Phosphogypsum slag field water treatment process based on membrane coupled chemical reactor

Abstract

The application provides a phosphogypsum residue field water treatment process based on a membrane coupling chemical reactor, relates to the technical field of wastewater resource treatment, and can selectively perform gradient precipitation on fluorine, phosphorus, nitrogen and sulfur, cooperates with a membrane filter for targeted design, realizes a membrane separation process resistant to high-concentration suspended solids, and can make the membrane flux attenuation rate less than 5% / day. The application can effectively separate fluorine, phosphorus and nitrogen from water to form mud with resource value, the value of the resource product per ton of water treatment is higher than the treatment cost, 80% of the recovered water meets the industrial recycling standard, and the phosphogypsum residue field water can be advantageously used for quasi-zero emission and deep high-value resource utilization.

Description

Technical Field

[0001] This invention relates to the technical field of wastewater resource utilization treatment, and in particular to a treatment process for phosphogypsum slag field water based on a membrane-coupled chemical reactor. Background Technology

[0002] Phosphogypsum is a major byproduct of wet-process phosphoric acid production, generating approximately 4-5 tons of phosphogypsum for every ton of phosphoric acid produced. The main water quality characteristics of phosphogypsum slag dumps are strong acidity and high concentrations of fluoride ions, phosphate, sulfate, ammonia nitrogen, silicon, and calcium. 2+ Mg 2+ And heavy metals such as As and Pb.

[0003] The existing water treatment processes for phosphogypsum slag disposal sites have the following core contradictions: (1) Bottlenecks in membrane separation technology: When treating slurry with suspended solids >3g / L, the membrane flux decay rate is >50% / h, the cleaning frequency is <2 days, and the membrane life is <6 months; (2) Insufficient purity of resource products: Traditional lime neutralization produces mixed mud with a purity of <60%, and the purity of the product as an industrial raw material needs to be improved; (3) Some resources are not recovered: ammonia nitrogen in water is either denitrified or directly discharged, sulfate is not effectively separated, and the produced water is difficult to treat and is directly discharged; (4) Lack of economic benefits: The cost of large-scale operation and maintenance and resource-based processing is greater than the value of the resource-based products, making it difficult to promote industrialization.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a treatment process for phosphogypsum slag plant water based on a membrane-coupled chemical reactor, which can effectively separate fluorine, phosphorus and nitrogen from the water to form sludge with resource value. The value of the resource products treated per ton of water is higher than the treatment cost, and 80% of the produced water is recovered to meet industrial reuse standards, which is conducive to achieving near-zero discharge and deep high-value resource utilization of phosphogypsum slag plant water.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A process for treating water from a phosphogypsum slag disposal site based on a membrane-coupled chemical reactor includes the following steps: The phosphogypsum slag site water is treated through a first-stage membrane coupled chemical reactor to induce the selective formation of calcium fluoride precipitate by fluoride ions, and the mud and water are separated to obtain acidic fluoride-containing mud and first-stage permeate. The first-stage permeate is treated through a second-stage membrane-coupled chemical reactor to generate dicalcium phosphate and silicate precipitates. The mud and water are then separated to obtain neutral phosphorus-containing silica mud and second-stage permeate. The second-stage permeate is treated through a third-stage membrane-coupled chemical reactor to generate magnesium ammonium phosphate and calcium fluorophosphate precipitates, and the mud and water are separated to obtain alkaline nitrogen- and phosphorus-containing mud and third-stage permeate. The membrane-coupled chemical reactor includes a reaction tank and a membrane filter; The membrane filter is equipped with a multi-ring pulse airflow disturbance device at the bottom sludge discharge port; The water inlet to the membrane filter enters the tank along the tangential direction of the inner wall of the tank, thereby producing a cleaning effect by rinsing the surface of the membrane tube. The mud-water separation includes mud-water separation through the membrane filter.

[0007] Furthermore, the inlet pipe of the membrane filter enters the tank along the tangential direction of the inner wall of the tank; Alternatively, a tangential guide vane can be installed at the vertical tangential inlet.

[0008] Furthermore, the inlet and outlet of the tangential guide vane are both welded inside the tank body; The inlet of the tangential guide vane is aligned with the water inlet, and the outlet is aligned with the tangential direction of the inner wall of the tank. The material of the tangential guide vane is the same as that of the tank body.

[0009] Furthermore, the multi-ring pulse airflow disturbance device is installed above the conical sludge hopper of the tank body; Preferably, the cone angle of the conical mud hopper is 40°-70°; Preferably, the multi-ring pulse airflow disturbance device includes multiple independent annular aeration pipes, and the air intake and shut-off of the annular aeration pipes are controlled by an automatic valve; Preferably, the annular aeration pipe is located at the top of the conical sludge hopper and is fixed by a pre-reserved gate on the inner wall of the tank; Preferably, the annular aeration pipe is provided with two rows of jet nozzles angled downwards at 45° to the vertical. The jetting frequency is synchronized with the backwashing cycle. The aeration pipe is activated by pulses from the inside out or from the outside in, generating multi-ring bubbles that rise from the center to the tank wall and then clean the membrane surface, thus avoiding dead zones that the bubbles cannot reach.

[0010] Furthermore, the membrane-coupled chemical reactor operates automatically via automatic valve switching; Preferably, the operation includes filtration, backwashing, sludge removal, and venting; Preferably, after filtration for 1 min to 60 min, pulse backwashing is performed for 1 s to 60 s, followed by sludge discharge for 1 h to 4 h; Preferably, the membrane-coupled chemical reactor is equipped with membrane fouling monitoring instruments and a chemical cleaning system to automatically identify backwashing and chemical cleaning cycles, with a membrane flux decay rate of <5% / day.

[0011] Furthermore, the processing technology includes the following steps: (a) Add the phosphogypsum slag water and lime slurry to the first-stage membrane coupled chemical reactor and mix them. Control the pH to 2.0-4.0 to induce the selective formation of calcium fluoride precipitate by fluoride ions. The reaction mixture is then separated into acidic fluoride-containing sludge and first-stage permeate by passing it through a membrane filter. (b) The first-stage permeate, lime slurry and silica removal agent are added to the second-stage membrane coupled chemical reactor and mixed. The pH is controlled at 7.5-8.5. Simultaneously, dicalcium phosphate and silicate precipitates are generated. The reaction mixture is then separated into sludge and water in the second stage through a membrane filter to obtain neutral phosphorus-containing silica sludge and second-stage permeate. (c) The second-stage permeate and complexing agent are added to the third-stage membrane coupled chemical reactor and mixed. The pH is controlled at 9.0-11.0. Magnesium ammonium phosphate and calcium fluorophosphate precipitates are generated by directional crystallization. The reaction mixture is then separated into sludge and water in the third stage through a membrane filter to obtain alkaline nitrogen and phosphorus sludge and third-stage permeate.

[0012] Furthermore, the water quality suitable for the treatment process is: F - The content is 0-8000 mg / L, SO4 2- The content is 0-30000 mg / L, PO4 3- The content is 0-15000 mg / L, NH4 + The content is 0-1000 mg / L, and the total silicon content is 0-5000 mg / L.

[0013] Furthermore, the silicon remover includes at least one of magnesium salt and aluminum salt; Preferably, the mass ratio of the magnesium salt to the aluminum salt is in the range of 0.1-10; Preferably, the complexing agent includes at least one selected from phosphate, sodium carbonate, and ferric sulfate; Preferably, the mass ratio of the phosphate to sodium carbonate is in the range of 1-10.

[0014] Furthermore, the processing technology also includes the following steps: The third-stage permeate is neutralized to a pH of 6-8, and then concentrated through a reverse osmosis membrane and / or separated through a nanofiltration membrane to obtain industrial recycled water and sulfate products.

[0015] Furthermore, the processing technology also includes the following steps: The mud obtained from each step is dewatered to obtain dewatered mud cake and filtrate.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention provides a treatment process for phosphogypsum slag wastewater based on a membrane-coupled chemical reactor. Through multi-stage chemical reactions, different precipitates are generated, and mud-water separation is achieved sequentially through multi-stage membrane filters. The tangential flow design and multi-ring pulsed airflow disturbance device inside the membrane filters facilitate hydraulic scouring and removal of the mud cake layer from the membrane tube surface. This process not only has a low membrane fouling rate (membrane flux decay rate <5% / day) and reduces the cleaning frequency to once a month, but also achieves thorough resource recovery of valuable elements, high product purity, and a resource recovery product value exceeding the treatment cost per ton of water treated. It realizes water resource recycling, generates no solid waste, and has significant environmental benefits. The process can obtain calcium fluoride sludge with a purity of 40%-60%, dicalcium phosphate sludge with a purity of 80%-95%, and magnesium ammonium phosphate sludge with a purity of 80%-90%. 80% of the wastewater is converted into high-quality reclaimed water with a conductivity of <100µS / cm, and the remaining 20% ​​concentrate contains industrial-grade sulfate with a purity of over 99%. Attached Figure Description

[0017] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 A flowchart of a water treatment process for phosphogypsum slag dumps according to one embodiment of the present invention; Figure 2 This is a schematic diagram of the connection of a membrane-coupled chemical reactor according to one embodiment of the present invention; Figure 3 This is the flux change curve of the second-stage membrane-coupled chemical reactor obtained from the experimental examples of this invention; Figure 4 The figure shows the flux variation curve of the third-stage membrane-coupled chemical reactor obtained from the experimental example of this invention.

[0019] Icons: 1-1-Membrane filter; 1-2-Reaction tank; 1-3-Agitator; 1-4-Inlet pump; 1-5-Chemical cleaning water tank; 1-6-Chemical cleaning pump; 1-7-Automatic inlet valve; 1-8-Automatic backwash drain valve; 1-9-Automatic exhaust valve; 1-10-Automatic sludge discharge valve; 1-11-Automatic chemical cleaning inlet valve; 1-12-Automatic chemical cleaning return valve; 1-13-Air inlet valve. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a process for treating water from a phosphogypsum slag disposal site based on a membrane-coupled chemical reactor, comprising the following steps: The phosphogypsum slag site water is treated through a first-stage membrane coupled chemical reactor to induce the selective formation of calcium fluoride precipitate by fluoride ions, and the mud and water are separated to obtain acidic fluoride-containing mud and first-stage permeate. The first-stage permeate is treated by a second-stage membrane-coupled chemical reactor to generate dicalcium phosphate and silicate precipitates, and the mud and water are separated to obtain neutral phosphorus-containing silica mud and second-stage permeate. The second-stage permeate is treated through a third-stage membrane-coupled chemical reactor to produce magnesium ammonium phosphate and calcium fluorophosphate precipitates, separating the sludge and water to obtain alkaline nitrogen- and phosphorus-containing sludge and third-stage permeate. Membrane-coupled chemical reactors include, but are not limited to, reaction tanks and membrane filters; The membrane filter is equipped with a multi-ring pulse airflow disturbance device above the bottom sludge hopper; The water inlet to the membrane filter enters the tank tangentially along the inner wall of the tank, thereby creating a cleaning effect by rinsing the surface of the membrane tube. Sludge-water separation includes sludge-water separation via membrane filters.

[0022] The process of this invention generates precipitates of different components through multi-stage chemical reactions, and then achieves mud-water separation through multi-stage membrane filters. The tangential flow design inside the membrane filters and the multi-ring pulsed airflow disturbance device facilitate the hydraulic flushing and shedding of the mud cake layer on the surface of the membrane tubes.

[0023] The process of this invention not only has a low membrane fouling rate and a membrane flux decline rate of <5% / day, reducing the cleaning frequency to once a month, but also achieves thorough resource recovery of valuable elements, high product purity, and the value of the resource-recovered products per ton of water treated exceeds the treatment cost. It realizes water resource recycling, generates no solid waste, and has significant environmental benefits.

[0024] The process of this invention can produce calcium fluoride sludge with a purity of 40%-60%, calcium hydrogen phosphate sludge with a purity of 80%-95%, and magnesium ammonium phosphate sludge with a purity of 80%-90%. 80% of the wastewater is converted into high-quality reclaimed water with a conductivity of <100µS / cm, and 20% of the concentrate contains industrial-grade sulfate with a purity of over 99%.

[0025] In this invention, a membrane-coupled chemical reactor resistant to high concentrations of suspended solids is introduced as the core of the treatment process. By combining chemical reactions with high-precision filtration, the problems of low mud-water separation efficiency and severe membrane fouling in conventional processes can be solved.

[0026] In a preferred embodiment, the bottom of the reaction tank can be a conical mud hopper with a cone angle of 40°-70°, which is more conducive to the accumulation of mud cake falling off the membrane surface and more thorough mud removal.

[0027] In a preferred embodiment, the inlet pipe of the membrane filter enters the tank along the tangential direction of the inner wall of the tank. Alternatively, a tangential guide vane can be installed at the vertical tangential inlet.

[0028] In this invention, the inlet and outlet of the tangential guide plate are both welded inside the tank; the inlet of the tangential guide plate is aligned with the water inlet, and the outlet is aligned with the tangential direction of the inner wall of the tank. Meanwhile, the material of the tangential guide vane can be the same as that of the tank body.

[0029] The tangential flow water inlet design of this invention changes the water inlet direction of the traditional vertical tank tangent, which is more conducive to the hydraulic flushing and shedding of the mud cake layer on the surface of the membrane tube.

[0030] In this invention, the multi-ring pulse airflow disturbance device includes, but is not limited to, multiple independent annular aeration pipes, and the air intake and shut-off of the annular aeration pipes are controlled by an automatic valve; the annular aeration pipes can be located at the top of the conical sludge hopper, with a diameter slightly smaller than the inner diameter of the tank.

[0031] In a preferred embodiment, the diameter of the annular aeration pipe can be 5mm-200mm, for example, 40mm, 80mm, or 100mm, but is not limited thereto; the spacing between each annular aeration pipe can be 50mm-500mm, for example, 100mm, 300mm, or 400mm, but is not limited thereto. The smaller the spacing, the fewer dead corners that the multi-ring pulse airflow disturbance device cannot reach, and the more thorough the membrane module cleaning.

[0032] In a preferred embodiment, each annular aeration pipe may be equipped with two rows of jet nozzles angled downwards at 45° to the vertical. The jetting frequency is synchronized with the backwashing cycle, and pulse aeration is performed sequentially from the outer ring to the inner ring. The air pressure can be 0.1MPa-0.3MPa, which is more conducive to the formation of turbulence on the membrane surface, thereby more thoroughly removing the mud cake layer and pollutants on the membrane surface.

[0033] In this invention, the outlet of the reaction tank of the membrane coupled chemical reactor is connected to the inlet of the inlet pump, and the outlet of the inlet pump is connected to the inlet of the membrane filter. The reaction tank can be equipped with a stirrer and online monitoring instruments to control the dosage of reagents and the reaction time. The online monitoring instruments include, but are not limited to, pH meters, thermometers and elemental analyzers.

[0034] In a preferred embodiment, the membrane-coupled chemical reactor can be operated automatically by switching automatic valves. The operation includes, but is not limited to, filtration, backwashing, sludge discharge, and venting. After filtration for 1 min to 60 min, pulse backwashing is performed for 1 s to 60 s, and sludge discharge is performed for 1 h to 4 h.

[0035] In a preferred embodiment, the membrane-coupled chemical reactor may also be equipped with membrane fouling monitoring instruments and a chemical cleaning system for automatically identifying backwashing and chemical cleaning cycles, with a membrane flux decay rate of <5% / day.

[0036] This invention employs a three-step sequential process for recovering fluorine via acid, phosphorus and silicon via neutral reaction, and nitrogen and magnesium via alkaline reaction. The entire process can be automated using a central control system. Firstly, each reactor stage is equipped with online pH monitoring, and a PID algorithm is used to control the lime slurry dosage, ensuring pH fluctuations are within ±0.2. Secondly, the transmembrane pressure difference and flux decay rate are linked as dual parameters to trigger backwashing or chemical cleaning procedures. Thirdly, the mud concentration and composition are analyzed online, and feedback is used to adjust reagent dosage and reaction time. This allows for more thorough resource conversion, avoids co-precipitation, and thus results in higher product purity.

[0037] In this invention, the permeate outlet of the first-stage membrane coupled chemical reactor is connected to the inlet of the second-stage membrane coupled chemical reactor, the permeate outlet of the second-stage membrane coupled chemical reactor is connected to the inlet of the third-stage membrane coupled chemical reactor, the permeate outlet of the third-stage membrane coupled chemical reactor is connected to the inlet of the reverse osmosis membrane concentration process, the permeate outlet of the reverse osmosis concentration process is connected to the external water point, the concentrate of the reverse osmosis concentration process is connected to the inlet of the nanofiltration membrane desalination system, the permeate of the nanofiltration membrane desalination system is connected to the external wastewater pool, and the concentrate of the nanofiltration membrane system is connected to the inlet of the evaporation crystallization process (if high-purity sulfate solids are required). Simultaneously, the sludge discharge outlets of the three-stage membrane coupled chemical reactors are respectively connected to the inlet of the corresponding sludge concentrate dewatering system, the filtrate outlet of the sludge dewatering device is connected to the inlet of the corresponding membrane coupled chemical reactor, and the sludge outlet of the sludge dewatering device is used to discharge sludge.

[0038] In a preferred embodiment, the processing method of the present invention includes the following steps: (a) First-stage acidic fluoride recovery process: Water from the phosphogypsum slag site and lime slurry are added to the first-stage membrane coupled chemical reactor and mixed. The calcium-fluoride molar ratio Ca / F is 1.0-1.2, the hydraulic retention time is 5 min-60 min, the stirring speed is 50 rpm-300 rpm, and the pH is controlled at 2.0-4.0 to induce the selective formation of calcium fluoride precipitate by fluoride ions. The reaction mixture is separated into sludge and water in the first stage by passing through a membrane filter and / or gravity sedimentation process to obtain acidic fluoride-containing sludge and first-stage permeate. (b) Second-stage neutral phosphorus and silicon recovery process: The first-stage product water, lime slurry and silicon removal agent are added to the second-stage membrane coupled chemical reactor and mixed. The magnesium / silicon molar ratio is 0.8-1.5, the hydraulic retention time is 5 min-60 min, the stirring speed is 50 rpm-300 rpm, and the pH is controlled at 7.5-8.5. Simultaneously, dicalcium phosphate and silicate precipitates are generated. The reaction mixture is passed through a membrane filter for second-stage mud-water separation to obtain neutral phosphorus and silicon sludge and second-stage product water. (c) Third-stage alkaline nitrogen and magnesium recovery process: The second-stage product water and complexing agent are added to the third-stage membrane coupled chemical reactor and mixed. The magnesium / nitrogen / phosphorus molar ratio is 1.2:1:1.1, the hydraulic retention time is 5min-60min, the stirring speed is 50rpm-300rpm, and the pH is controlled at 9.0-11.0. Magnesium ammonium phosphate and calcium fluorophosphate precipitates are generated by directional crystallization. The reaction mixture is then separated into third-stage mud and water through a membrane filter to obtain alkaline nitrogen and phosphorus sludge and third-stage product water.

[0039] F in the water of phosphogypsum slag dump - The content is 0-8000 mg / L, SO4 2- The content is 0-30000 mg / L, PO4 3- The content is 0-15000 mg / L, NH4 + The content is 0-1000 mg / L, and the total silicon content is 0-5000 mg / L.

[0040] In a preferred embodiment, the silicon remover includes, but is not limited to, at least one of magnesium salt and aluminum salt, and the mass ratio of magnesium salt to aluminum salt can be in the range of 0.1-10.

[0041] In a preferred embodiment, the complexing agent includes, but is not limited to, at least one of phosphate, sodium carbonate, and ferric sulfate, and the mass ratio of phosphate to sodium carbonate can be in the range of 1-10.

[0042] In a preferred embodiment, the process of the present invention further includes a neutralization and salt concentration step, wherein the third-stage permeate is neutralized with acid to a pH of 6-8, and then concentrated by reverse osmosis membrane and / or separated by nanofiltration membrane to obtain industrial recycled water and high-purity sulfate products.

[0043] The recovery rate of reverse osmosis membrane concentration is 70%-75%, and the conductivity of the product water is <100μS / cm; the sulfate rejection rate of nanofiltration membrane desalination is >98%, the chloride ion permeability is >90%, and the nanofiltration concentrate is evaporated and crystallized to obtain anhydrous sulfate crystals with a purity ≥99%.

[0044] Sulfate ions are recovered as anhydrous sulfate with a purity of >99%, while high-quality reclaimed water with a conductivity of <100µS / cm is obtained, achieving near-zero discharge and deep resource utilization with virtually no wastewater discharge.

[0045] In a preferred embodiment, the processing technology of the present invention further includes the steps of mud concentration and dewatering, wherein different mud materials are respectively fed into independent mud concentration tanks and mud dewatering machines for dewatering treatment to obtain dewatered mud cake and filtrate.

[0046] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0047] Example 1 A process for treating phosphogypsum slag wastewater based on a membrane-coupled chemical reactor is described below. Figure 1 This includes the following steps: (a) First-stage acidic fluoride recovery process: Water from the phosphogypsum slag site and lime slurry are added to the first-stage membrane coupled chemical reactor and mixed. The calcium-fluoride molar ratio Ca / F is 1.0-1.2, the hydraulic retention time is 5 min-60 min, the stirring speed is 50 rpm-300 rpm, and the pH is controlled at 2.0-4.0 to induce the selective formation of calcium fluoride precipitate by fluoride ions. The reaction mixture is then separated into first-stage mud and water through a membrane filter to obtain acidic fluoride-containing mud and first-stage permeate. (b) Second-stage neutral phosphorus and silicon recovery process: The first-stage product water, lime slurry and silicon removal agent are added to the second-stage membrane coupled chemical reactor and mixed. The magnesium / silicon molar ratio is 0.8-1.5, the hydraulic retention time is 5 min-60 min, the stirring speed is 50 rpm-300 rpm, and the pH is controlled at 7.5-8.5. Simultaneously, dicalcium phosphate and silicate precipitates are generated. The reaction mixture is passed through a membrane filter for second-stage mud-water separation to obtain neutral phosphorus and silicon sludge and second-stage product water. (c) Third-stage alkaline nitrogen and magnesium recovery process: The second-stage product water and complexing agent are added to the third-stage membrane coupled chemical reactor and mixed. The magnesium / nitrogen / phosphorus molar ratio is 1.2:1:1.1, the hydraulic retention time is 5min-60min, the stirring speed is 50rpm-300rpm, and the pH is controlled at 9.0-11.0. Magnesium ammonium phosphate and calcium fluorophosphate precipitates are generated by directional crystallization. The reaction mixture is then separated into third-stage mud and water through a membrane filter to obtain alkaline nitrogen and phosphorus sludge and third-stage product water.

[0048] In this embodiment, the membrane-coupled chemical reactor includes a reaction tank and a membrane filter; The bottom of the membrane filter is a conical sludge hopper with a cone angle of 50°. A multi-ring pulse airflow disturbance device is installed above the sludge hopper. The pulse airflow disturbance device includes annular aeration pipes with a diameter of φ50mm. The spacing between every two aeration rings is 300mm. There are a total of four annular aeration pipes, which are connected to four air inlet pipes and four automatic valves respectively. The annular aeration pipes are equipped with two rows of jet nozzles that are inclined downward at 45° to the vertical. The jet frequency is synchronized with the backwashing cycle. The inlet pipe of the membrane filter enters the tank tangentially, and its function is to generate tangential flow to wash the membrane surface.

[0049] For specific connection methods of membrane-coupled chemical reactors, please refer to Figure 2 The main equipment consists of membrane filter 1-1 and reaction tank 1-2. The reaction tank 1-2 is equipped with a stirrer 1-3 and a detection instrument. The membrane coupled chemical reactor is equipped with auxiliary systems such as a dosing device and a chemical cleaning device. The chemical cleaning device includes a chemical cleaning water tank 1-5 and a chemical cleaning pump 1-6. The outlet of reaction tank 1-2 is connected to the suction port of water pump 1-4. The outlet of water pump 1-4 is connected to the inlet of membrane filter 1-1. The product water port of membrane filter 1-1 is connected to the external water point of the system. The backwash outlet and exhaust port of membrane filter 1-1 are respectively connected to the return port of reaction tank 1-2. The return port of membrane filter 1-1 is connected to the suction pipe of water pump 1-4. The sludge discharge port of membrane filter 1-1 is connected to the external sludge storage system of the system. The auxiliary system is connected as follows: the outlet of the dosing device is connected to the inlet of reaction tank 1-2; the outlet of chemical cleaning water tank 1-5 is connected to the suction port of chemical cleaning pump 1-6; the outlet of chemical cleaning pump 1-6 is connected to the chemical cleaning inlet of membrane filter 1-1; the chemical cleaning return port of membrane filter 1-1 is connected to the return port of chemical cleaning water tank 1-5; the air inlet of membrane filter 1-1 is connected to an external air source pipe; and the air outlet of membrane filter 1-1 is connected to an external drainage system. The automatic valve group of the membrane coupled chemical reactor includes automatic inlet valve 1-7, automatic backwash drain valve 1-8, automatic vent valve 1-9, automatic sludge discharge valve 1-10, automatic chemical cleaning inlet valve 1-11, and automatic chemical cleaning return valve 1-12. The automatic valves are located as follows: automatic inlet valve 1-7 is located on the pipeline between the outlet and inlet of inlet pump 1-4; automatic backwash drain valve 1-8 is located on the pipeline between the backwash outlet and the return port of reaction tank 1-2; automatic vent valve 1-9 is located on the pipeline between the vent outlet and the return port of reaction tank 1-2; automatic sludge discharge valve 1-10 is located on the sludge discharge port pipeline; automatic chemical cleaning inlet valve 1-11 is located on the outlet pipeline of chemical cleaning pump 1-6; and automatic chemical cleaning return valve 1-12 is located on the chemical cleaning return port pipeline.

[0050] In this embodiment, the operating procedure of the membrane-coupled chemical reactor includes: (1) For conventional water inlet filtration, only open the automatic water inlet valves 1-7, turn on the water inlet pumps 1-4 and keep them running at the designed water production flow rate, and perform conventional vortex water inlet filtration mode for 10 minutes; (2) Backwashing: Open only the backwash drain automatic valve 1-8, exhaust automatic valve 1-9, and air inlet valve 1-13 until the liquid level in the product water area of ​​the cyclone membrane filter 1-1 drops to the set value and the backwashing ends. The four air inlet valves 1-13 are quickly switched on and off in sequence. (3) Sludge discharge: only open the sludge discharge automatic valve 1-10 and the exhaust automatic valve 1-9 to discharge the sludge accumulated in the sludge hopper area into the tank. (4) Exhaust air: Open only the automatic water inlet valve 1-7 and the automatic exhaust valve 1-9, and turn on the water inlet pump 1-4 to introduce liquid and exhaust air until the filter area is filled with mixed liquid; (5) Chemical cleaning: only open the chemical cleaning inlet automatic valve 1-11 and the chemical cleaning return automatic valve 1-12, and turn on the chemical cleaning pump 1-6. The chemical cleaning solution enters the membrane filter tank and cleans the contaminants on the membrane surface through a dual action of flushing and chemical dissolution. Run and cycle through steps (1), (2), (3) and (4) in sequence. By using the detection instruments, including pressure transmitters and differential pressure transmitters, the two parameters of transmembrane pressure difference and flux attenuation rate are linked to trigger the backwashing or chemical cleaning program.

[0051] The processing technology in this embodiment also includes the steps of mud concentration and dewatering, that is, different mud materials are respectively put into independent mud concentration tanks and mud dewatering machines for dewatering treatment to obtain dewatered mud cake and filtrate.

[0052] Example 2 The only difference between this embodiment and Embodiment 1 is that the treatment process also includes the steps of neutralization and salt concentration, that is, the third-stage permeate is neutralized with acid to a pH of 6-8, and then concentrated by reverse osmosis membrane and separated by nanofiltration membrane to obtain industrial recycled water and high-purity sulfate products. The recovery rate of reverse osmosis membrane concentration is 75%, and the conductivity of the product water is <100μS / cm; the sulfate rejection rate of nanofiltration membrane desalination is >98%, the chloride ion permeability is >90%, and the nanofiltration concentrate is evaporated and crystallized to obtain anhydrous sodium sulfate crystals with a purity ≥99%.

[0053] Comparative Example 1 The only difference between this comparative example and Example 1 is that step (a) of the first-stage acidic fluorine recovery process was not performed. Everything else is the same as in Example 1.

[0054] Compared with Example 1, the drawback of this comparative example is that calcium fluoride and calcium hydrogen phosphate are completely mixed together, and calcium fluoride becomes an impurity that affects the purity of calcium hydrogen phosphate, reducing the purity of calcium hydrogen phosphate by 50%. In addition, calcium fluoride partially dissolves in a slightly alkaline environment, which is not conducive to the removal of fluoride ions from water.

[0055] Comparative Example 2 The only difference between this comparative example and Example 1 is that step (b) of the second-stage neutral phosphorus and silicon recovery process was not performed. Everything else is the same as in Example 1.

[0056] Compared with Example 1, the drawback of this comparative example is that the mixing of calcium phosphate and magnesium ammonium phosphate affects the purity, and a large amount of silicate will partially redissolve in a slightly alkaline environment, resulting in excessive silicon concentration in the product water, which cannot meet the requirements of reverse osmosis membrane feed water.

[0057] Comparative Example 3 The only difference between this comparative example and Example 1 is that step (c) of the third-stage alkaline nitrogen and magnesium recovery process was not performed. Everything else is the same as in Example 1.

[0058] Compared with Example 1, the drawback of this comparative example is that the permeate contains high concentrations of calcium ions, magnesium ions, and dissolved silica, which does not meet the requirements for reverse osmosis membrane feed water.

[0059] Comparative Example 4 The only difference between this comparative example and Example 1 is that the sludge discharge port is not equipped with a pulse airflow disturbance device; Everything else is the same as in Example 1.

[0060] Compared with Example 1, the drawback of this comparative example is that it lacks bubble disturbance and scrubbing, the cleaning effect on the membrane surface is not thorough, and the chemical cleaning interval of the membrane system is shortened by 20% to 30%.

[0061] Test case The treatment process of this invention is used to treat the phosphogypsum slag wastewater generated from phosphoric acid production in a chemical plant for resource recovery. The resource recovery targets are fluoride-containing sludge, phosphorus-containing sludge, magnesium ammonium phosphate-containing sludge, and reclaimed water. The water quality of the phosphogypsum slag wastewater is as follows: F - Approximately 3100 mg / L, total phosphorus approximately 11000 mg / L, Ca 2+ Approximately 540 mg / L, Mg 2+ Approximately 500 mg / L, ammonia nitrogen approximately 320 mg / L, and total silicon approximately 2200 mg / L.

[0062] The first-stage acidic fluoride recovery process involves mixing phosphogypsum slag water and lime slurry in a first-stage membrane-coupled chemical reactor. The calcium-fluoride molar ratio (Ca / F) is 1.1, the hydraulic retention time is 30 min, the stirring speed is 200 rpm, and the pH is controlled at 3. This induces the selective formation of calcium fluoride from fluoride ions. The reaction mixture undergoes first-stage sludge-water separation through a membrane filter, yielding acidic sludge containing calcium fluoride, calcium dihydrogen phosphate, and calcium sulfate, as well as first-stage permeate. The acidic sludge is then fed into a sludge thickening and dewatering system for dewatering, resulting in a sludge cake (mainly containing calcium fluoride, calcium dihydrogen phosphate, and calcium sulfate in a mass ratio of approximately 4:4:1). The filtrate then enters the second-stage membrane-coupled chemical reactor. The second-stage neutral phosphorus and silicon recovery process involves mixing the first-stage permeate, lime slurry, and silicon removal agent in a second-stage membrane-coupled chemical reactor. The magnesium / silicon molar ratio (Mg / Si) is 1.2, the hydraulic retention time is 30 minutes, the stirring speed is 200 rpm, and the pH is controlled at 7.5. Simultaneously, dicalcium phosphate and silicate solids are generated. The reaction mixture undergoes second-stage sludge-water separation through a membrane filter to obtain neutral phosphorus- and silica-containing sludge and second-stage permeate. The neutral sludge is then fed into a sludge thickening and dewatering system for dewatering, yielding a sludge cake (mainly containing dicalcium phosphate and silica compounds in a mass ratio of approximately 6:1). The filtrate enters the third-stage membrane-coupled chemical reactor. The flux variation curve of the second-stage membrane-coupled chemical reactor is shown in the figure. Figure 3 It can be seen that the membrane specific flux is stable at 1.4m. 3 / m 2 The membrane flux decay rate can be controlled to below 5%, ensuring stable water production from the membrane system and long chemical cleaning intervals. The third-stage alkaline nitrogen and magnesium recovery process: The second-stage permeate and complexing agent are added to the third-stage membrane-coupled chemical reactor and mixed. The magnesium / nitrogen / phosphorus molar ratio (Mg / N / P) is 1.2:1:1.1, the hydraulic retention time is 20 min, the stirring speed is 200 rpm, and the pH is controlled at 9.5. Oriented crystallization produces magnesium ammonium phosphate and calcium fluorophosphate. The reaction mixture is then passed through a membrane filter for third-stage sludge-water separation to obtain alkaline nitrogen- and magnesium-containing sludge and third-stage permeate. The water quality is shown in Table 1. After acid neutralization, the sludge enters the reverse osmosis membrane system. The discharged sludge enters the sludge thickening and dewatering system for dewatering treatment, resulting in sludge cake (mainly containing magnesium ammonium phosphate, calcium fluorophosphate, and calcium carbonate, with a mass ratio of approximately 25:5:1). The flux variation curve of the third-stage membrane-coupled chemical reactor is shown in the figure. Figure 4 It can be seen that the membrane specific flux is stable at 0.7m. 3 / m 2 The membrane flux decay rate can be controlled to below 5%, ensuring stable water production from the membrane system and long chemical cleaning intervals. Table 1. Product water quality of the third-stage membrane-coupled chemical reactor

[0063] The reverse osmosis membrane system adopts a single-stage two-stage reverse osmosis membrane module arrangement, using DuPont BW30XFR-400 / 34i reverse osmosis membrane modules. The product water recovery rate is 70%, the product water conductivity is <100μS / cm, and the concentrate TDS is >20000mg / L.

[0064] In summary, the treatment process of this invention can effectively separate fluorine, phosphorus, and nitrogen from water to form sludge with resource value. The cost per ton of water treated is less than the value of the resource products. 80% of the produced water is recovered to meet industrial reuse standards, which is conducive to achieving near-zero discharge and deep high-value resource utilization of phosphogypsum slag.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A treatment process for phosphogypsum slag field water based on a membrane-coupled chemical reactor, characterized in that, Includes the following steps: The phosphogypsum slag site water is treated through a first-stage membrane coupled chemical reactor to induce the selective formation of calcium fluoride precipitate by fluoride ions, and the mud and water are separated to obtain acidic fluoride-containing mud and first-stage permeate. The first-stage permeate is treated through a second-stage membrane-coupled chemical reactor to generate dicalcium phosphate and silicate precipitates. The mud and water are then separated to obtain neutral phosphorus-containing silica mud and second-stage permeate. The second-stage permeate is treated through a third-stage membrane-coupled chemical reactor to generate magnesium ammonium phosphate and calcium fluorophosphate precipitates, and the mud and water are separated to obtain alkaline nitrogen- and phosphorus-containing mud and third-stage permeate. The membrane-coupled chemical reactor includes a reaction tank and a membrane filter; The membrane filter is equipped with a multi-ring pulse airflow disturbance device at the bottom sludge discharge port; The water inlet to the membrane filter enters the tank along the tangential direction of the inner wall of the tank, thereby producing a cleaning effect by rinsing the surface of the membrane tube. The mud-water separation includes mud-water separation through the membrane filter.

2. The processing technology according to claim 1, characterized in that, The inlet pipe of the membrane filter enters the tank along the tangential direction of the inner wall of the tank; Alternatively, a tangential guide vane can be installed at the vertical tangential inlet.

3. The processing technology according to claim 2, characterized in that, The inlet and outlet of the tangential guide vane are both welded inside the tank. The inlet of the tangential guide vane is aligned with the water inlet, and the outlet is aligned with the tangential direction of the inner wall of the tank. The material of the tangential guide vane is the same as that of the tank body.

4. The processing technology according to claim 1, characterized in that, The multi-ring pulse airflow disturbance device is installed above the conical mud hopper of the tank body; Preferably, the cone angle of the conical mud hopper is 40°-70°; Preferably, the multi-ring pulse airflow disturbance device includes multiple independent annular aeration pipes, and the air intake and shut-off of the annular aeration pipes are controlled by an automatic valve; Preferably, the annular aeration pipe is located at the top of the conical sludge hopper and is fixed by a pre-reserved gate on the inner wall of the tank; Preferably, the annular aeration pipe is provided with two rows of jet nozzles angled downwards at 45° to the vertical. The jetting frequency is synchronized with the backwashing cycle. The aeration pipe is activated by pulses from the inside out or from the outside in, generating multi-ring bubbles that rise from the center to the tank wall and then clean the membrane surface, thus avoiding dead zones that the bubbles cannot reach.

5. The processing method according to any one of claims 1-4, characterized in that, The membrane-coupled chemical reactor operates automatically via automatic valve switching; Preferably, the operation includes filtration, backwashing, sludge removal, and venting; Preferably, after filtration for 1 min to 60 min, pulse backwashing is performed for 1 s to 60 s, followed by sludge discharge for 1 h to 4 h; Preferably, the membrane-coupled chemical reactor is equipped with membrane fouling monitoring instruments and a chemical cleaning system to automatically identify backwashing and chemical cleaning cycles, with a membrane flux decay rate of <5% / day.

6. The processing method according to any one of claims 1-4, characterized in that, The processing technology includes the following steps: (a) Add the phosphogypsum slag water and lime slurry to the first-stage membrane coupled chemical reactor and mix them. Control the pH to 2.0-4.0 to induce the selective formation of calcium fluoride precipitate by fluoride ions. The reaction mixture is then separated into acidic fluoride-containing sludge and first-stage permeate by passing it through a membrane filter. (b) The first-stage permeate, lime slurry and silica removal agent are added to the second-stage membrane coupled chemical reactor and mixed. The pH is controlled at 7.5-8.

5. Simultaneously, dicalcium phosphate and silicate precipitates are generated. The reaction mixture is then separated into sludge and water in the second stage through a membrane filter to obtain neutral phosphorus-containing silica sludge and second-stage permeate. (c) The second-stage permeate and complexing agent are added to the third-stage membrane coupled chemical reactor and mixed. The pH is controlled at 9.0-11.

0. Magnesium ammonium phosphate and calcium fluorophosphate precipitates are generated by directional crystallization. The reaction mixture is then separated into sludge and water in the third stage through a membrane filter to obtain alkaline nitrogen and phosphorus sludge and third-stage permeate.

7. The processing technology according to claim 6, characterized in that, The water quality suitable for the treatment process is: F - The content is 0-8000 mg / L, SO4 2- The content is 0-30000 mg / L, PO4 3- The content is 0-15000 mg / L, NH4 + The content is 0-1000 mg / L, and the total silicon content is 0-5000 mg / L.

8. The processing technology according to claim 6, characterized in that, The silicon remover includes at least one of magnesium salt and aluminum salt; Preferably, the mass ratio of the magnesium salt to the aluminum salt is in the range of 0.1-10; Preferably, the complexing agent includes at least one selected from phosphate, sodium carbonate, and ferric sulfate; Preferably, the mass ratio of the phosphate to sodium carbonate is in the range of 1-10.

9. The processing technology according to claim 6, characterized in that, The processing technology further includes the following steps: The third-stage permeate is neutralized to a pH of 6-8, and then concentrated through a reverse osmosis membrane and / or separated through a nanofiltration membrane to obtain industrial recycled water and sulfate products.

10. The processing method according to claim 6, characterized in that, The processing technology further includes the following steps: The mud obtained from each step is dewatered to obtain dewatered mud cake and filtrate.

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