in terms of treatment efficiency, salt separation effect, energy efficiency and cost control. In order to effectively solve these problems, a new technical solution is urgently needed to overcome the bottleneck of the existing technology and improve the salt separation efficiency and resource recovery rate in the waste salt treatment process. In particular, technical improvements are needed in terms of the reaction efficiency and stability of the sedimentation tank, membrane pollution and energy efficiency of the membrane separation technology, and energy loss of the evaporation crystallization technology to meet the growing demand for waste salt resource treatment. Therefore, how to improve the overall efficiency of the waste salt treatment process and reduce energy consumption based on the optimization of the sedimentation tank and the reagent addition process, combined with the low-temperature multi-effect evaporation technology, has become an important issue that needs to be solved urgently in current technology. Summary of the invention

[0009] In order to achieve the above-mentioned purpose of the invention and to solve the above-mentioned technical problems, the present invention provides a process for treating salt separation of impurities based on low-temperature multi-effect evaporation, comprising the following steps:

[0010] S1 brine dissolution and pretreatment:

[0011] Adding the impurities into the dissolution regulating tank, stirring and regulating to obtain brine, and the brine enters the defluorination tank, sedimentation tank and multi-media filter for treatment to obtain pre-treated brine;

[0012] S2 catalytic oxidation and separation:

[0013] The pretreated brine enters the catalytic oxidation and nanofiltration device to remove total organic carbon and some dissolved substances, and separates the monovalent salt solution and the divalent salt solution through the nanofiltration device;

[0014] S3 reverse osmosis concentration and crystallization:

[0015] The monovalent salt solution is concentrated by a reverse osmosis device and then enters a low-temperature multiple-effect evaporation system, and then enters a sodium chloride crystallization device for sodium chloride crystallization.

[0016] The divalent salt solution passes through a secondary catalytic oxidation device and enters a low-temperature multiple-effect evaporation system, and then enters a sodium sulfate crystallization device for sodium sulfate crystallization;

[0017] S4 mother liquor reflux and treatment:

[0018] The mother liquor from the low-temperature multiple-effect evaporation device is refluxed to the low-temperature multiple-effect evaporation system for further concentration and treatment.

[0019] Preferably, the raw materials added to the defluorination sedimentation tank are 24-45ppm calcium chloride, 20-37.5ppm calcium hydrogen phosphate, 28-52.5ppm nano alumina, and 8-15ppm sodium lignin sulfonate, and the raw materials added to the high-efficiency sedimentation tank are 8-15ppm sodium polyacrylate, 2.4-4.5g / L diatomaceous earth, and 12-22.5ppm chlorine dioxide.

[0020] Preferably, the process for adding the raw materials is as follows: calcium chloride and calcium hydrogen phosphate are initially added, nano-alumina and sodium lignin sulfonate are added after the reaction, the initial stirring speed is 40-80 rpm, sodium polyacrylate and diatomaceous earth are added later, chlorine dioxide is added after the reaction, and the stirring speed is 20-40 rpm later.

[0021] Preferably, the pH value in step S1 is adjusted to 6.5-7.5.

[0022] Preferably, the reaction temperature in step S1 is 25-45°C.

[0023] Preferably, the concentration of the brine in step S1 is 80-150 g / L.

[0024] Preferably, the reaction process of the low-temperature multiple-effect evaporation system is:

[0025] First effect: The sodium chloride solution is heated to about 70-80°C through a heat exchanger to remove part of the water by evaporation;

[0026] Second effect: Use the steam generated by the first effect to further heat the solution through a heat exchanger to evaporate the water at a temperature of 50-60°C, further reducing the water content of the solution;

[0027] Third effect: In the third effect, the steam generated by the second effect is continued to be heated and evaporated through the exchanger at a temperature of 40-50°C to further remove moisture.

[0028] Preferably, in step S3, the nucleation rate of sodium chloride is controlled at 0.2-0.6 mm / h, and the supersaturation is controlled at 1.04-1.08; the nucleation rate of sodium sulfate is controlled at 0.04-0.06 mm / h, and the supersaturation is controlled at 1.06-1.10.

[0029] Preferably, the operating pressure of the multi-media filter is 0.2-0.5 MPa, and the filtration flow rate is 10-15 m / h.

[0030] Preferably, the membrane flux of the nanofiltration device is 10-13 L / m 2 , the water inlet pressure is 5-15MPa.

[0031] The technical solution provided by the present invention brings beneficial effects:

[0032] Improve waste salt treatment efficiency: The present invention significantly improves the reaction efficiency and treatment stability of the sedimentation tank by adopting an optimized sedimentation tank reagent addition process, can effectively remove impurities, fluoride ions and some dissolved substances in the brine, and enhances the overall treatment capacity.

[0033] Reducing membrane fouling and improving membrane separation efficiency: By removing organic matter in the catalytic oxidation stage and using a nanofiltration device to separate monovalent salt solution and divalent salt solution, the present invention effectively reduces the membrane fouling problem, ensures efficient operation of the membrane, avoids frequent cleaning caused by membrane fouling, and improves the service life and separation efficiency of the membrane.

[0034] Optimize energy efficiency and reduce energy loss: The present invention adopts low-temperature multi-effect evaporation technology, in which the cascade utilization of the three-effect evaporation process maximizes the recovery of steam heat in the evaporation process, thereby significantly improving energy utilization efficiency and reducing energy loss in the evaporation process. Compared with the existing technology, the present invention can complete the concentration and crystallization of brine at a lower operating temperature, further reducing energy consumption.

[0035] Improve resource recovery rate and salt separation effect: In the low-temperature multi-effect evaporation system, the present invention can effectively improve the crystallization efficiency of sodium chloride and sodium sulfate and achieve a higher resource recovery rate through reasonable nucleation rate control and supersaturation adjustment. Compared with the prior art, the crystallization process of the present invention can operate stably within a wider operating range, ensuring efficient recovery of salt.

[0036] Waste liquid reflux and reprocessing to reduce waste: The present invention further reduces waste liquid discharge and realizes the reprocessing of mother liquid through the mother liquid reflux mechanism in the low-temperature multi-effect evaporation system, which not only improves the resource utilization of waste salt treatment, but also reduces the burden on the environment. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. Of course, the specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.

[0038] Example 1

[0039] A process for treating impure salt by low-temperature multi-effect evaporation, the implementation steps are as follows:

[0040] S1 brine dissolution and pretreatment:

[0041] Dissolution adjustment tank: dissolve the miscellaneous salt in 15m 3 In water, by stirring and adjusting, a saline solution is obtained, and the concentration is controlled at 120g / L.

[0042] Dissolution adjustment post-processing:

[0043] Fluoride removal sedimentation tank:

[0044] Add 30ppm calcium chloride, 25ppm calcium hydrogen phosphate, 35ppm nano alumina, 10ppm sodium lignin sulfonate, 10ppm sodium polyacrylate, 3g / L diatomaceous earth, and 15ppm chlorine dioxide.

[0045] Reaction time control: add calcium chloride and calcium hydrogen phosphate in the initial stage, and react for 5 minutes; add nano-alumina and sodium lignin sulfonate in the later stage, and react for 10 minutes; finally add sodium polyacrylate and diatomaceous earth, and react for 10 minutes, and add chlorine dioxide and react for 15 minutes.

[0046] Stirring speed: 60 rpm at the beginning and 30 rpm at the end.

[0047] pH adjustment: The pH value in step S1 was adjusted to 6.8 to ensure the best effect of the dissolution process.

[0048] Reaction temperature control: The reaction temperature in step S1 was controlled at 35°C to ensure the stability of the reaction.

[0049] Filter by:

[0050] Multi-media filter operating pressure: 0.4MPa.

[0051] Filtration flow rate: 12m / h, pretreated brine is obtained after filtration.

[0052] S2 catalytic oxidation and separation:

[0053] Catalytic oxidation: The pretreated brine is fed into a catalytic oxidation system to remove total organic carbon (TOC) and dissolved matter.

[0054] Nanofiltration device:

[0055] Membrane flux: 12L / m 2 .

[0056] Water inlet pressure: 10MPa.

[0057] Separate the monovalent salt solution and the divalent salt solution.

[0058] S3 reverse osmosis concentration and crystallization:

[0059] Monovalent salt solution: enters the reverse osmosis device, is concentrated and then sent to the low-temperature multiple-effect evaporation system, and then enters the sodium chloride crystallization device.

[0060] The nucleation rate of sodium chloride is 0.4 mm / h and the supersaturation is 1.06.

[0061] Divalent salt solution: After entering the secondary catalytic oxidation device, it enters the low-temperature multiple-effect evaporation system and then enters the sodium sulfate crystallization device.

[0062] The sodium sulfate nucleation rate was 0.05 mm / h, and the supersaturation was controlled at 1.08.

[0063] S4 mother liquor reflux and treatment:

[0064] Mother liquor reflux: The mother liquor from the low-temperature multiple-effect evaporator is refluxed to the evaporation system for further concentration.

[0065] The reaction process is carried out through a triple-effect evaporator:

[0066] First effect: heating to 75℃, evaporation and removal of part of the water.

[0067] Second effect: Use the steam generated by the first effect to heat to 55°C and continue to evaporate water.

[0068] Third effect: Utilize the second effect steam to heat to 45°C to further remove moisture.

[0069] Test results

[0070] The defluorination sedimentation tank system undergoes a comprehensive reaction, and the overall reaction time is 40 minutes.

[0071] Sedimentation tank sediment: fluoride ion removal rate increased to 99%, fluorine content in the sediment decreased to 0.01%, and the effect of impurity removal increased to 90%.

[0072] The water pressure entering the reverse osmosis device dropped to 10MPa, the load on the reverse osmosis system was significantly reduced, and the water quality became more stable.

[0073] Daily energy consumption is reduced to 170kWh, saving the use of chemicals and reducing unnecessary heating, stirring and other processing processes.

[0074] Water quality, TOC concentration dropped to 85ppm, and suspended matter concentration dropped to 25mg / L.

[0075] The D50 of sodium chloride crystals is 50-100 μm, and the sodium chloride crystallization efficiency reaches 93%;

[0076] The D50 of sodium sulfate crystals is 30-60 μm, which is small and uniform.

[0077] The sodium sulfate crystallization efficiency reaches more than 87%, and this efficiency will be improved due to the optimized control of low-temperature multi-effect evaporation.

[0078] Example 2

[0079] According to the same process as in Example 1, the difference is that

[0080] S1 brine dissolution and pretreatment:

[0081] Dissolution and adjustment tank: dissolve the miscellaneous salts in 15m3 of water, and obtain a brine solution through stirring and adjustment, with the concentration controlled at 180g / L.

[0082] Dissolution adjustment post-processing:

[0083] Fluoride removal sedimentation tank:

[0084] Add 45ppm calcium chloride, 37.5ppm calcium hydrogen phosphate, 52.5ppm nano alumina, 15ppm sodium lignin sulfonate, 15ppm sodium polyacrylate, 4.5g / L diatomaceous earth, and 22.5ppm chlorine dioxide.

[0085] Test results

[0086] Sedimentation tank sediment: The fluoride ion removal rate increased to 98%, the fluorine content in the sediment decreased to 0.03%, and the impurity removal effect increased to 87%.

[0087] The water pressure entering the reverse osmosis device dropped to 10.5MPa, the load on the reverse osmosis system was significantly reduced, and the water quality was more stable. The daily energy consumption was reduced to 180kWh, saving the amount of chemicals and reducing unnecessary heating, stirring and other treatment processes.

[0088] Water quality, TOC concentration dropped to 95ppm, and suspended matter concentration dropped to 30mg / L.

[0089] Example 3

[0090] According to the same process as in Example 1, the difference is that

[0091] S1 brine dissolution and pretreatment:

[0092] Dissolution and adjustment tank: dissolve the miscellaneous salts in 15m3 of water, and obtain a brine solution through stirring and adjustment, with the concentration controlled at 80g / L.

[0093] Dissolution adjustment post-processing:

[0094] Fluoride removal sedimentation tank:

[0095] Add 24ppm calcium chloride, 20ppm calcium hydrogen phosphate, 28ppm nano alumina, 8ppm sodium lignin sulfonate, 8ppm sodium polyacrylate, 2.4g / L diatomaceous earth, and 12ppm chlorine dioxide.

[0096] Test results

[0097] Sedimentation tank sediment: The fluoride ion removal rate is increased to 98%, the fluorine content in the sediment is reduced to 0.02%, and the impurity removal effect is increased to 85%.

[0098] The water pressure entering the reverse osmosis device dropped to 10MPa, the load on the reverse osmosis system was significantly reduced, and the water quality became more stable.

[0099] Daily energy consumption is reduced to 175kWh, saving the use of chemicals and reducing unnecessary heating, stirring and other processing processes.

[0100] Water quality, TOC concentration dropped to 90ppm, and suspended matter concentration dropped to 30mg / L.

[0101] Comparative Example 1

[0102] According to the same process as in Example 1, the difference is that

[0103] Add the impure salts in a quantitative manner into the dissolution adjustment tank for dissolution, and prepare the brine with a concentration of 120g / L through stirring, adjustment, and pretreatment of the brine in turn through a defluorination sedimentation tank, a high-efficiency sedimentation tank and a multi-media filter. Add 30ppm of defluorination agent calcium chloride to the defluorination sedimentation tank, and add 30ppm of coagulant polyaluminum chloride, 1ppm of coagulant anionic polyacrylamide, and 10ppm of bactericide chlorine dioxide to the high-efficiency sedimentation tank. Contact time: Total time: The total reaction time of the defluorination tank + the high-efficiency sedimentation tank is 60 minutes.

[0104] Fluoride removal pool sediment: The fluoride ion removal effect is 80%, and the fluoride content in the sediment is 0.5%.

[0105] Sediment from high-efficiency sedimentation tank: impurity removal rate is 85%, and the sediment still contains impurities that have not been completely removed.

[0106] Since there are still high levels of impurities and unprocessed ions in the water, the water pressure entering the reverse osmosis device is 12MPa, and a higher pressure is required for filtration and separation.

[0107] Daily energy consumption: The total energy consumption of equipment and chemicals is 200kWh, including energy consumption such as adding chemicals, heating the reaction tank, stirring, etc.

[0108] The total organic carbon (TOC) in the water is 150ppm and the suspended matter concentration is 50mg / L

[0109] Comparative Example 2

[0110] According to the same process as in Example 1, the difference is that

[0111] Add the impure salts in a quantitative manner into the dissolution adjustment tank for dissolution, and prepare the brine with a concentration of 120g / L through stirring, adjustment, and pretreatment of the brine in turn through the defluorination sedimentation tank and the multi-media filter. Add 30ppm defluorination agent calcium chloride to the defluorination sedimentation tank, add 30ppm coagulant polyaluminum chloride, 1ppm coagulant aid anionic polyacrylamide, and 10ppm bactericide chlorine dioxide to the high-efficiency sedimentation tank. The contact time: total time: the reaction time in the defluorination tank is 90 minutes.

[0112] Fluoride removal pool sediment: The fluoride ion removal effect is 72%, and the fluoride content in the sediment is 0.8%.

[0113] Sedimentation in high-efficiency sedimentation tank: The impurity removal rate is 70%, and there are still more suspended matter and dissolved ions in the water.

[0114] Since there are still high levels of impurities and unprocessed ions in the water, the water pressure entering the reverse osmosis device is 13MPa, and a higher pressure is required for filtration and separation.

[0115] Daily energy consumption: The total energy consumption of equipment and chemicals is 220kWh, including energy consumption such as adding chemicals, heating the reaction tank, stirring, etc.

[0116] The total organic carbon (TOC) in the water is 200ppm and the suspended matter concentration is 70mg / L

[0117] Comparative Example 3

[0118] According to the same process as in Example 1, the difference is that

[0119] S3 reverse osmosis concentration and crystallization:

[0120] Monovalent salt solution: enters the reverse osmosis device, and after concentration, is sent to the sodium chloride crystallization device.

[0121] Sodium chloride nucleation rate: controlled at 0.6 mm / h, supersaturation controlled at 1.10.

[0122] Divalent salt solution: After entering the secondary catalytic oxidation device, it enters the sodium sulfate crystallization device.

[0123] Sodium sulfate nucleation rate: controlled at 0.08 mm / h, supersaturation controlled at 1.12.

[0124] Daily energy consumption: The total energy consumption of equipment and chemicals is 210kWh.

[0125] Sodium chloride crystals:

[0126] D50>150μm, crystallization efficiency is 70%.

[0127] Low-temperature multi-effect evaporation technology can control smaller and more uniform crystals, and its crystallization efficiency is higher than that of traditional methods.

[0128] Sodium sulfate crystals:

[0129] D50>100μm, crystallization efficiency is 60%.

[0130] 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 principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A process for treating salt separation of impurities based on low-temperature multi-effect evaporation, characterized in that: The following steps are involved: S1 brine dissolution and pretreatment: Adding the impurities into the dissolution regulating tank, stirring and regulating to obtain brine, and the brine enters the defluorination tank, sedimentation tank and multi-media filter for treatment to obtain pre-treated brine; S2 catalytic oxidation and separation: The pretreated brine enters the catalytic oxidation and nanofiltration device to remove total organic carbon and some dissolved substances, and separates the monovalent salt solution and the divalent salt solution through the nanofiltration device; S3 reverse osmosis concentration and crystallization: The monovalent salt solution is concentrated by a reverse osmosis device and then enters a low-temperature multiple-effect evaporation system, and then enters a sodium chloride crystallization device for sodium chloride crystallization. The divalent salt solution passes through a secondary catalytic oxidation device and enters a low-temperature multiple-effect evaporation system, and then enters a sodium sulfate crystallization device for sodium sulfate crystallization; S4 mother liquor reflux and treatment: The mother liquor from the low-temperature multiple-effect evaporation device is refluxed to the low-temperature multiple-effect evaporation system for further concentration and treatment.

2. The process according to claim 1, characterized in that: The raw materials added to the defluorination sedimentation tank are 24-45ppm calcium chloride, 20-37.5ppm calcium hydrogen phosphate, 28-52.5ppm nano alumina, and 8-15ppm sodium lignin sulfonate. The raw materials added to the high-efficiency sedimentation tank are 8-15ppm sodium polyacrylate, 2.4-4.5g / L diatomaceous earth, and 12-22.5ppm chlorine dioxide.

3. The process according to claim 2, characterized in that: The process of adding the raw materials is as follows: calcium chloride and calcium hydrogen phosphate are added initially, nano-alumina and sodium lignin sulfonate are added after the reaction, the initial stirring speed is 40-80 rpm, sodium polyacrylate and diatomaceous earth are added later, chlorine dioxide is added after the reaction, and the stirring speed is 20-40 rpm later.

4. The process according to claim 1, characterized in that: The pH value in step S1 is adjusted to 6.5-7.

5.

5. The process according to claim 1, characterized in that: The reaction temperature in step S1 is 25-45°C.

6. The process according to claim 1, characterized in that: The concentration of the brine in step S1 is 80-180 g / L.

7. The process according to claim 1, characterized in that: The reaction process of the low-temperature multiple-effect evaporation system is as follows: First effect: The sodium chloride solution is heated to about 70-80°C through a heat exchanger to remove part of the water by evaporation; Second effect: Use the steam generated by the first effect to further heat the solution through a heat exchanger to evaporate the water at a temperature of 50-60°C, further reducing the water content of the solution; Third effect: In the third effect, the steam generated by the second effect is continued to be heated and evaporated through the exchanger at a temperature of 40-50°C to further remove moisture.

8. The process according to claim 1, characterized in that: The operating pressure of the multi-media filter is 0.2-0.5 MPa, and the filtration flow rate is 10-15 m / h.

9. The process according to claim 1, characterized in that: The membrane flux of the nanofiltration device is 10-13 L / m 2 , the water inlet pressure is 5–15MPa.