A zero-discharge treatment system and method for magnesium-based desulfurization wastewater

Abstract

The present invention provides a zero-discharge treatment system and method for magnesium-based desulfurization wastewater. The system utilizes a wastewater pre-sedimentation tank, a wastewater collection tank, a flocculation reaction tank, an inclined tube clarifier, and a filtration device to remove impurities such as hardness, heavy metals, and suspended solids from the desulfurization wastewater. The concentrated solution then passes through a third-, first-, and second-effect evaporation crystallization device, sequentially passing through a freeze crystallization device and centrifuge I to produce magnesium sulfate heptahydrate. The mother liquor at the outlet enters a defluorination device. The defluorination device outlet is sequentially passed through a single-effect evaporation crystallization device and centrifuge II to produce magnesium chloride. The mother liquor at the outlet also enters the defluorination device. The first-, second-, and third-effect evaporation crystallization devices are connected to condensation device I, and the single-effect evaporation crystallization device is connected to condensation device II. Condensate is recycled. Sludge water from the bottom of the wastewater pre-sedimentation tank, inclined tube clarifier, and defluorination device is filtered through a plate and frame filter press. Advantages include the step-by-step removal of various harmful substances from magnesium-based desulfurization wastewater, achieving the rational resource utilization of wastewater and waste.

Description

Technical Field

[0001] The present invention belongs to the technical field of industrial wastewater treatment in steel enterprises, and in particular relates to a zero-discharge treatment system and method for magnesium-based desulfurization wastewater. Background Art

[0002] Industrial wastewater, especially the wastewater discharged from magnesium desulfurization in steel enterprises, is neutral, but its suspended solids, salt content, magnesium ion hardness and other indicators exceed the standards seriously, making it one of the most difficult terminal wastewaters to treat.

[0003] The current treatment method for desulfurization wastewater primarily relies on conventional chemical precipitation, but this method struggles to remove the concentrated chloride and sulfate ions. Because these chloride and sulfate ions are highly corrosive, they require further treatment before being recycled or discharged. Currently, most steel mills' desulfurization wastewater is primarily used for wet slag flushing or discharged after dilution. However, this utilization and discharge method carries certain risks and can easily cause secondary pollution, failing to truly achieve zero desulfurization wastewater discharge and comprehensive resource utilization. Summary of the Invention

[0004] In response to the above-mentioned problems existing in the prior art, the purpose of the present invention is to provide a zero-emission treatment system and method for magnesium-based desulfurization wastewater, which can not only achieve the step-by-step removal of various harmful substances in magnesium-based desulfurization wastewater to achieve the purpose of zero emission of such desulfurization wastewater, but also "turn waste into treasure" and realize the resource recycling of harmful substances.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] The invention provides a zero-discharge treatment system for magnesium-based desulfurization wastewater, characterized in that the treatment system comprises: a wastewater pre-sedimentation tank, a wastewater collection tank, a flocculation reaction tank, an inclined tube clarifier, a filtration device, a third-effect evaporation crystallization device, a first-effect evaporation crystallization device, a second-effect evaporation crystallization device, a freezing crystallization device, a centrifugal device I, a defluorination device, a single-effect evaporation crystallization device, a centrifugal device II, a condensation device I, a condensation device II, and a plate and frame filter press;

[0007] The wastewater pre-sedimentation tank, wastewater collection tank, flocculation reaction tank, inclined tube clarifier and filtration device are connected in series in sequence, the bottom outlet of the wastewater pre-sedimentation tank and the bottom outlet of the inclined tube clarifier are both connected to the plate and frame filter press, and the overflow outlet of the inclined tube clarifier is fed into the filtration device;

[0008] The third-effect evaporation crystallization device, the first-effect evaporation crystallization device, and the second-effect evaporation crystallization device are sequentially connected in series, the inlet of the third-effect evaporation crystallization device is connected to the outlet of the filtering device, and the steam outlet of the third-effect evaporation crystallization device, the steam outlet of the first-effect evaporation crystallization device, and the steam outlet of the second-effect evaporation crystallization device are all connected to the inlet of the condensing device I;

[0009] The freeze crystallization device and the centrifugal device I are connected in series in sequence, the freeze crystallization device inlet is connected to the concentrated liquid outlet of the second-effect evaporation crystallization device, and the solid outlet of the centrifugal device I is used to recover magnesium sulfate heptahydrate;

[0010] The defluorination device is connected to the mother liquor outlet of the centrifugal device I, the bottom outlet of the defluorination device is connected to the plate and frame filter press, the filtrate outlet of the plate and frame filter press is connected to the inlet of the wastewater pre-sedimentation tank, and a sludge outlet is provided at the bottom of the plate and frame filter press;

[0011] The single-effect evaporation crystallization device and the centrifugal device II are connected in series in sequence, the inlet of the single-effect evaporation crystallization device is connected to the mother liquor outlet of the defluorination device, the steam outlet of the single-effect evaporation crystallization device is connected to the inlet of the condensation device II, the mother liquor outlet of the centrifugal device II is connected to the inlet of the defluorination device, and the solid outlet of the centrifugal device II is used to recover impurities.

[0012] Furthermore, the condensing device I is used to cool the steam generated by the third-effect evaporation crystallization device, the first-effect evaporation crystallization device and the second-effect evaporation crystallization device into condensed water I; the condensing device II is used to cool the steam generated by the single-effect evaporation crystallization device into condensed water II; and the condensed water I and the condensed water II are combined and recycled.

[0013] The present invention provides a zero-discharge treatment method for magnesium-based desulfurization wastewater, which uses the above-mentioned treatment system and is characterized by comprising the following steps:

[0014] Step S1: feeding the magnesium desulfurization wastewater into a wastewater pre-sedimentation tank, feeding the overflow water after pre-sedimentation into a wastewater collection tank for homogenization and equalization treatment, then feeding into a flocculation reaction tank and adding PAM solution and PAC solution to carry out chemical reaction under stirring conditions, then feeding into an inclined tube clarifier for sedimentation and clarification, and finally feeding the sedimented and clarified overflow water into a filtration device to obtain a first neutral high-salt clear water after filtration;

[0015] Step S2: feeding the first neutral high-salt clean water into a third-effect evaporation crystallization device, a first-effect evaporation crystallization device, and a second-effect evaporation crystallization device in sequence for evaporation and concentration treatment, feeding the steam discharged from the steam outlets of the third-effect evaporation crystallization device, the first-effect evaporation crystallization device, and the second-effect evaporation crystallization device into a condensation device I to obtain condensed water I for reuse, and obtaining a first neutral concentrated liquid at the concentrated liquid outlet of the second-effect evaporation crystallization device;

[0016] Step S3: feeding the first neutral concentrated liquid into a freeze crystallization device to separate the magnesium chloride salt component and the magnesium sulfate salt component, and after freeze crystallization treatment, feeding the liquid into a centrifugal device I for centrifugal separation to obtain a magnesium sulfate heptahydrate product and a second neutral concentrated liquid;

[0017] Step S4: feeding the second neutral concentrated liquid into a defluorination device, and simultaneously adding a calcium chloride solution for chemical reaction precipitation, and then feeding the mother liquor at the outlet of the defluorination device into a single-effect evaporation crystallization device for further concentration to obtain a third neutral concentrated liquid; feeding the third neutral concentrated liquid into a centrifugal device II for centrifugal separation to obtain a miscellaneous salt product and a fourth neutral concentrated liquid, and returning the fourth neutral concentrated liquid to the defluorination device for coordinated defluorination; feeding the steam discharged from the steam outlet of the single-effect evaporation crystallization device into a condensation device II to obtain condensed water II for reuse;

[0018] Step S5: The sludge water from the bottom outlet of the wastewater pre-sedimentation tank, the bottom outlet of the inclined tube clarifier, and the bottom outlet of the defluorination device is fed into a plate-and-frame filter press for filter press treatment. The filtrate outlet of the plate-and-frame filter press is connected to the inlet of the wastewater pre-sedimentation tank 1.

[0019] Step S5: The sludge water from the bottom outlet of the wastewater pre-sedimentation tank, the bottom outlet of the inclined tube clarifier, and the bottom outlet of the defluorination device are respectively fed into the inlet of the plate and frame filter press device, and the sludge water generated by the above devices is collected and subjected to filter press treatment; the filtrate outlet of the plate and frame filter press device is connected to the inlet of the wastewater pre-sedimentation tank.

[0020] Furthermore, in step S1, the concentration range of the PAM solution is 0.05% to 0.1% by mass, and the addition amount is 0.6 ml / L to 1.2 ml / L; the concentration range of the PAC solution is 5% to 10% by mass, and the addition amount is 0.1 ml / L to 0.2 ml / L.

[0021] Furthermore, the suspended matter content in the first neutral high-salt water obtained after filtration in step S1 is less than 5 mg / L.

[0022] Furthermore, in step S4, the mass percentage concentration range of the calcium chloride solution is 5% to 10%.

[0023] Compared with the prior art, the advantages of the present invention are:

[0024] The present invention can effectively treat industrial wastewater desulfurized by magnesium method, especially magnesium desulfurization wastewater discharged from the steel industry, and realizes the step-by-step removal of multiple harmful substances in the magnesium desulfurization wastewater. At the same time, it can "turn waste into treasure". The generated by-products have a certain economic recovery value, realizing the resource recycling and utilization of harmful substances. The treated effluent can be recycled as new industrial water, achieving zero wastewater discharge.

[0025] The process of the present invention is simple to operate and has practical significance for achieving water saving, consumption reduction and revenue increase in steel enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic structural diagram of a zero-discharge treatment system for magnesium-based desulfurization wastewater according to the present invention.

[0027] Figure 1 In: 1. Wastewater pre-sedimentation tank, 2. Wastewater collection tank, 3. Flocculation reaction tank, 4. Inclined tube clarifier, 5. Filtration device, 6. Third-effect evaporation crystallization device, 7. First-effect evaporation crystallization device, 8. Second-effect evaporation crystallization device, 9. Freeze crystallization device, 10. Centrifugal device I, 11. Fluorination removal device, 12. Single-effect evaporation crystallization device, 13. Centrifugal device II, 14. Condensation device I, 15. Condensation device II, 16. Plate and frame filter press device. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and examples.

[0029] The present invention provides a zero-discharge treatment system and method for magnesium-based desulfurization wastewater. These systems are capable of effectively treating and recycling magnesium-based desulfurization wastewater from the steel industry, achieving zero discharge and resource recovery. To facilitate a comprehensive understanding of the present invention, the following description, in conjunction with the examples, sets forth numerous specific details. However, it will be apparent that these examples may be practiced without these specific details.

[0030] like Figure 1 As shown, in this embodiment, a zero-discharge treatment system for magnesium-based desulfurization wastewater is provided, which is used to treat magnesium-based desulfurization wastewater from steel enterprises. The treatment system includes: a wastewater pre-sedimentation tank 1, a wastewater collection tank 2, a flocculation reaction tank 3, an inclined tube clarifier 4, a filtration device 5, a third-effect evaporation crystallization device 6, a first-effect evaporation crystallization device 7, a second-effect evaporation crystallization device 8, a freeze crystallization device 9, a centrifugal device I 10, a defluorination device 11, a single-effect evaporation crystallization device 12, a centrifugal device II 13, a condensation device I 14, a condensation device II 15, and a plate and frame filter press 16, which are connected in sequence.

[0031] The wastewater pre-sedimentation tank 1, the wastewater collection tank 2, the flocculation reaction tank 3, the inclined tube clarifier 4 and the filter device 5 are connected in series in sequence. The bottom outlet of the wastewater pre-sedimentation tank 1 and the bottom outlet of the inclined tube clarifier 4 are both connected to the plate and frame filter press 16, and the overflow outlet of the inclined tube clarifier 4 is fed into the filter device 5;

[0032] The third-effect evaporation crystallization device 6, the first-effect evaporation crystallization device 7, and the second-effect evaporation crystallization device 8 are sequentially connected in series, the inlet of the third-effect evaporation crystallization device 6 is connected to the outlet of the filtering device 5, and the steam outlet of the third-effect evaporation crystallization device, the steam outlet of the first-effect evaporation crystallization device, and the steam outlet of the second-effect evaporation crystallization device are all connected to the inlet of the condensing device I 14;

[0033] The freeze crystallization device 9 and the centrifugal device I 10 are connected in series in sequence, the inlet of the freeze crystallization device 9 is connected to the concentrated liquid outlet of the second-effect evaporation crystallization device 8, and the solid outlet of the centrifugal device I 10 is used to recover magnesium sulfate heptahydrate;

[0034] The defluorination device 11 is connected to the mother liquor outlet of the centrifugal device I 10, the bottom outlet of the defluorination device 11 is connected to the plate and frame filter press 16, the filtrate outlet of the plate and frame filter press 16 is connected to the inlet of the wastewater pre-sedimentation tank 1, and a sludge outlet is provided at the bottom of the plate and frame filter press 16;

[0035] The single-effect evaporation crystallization device 12 and the centrifugal device II 13 are connected in series in sequence, the inlet of the single-effect evaporation crystallization device 12 is connected to the mother liquor outlet of the defluorination device 11, the steam outlet of the single-effect evaporation crystallization device 12 is connected to the inlet of the condensation device II 15, the mother liquor outlet of the centrifugal device II 13 is connected to the inlet of the defluorination device 11, and the solid outlet of the centrifugal device II 13 is used to recover miscellaneous salts.

[0036] Since the wastewater pre-sedimentation tank 1, the inclined tube clarifier 4 and the defluorination device 11 will produce sludge during use, the bottom outlets of the wastewater pre-sedimentation tank 1, the inclined tube clarifier 4 and the defluorination device 11 are respectively connected to the inlet of the plate and frame filter press device 16 to collect the sludge.

[0037] The preferred reference suggestions for the various components and equipment of the processing system of the present invention are as follows:

[0038] The wastewater pre-sedimentation tank 1 is preferably a closed square or circular lower cone structure, and the wastewater pre-sedimentation tank 1 is preferably equipped with a guide tube type mechanical slow scraper device.

[0039] Wastewater collection tank 2 is a closed circular lower cone structure with a guide tube type mechanical stirring equipment.

[0040] The flocculation reaction tank 3 is of a closed square structure, and the stirring device is preferably a mechanical rapid stirring device with a guide tube.

[0041] The inclined tube clarifier 4 preferably has an open square lower cone structure.

[0042] The filter device 5 is preferably a multimedia filter device or a ceramic membrane filter device.

[0043] The third-effect evaporation crystallization device 6 , the first-effect evaporation crystallization device 7 , the second-effect evaporation crystallization device 8 and the single-effect evaporation crystallization device 12 are preferably made of metal materials resistant to chloride ion and sulfate ion corrosion.

[0044] The freezing crystallization device 9 is preferably a DTB variable frequency crystallization device with a chilled water system.

[0045] The centrifugal device I10 is preferably a horizontal screw centrifugal device.

[0046] The defluorination device 11 is preferably a closed clarification and filtration integrated closed device.

[0047] The centrifugal device II13 is preferably a horizontal screw type or a flat plate type centrifugal device.

[0048] Condensation device I14 and condensation device II15 are preferably tubular condensation devices with circulating water cooling.

[0049] The plate and frame filter press device 16 is preferably a high-pressure diaphragm plate and frame filter press device.

[0050] In this embodiment, a method for treating magnesium-based desulfurization wastewater with zero discharge is also provided. The above-mentioned treatment system is used, and the specific steps of the treatment method are as follows:

[0051] Step S1: The magnesium desulfurization wastewater is fed into a wastewater pre-sedimentation tank 1, and the overflow water after pre-sedimentation is fed into a wastewater collection tank 2 for homogenization and equalization treatment, and then fed into a flocculation reaction tank 3 and added with a PAM solution and a PAC solution to carry out a chemical reaction under stirring conditions, and then fed into an inclined tube clarifier 4 for precipitation and clarification, and finally the precipitated and clarified overflow water is fed into a filter device 5 to obtain a first neutral high-salt clear water after filtration, wherein the suspended matter content in the first neutral high-salt clear water is less than 5 mg / L.

[0052] Among them, the PAM and PAC solutions used in the flocculation reaction tank 3 have a configuration concentration of the PAM solution in the range of 0.05% to 0.1% by mass, and a concentration of the PAC solution in the range of 5% to 10% by mass. The chemical reaction precipitates in this step are mainly calcium magnesium sludge, fluoride sludge, and a small amount of heavy metal sludge.

[0053] Step S2: The first neutral high-salt clean water is sequentially fed into the third-effect evaporation crystallization device 6, the first-effect evaporation crystallization device 7, and the second-effect evaporation crystallization device 8 for evaporation and concentration treatment. The steam discharged from the steam outlet of the third-effect evaporation crystallization device 6, the first-effect evaporation crystallization device 7, and the second-effect evaporation crystallization device 8 is fed into the condensation device I 14 to obtain condensed water I for reuse, and a first neutral concentrated liquid is obtained at the concentrated liquid outlet of the second-effect evaporation crystallization device 8.

[0054] Step S3: feeding the first neutral concentrated liquid into a freeze crystallization device 9 to separate the magnesium chloride salt component and the magnesium sulfate salt component, and after freeze crystallization treatment, feeding the liquid into a centrifugal device I 10 for centrifugal separation to obtain a magnesium sulfate heptahydrate product and a second neutral concentrated liquid;

[0055] Step S4: feeding the second neutral concentrated liquid into the defluorination device 11, and adding calcium chloride solution at the same time for chemical reaction precipitation, and then feeding the mother liquor at the outlet of the defluorination device 11 into the single-effect evaporation crystallization device 12 for further concentration to obtain a third neutral concentrated liquid; feeding the third neutral concentrated liquid into the centrifugal device II 13 for centrifugal separation to obtain a magnesium chloride salt product and a fourth neutral concentrated liquid, and the fourth neutral concentrated liquid is returned to the defluorination device 11 for coordinated defluorination; feeding the steam discharged from the steam outlet of the single-effect evaporation crystallization device 12 into the condensation device II 15 to obtain condensed water II for reuse;

[0056] Step S5: The sludge water from the bottom outlet of the wastewater pre-sedimentation tank 1, the bottom outlet of the inclined tube clarifier 4, and the bottom outlet of the defluorination device 11 are respectively fed into the inlet of the plate and frame filter press device 16, and the sludge water generated by the above devices is collected and subjected to filter press treatment; the filtrate outlet of the plate and frame filter press device 16 is connected to the inlet of the wastewater pre-sedimentation tank 1.

[0057] The method of the present invention is further described below based on the specific data of treating magnesium-process desulfurization wastewater from a steel enterprise with known initial water quality.

[0058] The initial water quality of the magnesium desulfurization wastewater from the steel enterprise is shown in Table 1.

[0059] Table 1: Initial water quality of magnesium desulfurization wastewater

[0060]

[0061] The specific processing methods are as follows:

[0062] Step S1: After the magnesium desulfurization wastewater from the steel industry is passed into a wastewater pre-sedimentation tank 1 with a closed circular lower cone structure and a guide tube mechanical slow scraper, it overflows into a wastewater collection tank 2 with a closed circular lower cone structure and a guide tube mechanical stirrer. After being homogenized and evenly distributed in the wastewater collection tank 2, it is passed into a flocculation reaction tank 3. 0.1% PAM solution is added in an amount of 0.6 ml / L to 1.2 ml / L, and 5% PAC solution is added in an amount of 0.1 ml / L to 0.2 ml / L to the flocculation reaction tank 3. The wastewater is fully mixed and stirred. After a chemical reaction, the wastewater enters an open square lower cone structure inclined tube clarifier 4 for sedimentation and clarification. The water produced by the inclined tube clarifier 4 is sent to a ceramic membrane filtration device 5. At the outlet of the ceramic membrane filtration device 5, a first neutral high-salt clear water is obtained.

[0063] Step S2: The first neutral high-salt clean water obtained in step S1 is sequentially passed into the third-effect evaporation crystallization device 6, the first-effect evaporation crystallization device 7 and the second-effect evaporation crystallization device 8 to evaporate and concentrate the first neutral high-salt clean water. The steam outlets of the third-effect evaporation crystallization device 6, the first-effect evaporation crystallization device 7 and the second-effect evaporation crystallization device 8 are connected to a tubular condenser device I14 with circulating water cooling, and the steam generated by the above devices is collected. Qualified condensed water I is obtained by condensation for reuse, and a first neutral concentrated liquid is obtained at the outlet of the second-effect evaporation crystallization device 8.

[0064] Step S3: The first neutral concentrated solution obtained in step S2 is passed into a DTB variable frequency freezing crystallization device 9 with a chilled water system to separate the magnesium chloride salt component and the magnesium sulfate salt component. After freeze crystallization treatment in the DTB variable frequency freezing crystallization device 9 with a chilled water system, the first neutral concentrated solution enters a horizontal screw centrifuge I10 for centrifugal separation to obtain magnesium sulfate heptahydrate with a purity of 95% and a second neutral concentrated solution.

[0065] Step S4: The second neutral concentrated liquid obtained in step S3 is passed through a closed, integrated clarification and filtration defluorination device 11. A calcium chloride solution with a mass concentration of 5% to 10% is added to the defluorination device 11 in an amount of 20 ml / L to 40 ml / L for chemical reaction precipitation. The liquid output from the defluorination device 11 is passed through a single-effect evaporation crystallization device 12 for further concentration to obtain a third neutral concentrated liquid. The third neutral concentrated liquid is passed through a flat-plate centrifuge II 13 for centrifugal separation to obtain a 90% pure mixed salt and a fourth neutral concentrated liquid. The mixed salt is a mixture of magnesium chloride and magnesium sulfate. The fourth neutral concentrated liquid is sent to the defluorination device 11 for synergistic defluorination. The outlet of the single-effect evaporation crystallization device 12 is connected to a tubular condenser II 15 with circulating water cooling. The steam generated by the single-effect evaporation crystallization device 12 is collected and condensed to obtain qualified condensate II for reuse.

[0066] Step S5: The bottom outlets of the wastewater pre-sedimentation tank 1, the inclined tube clarifier 4 and the defluorination device 11 are respectively connected to the inlet of the high-pressure diaphragm plate and frame filter press device 16 to collect the sludge water generated by the devices; the filtrate outlet of the high-pressure diaphragm plate and frame filter press device 16 is connected to the inlet of the wastewater pre-sedimentation tank 1.

[0067] The present invention can effectively treat magnesium-based desulfurization wastewater from steel enterprises, achieving the purpose of step-by-step removal of multiple harmful substances in the magnesium-based desulfurization wastewater. The process of the present invention is simple to operate, and the treated effluent can be recycled as new industrial water, achieving zero wastewater discharge. The generated by-products have certain economic recovery value, thereby achieving resource utilization.

[0068] In this embodiment, impurity salts with a purity of ≥90% and magnesium sulfate heptahydrate with a purity of ≥95% can be produced and sold. This system can achieve automated and stable operation, and the generated condensed water can be reused in the wastewater treatment system and also used in the circulating cooling water section.

[0069] The embodiments of the present invention are described in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas. At the same time, for those skilled in the art, according to the ideas of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A zero-discharge treatment system for magnesium desulfurization wastewater, characterized in that: The treatment system includes: a wastewater pre-sedimentation tank, a wastewater collection tank, a flocculation reaction tank, an inclined tube clarifier, a filtration device, a third-effect evaporation crystallization device, a first-effect evaporation crystallization device, a second-effect evaporation crystallization device, a freezing crystallization device, a centrifugal device I, a defluorination device, a single-effect evaporation crystallization device, a centrifugal device II, a condensation device I, a condensation device II and a plate and frame filter press; The wastewater pre-sedimentation tank, wastewater collection tank, flocculation reaction tank, inclined tube clarifier and filtration device are connected in series in sequence, the bottom outlet of the wastewater pre-sedimentation tank and the bottom outlet of the inclined tube clarifier are both connected to the plate and frame filter press, and the overflow outlet of the inclined tube clarifier is fed into the filtration device; The third-effect evaporation crystallization device, the first-effect evaporation crystallization device, and the second-effect evaporation crystallization device are sequentially connected in series, the inlet of the third-effect evaporation crystallization device is connected to the outlet of the filtering device, and the steam outlet of the third-effect evaporation crystallization device, the steam outlet of the first-effect evaporation crystallization device, and the steam outlet of the second-effect evaporation crystallization device are all connected to the inlet of the condensing device I; The freeze crystallization device and the centrifugal device I are connected in series in sequence, the freeze crystallization device inlet is connected to the concentrated liquid outlet of the second-effect evaporation crystallization device, and the solid outlet of the centrifugal device I is used to recover magnesium sulfate heptahydrate; The defluorination device is connected to the mother liquor outlet of the centrifugal device I, the bottom outlet of the defluorination device is connected to the plate and frame filter press, the filtrate outlet of the plate and frame filter press is connected to the inlet of the wastewater pre-sedimentation tank, and a sludge outlet is provided at the bottom of the plate and frame filter press; The single-effect evaporation crystallization device and the centrifugal device II are connected in series in sequence, the inlet of the single-effect evaporation crystallization device is connected to the mother liquor outlet of the defluorination device, the steam outlet of the single-effect evaporation crystallization device is connected to the inlet of the condensation device II, the mother liquor outlet of the centrifugal device II is connected to the inlet of the defluorination device, and the solid outlet of the centrifugal device II is used to recover impurities.

2. A zero-discharge treatment system for magnesium-based desulfurization wastewater according to claim 1, characterized in that: The condensing device I is used to cool the steam generated by the third-effect evaporation crystallization device, the first-effect evaporation crystallization device and the second-effect evaporation crystallization device into condensed water I; the condensing device II is used to cool the steam generated by the single-effect evaporation crystallization device into condensed water II; condensed water I and condensed water II are combined and recycled.

3. A zero-discharge treatment method for magnesium desulfurization wastewater, using the above-mentioned treatment system, characterized in that: The following steps are involved: Step S1: feeding the magnesium desulfurization wastewater into a wastewater pre-sedimentation tank, feeding the overflow water after pre-sedimentation into a wastewater collection tank for homogenization and equalization treatment, then feeding into a flocculation reaction tank and adding PAM solution and PAC solution to carry out chemical reaction under stirring conditions, then feeding into an inclined tube clarifier for sedimentation and clarification, and finally feeding the sedimented and clarified overflow water into a filtration device to obtain a first neutral high-salt clear water after filtration; The PAM solution has a concentration range of 0.05% to 0.1% by mass, and the addition amount is 0.6 ml / L to 1.2 ml / L. The PAC solution has a concentration range of 5% to 10% by mass, and the addition amount is 0.1 ml / L to 0.2 ml / L. The suspended solids content in the first filtered neutral high-salt water is less than 5 mg / L. Step S2: feeding the first neutral high-salt clean water into a third-effect evaporation crystallization device, a first-effect evaporation crystallization device, and a second-effect evaporation crystallization device in sequence for evaporation and concentration treatment, feeding the steam discharged from the steam outlets of the third-effect evaporation crystallization device, the first-effect evaporation crystallization device, and the second-effect evaporation crystallization device into a condensation device I to obtain condensed water I for reuse, and obtaining a first neutral concentrated liquid at the concentrated liquid outlet of the second-effect evaporation crystallization device; Step S3: feeding the first neutral concentrated liquid into a freeze crystallization device to separate the magnesium chloride salt component and the magnesium sulfate salt component, and after freeze crystallization treatment, feeding the liquid into a centrifugal device I for centrifugal separation to obtain a magnesium sulfate heptahydrate product and a second neutral concentrated liquid; Step S4: feeding the second neutral concentrated liquid into a defluorination device, and simultaneously adding a calcium chloride solution for chemical reaction precipitation, and then feeding the mother liquor at the outlet of the defluorination device into a single-effect evaporation crystallization device for further concentration to obtain a third neutral concentrated liquid; feeding the third neutral concentrated liquid into a centrifugal device II for centrifugal separation to obtain a miscellaneous salt product and a fourth neutral concentrated liquid, and the fourth neutral concentrated liquid is returned to the defluorination device for coordinated defluorination; feeding the steam discharged from the steam outlet of the single-effect evaporation crystallization device into a condensation device II to obtain condensed water II for reuse; the mass percentage concentration range of the calcium chloride solution is 5% to 10%; Step S5: The sludge water from the bottom outlet of the wastewater pre-sedimentation tank, the bottom outlet of the inclined tube clarifier, and the bottom outlet of the defluorination device are respectively fed into the inlet of the plate and frame filter press device, and the sludge water generated by the above devices is collected and subjected to filter press treatment; the filtrate outlet of the plate and frame filter press device is connected to the inlet of the wastewater pre-sedimentation tank.

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