A system and method for treating inorganic salt-containing wastewater

Abstract

This invention discloses a treatment system and method for wastewater containing inorganic salts. The treatment system includes: a sodium chloride evaporation and crystallization unit to obtain sodium chloride crystals B and evaporation and crystallization mother liquor; a salting-out unit connected to the sodium chloride evaporation and crystallization unit to obtain sodium chloride crystals D and salting-out separation mother liquor, wherein the salting-out separation mother liquor is discharged to a potassium nitrate freeze crystallization unit; a potassium nitrate freeze crystallization unit connected to the salting-out unit to obtain potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor; and a nanofiltration unit connected to the potassium nitrate freeze crystallization unit to perform nanofiltration treatment on the effluent from the potassium nitrate freeze crystallization unit, while simultaneously discharging the nanofiltration permeate to the sodium chloride evaporation and crystallization unit.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment, specifically relating to a treatment system and method for wastewater containing inorganic salts. Background Technology

[0002] In wastewater zero-discharge or waste salt comprehensive utilization projects, high-salinity wastewater is ultimately formed. Evaporation and crystallization are necessary to crystallize the inorganic salts from the wastewater to obtain products such as sodium chloride and sodium sulfate. A portion of the evaporation and crystallization mother liquor is discharged, or the mother liquor is further evaporated and crystallized to obtain a mixed salt. Typically, the cations in the wastewater, except for sodium... + In addition, it also contains some K + Ions, typically Na + The content must be greater than K + The anions in the wastewater, except for Cl... - In addition, it also contains SO4. 2- and NO3 - Ions, these K + NO3 - SO4 2- The presence of ions can result in a final mixed salt concentration of over ten percent, which not only makes the waste unusable as a product but also requires expensive additional treatment. This severely impacts the application and promotion of zero-discharge wastewater treatment and the economic benefits of waste salt comprehensive utilization projects, making an economical and efficient treatment technology urgently needed. Summary of the Invention

[0003] In view of the defects and needs existing in the treatment of wastewater containing inorganic salts, the present invention provides a treatment system for wastewater containing inorganic salts, specifically for treating Cl-containing wastewater. - SO4 2- NO3 - Na + and K + The saline wastewater is subjected to salt recovery to prepare high-purity sodium chloride and potassium nitrate, specifically including the following treatment units:

[0004] The sodium chloride evaporation and crystallization unit is used to treat the wastewater containing inorganic salts by evaporation and crystallization to obtain sodium chloride crystals B and evaporation and crystallization mother liquor;

[0005] A salting-out unit, connected to the sodium chloride evaporation and crystallization unit, is used to treat the mother liquor of the sodium chloride evaporation and crystallization unit, add potassium chloride to the mother liquor to obtain sodium chloride crystals D and salting-out separation mother liquor, and discharge the salting-out separation mother liquor to the potassium nitrate freeze crystallization unit.

[0006] A potassium nitrate freeze crystallization unit, connected to the salting-out unit, is used to add dilution water to the effluent of the salting-out unit, the salting-out separation mother liquor, and perform freeze crystallization treatment to obtain potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor.

[0007] The nanofiltration unit is connected to the potassium nitrate freeze crystallization unit and is used to perform nanofiltration treatment on the effluent from the potassium nitrate freeze crystallization unit, while simultaneously discharging the nanofiltration permeate to the sodium chloride evaporation crystallization unit.

[0008] According to a specific embodiment of the present invention, the sodium chloride evaporation and crystallization unit is equipped with an evaporator crystallizer for evaporating and crystallizing the wastewater containing inorganic salts.

[0009] According to a specific embodiment of the present invention, the sodium chloride evaporation crystallization unit is equipped with a centrifugal separation device for separating sodium chloride crystals from the solution to obtain solid sodium chloride crystals B and the evaporation crystallization mother liquor.

[0010] According to a specific embodiment of the present invention, the salting-out unit is provided with a salting-out crystallizer for salting out the evaporated crystallization mother liquor.

[0011] According to a specific embodiment of the present invention, the salting-out unit is equipped with a crystal separation device for separating sodium chloride crystals from the solution to obtain sodium chloride crystal D and the salting-out mother liquor.

[0012] According to a specific embodiment of the present invention, the potassium nitrate freeze crystallization unit is equipped with a freeze crystallizer for freeze crystallizing the salting-out mother liquor.

[0013] According to a specific embodiment of the present invention, the potassium nitrate freeze crystallization unit is equipped with a centrifugal separation device for separating potassium nitrate crystals from the solution to obtain potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor.

[0014] According to a specific embodiment of the present invention, in the potassium nitrate freeze crystallization unit, the amount of dilution water added accounts for 5% to 400% of the volume percentage of the water output from the salting-out unit, preferably 5% to 100%, and more preferably 5% to 20%.

[0015] According to a specific embodiment of the present invention, the freezing crystallization temperature of the potassium nitrate freezing crystallization unit is -5 to 30°C, preferably 0 to 10°C.

[0016] According to a specific embodiment of the present invention, the processing system further includes a media filtration unit, which is connected to the potassium nitrate freeze crystallization unit and the nanofiltration unit, for performing media filtration treatment on the potassium nitrate freeze crystallization mother liquor effluent from the potassium nitrate freeze crystallization unit to remove suspended solids in the effluent from the potassium nitrate freeze crystallization unit, and transporting the effluent from the media filtration unit to the nanofiltration unit.

[0017] According to a specific embodiment of the present invention, the processing system further includes a silicon removal unit, which is connected to the potassium nitrate freeze crystallization unit and the media filtration unit, for treating the effluent potassium nitrate freeze crystallization mother liquor from the potassium nitrate freeze crystallization unit. By adding a silicon removal chemical agent, silicon undergoes a chemical reaction to form a precipitate and is removed. The silicon-removed effluent is then transported to the media filtration unit.

[0018] According to a specific embodiment of the present invention, the silicon removal unit is provided with a chemical silicon removal reaction precipitation device for treating the potassium nitrate freeze crystallization mother liquor to obtain supernatant and precipitated sludge, and the supernatant is transported to the media filtration unit.

[0019] According to a specific embodiment of the present invention, the silicon removal unit is equipped with a precipitated sludge dewatering device for filtering and separating the precipitated sludge. The separated dewatered sludge is transported off-site, and the sludge dewatering filtrate is returned to the inlet of the chemical silicon removal reaction precipitation device for further processing.

[0020] According to a specific embodiment of the present invention, the influent of the nanofiltration unit, in addition to coming from the media filtration unit, also receives a portion of the influent from the sodium chloride evaporation and crystallization unit, i.e., the inorganic salt-containing wastewater to be treated, in order to reduce the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of.

[0021] According to a specific embodiment of the present invention, the flow rate of the inorganic salt-containing wastewater to be treated received by the nanofiltration unit is 0.5 to 10 times the effluent flow rate of the media filtration unit.

[0022] Another object of the present invention is to provide a method for treating wastewater containing inorganic salts, specifically for treating wastewater containing Cl... - SO4 2- NO3 - Na + and K + The saline wastewater is subjected to salt recovery to prepare high-purity sodium chloride and potassium nitrate, specifically including the following steps:

[0023] The sodium chloride evaporation and crystallization step involves evaporating and crystallizing the wastewater containing inorganic salts to obtain sodium chloride crystals B and evaporation and crystallization mother liquor.

[0024] In the salting-out step, the mother liquor from the sodium chloride evaporation and crystallization step is treated by adding potassium chloride to the mother liquor to obtain sodium chloride crystals D and salting-out separation mother liquor. At the same time, the salting-out separation mother liquor is discharged to the potassium nitrate freeze crystallization step.

[0025] In the potassium nitrate freeze crystallization step, dilution water containing sodium chloride is added to the effluent of the salting-out step, the mother liquor from the salting-out separation, and then the liquid is freeze crystallized to recover potassium nitrate crystals.

[0026] The nanofiltration step treats the effluent from the potassium nitrate freeze crystallization step with nanofiltration, while the nanofiltration permeate is discharged to the sodium chloride evaporation crystallization step.

[0027] According to a specific embodiment of the present invention, in the potassium nitrate freeze crystallization step, the amount of dilution water added is 5 wt% to 20 wt% of the effluent from the salting-out step.

[0028] According to a specific embodiment of the present invention, the freezing crystallization temperature of the potassium nitrate freezing crystallization step is -5 to 60°C, preferably 0 to 20°C, and more preferably 0 to 10°C.

[0029] According to a specific embodiment of the present invention, in the potassium nitrate freeze crystallization step, a portion of the potassium nitrate freeze crystallization mother liquor is recycled to the inlet of the potassium nitrate freeze crystallization step.

[0030] According to a specific embodiment of the present invention, the processing method further includes a media filtration step, wherein the effluent potassium nitrate cryo-crystallization mother liquor from the potassium nitrate cryo-crystallization step is subjected to media filtration treatment to remove suspended solids from the potassium nitrate cryo-crystallization mother liquor, and the effluent from the media filtration step is transported to the nanofiltration step.

[0031] According to a specific embodiment of the present invention, the processing method further includes a silicon removal step, in which the effluent potassium nitrate freeze crystallization mother liquor from the potassium nitrate freeze crystallization step is treated by adding a silicon removal chemical agent to cause the silicon to react chemically and form a precipitate to remove the silicon, and the silicon-removed effluent is then transported to the media filtration step.

[0032] According to a specific embodiment of the present invention, the silicon removal unit includes a chemical silicon removal reaction precipitation device, the chemical reaction is carried out in the chemical silicon removal reaction precipitation device, and the precipitated sludge generated by the chemical reaction is dewatered, and the filtrate is returned to the inlet end of the chemical silicon removal reaction precipitation device.

[0033] According to a specific embodiment of the present invention, the influent of the nanofiltration step, in addition to coming from the media filtration step, also receives a portion of the influent from the sodium chloride evaporation and crystallization step, i.e., the inorganic salt-containing wastewater to be treated, in order to reduce the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of.

[0034] According to a specific embodiment of the present invention, the flow rate of the inorganic salt-containing wastewater to be treated received in the nanofiltration step is 0.5 to 10 times the effluent flow rate of the media filtration unit.

[0035] Beneficial effects:

[0036] The present invention provides a method for treating Cl-containing compounds - SO4 2- NO3 - Na + and K + The saline wastewater treatment system is configured with a sodium chloride evaporation and crystallization unit, a salting-out unit, a potassium nitrate freeze crystallization unit, and a nanofiltration unit. Through the design of the type and sequence of each treatment unit, high-purity sodium chloride and potassium nitrate products with high yields were obtained.

[0037] The system and method provided by this invention can recover salts from wastewater containing inorganic salts in the form of sodium chloride and potassium nitrate products, remove impurities such as silicon to prevent their accumulation, and remove SO4. 2- After ion concentration, the wastewater is discharged for separate treatment and recovery, while minimizing the discharge of other inorganic salts. This improves salt recovery efficiency, saves energy, and realizes the resource-based treatment of wastewater containing inorganic salts. There is no secondary pollution from waste salts, resulting in significant environmental and economic benefits. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the wastewater treatment system containing inorganic salts provided by the present invention.

[0039] The reference numerals in the attached figures are explained as follows:

[0040] 1. Sodium chloride evaporation and crystallization unit; 2. Evaporation and crystallization mother liquor; 3. Salting-out unit; 4. Potassium nitrate freeze crystallization unit; 5. Potassium nitrate freeze crystallization mother liquor; 6. Silicon removal unit; 7. Sedimented sludge; 8. Supernatant; 9. Sedimented sludge dewatering device; 10. Sludge dewatering filtrate; 11. Media filtration unit; 12. Nanofiltration unit; 13. Nanofiltration rinsing solution; 14. Nanofiltration permeate; 15. Nanofiltration retentate;

[0041] A. Wastewater containing inorganic salts; B. Sodium chloride crystals; C. Potassium chloride; D. Sodium chloride crystals; E. Potassium nitrate crystals; F. Dilution water; G. Chemical reagents; H. Dewatered sludge. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the invention will be thorough and complete, and the concept of the exemplary embodiments will be fully conveyed to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed descriptions will be omitted.

[0043] The first aspect of the present invention provides a treatment system for wastewater containing inorganic salts, for treating wastewater containing Cl... - SO4 2- NO3 - Na + and K + The saline wastewater is subjected to salt recovery to prepare high-purity sodium chloride and potassium nitrate, specifically including the following treatment units:

[0044] Sodium chloride evaporation and crystallization unit 1 is used to perform evaporation and crystallization treatment on the wastewater containing inorganic salts to obtain sodium chloride crystals B and evaporation and crystallization mother liquor;

[0045] Salting out unit 3 is connected to sodium chloride evaporation and crystallization unit 1 and is used to treat the mother liquor of evaporation and crystallization of sodium chloride evaporation and crystallization unit. Potassium chloride is added to the mother liquor of evaporation and crystallization to obtain sodium chloride crystals D and salting out mother liquor. The salting out mother liquor is discharged to the potassium nitrate freeze crystallization unit.

[0046] Potassium nitrate freeze crystallization unit 4 is connected to the salting-out unit 3 and is used to add dilution water to the effluent of the salting-out unit 3, which is then subjected to freeze crystallization treatment to obtain potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor.

[0047] Nanofiltration unit 12 is connected to potassium nitrate freeze crystallization unit 4 and is used to perform nanofiltration treatment on the effluent of potassium nitrate freeze crystallization unit, while discharging the nanofiltration permeate to sodium chloride evaporation crystallization unit.

[0048] This invention incorporates a salting-out unit between the sodium chloride evaporation and crystallization unit and the potassium nitrate freeze crystallization unit. By adding potassium chloride to the evaporation and crystallization mother liquor, more sodium ions can precipitate as NaCl, thus improving the purity and yield of NaCl. Furthermore, it also causes nitrates to precipitate as KNO3 during freeze crystallization, thereby reducing the NaNO3 content and improving the purity and yield of the product KNO3.

[0049] This invention, by adding dilution water to the effluent of the salting-out separation mother liquor from the salting-out unit, can prevent the crystallization of other salts besides potassium nitrate during subsequent freeze-crystallization, thus helping to improve the purity and yield of the product KNO3.

[0050] In an optional embodiment, the sodium chloride evaporation and crystallization unit is equipped with an evaporator crystallizer for evaporating and crystallizing the wastewater containing inorganic salts.

[0051] The present invention does not have any particular limitation on the sodium chloride evaporation crystallizer, for example, multi-effect evaporation or MVR evaporation crystallization known in the art can be used.

[0052] In an optional embodiment, the sodium chloride evaporation crystallization unit is equipped with a centrifugal separation device for separating sodium chloride crystals from the solution to obtain solid sodium chloride crystals B and the evaporation crystallization mother liquor.

[0053] This invention does not impose any special limitations on the centrifugal separation equipment. For example, a horizontal screw centrifuge or a pusher centrifuge and its auxiliary equipment commonly used in the field can be used, as long as the separation of sodium chloride crystals and solution is achieved.

[0054] In an optional embodiment, the amount of potassium chloride added in the salting-out unit is based on satisfying the K content in the evaporation and crystallization mother liquor. + With NO3 - The appropriate amount is added to achieve the goal of equal molar concentration.

[0055] In an optional embodiment, the salting-out unit is equipped with a salting-out crystallizer for salting out the evaporated crystallization mother liquor.

[0056] This invention does not impose any special limitations on the salting-out crystallizer; salting-out crystallizers known in the art can be used.

[0057] In an optional embodiment, the salting-out unit is equipped with a crystal separation device for separating sodium chloride crystals from the solution to obtain sodium chloride crystal D and the salting-out mother liquor.

[0058] The present invention does not impose any special limitations on the crystal separation equipment. For example, a horizontal screw centrifuge or pusher centrifuge and its auxiliary equipment commonly used in the field can be used, as long as the separation of sodium chloride crystals from the solution is achieved.

[0059] In an optional embodiment, the potassium nitrate freeze crystallization unit is equipped with a freeze crystallizer for freeze crystallizing the salting-out mother liquor.

[0060] The present invention does not have any particular limitation on the cryogenic crystallizer, and can use common crystallizers in the art such as OSLO crystallizer or DTB crystallizer.

[0061] In an optional embodiment, the potassium nitrate freeze crystallization unit is equipped with a centrifugal separation device for separating potassium nitrate crystals from the solution to obtain potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor.

[0062] The present invention does not impose any special limitations on the centrifugal separation device. For example, a horizontal screw centrifuge or a pusher centrifuge and its auxiliary equipment commonly used in the field can be used, as long as the separation of potassium nitrate crystals and solution is achieved.

[0063] In an optional embodiment, in the potassium nitrate freeze crystallization unit, the amount of dilution water added accounts for 5% to 400% of the volume percentage of the water output from the salting-out unit, preferably 5% to 100%, and more preferably 5% to 20%.

[0064] The present invention does not have any special limitations on the dilution water, as long as it contains a lower concentration of sodium chloride than the separation mother liquor of the salting-out crystallization unit and does not introduce any additional impurities. For example, the inlet water A of the evaporation crystallization unit can be used, or demineralized water can be used.

[0065] In an optional embodiment, the freezing crystallization temperature of the potassium nitrate freezing crystallization unit is -5 to 60°C, preferably 0 to 20°C, and more preferably 0 to 10°C.

[0066] In an optional embodiment, the processing system further includes a media filtration unit connected to the potassium nitrate freeze crystallization unit and the nanofiltration unit, for media filtration treatment of the potassium nitrate freeze crystallization mother liquor effluent from the potassium nitrate freeze crystallization unit, removing suspended solids from the effluent from the potassium nitrate freeze crystallization unit, and conveying the effluent from the media filtration unit to the nanofiltration unit.

[0067] In an optional embodiment, the processing system further includes a silicon removal unit connected to the potassium nitrate freeze crystallization unit and the media filtration unit. The silicon removal unit is used to treat the potassium nitrate freeze crystallization mother liquor effluent from the potassium nitrate freeze crystallization unit. By adding silicon removal chemical agents, silicon undergoes a chemical reaction to form a precipitate and is removed. The silicon-removed effluent is then transported to the media filtration unit.

[0068] This invention does not have any special limitations on the silicon removal unit. Common equipment and agents used in the field for removing impurities such as silicon and hardness can be used. For example, sodium aluminate, PAC and PAM can be used for chemical silicon removal, or magnesium silicate can be used for clarification and softening treatment. As long as these devices achieve the same treatment effect, they are acceptable.

[0069] In an optional embodiment, the silicon removal unit is equipped with a chemical silicon removal reaction precipitation device for treating the potassium nitrate freeze crystallization mother liquor to obtain supernatant and precipitated sludge, and the supernatant is transported to the media filtration unit.

[0070] In an optional embodiment, the desiliconization unit is equipped with a sludge dewatering device for filtering and separating the sludge. The separated dewatered sludge is transported off-site, and the sludge dewatering filtrate is returned to the inlet of the chemical desiliconization reaction precipitation device for further processing.

[0071] This invention does not impose any special limitations on sludge dewatering equipment; common filter presses or centrifugal separation equipment in the field can be used.

[0072] In an optional embodiment, the nanofiltration unit receives not only water from the media filtration unit but also a portion of the water from the sodium chloride evaporation and crystallization unit, i.e., the inorganic salt-containing wastewater to be treated, in order to reduce the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of.

[0073] In an optional embodiment, the flow rate of the inorganic salt-containing wastewater to be treated received by the nanofiltration unit is 0.5 to 10 times the effluent flow rate of the media filtration unit.

[0074] The following combination Figure 1 The processing system of the present invention will be explained in detail.

[0075] Wastewater A containing inorganic salts to be treated enters sodium chloride evaporation and crystallization unit 1. Sodium chloride evaporation and crystallization unit 1 is used to treat the wastewater containing inorganic salts by evaporation and crystallization, recovering sodium chloride crystals while simultaneously discharging the evaporation and crystallization mother liquor. Sodium chloride evaporation and crystallization unit 1 is equipped with an evaporator and a centrifugal separator. Steam heating is used in the evaporator to evaporate the water in the wastewater containing inorganic salts. As evaporation continues, the concentration of sodium chloride in the solution continuously increases until it reaches a supersaturated state and precipitates as crystals. The sodium chloride crystals are then separated from the solution using the centrifugal separator, yielding solid sodium chloride product B and the evaporation and crystallization mother liquor. The obtained sodium chloride has high purity (dry basis purity can reach over 99%), meeting industrial-grade standards. The evaporated gas is condensed into evaporation condensate for recycling.

[0076] As evaporation proceeds, the SO4 content in the solution decreases, except for sodium chloride, which remains saturated. 2- NO3 - and K + As the concentration of sodium chloride increases, silicon in the wastewater also gradually accumulates. To prevent these ions from crystallizing as sodium sulfate, sodium nitrate, or potassium nitrate, or from reaching a concentration high enough to affect the quality of sodium chloride products, a portion of the evaporation and crystallization mother liquor from the sodium chloride evaporation and crystallization unit needs to be discharged. Sodium chloride evaporation and crystallization mother liquor 2 is sent to salting-out unit 3.

[0077] Sodium chloride evaporation and crystallization mother liquor 2 enters salting-out unit 3. Salting-out unit 3 is connected to sodium chloride evaporation and crystallization unit 1, and is equipped with a salting-out crystallizer and crystal separation equipment for salting out the sodium chloride centrifugal separation mother liquor 2. First, potassium chloride crystals C are added to the evaporation and crystallization mother liquor 2. Since the evaporation and crystallization mother liquor is saturated with sodium chloride and unsaturated with KCl, as Cl... - With the addition of potassium chloride, the sodium chloride dissolved in the solution continues to precipitate out in crystal form, while the added potassium chloride dissolves. Then, the sodium chloride crystals are separated from the solution using a crystal separation device to obtain sodium chloride crystals D and salting-out mother liquor. The salting-out mother liquor is discharged to the potassium nitrate freeze crystallization unit 4.

[0078] The salting-out mother liquor enters the potassium nitrate freeze crystallization unit 4, which is connected to the salting-out unit 3. Unit 4 is used to dilute the discharged salting-out mother liquor from unit 3 with dilution water containing a low sodium chloride content, and then perform freeze crystallization to recover potassium nitrate crystals. Simultaneously, the potassium nitrate freeze crystallization mother liquor is discharged. While the salting-out mother liquor enters the potassium nitrate freeze crystallization unit 4, dilution water F is added to prevent the crystallization of other salts besides potassium nitrate during subsequent freeze crystallization. The amount of dilution water F added can be 5-400% of the volume of the salting-out mother liquor, preferably 5-100%, and more preferably 5-20%. The potassium nitrate freeze crystallization unit 4 includes a freeze crystallizer and a centrifugal separator. The salting-out centrifugal mother liquor is cooled by an external cooling medium. As the temperature decreases, the potassium nitrate dissolved in the water reaches a supersaturated state and precipitates out in crystal form, separating from the solution. Other ions in the solution remain in solution form and do not precipitate. After centrifugation, potassium nitrate crystals E and potassium nitrate freeze crystallization mother liquor 5 are obtained. The freezing temperature is -5 to 60°C, preferably 0 to 20°C, and more preferably 0 to 10°C. The purpose is to allow as much potassium nitrate as possible to crystallize from the solution while minimizing energy consumption, and to prevent other impurities from precipitating. The potassium nitrate freezing crystallization mother liquor 5 from the potassium nitrate freezing crystallization unit is transported to the silicon removal unit 6 for further processing.

[0079] Potassium nitrate cryogenic crystallization mother liquor 5 enters the silicon removal unit 6, which is connected to the potassium nitrate cryogenic crystallization unit 4, and is used to chemically remove silicon from the potassium nitrate cryogenic crystallization mother liquor 5. The silicon removal unit 6 includes a chemical silicon removal reaction precipitation device and a sludge dewatering device. By adding chemical agent G, impurities such as silicon are removed, thereby reducing the SiO2 concentration in the wastewater to less than 20 mg / L. Simultaneously, a hardness-removing chemical agent can be added to remove calcium, magnesium, hardness, and other impurities, as well as some organic matter, suspended solids, and colloids. This results in a hardness concentration in the wastewater of less than 150 mg / L (calculated as CaCO3) and an SDI of less than 3, ensuring that the concentration of these impurities does not affect the operation of subsequent nanofiltration, cryogenic crystallization, and evaporative crystallization units, or the quality of the products.

[0080] The supernatant 8 from the silicon removal reaction precipitation device is transported to the media filtration unit 11 for further processing. The resulting precipitated sludge 7 is filtered and separated by the sludge dewatering device 9. The separated dewatered sludge H is transported off-site, and the sludge dewatering filtrate 10 can be returned to the inlet of the silicon removal reaction precipitation device for further processing.

[0081] The media filtration unit 11 is connected to the desiliconization unit 6 and is used to perform media filtration treatment on the effluent from the desiliconization unit 6 to remove suspended solids, and then transport the effluent from the media filtration unit 11 to the nanofiltration unit 12. The media filtration unit 11 is equipped with a media filter, which performs filtration in the presence of one or more filter media to further reduce the suspended solids content in the wastewater to below 5 mg / L and the turbidity to below 5 NTU.

[0082] The present invention does not have any particular limitation on the form of the media filter. A multi-media filter commonly used in the art can be used. The multi-media filter can be vertical, single-layer horizontal or double-layer horizontal, or a filter bed.

[0083] This invention does not specifically limit the type of filter medium; one or more commonly used filter media in the art can be used, such as quartz sand, anthracite, etc. In a preferred embodiment, a dual-media filtration method is used, and the filter media can be quartz sand and anthracite. More preferably, the particle size of the filter media can be: 0.4-0.6 mm for quartz sand and 0.8-1.2 mm for anthracite. The filter layer thickness is preferably: 800 mm for quartz sand and 400 mm for anthracite, and can also be adjusted appropriately according to the water quality.

[0084] The effluent from the media filtration unit 11 enters the nanofiltration unit 12, which is connected to the media filtration unit 11. The nanofiltration unit 12 performs nanofiltration treatment on the effluent from the media filtration unit 11 and simultaneously discharges the nanofiltration permeate to the sodium chloride evaporation and crystallization unit 1. The nanofiltration unit 12 is equipped with a nanofiltration membrane assembly, which filters out most monovalent ions such as Cl. - NO3 - Na+ K + Water can pass through the nanofiltration membrane, while most organic matter and SO4 can pass through. 2- Since nanofiltration membranes cannot pass through them, nanofiltration membrane modules separate the effluent from the media filtration unit into two streams: one rich in organic matter and the other rich in SO4. 2- Nanofiltration retentate 15 and Cl-rich - NO3 - The nanofiltration permeate 14 is used as the nanofiltration retentate 15. The retentate 15 is sent externally for separate sulfate recovery, removal of other enriched impurities, and then reused. The nanofiltration permeate 14 then enters the sodium chloride evaporation and crystallization unit 1.

[0085] Because nanofiltration membranes are effective against Cl - NO3 - The rejection rate is very low, and the Cl in the nanofiltration retentate and nanofiltration permeate is low. - and NO3 - The concentration change is very small, while the Cl in the effluent of the media filtration unit 11 is very high. - NO3 - Na + and K + The concentration of SO4 is also very high, therefore, in order to reduce SO4... 2- The removal of other salts reduces the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of inorganic salt wastewater A (i.e. nanofiltration backwash liquid 13) to be treated is diverted into nanofiltration unit 12, and filtered together with the effluent from media filtration unit 11 through nanofiltration membrane. The filtered permeate then enters sodium chloride evaporation crystallization device 1.

[0086] This invention does not impose any particular limitation on the nanofiltration membrane module; nanofiltration membrane modules commonly used in the art can be employed. In a preferred embodiment, a nanofiltration membrane module suitable for organic matter and SO4 can be selected. 2- A nanofiltration membrane module with high removal efficiency. In a preferred embodiment, the influent temperature of the nanofiltration membrane can be 5–40°C, preferably 20–25°C, so as to remove SO4 from the media filtration unit 11. 2- The concentration of the nanofiltration membrane is concentrated to an ideal value without saturation, thereby reducing the amount of wastewater that needs to be discharged (i.e., nanofiltration retentate) and saving energy. The nanofiltration membrane module can be single-stage or multi-stage; in a preferred embodiment, single-stage nanofiltration is used.

[0087] This invention also provides a method for treating wastewater containing inorganic salts, specifically for treating wastewater containing Cl... - SO4 2- NO3 - Na + and K +The saline wastewater is subjected to salt recovery to prepare high-purity sodium chloride and potassium nitrate, specifically including the following steps:

[0088] The sodium chloride evaporation and crystallization step involves evaporating and crystallizing the wastewater containing inorganic salts to obtain sodium chloride crystals B and evaporation and crystallization mother liquor.

[0089] In the salting-out step, the mother liquor from the sodium chloride evaporation and crystallization step is treated by adding potassium chloride to the mother liquor to obtain sodium chloride crystals D and salting-out separation mother liquor. At the same time, the salting-out separation mother liquor is discharged to the potassium nitrate freeze crystallization step.

[0090] In the potassium nitrate freeze crystallization step, dilution water containing sodium chloride is added to the effluent of the salting-out step, the mother liquor from the salting-out separation, and then the liquid is freeze crystallized to recover potassium nitrate crystals.

[0091] The nanofiltration step treats the effluent from the potassium nitrate freeze crystallization step with nanofiltration, while the nanofiltration permeate is discharged to the sodium chloride evaporation crystallization step.

[0092] In an optional embodiment, in the potassium nitrate freeze crystallization step, the amount of dilution water added is 5 wt% to 20 wt% of the effluent from the salting-out step.

[0093] In an optional embodiment, the freezing crystallization temperature of the potassium nitrate freezing crystallization step is -5 to 60°C, preferably 0 to 20°C, and more preferably 0 to 10°C.

[0094] In an optional embodiment, the dilution water can be pure water or an aqueous solution containing a low concentration of sodium chloride; when the dilution water is an aqueous solution containing a low concentration of sodium chloride, the concentration of sodium chloride is lower than the concentration of sodium chloride in the salting-out mother liquor. Adding dilution water to the effluent of the salting-out unit prevents the crystallization of other salts besides potassium nitrate during subsequent freeze-crystallization.

[0095] In an optional embodiment, the processing method further includes a media filtration step, in which the effluent potassium nitrate cryo-crystallization mother liquor from the potassium nitrate cryo-crystallization step is subjected to media filtration to remove suspended solids from the potassium nitrate cryo-crystallization mother liquor, and the effluent from the media filtration step is then transported to the nanofiltration step.

[0096] In an optional embodiment, the treatment method further includes a silicon removal step, in which the effluent from the potassium nitrate freeze crystallization step is treated by adding a silicon removal chemical agent to cause the silicon to react chemically and form a precipitate, thereby removing the silicon. The silicon-removed effluent is then transported to the media filtration step.

[0097] In an optional embodiment, the desiliconization unit includes a chemical desiliconization reaction precipitation device, wherein the chemical reaction is carried out in the chemical desiliconization reaction precipitation device, and the precipitated sludge generated by the chemical reaction is dewatered, and the filtrate is returned to the inlet end of the chemical desiliconization reaction precipitation device.

[0098] In an optional embodiment, the influent to the nanofiltration step, in addition to coming from the media filtration step, also receives a portion of the influent from the sodium chloride evaporation and crystallization step, i.e., the inorganic salt-containing wastewater to be treated, in order to reduce the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of.

[0099] In an optional embodiment, the flow rate of the inorganic salt-containing wastewater to be treated received in the nanofiltration step is 0.5 to 10 times the effluent flow rate of the media filtration unit.

[0100] The inorganic salt-containing wastewater treatment system and method of this invention are applicable to wastewater containing inorganic salts generated in various industries such as petrochemicals, coal chemicals, fine chemicals, and power plants. These industries typically generate large quantities of wastewater containing a certain amount of Cl. - SO4 2- NO3 - Na + and K + Wastewater is very suitable for treatment using the treatment system and method of the present invention.

[0101] The present invention will be further described in detail below through examples.

[0102] Example 1

[0103] The following steps should be followed to treat wastewater containing inorganic salts.

[0104] a. Wastewater containing inorganic salts to be treated, with a flow rate of 21.03 t / h, of which Na... + Concentration 37665 mg / L, K + Concentration 440 mg / L, Cl - Concentration 54939 mg / L, SO4 2- Concentration 64.7 mg / L, NO3 - The concentration was 6251 mg / L, and the SiO2 concentration was 43 mg / L.

[0105] b. 5.38 t / h of inorganic salt-containing wastewater is diverted to the inlet of the downstream nanofiltration unit. The remaining portion directly enters the sodium chloride evaporation and crystallization unit, where it is mixed with the permeate from the nanofiltration unit and then treated by triple-effect vacuum evaporation. The evaporator outlet temperature is 71℃. After centrifugation, 1744 kg / h of sodium chloride crystals B are obtained. The dry basis purity of sodium chloride product B reaches 99.5%, meeting industrial application requirements. The resulting evaporation and crystallization mother liquor contains Na... + Concentration 166546 mg / L, K + Concentration 29208 mg / L, Cl - Concentration 172159 mg / L, SO4 2- Concentration 908 mg / L, NO3 - Concentration 211842 mg / L.

[0106] c. 1.55 t / h of the mother liquor from evaporation and crystallization is fed into the salting-out reactor. 114.8 kg / h of KCl crystals are added to the reactor. After the reaction, centrifugation is performed to obtain sodium chloride crystals (D) at a rate of 48.2 kg / h. The dry basis purity of sodium chloride product D reaches 99.5%. The Na... + Concentration 146607 mg / L, K + Concentration 78754 mg / L, Cl - Concentration 189092 mg / L, SO4 2- Concentration 886 mg / L, NO3 - Concentration 206590 mg / L, SiO2 concentration 800 mg / L.

[0107] d. Add 0.24 t / h of sodium chloride evaporation crystallization unit feed water to the mother liquor from salting-out separation, then send it to the freeze crystallization separation device. In the freeze crystallization separation device, first cool the water temperature to 7-10℃, causing potassium nitrate to reach a supersaturated state and precipitate as crystals. Then, separate the potassium nitrate crystals from the solution by centrifugation. The yield of potassium nitrate crystals E is 168.6 kg / h. The Na content in the freeze crystallization mother liquor... + Concentration 139829 mg / L, K + Concentration 22254 mg / L, Cl - Concentration 181389 mg / L, SO4 2- Concentration 810 mg / L, NO3 - Concentration 110753 mg / L.

[0108] e. Add sodium aluminate 830 mg / L, PAC 20 mg / L, and PAM 1.0 mg / L to the discharged frozen crystallization mother liquor. Then, let it enter the reaction sedimentation tank for silicon removal reaction and precipitation separation. The SiO2 concentration in the effluent of the sedimentation tank is 20 mg / L. The sludge settled at the bottom of the reaction sedimentation tank is dewatered by a filter press, and the filtrate is returned to the inlet of the reaction sedimentation tank.

[0109] f. The supernatant from the reaction sedimentation tank is pumped into a multi-media filter. The multi-media filter is filled with quartz sand and anthracite, with the quartz sand filling height at 800 mm and the anthracite filling height at 400 mm. The filtration rate is 7.0 m / h. The suspended solids in the effluent from the multi-media filter are less than 2 mg / L, and the turbidity is less than 5 NTU.

[0110] g. The effluent from the multi-media filter is mixed with the inorganic salt-containing wastewater to be treated, which is directly diverted to the nanofiltration unit, and then enters the membrane module system of the nanofiltration unit. The nanofiltration membrane module is a GE DK8040 model. SO42-containing wastewater in the nanofiltration membrane retentate... 2- The concentration reached approximately 2181 mg / L, and it was discharged externally. The Na+ permeate from the nanofiltration membrane... + Concentration 61500 mg / L, K + Concentration 5810 mg / L, Cl - Concentration 84294 mg / L, SO4 2- Concentration 10 mg / L, NO3 - The concentration was 31840 mg / L, and it was transported to a sodium chloride evaporation and crystallization device for evaporation and crystallization treatment.

[0111] Comparative Example 1

[0112] Other conditions are the same as in Example 1, except that no salting-out unit is set up, and the mother liquor obtained from the sodium chloride evaporation crystallization unit is directly fed into the potassium nitrate freeze crystallization unit.

[0113] Comparative Example 2

[0114] Other conditions are the same as in Example 1, except that no dilution water or sodium chloride evaporation crystallization unit inlet water was added to the salting-out mother liquor in step d.

[0115] The purity and yield of the products obtained in Example 1 and Comparative Examples 1-2 are shown in Table 1:

[0116] Table 1

[0117]

[0118] As shown in the examples, sodium chloride solid product was obtained by evaporation and crystallization separation of wastewater containing inorganic salts. Further sodium chloride solid product was obtained by adding potassium chloride for salting-out and crystallization separation, while maintaining the Cl concentration in the solution. -Concentration remains constant while K + The concentration was significantly increased, and potassium nitrate solid product was obtained by freeze crystallization. Adding dilution water before freeze crystallization prevented sodium chloride saturation precipitation during the process. Furthermore, the reflux of part of the freeze crystallization mother liquor lowered the potassium content in the solution. + While reducing the concentration, the flow rate of dilution water was also reduced, preventing the precipitation of potassium chloride during the freeze crystallization process. Silicon in the wastewater was removed by adding a magnesium silicate remover, preventing its accumulation during evaporation. Most of the suspended solids and colloids in the wastewater after silica removal were removed by media filtration. Subsequent treatment by a nanofiltration unit achieved sulfate retention and concentration. By diverting a portion of the salt-containing wastewater to the inlet side of the nanofiltration membrane, the Cl- concentration in the nanofiltration retentate was reduced. - NO3 - Na + and K + The concentration of these ions is increased, thereby improving their recovery rate. Through these treatment steps, salt and water can be separated from inorganic salt-containing wastewater to obtain high-quality sodium chloride, potassium nitrate, and SO4 products. 2- The enriched and separated components are then sent out for separate processing. The method and system of this invention also ensure long-term, stable, economical, and reliable operation without secondary pollution, resulting in significant economic and environmental benefits.

[0119] Although the invention has been described with reference to exemplary embodiments, it should be understood that the terminology used is descriptive and exemplary, and not restrictive. Since the invention can be embodied in many forms without departing from its spirit or essence, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be interpreted broadly within the spirit and scope defined by the appended claims. Therefore, all variations and modifications falling within the scope of the claims or their equivalents should be covered by the appended claims.

Claims

1. A method for treating wastewater containing inorganic salts, used for treating wastewater containing Cl... - SO4 2- NO3 - Na + and K + Salt recovery is performed on saline wastewater to prepare high-purity sodium chloride and potassium nitrate, characterized in that... include: The sodium chloride evaporation and crystallization step involves evaporating and crystallizing the wastewater containing inorganic salts to obtain sodium chloride crystals B and evaporation and crystallization mother liquor. In the salting-out step, the mother liquor from the sodium chloride evaporation and crystallization step is treated by adding potassium chloride to the mother liquor to obtain sodium chloride crystals D and a salting-out separation mother liquor. Simultaneously, the salting-out separation mother liquor is discharged to the potassium nitrate freeze crystallization step. The amount of potassium chloride added satisfies the requirement of K in the mother liquor. + With NO3 - The molar concentrations are equal; In the potassium nitrate freeze crystallization step, dilution water containing sodium chloride is added to the effluent of the salting-out step, the mother liquor from the salting-out separation, and then the liquid is freeze crystallized to recover potassium nitrate crystals. The nanofiltration step treats the effluent from the potassium nitrate freeze crystallization step with nanofiltration, while the nanofiltration permeate is discharged to the sodium chloride evaporation crystallization step; the nanofiltration retentate is sent out separately for sulfate recovery. In the potassium nitrate freeze crystallization step, a portion of the potassium nitrate freeze crystallization mother liquor is recycled back to the inlet of the potassium nitrate freeze crystallization step.

2. The processing method according to claim 1, characterized in that, In the potassium nitrate freeze crystallization step, the amount of dilution water added is 5wt% to 20wt% of the effluent from the salting-out step.

3. The processing method according to claim 1, characterized in that, The freezing crystallization temperature for the potassium nitrate freezing crystallization step is -5~60℃.

4. The processing method according to claim 3, characterized in that, The freezing crystallization temperature for the potassium nitrate freezing crystallization step is 0~20℃.

5. The processing method according to claim 3, characterized in that, The freezing crystallization temperature for the potassium nitrate freezing crystallization step is 0~10℃.

6. The processing method according to claim 1, characterized in that, The treatment method further includes a media filtration step, in which the effluent potassium nitrate cryo-crystallization mother liquor from the potassium nitrate cryo-crystallization step is subjected to media filtration treatment to remove suspended solids from the potassium nitrate cryo-crystallization mother liquor, and the effluent from the media filtration step is transported to the nanofiltration step.

7. The processing method according to claim 6, characterized in that, The treatment method further includes a silicon removal step, in which the effluent from the potassium nitrate freeze crystallization step is treated by adding a silicon removal chemical agent to cause the silicon to react chemically and form a precipitate, thereby removing the silicon. The silicon-removed effluent is then transported to the media filtration step.

8. The processing method according to claim 7, characterized in that, The chemical reaction is carried out in a chemical desiliconization reaction precipitation device. The precipitated sludge generated by the chemical reaction is dewatered, and the filtrate is returned to the inlet of the chemical desiliconization reaction precipitation device.

9. The processing method according to claim 6, characterized in that, The nanofiltration step receives water from the media filtration step, as well as a portion of the water from the sodium chloride evaporation and crystallization step, i.e., the inorganic salt-containing wastewater to be treated, in order to reduce the Cl content in the nanofiltration retentate. - NO3 - Na + and K + The content of.

10. The processing method according to claim 9, characterized in that, The flow rate of the inorganic salt-containing wastewater received in the nanofiltration step is 0.5 to 10 times the effluent flow rate of the media filtration unit.

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