Method for treating wastewater generated in preparation of phosphate from phosphoric acid by wet process

Abstract

The invention discloses a method for treating wastewater generated in preparation of phosphate from wet-process phosphoric acid, and belongs to the technical field of industrial wastewater treatment. The method sequentially comprises the following four steps: pretreatment recovery, adsorption impurity removal, catalytic oxidation and membrane filtration reuse: firstly, adjusting the pH value of the wastewater to 8.5-9.5, and adding magnesium salt and ammonium salt to generate struvite to recover phosphorus so as to obtain primary treatment water; adding 20-30 g / L of modified ceramsite, adjusting the pH value to 10-11, and adsorbing for 4-6 hours to obtain secondary treated water; adding 2-2.5 g / L of a composite photocatalytic material, and irradiating with visible light for 2-4 hours to degrade organic matters, so as to obtain tertiary treated water; and finally, recycling produced water through membrane filtration, and evaporating and crystallizing concentrated water. The method has the advantages of small sludge amount, high resource recovery rate, synchronous deep removal of phosphorus, fluorine, heavy metals and organic matters, up-to-standard reuse of effluent, and substantial environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, specifically relating to a method for treating high-concentration phosphorus- and fluoride-containing acidic wastewater generated during the wet-process phosphoric acid production of phosphates. Background Technology

[0002] Phosphate is an important raw material in industrial production, widely used in fertilizers, food additives, detergents, and other fields. Wet-process phosphoric acid refers to phosphoric acid produced by decomposing phosphate rock with inorganic acids such as sulfuric acid, nitric acid, or hydrochloric acid. Wet-process phosphoric acid is the main raw material for producing phosphate fertilizers and various phosphates. However, the production process of phosphates using wet-process phosphoric acid generates a large amount of high-concentration wastewater. Due to the large amount of impurities in wet-process phosphoric acid, purification operations such as chemical precipitation and solvent extraction are often required to produce high-purity phosphates. These processes generate wastewater with high concentrations of organic matter, heavy metals, and phosphorus. The neutralization reaction of phosphoric acid with alkalis (such as sodium hydroxide, sodium carbonate, ammonia, etc.) to produce phosphates also generates a large amount of wastewater containing phosphates, fluoride ions, heavy metal ions (such as cadmium, lead, and zinc), suspended solids, and acidic components. This wastewater typically has the significant characteristics of "high turbidity, high hardness, high fluoride, high phosphorus, and strong acidity." Direct discharge will lead to eutrophication of water bodies, soil pollution, and ecological damage, while also causing a serious waste of phosphorus resources.

[0003] Currently, the traditional process of "lime neutralization + precipitation" is commonly used to treat this type of wastewater. This method neutralizes the acidity by adding lime slurry (Ca(OH)2) and removes phosphorus and fluoride by forming precipitates such as fluorapatite. However, this method has the following significant drawbacks: it generates a large amount of sludge with high water content, which easily causes secondary pollution and results in high disposal costs; the effluent has high hardness (high calcium ion concentration), is prone to scaling, and is difficult to reuse in production systems; its removal effect on low-concentration phosphates and specific forms of phosphorus (such as polyphosphates) is limited, making it difficult to consistently meet increasingly stringent emission standards; and it fails to achieve effective recovery of valuable resources such as phosphorus and fluoride from the wastewater.

[0004] Therefore, developing a new treatment process that can efficiently remove pollutants, reduce sludge production, and enable the reuse of water resources and some valuable resources is of great environmental and economic significance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for treating wastewater generated during the wet-process phosphoric acid preparation of phosphates, which has high treatment efficiency, low sludge production, and enables wastewater reuse and resource recovery.

[0006] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution: A method for treating wastewater generated during wet-process phosphoric acid production of phosphates includes the following steps: a) After passing through the screen and equalization tank, the wastewater enters the struvite reaction tank. The pH of the wastewater is adjusted to 8.5-9.5 with NaOH, magnesium salt and ammonium salt are added, and the reaction is carried out for 30-45 minutes. The reaction mixture enters the sedimentation tank, and after solid-liquid separation, struvite crystals and primary treated water are obtained. b) Add modified ceramsite to the primary treated water, adjust the pH to 10-11, carry out the adsorption reaction for 4-6 hours, and then separate the solid and liquid to obtain secondary treated water; c) Add composite photocatalytic material to the secondary treated water, and use a xenon lamp with a filter to simulate visible light (420~800nm) irradiation for catalytic degradation treatment for 2-4 hours to obtain tertiary treated water; d) The tertiary treated water is subjected to membrane filtration, the permeate is reused, and the concentrate is subjected to evaporation and crystallization treatment.

[0007] Preferably, in step a), the magnesium salt is selected from one or more of magnesium chloride, magnesium sulfate or magnesium oxide; the ammonium salt is selected from ammonium chloride or ammonium sulfate; and the molar ratio of magnesium, phosphorus and nitrogen is (1.0-1.5):1:(1.0-1.2).

[0008] Preferably, the amount of modified ceramsite added is 20-30 g / L.

[0009] Preferably, the dosage of the composite photocatalytic material is 2-2.5 g / L.

[0010] Preferably, the modified ceramsite is prepared by the following method: (1) Mix red mud, river sludge and kaolin according to the mass ratio, slowly add water to the mixture, roll it into balls in a granulator, and screen out raw material balls of 2-5 mm. (2) In an air atmosphere, heat the material in a muffle furnace from room temperature to 500-600℃ at 3-5℃ / min, hold for 20-30 minutes, then heat it to 1050-1150℃ at 8-10℃ / min, hold for 20-30 minutes, and remove it after the temperature in the furnace drops to room temperature to obtain ceramsite. (3) The obtained ceramsite was immersed in ZrOCl2 solution according to the solid-liquid ratio. The pH of the solution was adjusted to 3-4 with 0.1 mol / L dilute hydrochloric acid solution. Then the mixture was transferred to a high-pressure reactor with polytetrafluoroethylene lining and reacted at 120-130℃ for 6-7 hours. After natural cooling, the ceramsite was taken out, rinsed with deionized water until neutral, and dried to obtain Zr-modified ceramsite. (4) The obtained Zr-modified ceramsite and FeCl3 solution are mixed at a solid-liquid ratio and then transferred to a high-pressure reactor. The reaction is carried out at 100-120℃ for 4-6 hours. After natural cooling, the ceramsite is taken out, rinsed with deionized water until neutral, and dried to obtain the modified ceramsite.

[0011] Preferably, the mass ratio of red mud, river sludge and kaolin in step (1) is 1:2:(2-3).

[0012] Preferably, in step (3), the solid-liquid ratio of the ceramsite to the ZrOCl2 solution is 1g:10mL; and the concentration of the ZrOCl2 solution is 0.05mol / L.

[0013] Preferably, in step (4), the solid-liquid ratio of Zr-modified ceramsite to FeCl3 solution is 1g:10mL; and the concentration of FeCl3 solution is 0.1mol / L.

[0014] Preferably, the composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred into a muffle furnace and heated to 550°C at a rate of 5°C / min, held for 2 hours, and cooled to room temperature to obtain P-doped g-C3N4. S2: P-doped g-C3N4 is dispersed in deionized water, nano-titanium dioxide and Fe(NO3)3·9H2O are added, stirred evenly, and ultrasonically dispersed for 30-60 min. Then, it is transferred to a high-pressure reactor and reacted at 180℃ for 10-12 h. After the reaction is completed, it is cooled to room temperature and centrifuged. The solid product is washed with deionized water and anhydrous ethanol alternately more than 3 times, and vacuum dried to obtain the composite photocatalytic material.

[0015] Preferably, in step S1, the amount of ammonium dihydrogen phosphate used is 5-8% of the mass of melamine; in step S2, the ratio of P-doped g-C3N4, deionized water, nano titanium dioxide, and Fe(NO3)3·9H2O used is 1g:30mL:(0.5-1)g:0.1g.

[0016] The membrane filtration used in this invention can be any one of microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes.

[0017] This method is applicable to TP 500-5000 mg / L, F - For wastewater with concentrations of 200-1500 mg / L and COD of 200-1000 mg / L, the dosage of reagents or reaction time can be adjusted if the concentrations exceed the specified range.

[0018] The beneficial effects of this invention are as follows: (1) The present invention constructs a multi-stage high-efficiency treatment system of “pretreatment recovery - adsorption and impurity removal - catalytic oxidation - membrane filtration and reuse”. The struvite crystallization process in step a efficiently recovers most of the soluble phosphorus in the wastewater in the pretreatment stage, generating economically valuable magnesium ammonium phosphate fertilizer, realizing the recycling of resources, and significantly reducing the phosphorus load of subsequent treatment units.

[0019] (2) Deep purification effect of modified ceramsite: The modified ceramsite added in step b played a key role. Through Zr-Fe bimetallic modification, the ceramsite formed a porous structure with more active sites on its surface, exhibiting extremely strong specific adsorption capacity for various heavy metal ions (such as arsenic and lead) and fluoride ions in wastewater. Under alkaline conditions, it can achieve deep and simultaneous removal of heavy metals and fluorides through surface complexation, ion exchange, and co-precipitation, effectively solving the problems of incomplete fluoride removal and heavy metal residues in traditional methods, and creating low-interference and low-toxicity influent conditions for subsequent photocatalytic oxidation steps.

[0020] (3) Highly efficient degradation effect of composite photocatalytic material: The composite photocatalytic material added in step c is excited under visible light irradiation, generating highly oxidizing holes (H+). + This material effectively degrades trace organic pollutants (COD), complexed organic matter, and some recalcitrant toxic substances remaining in the effluent from step b, significantly improving the biochemical safety and water quality of the effluent. This step specifically targets dissolved organic pollutants that are difficult to treat by adsorption methods, overcoming the limitations of physical adsorption.

[0021] (4) Synergistic Effect and Enhanced Efficiency: The entire process exhibits significant synergistic effects. Phosphorus recovery in step a reduces the burden on all subsequent units; adsorption treatment in step b removes heavy metal ions that are toxic to or deactivate the photocatalyst and reduces the turbidity of the water, thus ensuring the efficiency and stability of the photocatalytic reaction in step c; while step c further removes organic pollutants that cannot be addressed in the preceding steps. This step-by-step, targeted pollutant removal model results in an overall removal rate of heavy metal ions, fluoride ions, phosphates, and organic pollutants that far exceeds that of a single treatment method, ultimately producing excellent effluent quality that meets high-standard reuse requirements.

[0022] (5) Sludge reduction and resource recycling: Compared with the traditional lime process, this process produces less sludge, and the struvite and modified ceramsite have the potential for resource utilization, resulting in lower operating costs. The end-of-pipe membrane system enables water reuse, and the concentrated water is evaporated and crystallized to ultimately achieve zero wastewater discharge, resulting in significant environmental and economic benefits. Attached Figure Description

[0023] Figure 1 The images show SEM images of the modified ceramsite used in this invention before and after modification, where a represents the unmodified ceramsite and b represents the Zr-Fe modified ceramsite. Figure 2 The images show SEM images of the composite photocatalytic material used in this invention, where a is P-doped g-C3N4 and b is the composite photocatalytic material of this invention. Figure 3This figure shows the effect of different dosages of the modified ceramsite of this invention on the treatment effect of phosphate production wastewater. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to specific embodiments, but it is not limited thereto.

[0025] Example 1 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates includes the following steps: a) After passing through a screen and equalization tank, the wastewater enters the struvite reaction tank. The pH of the wastewater is adjusted to 8.5 with NaOH, magnesium salt and ammonium salt are added, and the reaction is carried out for 30 minutes. The reaction mixture enters the sedimentation tank, and after solid-liquid separation, struvite crystals and primary treated water are obtained. b) Add modified ceramsite to the primary treated water, adjust the pH to 10, and carry out the adsorption reaction for 4 hours. Then, separate the solid and liquid to obtain secondary treated water. The dosage of modified ceramsite is 20 g / L. c) Add composite photocatalytic material to the secondary treated water, and use a xenon lamp with a filter to simulate visible light (420~800nm) for catalytic degradation treatment for 2 hours to obtain tertiary treated water; the dosage of composite photocatalytic material is 2g / L; d) The tertiary treated water is subjected to membrane filtration, the permeate is reused, and the concentrate is subjected to evaporation and crystallization treatment.

[0026] In step a), the magnesium salt is magnesium chloride; the ammonium salt is ammonium chloride; and the molar ratio of magnesium, phosphorus, and nitrogen is 1:1:1.

[0027] The modified ceramsite was prepared using the following method: (1) Mix red mud, river sludge and kaolin in a mass ratio of 1:2:2, slowly add water to the mixture, roll it into balls in a granulator, and screen out raw material balls of 2-5 mm. (2) In an air atmosphere, the temperature is increased from room temperature to 500℃ at 3℃ / min in a muffle furnace and held for 20 minutes. Then the temperature is increased to 1050℃ at 8℃ / min and held for 30 minutes. After the temperature in the furnace drops to room temperature, the ceramsite is obtained. (3) The obtained ceramsite was immersed in a 0.05 mol / L ZrOCl2 solution at a solid-liquid ratio of 1 g: 10 mL. The pH of the solution was adjusted to 4 with a 0.1 mol / L dilute hydrochloric acid solution. The mixture was then transferred to a high-pressure reactor lined with polytetrafluoroethylene and reacted at 130 °C for 6 hours. After natural cooling, the ceramsite was removed, rinsed with deionized water until neutral, and dried to obtain Zr-modified ceramsite. (4) The obtained Zr-modified ceramsite was mixed with 0.1mol / L FeCl3 solution at a solid-liquid ratio of 1g:10mL and then transferred to a high-pressure reactor. The mixture was reacted at 100℃ for 4h, cooled naturally, and the ceramsite was taken out. It was rinsed with deionized water until neutral and then dried to obtain the modified ceramsite.

[0028] The composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred to a muffle furnace, heated to 550°C at a rate of 5°C / min, held at that temperature for 2 hours, and then cooled to room temperature to obtain P-doped g-C3N4; wherein the amount of ammonium dihydrogen phosphate was 5% of the mass of melamine. S2: P-doped g-C3N4 was dispersed in deionized water, and nano-titanium dioxide and Fe(NO3)3·9H2O were added. After stirring evenly, the mixture was ultrasonically dispersed for 30 min, and then transferred to a high-pressure reactor and reacted at 180℃ for 10 h. After the reaction was completed, the mixture was cooled to room temperature and centrifuged. The solid product was washed alternately with deionized water and anhydrous ethanol more than 3 times, and then vacuum dried to obtain the composite photocatalytic material. The ratio of P-doped g-C3N4, deionized water, nano-titanium dioxide, and Fe(NO3)3·9H2O was 1 g: 30 mL: 0.5 g: 0.1 g.

[0029] Example 2 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates includes the following steps: a) After passing through a screen and equalization tank, the wastewater enters the struvite reaction tank. The pH of the wastewater is adjusted to 9.5 with NaOH, magnesium salt and ammonium salt are added, and the reaction is carried out for 45 minutes. The reaction mixture enters the sedimentation tank, and after solid-liquid separation, struvite crystals and primary treated water are obtained. b) Add modified ceramsite to the primary treated water, adjust the pH to 11, and carry out the adsorption reaction for 6 hours. Then, separate the solid and liquid to obtain secondary treated water. The dosage of modified ceramsite is 30 g / L. c) Add composite photocatalytic material to the secondary treated water, and use a xenon lamp with a filter to simulate visible light (420~800nm) irradiation for 4 hours to carry out catalytic degradation treatment to obtain tertiary treated water; the dosage of composite photocatalytic material is 2.5g / L; d) The tertiary treated water is subjected to membrane filtration, the permeate is reused, and the concentrate is subjected to evaporation and crystallization treatment.

[0030] In step a), the magnesium salt is selected from one or more of magnesium chloride, magnesium sulfate, or magnesium oxide; the ammonium salt is selected from ammonium chloride or ammonium sulfate; and the molar ratio of magnesium, phosphorus, and nitrogen is 1.5:1:1.2.

[0031] The modified ceramsite was prepared using the following method: (1) Mix red mud, river sludge and kaolin in a mass ratio of 1:2:3, slowly add water to the mixture, roll it into balls in a granulator, and screen out raw material balls of 2-5 mm. (2) In an air atmosphere, the temperature is increased from room temperature to 600℃ at 3℃ / min in a muffle furnace and held for 20 minutes. Then the temperature is increased to 1150℃ at 10℃ / min and held for 20 minutes. After the temperature in the furnace drops to room temperature, the ceramsite is taken out. (3) The obtained ceramsite was immersed in a 0.05 mol / L ZrOCl2 solution at a solid-liquid ratio of 1 g: 10 mL. The pH of the solution was adjusted to 4 with a 0.1 mol / L dilute hydrochloric acid solution. The mixture was then transferred to a high-pressure reactor lined with polytetrafluoroethylene and reacted at 120 °C for 7 hours. After natural cooling, the ceramsite was removed, rinsed with deionized water until neutral, and dried to obtain Zr-modified ceramsite. (4) The obtained Zr-modified ceramsite was mixed with 0.1mol / L FeCl3 solution at a solid-liquid ratio of 1g:10mL and then transferred to a high-pressure reactor. The mixture was reacted at 120℃ for 6h, cooled naturally, and the ceramsite was taken out. It was rinsed with deionized water until neutral and then dried to obtain the modified ceramsite.

[0032] The composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred to a muffle furnace, heated to 550°C at a rate of 5°C / min, held at that temperature for 2 hours, and then cooled to room temperature to obtain P-doped g-C3N4; wherein the amount of ammonium dihydrogen phosphate was 8% of the mass of melamine. S2: P-doped g-C3N4 was dispersed in deionized water, and nano-titanium dioxide and Fe(NO3)3·9H2O were added. After stirring evenly, the mixture was ultrasonically dispersed for 60 min, and then transferred to a high-pressure reactor and reacted at 180℃ for 12 h. After the reaction was completed, the mixture was cooled to room temperature and centrifuged. The solid product was washed alternately with deionized water and anhydrous ethanol more than 3 times, and then vacuum dried to obtain the composite photocatalytic material. The ratio of P-doped g-C3N4, deionized water, nano-titanium dioxide, and Fe(NO3)3·9H2O was 1 g: 30 mL: 1 g: 0.1 g.

[0033] Example 3 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates includes the following steps: a) After passing through a screen and equalization tank, the wastewater enters the struvite reaction tank. The pH of the wastewater is adjusted to 9.0 with NaOH, magnesium salt and ammonium salt are added, and the reaction is carried out for 40 minutes. The reaction mixture enters the sedimentation tank, and after solid-liquid separation, struvite crystals and primary treated water are obtained. b) Add modified ceramsite to the primary treated water, adjust the pH to 10.5, carry out the adsorption reaction for 5 hours, and then separate the solid and liquid to obtain secondary treated water; the dosage of modified ceramsite is 25 g / L; c) Add composite photocatalytic material to the secondary treated water, and use a xenon lamp with a filter to simulate visible light (420~800nm) irradiation for catalytic degradation treatment for 3 hours to obtain tertiary treated water; the dosage of composite photocatalytic material is 2.2g / L; d) The tertiary treated water is subjected to membrane filtration, the permeate is reused, and the concentrate is subjected to evaporation and crystallization treatment.

[0034] In step a), the magnesium salt is selected from one or more of magnesium chloride, magnesium sulfate, or magnesium oxide; the ammonium salt is selected from ammonium chloride or ammonium sulfate; and the molar ratio of magnesium, phosphorus, and nitrogen is 1.2:1:1.2.

[0035] The modified ceramsite was prepared using the following method: (1) Mix red mud, river sludge and kaolin in a mass ratio of 1:2:2.5, slowly add water to the mixture, roll it into balls in a granulator, and screen out raw material balls of 2-5 mm. (2) In an air atmosphere, the temperature is increased from room temperature to 550°C at 5°C / min in a muffle furnace and held for 30 minutes. Then the temperature is increased to 1100°C at 8°C / min and held for 30 minutes. After the temperature in the furnace drops to room temperature, the ceramsite is obtained. (3) The obtained ceramsite was immersed in a 0.05 mol / L ZrOCl2 solution at a solid-liquid ratio of 1 g: 10 mL. The pH of the solution was adjusted to 3.8 with a 0.1 mol / L dilute hydrochloric acid solution. The mixture was then transferred to a high-pressure reactor lined with polytetrafluoroethylene and reacted at 125 °C for 6 hours. After natural cooling, the ceramsite was removed, rinsed with deionized water until neutral, and dried to obtain Zr-modified ceramsite. (4) The obtained Zr-modified ceramsite was mixed with 0.1mol / L FeCl3 solution at a solid-liquid ratio of 1g:10mL and then transferred to a high-pressure reactor. The mixture was reacted at 110℃ for 5h, cooled naturally, and the ceramsite was taken out. It was rinsed with deionized water until neutral and then dried to obtain the modified ceramsite.

[0036] The composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred to a muffle furnace, heated to 550°C at a rate of 5°C / min, held at that temperature for 2 hours, and then cooled to room temperature to obtain P-doped g-C3N4; wherein the amount of ammonium dihydrogen phosphate was 6% of the mass of melamine. S2: P-doped g-C3N4 was dispersed in deionized water, and nano-titanium dioxide and Fe(NO3)3·9H2O were added. After stirring evenly, the mixture was ultrasonically dispersed for 45 min, and then transferred to a high-pressure reactor and reacted at 180℃ for 11 h. After the reaction was completed, the mixture was cooled to room temperature and centrifuged. The solid product was washed alternately with deionized water and anhydrous ethanol more than 3 times, and then vacuum dried to obtain the composite photocatalytic material. The ratio of P-doped g-C3N4, deionized water, nano-titanium dioxide, and Fe(NO3)3·9H2O was 1 g: 30 mL: 0.8 g: 0.1 g.

[0037] Comparative Example 1 A method for treating wastewater generated during wet phosphoric acid preparation of phosphates is basically the same as in Example 1, except that the modified ceramsite is only modified with ZrOCl2 solution, without the FeCl3 modification step.

[0038] Comparative Example 2 A method for treating wastewater generated during wet-process phosphoric acid preparation of phosphates is basically the same as that in Example 1, except that the modified ceramsite is only modified with FeCl3 solution, without the ZrOCl2 modification step.

[0039] Comparative Example 3 A method for treating wastewater generated during wet phosphoric acid preparation of phosphates is basically the same as that in Example 1, except that: in step b), unmodified ceramsite is added to the primary treated water, and the ceramsite is not subjected to any modification treatment.

[0040] The ceramsite was prepared using the following method: (1) Mix red mud, river sludge and kaolin in a mass ratio of 1:2:2, slowly add water to the mixture, roll it into balls in a granulator, and screen out raw material balls of 2-5 mm. (2) In an air atmosphere, the temperature is increased from room temperature to 500℃ at 3℃ / min in a muffle furnace and held for 20 minutes. Then the temperature is increased to 1050℃ at 8℃ / min and held for 30 minutes. After the temperature in the furnace drops to room temperature, the ceramsite is taken out.

[0041] Comparative Example 4 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates is basically the same as in Example 1, except that the composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred to a muffle furnace, heated to 550°C at a rate of 5°C / min, held at that temperature for 2 hours, and then cooled to room temperature to obtain P-doped g-C3N4; wherein the amount of ammonium dihydrogen phosphate was 5% of the mass of melamine. S2: P-doped g-C3N4 was dispersed in deionized water, and nano-titanium dioxide was added. After stirring evenly, the mixture was ultrasonically dispersed for 30 min, then transferred to a high-pressure reactor and reacted at 180℃ for 10 h. After the reaction, the mixture was cooled to room temperature and centrifuged. The solid product was washed alternately with deionized water and anhydrous ethanol at least three times, and then vacuum dried to obtain the composite photocatalytic material. The ratio of P-doped g-C3N4, deionized water, and nano-titanium dioxide was 1 g: 30 mL: 0.5 g.

[0042] Comparative Example 5 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates is basically the same as in Example 1, except that the composite photocatalytic material is prepared using the following method: S1: Melamine and ammonium dihydrogen phosphate were mixed, transferred to a muffle furnace, heated to 550°C at a rate of 5°C / min, held at that temperature for 2 hours, and then cooled to room temperature to obtain P-doped g-C3N4; wherein the amount of ammonium dihydrogen phosphate was 5% of the mass of melamine. S2: P-doped g-C3N4 was dispersed in deionized water, and Fe(NO3)3·9H2O was added and stirred until homogeneous. The mixture was then ultrasonically dispersed for 30 min, transferred to a high-pressure reactor, and reacted at 180℃ for 10 h. After the reaction, the mixture was cooled to room temperature and centrifuged. The solid product was washed alternately with deionized water and anhydrous ethanol at least three times, and then vacuum dried to obtain the composite photocatalytic material. The ratio of P-doped g-C3N4, deionized water, and Fe(NO3)3·9H2O was 1 g: 30 mL: 0.1 g.

[0043] Comparative Example 6 A method for treating wastewater generated during wet-process phosphoric acid production of phosphates is basically the same as in Example 1, except that the composite photocatalytic material is prepared using the following method: S1: Melamine was placed in a muffle furnace and heated to 550°C at a rate of 5°C / min, then held at that temperature for 2 hours and cooled to room temperature to obtain g-C3N4. S2: Disperse g-C3N4 in deionized water, add nano-titanium dioxide and Fe(NO3)3·9H2O, stir evenly, and then ultrasonically disperse for 30 min. Transfer to a high-pressure reactor and react at 180℃ for 10 h. After the reaction, cool to room temperature and centrifuge. Wash the solid product alternately with deionized water and anhydrous ethanol at least three times, and then vacuum dry to obtain the composite photocatalytic material. The ratio of g-C3N4, deionized water, nano-titanium dioxide, and Fe(NO3)3·9H2O is 1 g: 30 mL: 0.5 g: 0.1 g.

[0044] Application test cases Wastewater from a phosphate chemical plant was treated using the methods described in Examples 1-3 and Comparative Examples 1-6. The raw water quality was as follows: pH = 1.5, total phosphorus (TP) = 2500 mg / L, and fluoride (F... - The concentrations of Fe=800 mg / L, COD=500 mg / L, As=0.5 mg / L, and Pb=0.2 mg / L are as follows: Total phosphorus (TP): determined by ammonium molybdate spectrophotometry; Fluorides (F) - ): Determined using the ion-selective electrode method; COD: determined by potassium dichromate digestion method; Heavy metals (arsenic, lead): determined by inductively coupled plasma mass spectrometry (ICP-MS); Reuse Compliance: The feasibility of product water reuse was assessed in accordance with the "Design Code for Industrial Circulating Water and Cooling Water Treatment" (GB / T 50050-2017). The test results are shown in Table 1 below.

[0045] Table 1. Processing effects of each embodiment and comparative example The data results above show that the methods in Examples 1-3 of this invention are effective in controlling total phosphorus (TP) and phosphorus (F) in wastewater. - The removal efficiency of COD and heavy metals arsenic and lead was significantly better than that of comparative examples 1-6. The results of comparative examples 1-3 show that after replacing the modified ceramsite composition, the removal efficiency of TP and F was significantly improved. - The removal efficiency of heavy metals arsenic and lead was significantly reduced. This is because the modified ceramic particles prepared in this invention, through Zr-Fe bimetallic modification, formed a porous structure with more active sites on the surface (such as...). Figure 1 As shown), unmodified ceramsite ( Figure 1 a) The surface is relatively dense with an underdeveloped pore structure, while the Zr-Fe modified ceramsite ( Figure 1 (b) A more porous and rough surface structure was formed, exposing more active sites. Zr has a strong complexing ability for fluoride ions, and Fe can efficiently adsorb heavy metal ions through redox reactions. The synergistic effect of these two elements enables the modified ceramic particles to achieve simultaneous deep removal of fluorides and heavy metals. Meanwhile, the results of Comparative Examples 4-6 show that the COD removal rate is only 71.9%-82.3%, far lower than that of Examples 1-3, indicating that the components of the composite photocatalytic material of this invention have a synergistic effect. Figure 2 As can be seen from the SEM image, the unrecombined P-doped g-C3N4 ( Figure 2 a) It exhibits aggregated blocky and layered structures, while the composite nano-titanium dioxide with Fe element ( Figure 2(b) The material exhibits almost no sheet-like structure, forming a porous heterojunction structure, which effectively suppresses photogenerated carrier recombination. P doping optimizes the band structure of g-C3N4, nano-titanium dioxide broadens the photoresponse range, and Fe acts as an electron trapping site to enhance catalytic activity. The synergistic effect of these three elements enables the material to efficiently generate free radicals such as ·OH under visible light, achieving deep degradation of recalcitrant organic matter.

[0046] This invention also investigated the effects of different dosages of modified ceramsite on the total phosphorus (TP) and fluoride ion content in the wastewater generated by the aforementioned phosphate chemical plant. Specifically, the method described in Example 1 was used to treat the wastewater, and the results are as follows: Figure 3 As shown. From Figure 3 It can be seen that the amount of modified ceramsite added is related to TP and F. - The removal rate showed a positive correlation. When the dosage increased from below 20 g / L to 20-30 g / L, the removal rate increased rapidly; at a dosage of 20 g / L, the TP removal rate reached 99.0%, and the F removal rate... - The removal rate reached 98.7%; at a dosage of 25 g / L, the removal rates increased to 99.5% and 99.1%, respectively; and stabilized at 30 g / L. This is because sufficient modified ceramsite provides ample adsorption sites, ensuring sufficient contact and reaction with pollutants in the wastewater; while insufficient dosage results in limited active sites, and the removal rate no longer increases after adsorption reaches saturation. In the example, a dosage of 20-30 g / L was selected, which ensured the treatment effect while avoiding the cost waste caused by excessive dosage, achieving a balance between efficiency and economy.

[0047] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. A method for treating wastewater generated from the production of phosphates by the wet process phosphoric acid, characterized in that, The method comprises the following steps: a) After the wastewater is screened and adjusted, the wastewater is introduced into a struvite reaction tank, the pH of the wastewater is adjusted to 8.5-9.5 by using NaOH, magnesium salt and ammonium salt are added, and the reaction is carried out for 30-45 min, then the reaction mixture is introduced into a sedimentation tank, and struvite crystals and first-stage treated water are obtained after solid-liquid separation; b) Modified ceramsite is added into the first-stage treated water, the pH is adjusted to 10-11, and the adsorption reaction is carried out for 4-6 h, then the solid-liquid separation is carried out, and second-stage treated water is obtained; c) A composite photocatalytic material is added into the second-stage treated water, visible light (420-800 nm) is simulated by using a xenon lamp with a filter, and the catalytic degradation treatment is carried out for 2-4 h, and third-stage treated water is obtained; d) The third-stage treated water is subjected to membrane filtration, the produced water is reused, and the concentrated water is subjected to evaporation crystallization treatment.

2. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 1, characterized in that, In the step a), the magnesium salt is selected from one or more of magnesium chloride, magnesium sulfate or magnesium oxide; the ammonium salt is selected from ammonium chloride or ammonium sulfate; and the molar addition ratio of magnesium, phosphorus and nitrogen is (1.0-1.5):1:(1.0-1.2).

3. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 1, characterized in that, The addition amount of the modified ceramsite is 20-30 g / L.

4. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 1, characterized in that, The addition amount of the composite photocatalytic material is 2-2.5 g / L.

5. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 1 or 3, characterized by, The modified ceramsite is prepared by the following method: (1) Red mud, river sludge and kaolin are mixed according to a mass ratio, water is slowly added in the mixture, and the mixture is rolled into balls in a granulator, and the raw balls with a size of 2-5 mm are screened out; (2) The temperature is raised to 500-600 ℃ at a rate of 3-5 ℃ / min from room temperature under air atmosphere in a muffle furnace, and then the temperature is raised to 1050-1150 ℃ at a rate of 8-10 ℃ / min, and the temperature is kept for 20-30 min, and then the temperature in the furnace is reduced to room temperature, and the ceramsite is obtained; (3) The obtained ceramsite is immersed in a ZrOCl2 solution according to a solid-liquid ratio, a 0.1 mol / L dilute hydrochloric acid solution is used to adjust the pH of the solution to 3-4, and then the mixture is transferred into a polytetrafluoroethylene-lined high-pressure reaction kettle, and the reaction is carried out at 120-130 ℃ for 6-7 h, and then the ceramsite is naturally cooled, washed with deionized water until neutral, and dried to obtain Zr-modified ceramsite; (4) The obtained Zr-modified ceramsite is mixed with a FeCl3 solution according to a solid-liquid ratio, and then the mixture is transferred into a high-pressure reaction kettle, and the reaction is carried out at 100-120 ℃ for 4-6 h, and then the ceramsite is naturally cooled, washed with deionized water until neutral, and dried to obtain the modified ceramsite.

6. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 5, characterized by, In the step (1), the mass ratio of the red mud, the river sludge and the kaolin is 1:2:(2-3).

7. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 5, characterized by, In the step (3), the solid-liquid ratio of the ceramsite to the ZrOCl2 solution is 1 g:10 mL, and the concentration of the ZrOCl2 solution is 0.05 mol / L.

8. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 5, characterized by, In the step (4), the solid-liquid ratio of the Zr-modified ceramsite to the FeCl3 solution is 1 g:10 mL, and the concentration of the FeCl3 solution is 0.1 mol / L.

9. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 1, characterized in that, The composite photocatalytic material is prepared by the following method: S1: Melamine and ammonium dihydrogen phosphate are mixed, and then the mixture is transferred into a muffle furnace, the temperature is raised to 550 ℃ at a rate of 5 ℃ / min, and then the temperature is kept for 2 h, and then the mixture is cooled to room temperature to obtain P-doped g-C3N4. S2: the P-doped g-C3N4 is dispersed in deionized water, nano-titanium dioxide and Fe(NO3)3·9H2O are added, after stirring uniformly, ultrasonic dispersion is carried out for 30-60 min, then it is transferred into a high-pressure reaction kettle, and reaction is carried out at 180℃ for 10-12 h, after reaction is finished, it is cooled to room temperature and centrifugal separation is carried out, the solid product is washed with deionized water and anhydrous ethanol alternately for more than 3 times, and vacuum drying is carried out, and a composite photocatalytic material is obtained.

10. The treatment method of wet-process phosphoric acid manufacturing phosphate producing wastewater according to claim 9, characterized by, The amount of the ammonium dihydrogen phosphate in the step S1 is 5-8% of the mass of the melamine; the amount ratio of the P-doped g-C3N4, deionized water, nano-titanium dioxide and Fe(NO3)3·9H2O in the step S2 is 1g:30mL:(0.5-1)g:0.1g.

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