Environment-friendly treatment method for recovering industrial salt through crystallization of chemical salt-containing wastewater

Abstract

This invention discloses an environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater through crystallization, relating to the field of industrial wastewater treatment and resource utilization technology. The invention includes the following steps: Step 1, gradient pre-softening treatment: Sodium hydroxide and sodium carbonate are added sequentially to the saline wastewater to precipitate calcium and magnesium ions, and sodium sulfide is added to precipitate heavy metal ions. The wastewater is then filtered through a tubular microfiltration membrane, resulting in an effluent with a total hardness ≤20 mg / L; Step 2, selective adsorption and membrane separation of organic matter: The effluent from Step 1 undergoes COD reduction through activated carbon fiber adsorption, and then enters a nanofiltration membrane system, enriching sodium chloride in the permeate and sodium sulfate in the concentrate; Step 3, multi-temperature zone coupled crystallization and salt separation: After the nanofiltration permeate is concentrated by MVR evaporation, sodium chloride crystallization is carried out in an Oslo crystallizer at 65-75°C. During crystallization, sodium chloride seeds are added and the solution is washed using a washing column; Step 4, seed-induced circulating mother liquor treatment; Step 5, waste heat recovery and tail gas purification.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment and resource utilization technology, specifically an environmentally friendly treatment method for crystallizing and recovering industrial salt from chemical saline wastewater. Background Technology

[0002] Chemical production processes generate large amounts of saline wastewater, such as those in pesticide, dye, fine chemical, coal chemical, and chlor-alkali chemical industries. This type of wastewater typically contains high concentrations of inorganic salts like sodium chloride and sodium sulfate, along with small amounts of organic matter, hardness ions, and heavy metal ions. Direct discharge not only causes severe soil salinization and water pollution but also wastes valuable salt resources. With increasingly stringent environmental regulations and the implementation of "zero discharge" policies, the advanced treatment and resource recovery of saline wastewater have become critical issues that the industry urgently needs to address.

[0003] Currently, the mainstream technologies for treating saline wastewater in industry include membrane separation, evaporation crystallization, and their combined processes. Among these, multi-effect evaporation (MED) and mechanical vapor recompression (MVR) evaporation crystallization are the most widely used technologies. However, existing technologies still have the following significant shortcomings:

[0004] Low crystallization purity: During the conventional evaporation and crystallization process, organic matter and impurity ions such as calcium, magnesium and sulfate in the wastewater will precipitate out along with the salt, resulting in the purity of the recovered industrial salt usually being less than 90%, which is difficult to meet the standard of "Industrial Salt" (GB / T 5462-2015) and cannot be directly reused in industrial production.

[0005] Scaling and corrosion are prominent issues: calcium and magnesium ions easily form insoluble scale such as calcium sulfate and calcium carbonate during the evaporation and concentration process, which adhere to the surface of the heat exchange tubes, resulting in decreased heat transfer efficiency, increased energy consumption, and the need for frequent equipment shutdowns for cleaning. In severe cases, it can cause corrosion and perforation.

[0006] High cost of treating mixed salts: Conventional processes cannot effectively separate salts from wastewater containing multiple salts, resulting in mixed salts that are classified as hazardous waste. The disposal cost is as high as 3,000-5,000 yuan / ton, which greatly increases the burden on enterprises.

[0007] High energy consumption: Traditional single-effect evaporation consumes about 0.6-0.8 tons of standard coal per ton of water. Even with multi-effect evaporation, there is still considerable room for energy saving.

[0008] The cumulative effect of mother liquor circulation: After repeated circulation of mother liquor, impurities accumulate continuously, leading to a decrease in crystallization rate and deterioration of salt quality. It is necessary to periodically discharge high-concentration mother liquor, causing secondary pollution.

[0009] To address the aforementioned issues, existing research has attempted to employ combined processes such as pretreatment softening, multi-effect evaporation, thermal crystallization or nanofiltration separation, and evaporation crystallization. However, these methods still suffer from drawbacks such as high investment costs, unstable operation, and limited separation efficiency. Therefore, developing a method for treating chemical wastewater with low energy consumption, high salt purity, effective separation of sodium chloride and sodium sulfate, and strong anti-scaling capabilities has significant industrial application value.

[0010] The information disclosed above in this background section is only for enhancing the understanding of the background section of this invention, and therefore may include prior art that is not known to those skilled in the art. Summary of the Invention

[0011] The purpose of this invention is to provide an environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization, in order to solve the problems in the prior art.

[0012] To achieve the above objectives, the present invention provides the following technical solution: an environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization, comprising the following steps:

[0013] Step 1, Gradient pre-softening treatment: Sodium hydroxide and sodium carbonate are added sequentially to the saline wastewater to precipitate calcium and magnesium ions, and sodium sulfide is added to precipitate heavy metal ions. Then, the wastewater is filtered through a tubular microfiltration membrane. The total hardness of the effluent is ≤20mg / L.

[0014] Step 2, Selective adsorption and membrane separation of organic matter: The effluent from Step 1 is adsorbed by activated carbon fiber to reduce COD, and then enters the nanofiltration membrane system to enrich sodium chloride in the permeate and sodium sulfate in the concentrate.

[0015] Step 3, Multi-temperature zone coupled crystallization and salt separation: After the nanofiltration permeate is concentrated by MVR evaporation, sodium chloride crystallization is carried out in an Oslo crystallizer at 65-75℃. Sodium chloride seed crystals are added during the crystallization process and the solution is washed with a washing column. The nanofiltration concentrate is passed through a freeze crystallizer at 0-5℃ to precipitate sodium sulfate decahydrate, which is then melted and evaporated to obtain anhydrous sodium sulfate.

[0016] Step 4, Seed-induced circulating mother liquor treatment: The mother liquor generated in step 3 is returned to the front end of the nanofiltration system in step 2 for circulation treatment. When the total concentration of impurity ions in the mother liquor exceeds 5 g / L, the mother liquor is switched to spray drying to obtain impurity salt powder.

[0017] Step 5, Waste heat recovery and exhaust gas purification: The secondary steam condensate from Step 3 is preheated, and the nanofiltration inlet water from Step 2 is used to spray-dry the exhaust gas before it is discharged after passing through a water film dust collector.

[0018] Preferably, in step 1, sodium hydroxide is used to adjust the pH to 10.5-11.5, the amount of sodium carbonate added is 1.2-1.5 times the total molar amount of calcium and magnesium ions in the wastewater, and the amount of sodium sulfide added is 1.1-1.3 times the total molar amount of heavy metal ions; the tubular microfiltration membrane has a pore size of 0.05-0.1 μm and an operating pressure of 0.1-0.3 MPa.

[0019] Preferably, in step 2, the empty column flow rate of the activated carbon fiber adsorption tower is 4-8 m / h, the residence time is 15-25 minutes, the molecular weight cutoff of the nanofiltration membrane is 150-300 Da, the operating pressure is 0.8-1.5 MPa, and the temperature is 25-35℃.

[0020] Preferably, in step 3, the MVR evaporation temperature for sodium chloride crystallization is 90-105℃, and the final concentration of NaCl is 26%-28%; the sodium chloride seed crystals in the Oslo crystallizer have a particle size of 50-100μm, and the addition amount is 0.5%-1.5% (mass-volume ratio) of the effective volume of the crystallizer; the crystallization temperature is 65-75℃, the stirring speed is 5-15r / min, and the crystallization time is 2-3 hours; the saturated brine concentration used in the elution column is 22%-24% (mass fraction), and the flow rate is 0.5-1.0m / s.

[0021] Preferably, in step 3, the temperature of the freeze crystallizer for sodium sulfate crystallization is 0-5℃, the residence time is 2-4 hours, the particle size of the added anhydrous sodium sulfate seed crystals is 80-150μm, and the addition amount is 0.3%-1.0%; the melting temperature of sodium sulfate decahydrate is 32-35℃; the forced circulation evaporation crystallization temperature is 50-60℃, and the supersaturation is controlled at 1.05-1.15.

[0022] Preferably, in step 4, the volume ratio of mother liquor returned to the nanofiltration system is 3:1-5:1; the inlet temperature of the spray dryer is 200-250℃, the outlet temperature is 80-100℃, and the moisture content of the resulting mixed salt powder is ≤3%.

[0023] Preferably, in step 5, the condensate from the secondary steam is preheated and then cooled to 35-40°C, and reused to replenish the circulating cooling water; the circulating water from the water film dust collector is returned to step 1 for softening treatment.

[0024] Preferably, in step 3, after sodium chloride crystallization, the centrifugation speed is 1200-1500 r / min, the dehydration time is 5-8 minutes, the hot air drying temperature is 110-130℃, and the moisture content after drying is ≤0.5%.

[0025] Preferably, in step 3, the drying temperature after anhydrous sodium sulfate crystallization is 105-120℃, and the purity of the dried product is ≥97.5% and the moisture content is ≤0.5%.

[0026] Preferably, the total dissolved solids content in the chemical saline wastewater is 30-200 g / L, wherein the mass ratio of sodium chloride to sodium sulfate is 0.2:1 to 5:1, the COD is 500-5000 mg / L, and the total hardness (calculated as CaCO3) is 50-2000 mg / L.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. This invention efficiently separates monovalent and divalent ions using nanofiltration membranes, combined with multi-temperature zone coupled crystallization. Sodium chloride is thermally crystallized at 65-75℃ and sodium sulfate is cold-crystallized at 0-5℃. With the addition of seed induction and washing with a washing column, the purity of sodium chloride products is ≥98.5% and sodium sulfate products are ≥97.5%, fully meeting the standards for industrial salt. They can be directly recycled in industries such as chlor-alkali, soda ash, and dyeing, realizing the resource utilization of waste.

[0029] 2. This invention reduces the total hardness to below 20 mg / L through gradient pre-softening treatment, and, combined with precision filtration using tubular microfiltration membranes, reduces calcium and magnesium scale buildup at the source. Meanwhile, the sodium chloride crystallization stage is controlled at 65-75℃ rather than high-temperature evaporation, effectively preventing the deposition of calcium sulfate on the heat exchange surface. Comparative experiments have shown that continuous operation for 90 days requires no acid washing, extending the equipment lifespan by more than 2 times.

[0030] 3. This invention uses MVR technology to recover the latent heat of secondary steam, with an evaporation power consumption of only 45 kW·h per ton of water, which is more than 35% more energy-efficient than traditional multi-effect evaporation. At the same time, it uses the evaporation condensate to preheat the nanofiltration inlet water, further reducing the system's heat consumption. The freezing and crystallization section uses cascade precooling and a high-efficiency refrigeration unit, which reduces the unit cooling capacity consumption by 20% compared to conventional processes.

[0031] 4. This invention uses seed crystals to induce the circulation of mother liquor, controlling the concentration of impurity ions to below 5g / L, so that the mother liquor can be stably reused. Only when the accumulation of impurities exceeds the standard is a small amount of mother liquor discharged for spray drying. The final amount of impurities is only 3%-8% of the total salt content of the raw water, which is more than 90% lower than the traditional process, and significantly saves the cost of hazardous waste disposal.

[0032] 5. This invention effectively controls the supersaturation of crystallization by introducing seed crystals, avoiding crystal refinement and impurity encapsulation caused by explosive nucleation, resulting in uniform product particle size and good flowability; the Oslo crystallizer combined with the elution column design ensures continuous and stable product output.

[0033] 6. This invention is environmentally friendly throughout the entire process with no secondary pollution. The exhaust gas is discharged in compliance with standards after being treated by water film dust removal, and the dust removal wastewater is returned to the pretreatment stage. The condensate is reused in production. The miscellaneous salts are finally treated in the cement kiln to achieve harmlessness. The entire process has no waste liquid or waste gas directly discharged, meeting the zero-emission requirements of the chemical industry. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0035] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] Example 1:

[0038] An environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization includes the following steps:

[0039] Step 1: Gradient Pre-softening Process

[0040] Chemical wastewater containing saline ions first enters the primary reaction tank, where sodium hydroxide is added to adjust the pH to 10.5-11.5, and sodium carbonate is added simultaneously. The dosage is 1.2-1.5 times the total molar amount of calcium and magnesium ions in the wastewater. The reaction time is 20-40 minutes, and the reaction temperature is 30-50℃. Calcium carbonate and magnesium hydroxide are precipitated and removed. The effluent then enters the secondary reaction tank, where sodium sulfide is added. The dosage is 1.1-1.3 times the total molar amount of heavy metal ions in the wastewater. The reaction time is 15-30 minutes to remove heavy metal sulfide precipitates. Subsequently, the effluent is filtered through a tubular microfiltration membrane with a pore size of 0.05-0.1 μm and an operating pressure of 0.1-0.3 MPa. The total hardness of the effluent (calculated as CaCO3) is ≤20 mg / L, and the heavy metal ion concentration is ≤0.5 mg / L.

[0041] Step 2: Selective adsorption and membrane separation of organic matter

[0042] Step 1: The effluent enters an activated carbon fiber adsorption tower. The empty tower flow rate is 4-8 m / h, and the residence time is 15-25 minutes. This adsorbs and removes organic pollutants with a molecular weight greater than 200 Da, achieving a COD removal rate of ≥60%. After adsorption, the effluent enters a nanofiltration membrane system. The nanofiltration membrane retains molecular weights of 150-300 Da, operating at a pressure of 0.8-1.5 MPa and a temperature of 25-35℃. This removes monovalent ions (Na+). + Cl - ) and divalent ions (SO4) 2-The nanofiltration permeate is rich in sodium chloride (NaCl purity ≥ 95%), and the nanofiltration concentrate is rich in sodium sulfate (Na2SO4 purity ≥ 90%).

[0043] Step 3: Multi-temperature zone coupled crystallization and salt separation

[0044] (1) Sodium chloride crystallization section: The nanofiltration permeate enters the MVR multi-effect evaporation crystallization system at an evaporation temperature of 90-105℃. When the sodium chloride mass concentration reaches 26%-28%, it is transferred to the Oslo crystallizer. Pure sodium chloride seed crystals with a particle size of 50-100μm are added to the crystallizer in advance. The amount added is 0.5%-1.5% (mass-volume ratio) of the effective volume of the crystallizer. The crystallization temperature is controlled at 65-75℃, the stirring speed is 5-15r / min, and the crystallization time is 2-3 hours to precipitate sodium chloride crystals. A washing column is set at the top of the crystallizer to wash away trace impurities attached to the crystal surface using saturated brine. The washing solution has a salt concentration of 22%-24% (mass fraction) and a flow rate of 0.5-1.0m / s. The sodium chloride crystals are centrifuged at a speed of 1200-1500r / min for 5-8 minutes and dried with hot air at a temperature of 110-130℃. The water content is ≤0.5% to obtain industrial grade sodium chloride.

[0045] (2) Sodium sulfate crystallization section: The nanofiltration concentrate enters the cryogenic crystallization system, is pre-cooled to 20-25℃ by a plate heat exchanger, and then enters the external circulation cryogenic crystallizer. The cooling medium is a 30%-40% ethylene glycol aqueous solution. The crystallization temperature is controlled at 0-5℃ and the residence time is 2-4 hours. Anhydrous sodium sulfate seed crystals are added to the crystallizer. The seed crystals have a particle size of 80-150μm and the addition amount is 0.3%-1.0%. The precipitated sodium sulfate decahydrate crystals are separated by centrifugation and then enter a melting tank to be heated and melted at 32-35℃. Then, they are sent to a forced circulation evaporation crystallizer. The evaporation temperature is 50-60℃. The solution is concentrated to a sodium sulfate supersaturation of 1.05-1.15, and anhydrous sodium sulfate crystals are precipitated. After centrifugation and drying at 105-120℃, industrial-grade anhydrous sodium sulfate is obtained.

[0046] Step 4: Treatment of mother liquor for seed crystal induction circulation

[0047] In step three, the mother liquor discharged from the sodium chloride crystallization section and the sodium sulfate crystallization section are combined and then enter the mother liquor tank to control the total concentration of impurity ions in the mother liquor (expressed as K). + Mg 2+ Ca 2+If the total concentration of impurity ions in the mother liquor is ≤5g / L, the mother liquor is returned to the front end of the nanofiltration membrane system in step two at a volume ratio of 3:1-5:1 to be mixed with the effluent from step one for reprocessing. When the total concentration of impurity ions in the mother liquor exceeds 5g / L, the direction of the mother liquor is switched, and the mother liquor is introduced into the spray dryer with an inlet temperature of 200-250℃ and an outlet temperature of 80-100℃. Spray drying yields impurity salt powder (moisture content ≤3%), with the amount of impurity salt accounting for 3%-8% of the total salt content of the raw water. This powder is then used for co-processing in cement kilns or landfill disposal.

[0048] Step 5: Waste heat recovery and exhaust gas purification

[0049] In step three, the secondary steam condensate (temperature 70-85℃) generated by the MVR multi-effect evaporation system is preheated by a plate heat exchanger to the nanofiltration inlet water from step two. After preheating, the condensate is cooled to 35-40℃ and reused as makeup water for the circulating cooling water in chemical production. The exhaust gas discharged from the spray dryer is treated by a water film dust collector and discharged in compliance with standards. The dust removal wastewater is returned to step one for softening treatment.

[0050] Example 2:

[0051] Treatment target: Saline wastewater from a fine chemical enterprise, with the following water quality indicators: Total dissolved solids (TDS) 85 g / L, including NaCl 62 g / L, Na₂SO₄ 18 g / L, and Ca... 2+ 320 mg / L, Mg 2+ 180 mg / L, COD 1200 mg / L, pH 6.8. Treatment scale: 50 m³ / d.

[0052] Step 1: Wastewater is pumped into the primary reaction tank at a flow rate of 2.1 m³ / h. A 30% sodium hydroxide solution is added to adjust the pH to 11.2. Sodium carbonate (solid) is added at a rate of 1.35 times the total molar amount of calcium and magnesium ions, i.e., 4.2 kg per hour. The reaction time is 30 minutes. The effluent enters the secondary reaction tank, where sodium sulfide (60% mass fraction solution) is added at a rate of 1.2 times the total molar amount of heavy metal ions. Testing revealed that the main heavy metals were copper ions (12 mg / L) and lead ions (5 mg / L). 0.9 L of sodium sulfide solution is added per hour, and the reaction time is 20 minutes. Subsequently, the effluent enters a tubular microfiltration membrane with a membrane area of ​​100 m², a pore size of 0.05 μm, an operating pressure of 0.2 MPa, a product water turbidity ≤0.5 NTU, and an effluent total hardness (calculated as CaCO₃) of 18 mg / L. Heavy metal ions were not detected.

[0053] Step 2: The microfiltration effluent enters an activated carbon fiber adsorption tower with a packing height of 1.8m and a diameter of 0.8m. The empty tower flow rate is 6m / h, and the residence time is 18 minutes. The effluent COD is reduced to 430mg / L, with a removal rate of 64.2%. Then, it enters a nanofiltration membrane system using a DK type nanofiltration membrane with a membrane area of ​​150m², arranged in three stages in series. The operating pressure is 1.2MPa, and the temperature is 30℃. The permeate (NaCl enriched solution) has a flow rate of 1.3m³ / h, a NaCl concentration of 118g / L, and SO₄²⁻ concentration of 118g / L. 2- Concentration 2.1 g / L; concentrated solution (Na2SO4 enriched solution) flow rate 0.8 m³ / h, Na2SO4 concentration 82 g / L, NaCl concentration 9.5 g / L.

[0054] Step 3:

[0055] Sodium chloride crystallization: The nanofiltration permeate enters an MVR evaporator at 98℃ and is concentrated to a NaCl concentration of 27%. It is then fed into an Oslo crystallizer, with 60 kg of industrial sodium chloride seed crystals (60-80 μm particle size) added beforehand. The crystallization temperature is controlled at 70℃, the stirring speed at 10 r / min, and the crystallization time at 2.5 hours. A saturated brine column with a concentration of 23% and a flow rate of 0.8 m / s is used for precipitation. Dehydration is performed in a centrifuge at 1400 r / min for 6 minutes, followed by hot air drying at 120℃. Sodium chloride crystals are obtained. Sampling analysis results are as follows: NaCl purity 98.6%, whiteness 82, sulfate content 0.25%, total calcium and magnesium ion content 0.08%, and moisture content 0.3%, meeting the secondary standard for refined industrial salt in "Industrial Salt" (GB / T5462-2015).

[0056] Sodium sulfate crystallization: The nanofiltration concentrate, pre-cooled to 22°C by a plate heat exchanger, enters a cryogenic crystallizer with an effective volume of 5 m³, an ethylene glycol aqueous solution concentration of 35%, a crystallization temperature of 3°C, and a residence time of 3 hours. 30 g / L of anhydrous sodium sulfate seed crystals (100 μm particle size) are added. Sodium sulfate decahydrate is separated by centrifugation at 1200 r / min for 5 minutes, then transferred to a melting tank and heated to 34°C to melt. The melt is then pumped into a forced circulation evaporation crystallizer at 55°C and a vacuum of -0.085 MPa, concentrating to a supersaturation of 1.1, precipitating anhydrous sodium sulfate. After centrifugation and drying at 110°C, anhydrous sodium sulfate product is obtained with a purity (based on Na₂SO₄) of 97.8%, a sodium chloride content of 0.8%, and a moisture content of 0.4%, meeting the Class II Grade I standard in "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014).

[0057] Step 4: Combine approximately 0.15 m³ / h of sodium chloride crystallization mother liquor with approximately 0.1 m³ / h of sodium sulfate crystallization mother liquor, and determine the total concentration of impurity ions to be 3.8 g / L (of which K... + 2.1 g / L, Mg 2+ 1.2 g / L, Ca 2+The mother liquor (0.5 g / L) was returned to the front end of the nanofiltration system at a volume ratio of 4:1, i.e., the total volume after mixing 1.0 m³ / h of mother liquor with 2.1 m³ / h of new wastewater was 3.1 m³ / h. After 15 days of continuous operation, the total concentration of impurity ions in the mother liquor rose to 5.6 g / L. At this point, the mother liquor was switched to a spray dryer with an inlet temperature of 230℃ and an outlet temperature of 90℃, yielding approximately 45 kg / day of mixed salt powder with a moisture content of 2.6%, and the amount of mixed salt accounted for 5.3% of the total salt content of the raw water.

[0058] Step 5: The condensate from the 82℃ secondary steam generated by the MVR evaporation system is preheated to 30℃ by a plate heat exchanger and then cooled to 38℃ before being reused as makeup water for the cooling tower. The particulate matter emission concentration of the spray drying exhaust gas is ≤20mg / m³ after being treated by a water film dust collector with a circulating water volume of 2m³ / h, which meets the "Integrated Emission Standard for Air Pollutants".

[0059] Example 3: Comparative Verification

[0060] To verify the advancement of this invention, a control group was set up: saline wastewater with the same water quality as in Example 1 was treated using a conventional "chemical softening + multi-effect evaporation crystallization" process. The results are as follows:

[0061] Product purity Sodium chloride 98.6%, sodium sulfate 97.8% The purity of the mixed salt is only 54%. Salt separation effect Effective separation of sodium chloride and sodium sulfate Unable to separate salts, producing mixed salts. Salt yield 5.3% (of the total salt content of the raw water) 100% (all mixed salts) Heat exchanger tube scaling cycle No significant scaling was observed after 90 days of continuous operation. Pickling is required once every 7 days. Energy consumption per ton of water 45 kW·h (MVR power consumption) 0.45 tons of steam (approximately 48 kg of standard coal) Mother liquor disposal costs The amount of miscellaneous salts is small, and the processing cost is low. Approximately 3800 yuan / ton of miscellaneous salt

[0062] By comparing the conventional "chemical softening + multi-effect evaporation crystallization" process, the significant advantages of the method of this invention were verified. Under the same water quality conditions, this invention can achieve highly efficient separation of sodium chloride and sodium sulfate, with sodium chloride purity reaching 98.6% and sodium sulfate purity reaching 97.8%, while the conventional process can only produce mixed salts with a purity of 54%. The mixed salt yield of this invention is only 5.3%, far lower than the 100% of the conventional process, significantly reducing the cost of hazardous waste disposal. At the same time, this invention effectively inhibits scaling through gradient softening and low-temperature crystallization, and can operate continuously for 90 days without acid washing, while the conventional process requires shutdown and cleaning every 7 days. In terms of energy consumption, the MVR system of this invention consumes 45 kW·h of electricity per ton of water, which is about 35% more energy-efficient than conventional steam evaporation.

[0063] Example 4: Adaptability verification to different water qualities

[0064] This invention was used to treat saline wastewater from a coal chemical plant. The wastewater had a TDS of 120 g / L (NaCl 45 g / L, Na₂SO₄ 68 g / L), COD of 800 mg / L, and total hardness of 450 mg / L. The parameters were adjusted as follows: Step 1: pH adjusted to 11.0, sodium carbonate dosage increased by 1.4 times; Step 2: nanofiltration pressure 1.4 MPa; Step 3: sodium chloride crystallization temperature 68℃, sodium sulfate crystallization temperature 2℃. The results are as follows:

[0065] Sodium chloride purity 97.9% Sodium sulfate purity 96.8% Total salt recovery rate 89.3% Salt content 6.1%

[0066] As shown above, this further verifies the adaptability of the present invention to different water qualities. For high-sodium sulfate wastewater (Na₂SO₄ 68 g / L, NaCl 45 g / L), by appropriately adjusting the softening pH, nanofiltration pressure, and crystallization temperature, high-quality industrial salt with a sodium chloride purity of 97.9% and a sodium sulfate purity of 96.8% can still be obtained, with a total salt recovery rate of 89.3% and an impurity salt rate of only 6.1%. This indicates that the method of the present invention has good universality and operational stability for chemical saline wastewater with a wide range of salt-to-nitrate ratios.

[0067] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. An environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization, characterized in that: Includes the following steps: Step 1, Gradient pre-softening treatment: Sodium hydroxide and sodium carbonate are added sequentially to the saline wastewater to precipitate calcium and magnesium ions, and sodium sulfide is added to precipitate heavy metal ions. Then, the wastewater is filtered through a tubular microfiltration membrane. The total hardness of the effluent is ≤20mg / L. Step 2, Selective adsorption and membrane separation of organic matter: The effluent from Step 1 is adsorbed by activated carbon fiber to reduce COD, and then enters the nanofiltration membrane system to enrich sodium chloride in the permeate and sodium sulfate in the concentrate. Step 3, Multi-temperature zone coupled crystallization and salt separation: After the nanofiltration permeate is concentrated by MVR evaporation, sodium chloride crystallization is carried out in an Oslo crystallizer at 65-75℃. Sodium chloride seed crystals are added during the crystallization process and the solution is washed with a washing column. The nanofiltration concentrate is passed through a freeze crystallizer at 0-5℃ to precipitate sodium sulfate decahydrate, which is then melted and evaporated to obtain anhydrous sodium sulfate. Step 4, Seed-induced circulating mother liquor treatment: The mother liquor generated in step 3 is returned to the front end of the nanofiltration system in step 2 for circulation treatment. When the total concentration of impurity ions in the mother liquor exceeds 5 g / L, the mother liquor is switched to spray drying to obtain impurity salt powder. Step 5, Waste heat recovery and exhaust gas purification: The secondary steam condensate from Step 3 MVR system is preheated, and the nanofiltration inlet water from Step 2 is used for spray drying. The exhaust gas is then discharged after passing through a water film dust collector.

2. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 1, sodium hydroxide is used to adjust the pH to 10.5-11.5, sodium carbonate is added at a rate of 1.2-1.5 times the total molar amount of calcium and magnesium ions in the wastewater, and sodium sulfide is added at a rate of 1.1-1.3 times the total molar amount of heavy metal ions. The tubular microfiltration membrane has a pore size of 0.05-0.1 μm and an operating pressure of 0.1-0.3 MPa.

3. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 2, the empty column flow rate of the activated carbon fiber adsorption tower is 4-8 m / h, and the residence time is 15-25 minutes; the nanofiltration membrane has a molecular weight cutoff of 150-300 Da, an operating pressure of 0.8-1.5 MPa, and a temperature of 25-35℃.

4. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 3, the MVR evaporation temperature for sodium chloride crystallization is 90-105℃, and the final concentration of NaCl is 26%-28%. The sodium chloride seed crystals in the Oslo crystallizer have a particle size of 50-100μm, and the addition amount is 0.5%-1.5% of the effective volume of the crystallizer. The crystallization temperature is 65-75℃, the stirring speed is 5-15r / min, and the crystallization time is 2-3 hours. The saturated brine concentration used in the elution column is 22%-24%, and the flow rate is 0.5-1.0m / s.

5. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 3, the temperature of the freeze crystallizer for sodium sulfate crystallization is 0-5℃, the residence time is 2-4 hours, the particle size of the added anhydrous sodium sulfate seed crystals is 80-150μm, and the addition amount is 0.3%-1.0%; the melting temperature of sodium sulfate decahydrate is 32-35℃; the forced circulation evaporation crystallization temperature is 50-60℃, and the supersaturation is controlled at 1.05-1.

15.

6. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 4, the volume ratio of mother liquor returned to the nanofiltration system is 3:1-5:1; the inlet temperature of the spray dryer is 200-250℃, the outlet temperature is 80-100℃, and the moisture content of the resulting mixed salt powder is ≤3%.

7. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 5, the condensate from the secondary steam is preheated and then cooled to 35-40℃, and reused to replenish the circulating cooling water; the circulating water from the water film dust collector is returned to step 1 for softening treatment.

8. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 3, after sodium chloride crystallization, the centrifugation speed is 1200-1500 r / min, the dehydration time is 5-8 minutes, the hot air drying temperature is 110-130℃, and the moisture content after drying is ≤0.5%.

9. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: In step 3, the anhydrous sodium sulfate crystals are dried at a temperature of 105-120℃. After drying, the product has a purity of ≥97.5% and a moisture content of ≤0.5%.

10. The environmentally friendly treatment method for recovering industrial salt from chemical saline wastewater by crystallization according to claim 1, characterized in that: The total dissolved solids content in the chemical saline wastewater is 30-200 g / L, wherein the mass ratio of sodium chloride to sodium sulfate is 0.2:1 to 5:1, the COD is 500-5000 mg / L, and the total hardness is 50-2000 mg / L.

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