Multistage membrane separation method for high-phosphorus and fluorine-containing wastewater under acidic conditions

Abstract

The application provides a multistage membrane separation method for high-phosphorus and fluorine wastewater under acidic conditions, and belongs to the technical field of industrial wastewater treatment and resource utilization. The method sequentially undergoes three main steps of primary acid-resistant reverse osmosis separation, acid-resistant nanofiltration separation and secondary acid-resistant reverse osmosis under acidic conditions with pH being 1.0-4.0. The primary reverse osmosis separates the acidic raw water into primary product water and primary concentrated water. The nanofiltration step separates the primary concentrated water into nanofiltration product water and nanofiltration concentrated water, and the obtained nanofiltration product water can be directly used for preparing high-value products such as ferric phosphate and monoammonium phosphate. The secondary reverse osmosis separates the primary product water into secondary product water and secondary concentrated water, and the secondary product water is recycled for production process. The method does not need to add lime, does not produce phosphogypsum solid waste, has high water recovery rate, high-value recovery of phosphoric acid resources, and has significant economic and environmental benefits.

Description

Technical Field

[0001] This application relates to the field of industrial wastewater treatment and resource utilization technology, specifically to a multi-stage membrane separation method for high-phosphorus and high-fluoride wastewater under acidic conditions. Background Technology

[0002] The phosphorus chemical industry generates a large amount of high-phosphorus and high-fluoride acidic wastewater during production. Typical characteristics of this type of wastewater include high concentrations of phosphate, fluoride, and sulfate, as well as a certain amount of metal ions such as calcium, magnesium, iron, and aluminum. The pH of the wastewater is usually between 1.0 and 4.0. If it is discharged directly without effective treatment, it will cause serious phosphorus and fluoride pollution to water bodies and soil, and at the same time, it will result in a large waste of phosphorus resources.

[0003] Traditional methods for treating high-phosphorus and fluoride wastewater primarily rely on lime neutralization and precipitation. This method involves adding lime to the wastewater to raise the pH to above 7.0, causing phosphate ions to precipitate as calcium phosphate and fluoride ions as calcium fluoride, which are then removed via solid-liquid separation. However, the lime neutralization and precipitation method has several drawbacks: (1) The amount of sludge is large and difficult to dispose of. Approximately 20-50 kg of phosphogypsum sludge is generated for each cubic meter of wastewater treated. The mass fraction of phosphorus pentoxide (P2O5) contained in the sludge is usually less than 10%. The phosphorus resources are solidified in the sludge and cannot be recovered. It must be disposed of as solid waste through landfill, which poses a high environmental risk.

[0004] (2) The water recovery rate is low. After sedimentation treatment, the effluent still contains a high concentration of dissolved salts and the conductivity is often above 1000 μS / cm. It is difficult to use it directly as process water. The overall water recovery rate of the system is usually less than 50%.

[0005] (3) Phosphorus resources cannot be effectively recovered. This method only transfers phosphorus from the liquid phase to the solid phase, and the recovery rate of phosphoric acid resources is almost zero.

[0006] (4) The application of membrane technology is limited. In recent years, some studies have attempted to introduce membrane separation technology into the treatment of phosphorus-containing wastewater. However, under neutral or alkaline conditions, slightly soluble salts such as calcium fluoride and calcium phosphate are very easy to form scale on the membrane surface, which leads to a rapid decline in membrane flux and poor operational stability, and the actual engineering application is greatly limited.

[0007] In addition, some existing all-membrane phosphoric acid recovery technologies employ a process route that combines nanofiltration staged separation with acid dialysis. These technologies are mainly suitable for purifying and refining high-concentration phosphoric acid. They are not well-suited for treating low- to medium-concentration acidic wastewater generated during the phosphoric acid chemical process. Furthermore, their process designs do not include a high water recovery rate target for this type of wastewater, nor do they mention a scheme to reuse the treated clarified liquid in the process system.

[0008] Therefore, developing a membrane separation treatment method that can operate stably under acidic conditions and has both high water recovery rate and high-value phosphoric acid recovery capability to overcome the shortcomings of existing technologies is an urgent technical need and has important industrial application value. Summary of the Invention

[0009] This application provides a multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions, aiming to solve the technical problems of large sludge production, inability to recover phosphoric acid resources, and difficulty in stable operation of membrane technology in acidic phosphorus-containing wastewater in the prior art.

[0010] In a first aspect, this application provides a multi-stage membrane separation method for high-phosphorus and high-fluoride wastewater under acidic conditions, comprising the following steps: (1) Adjustment of acidic raw water Acidic industrial wastewater containing phosphorus and fluoride is used as raw water, and the pH value of the raw water is controlled within the range of 1.0 to 4.0 to obtain adjusted acidic raw water; (2) First-stage reverse osmosis separation The acidic raw water adjusted in step (1) is sent into the first-stage acid-resistant reverse osmosis membrane module for separation treatment to obtain first-stage permeate and first-stage concentrate. The first-stage acid-resistant reverse osmosis membrane retains most of the dissolved salts, including phosphate, sulfate, fluoride, calcium, magnesium, iron, and aluminum ions, through a dissolution-diffusion mechanism, which significantly reduces the salt content of the first-stage permeate and correspondingly increases the concentration of each solute in the first-stage concentrate. (3) Nanofiltration separation The primary concentrate is fed into an acid-resistant nanofiltration membrane module for separation treatment to obtain nanofiltration permeate and nanofiltration concentrate. Acid-resistant nanofiltration membranes utilize both the Donnan effect and size sieving mechanisms to filter high-valence sulfate anions and high-valence metal cations (Ca²⁺). + Mg² + Fe³ + Al³ + The retention rate is relatively high, while for monovalent or low-valent ions, including dihydrogen phosphate (H2PO4), it is relatively high. - and fluoride ions F - It has high transmittance; Under acidic conditions with pH < 4, phosphoric acid exists primarily as neutral H3PO4 molecules and H2PO4. - Monovalent anions are less affected by the Donnan repulsion effect and have a permeability of 30% to 50% through nanofiltration membranes, thus achieving effective separation of phosphoric acid from impurities such as sulfate and heavy metals.

[0011] (4) Secondary reverse osmosis separation The primary permeate is fed into a secondary acid-resistant reverse osmosis membrane module for deep desalination to obtain secondary permeate and secondary concentrate. The conductivity of the secondary product water does not exceed 50 μS / cm, the phosphate concentration does not exceed 5 mg / L, and the fluoride concentration does not exceed 1 mg / L, meeting the process water reuse quality standards.

[0012] In conjunction with the first aspect, in one embodiment, the pH value of the acidic raw water after adjustment in step (1) is 1.0 to 3.5.

[0013] In conjunction with the first aspect, in one embodiment, the phosphate concentration in the raw water in step (1) is 1000-15000 mg / L, the fluoride concentration is 500-8000 mg / L, the sulfate concentration is 2000-25000 mg / L, and the conductivity is 5000-35000 μS / cm.

[0014] In conjunction with the first aspect, in one embodiment, the operating pressure of the first-stage reverse osmosis separation in step (2) is 1.0 to 8.0 MPa, preferably 2.0 to 6.0 MPa, and more preferably 3.0 to 5.0 MPa.

[0015] In conjunction with the first aspect, in one embodiment, the temperature of the first-stage reverse osmosis separation in step (2) is 10-45°C, preferably 20-35°C, and more preferably 25-30°C.

[0016] In conjunction with the first aspect, in one embodiment, the membrane surface flow rate in step (2) is 0.08 to 0.20 m / s.

[0017] In conjunction with the first aspect, in one embodiment, the water recovery rate in step (2) is 55% to 75%, preferably 60% to 70%.

[0018] Furthermore, the first-stage acid-resistant reverse osmosis membrane module described in step (2) retains dissolved salts, including phosphate, sulfate, fluoride, calcium, magnesium, iron, and aluminum ions, thereby significantly reducing the salt content of the first-stage permeate and increasing the salt content of the first-stage concentrate.

[0019] In conjunction with the first aspect, in one embodiment, the molecular weight cutoff of the acid-resistant nanofiltration membrane module in step (3) is 200-800 Da, preferably 300-500 Da.

[0020] In conjunction with the first aspect, in one embodiment, the operating pressure of nanofiltration separation in step (3) is 0.5 to 4.0 MPa, preferably 1.0 to 3.0 MPa, and more preferably 1.5 to 2.5 MPa.

[0021] In conjunction with the first aspect, in one embodiment, the temperature of nanofiltration separation in step (3) is 10-40°C, preferably 20-35°C, and more preferably 25-30°C.

[0022] In conjunction with the first aspect, in one embodiment, the nanofiltration separation in step (3) has a rejection rate of not less than 95% for sulfate ions, calcium ions, magnesium ions, iron ions, and aluminum ions; Step (3) Nanofiltration separation has high permeability to low-valence ions; the low-valence ions include dihydrogen phosphate ions and fluoride ions.

[0023] In conjunction with the first aspect, in one embodiment, the nanofiltration separation in step (3) has a permeability of 30% to 50% for phosphate and dihydrogen phosphate ions.

[0024] In conjunction with the first aspect, in one implementation, the nanofiltration permeate obtained in step (3) satisfies the following conditions: The phosphate concentration is not less than 5000 mg / L, the fluoride concentration is not less than 2000 mg / L, and the total concentration of impurity ions such as calcium, magnesium, iron, aluminum and sulfate does not exceed 500 mg / L.

[0025] Furthermore, the operating pressure of the secondary reverse osmosis separation in step (4) is 1.0 to 6.0 MPa, preferably 1.5 to 4.0 MPa.

[0026] Furthermore, the temperature for the secondary reverse osmosis separation in step (4) is 10–40°C, preferably 20–35°C.

[0027] Furthermore, the water recovery rate of the secondary reverse osmosis separation in step (4) is 70% to 85%.

[0028] Furthermore, the total water recovery rate of the multi-stage membrane separation method is 65%~70%.

[0029] In conjunction with the first aspect, in one embodiment, a pretreatment step is included before step (1): the acidic raw water is filtered through a filtration device to remove suspended particles.

[0030] Furthermore, the effluent turbidity of the pretreatment step does not exceed 1.0 NTU and the SDI value does not exceed 5.0.

[0031] Furthermore, the filtration device is a bag filter or a security filter with a filtration accuracy of 5–50 μm.

[0032] In conjunction with the first aspect, in one embodiment, before step (2), an acid-resistant scale inhibitor is added to the adjusted acidic raw water at a dosage of 2 to 15 mg / L; the acid-resistant scale inhibitor is selected from at least one of organophosphonate scale inhibitors, polycarboxylic acid scale inhibitors, or sulfonated copolymer scale inhibitors.

[0033] In conjunction with the first aspect, in one embodiment, the membrane material of the primary acid-resistant reverse osmosis membrane module and / or the secondary acid-resistant reverse osmosis membrane module is selected from at least one of polyamide composite acid-resistant reverse osmosis membrane, sulfonated polyethersulfone acid-resistant reverse osmosis membrane, or polypiperazine amide acid-resistant reverse osmosis membrane; the membrane material of the acid-resistant nanofiltration membrane module is selected from at least one of polyamide composite acid-resistant nanofiltration membrane, sulfonated polyethersulfone acid-resistant nanofiltration membrane, or polypiperazine amide acid-resistant nanofiltration membrane.

[0034] In conjunction with the first aspect, in one embodiment, the nanofiltration concentrate and the secondary concentrate are discharged as concentrated brine from the system.

[0035] In conjunction with the first aspect, in one embodiment, the nanofiltration permeate is concentrated by evaporation to recover a phosphoric acid solution with a mass fraction of 10% to 30%.

[0036] In conjunction with the first aspect, in one embodiment, the recovered phosphoric acid solution is reacted with an iron salt to produce iron phosphate.

[0037] In conjunction with the first aspect, in one embodiment, the nanofiltration permeate is reacted with ammonium salts to produce monoammonium phosphate and diammonium phosphate products.

[0038] In conjunction with the first aspect, in one embodiment, when the standardized flux of the primary acid-resistant reverse osmosis membrane module and / or the secondary acid-resistant reverse osmosis membrane module drops to below 80% of the initial flux, chemical cleaning is performed using an acidic cleaning solution; the acidic cleaning solution is a hydrochloric acid solution or citric acid solution with a mass fraction of 0.5% to 2.0%, the cleaning temperature is 25 to 40°C, and the cleaning time is 30 to 120 minutes.

[0039] Compared with the prior art, the beneficial effects of the technical solution provided in this application include: This application operates entirely under acidic conditions (pH 1.0–4.0), where the solubility of phosphates and fluorides of metal ions such as calcium, magnesium, and iron is high, preventing the formation of scale deposits on the membrane surface. This significantly reduces the risk of fouling in reverse osmosis and nanofiltration membranes, and the membrane flux decay rate is reduced by more than 40% compared to neutral or alkaline operating conditions. The continuous operation cycle can exceed 45 days, and the membrane lifespan is correspondingly extended.

[0040] This application utilizes the characteristic of phosphoric acid under acidic conditions, where it is predominantly composed of dihydrogen phosphate ions, to achieve efficient separation of phosphoric acid from high-valence impurities such as sulfate and heavy metal ions. The nanofiltration membrane has a sulfate rejection rate of no less than 95%, and the total amount of impurity ions in the nanofiltration permeate does not exceed 500 mg / L. This provides a high-purity feedstock for the subsequent preparation of high-value chemicals such as iron phosphate and ammonium phosphate. The purity of the recovered phosphoric acid is improved to an industrially usable level compared to the traditional lime method.

[0041] This application recovers phosphoric acid resources through evaporation and concentration of nanofiltration permeate, while also recovering evaporation condensate. The overall water recovery rate of the system reaches over 65%, reducing makeup water consumption and wastewater discharge.

[0042] This application requires no lime addition throughout the entire process, does not generate difficult-to-dispose-of solid wastes such as phosphogypsum, and eliminates the problem of secondary solid waste pollution.

[0043] The secondary permeate produced by this application has a conductivity of no more than 50 μS / cm, a phosphate concentration of no more than 5 mg / L, and a fluoride concentration of no more than 1 mg / L, meeting the standards for process water reuse. It can be directly reused in the phosphate chemical production system, reducing water intake costs. Attached Figure Description

[0044] The present application will be described in further detail below with reference to the accompanying drawings and preferred embodiments. However, those skilled in the art will understand that these drawings are drawn only for the purpose of explaining the preferred embodiments and therefore should not be construed as limiting the scope of this disclosure. Furthermore, unless specifically indicated, the drawings are only schematic representations of the composition or structure of the described objects and may contain exaggerated depictions, and the drawings are not necessarily drawn to scale.

[0045] Figure 1 This is a flowchart of a multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions, as provided in Example 1 of this application.

[0046] Figure 2 This is a schematic diagram of the process flow for the multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions provided in Example 1 of this application. Detailed Implementation

[0047] The present application will be further illustrated below with reference to the embodiments. These embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present application. The reagents and raw materials used in the embodiments are commercially available products or can be prepared by known methods. Unless otherwise specified, all percentage contents are by mass percentage.

[0048] Detection methods pH values ​​were measured using a pH meter (glass electrode method), in accordance with GB / T 6920-1986.

[0049] Conductivity was measured using a conductivity meter, referring to the "Methods for Monitoring and Analysis of Water and Wastewater" (4th Edition).

[0050] Turbidity was measured using a turbidimeter, in accordance with HJ 1075-2019; NTU (Nephelometric Turbidity Unit) is a unit of scattering turbidity used to measure the degree of scattering of light by suspended particulate matter in water. It is a standard unit of measurement that characterizes the turbidity of water bodies. The higher the value, the more turbid the water.

[0051] SDI values ​​are determined according to ASTM D4189 method; SDI (Silt Density Index), also known as silt density index, is an indicator that measures the potential for filterable substances such as colloids, fine particles, and microorganisms in water to foul reverse osmosis / nanofiltration membranes.

[0052] The concentrations of phosphate, fluoride, and sulfate were determined by ion chromatography, referring to HJ 84-2016.

[0053] The concentrations of calcium and magnesium ions were determined by atomic absorption spectrophotometry, in accordance with GB / T 11905-1989.

[0054] The determination of iron ion concentration shall be in accordance with HJ / T 345-2007.

[0055] Aluminum ion concentration was determined according to HJ 1452-2026.

[0056] For phosphoric acid detection, thymolphthalein was used as an indicator, and the solution was titrated to the endpoint with a standard sodium hydroxide solution. The mass fraction of phosphoric acid was calculated based on the volume of NaOH consumed.

[0057] The testing of ferric phosphate shall be performed in accordance with HG / T 4701-2021.

[0058] The iron / phosphorus molar ratio was determined by measuring the iron and phosphorus content in the ferric phosphate product using the formula Fe / P = (m Fe / 55.85) / (m P / 30.97) Calculate the molar ratio.

[0059] The total amount of impurity ions is calculated by taking a nanofiltration permeate water sample and measuring the concentrations of calcium, magnesium, iron, aluminum, and sulfate ions, and then summing them up.

[0060] The purity and nitrogen content of monoammonium phosphate products shall comply with GB / T 10209.1-2008.

[0061] The phosphorus content in monoammonium phosphate products shall comply with GB / T 10209.2-2010.

[0062] Calculation method Retention rate (R) = (C f C p ) / C f × 100%, where C f C represents the target ion mass concentration in the feed water of the membrane module. p The target ion mass concentration in the permeate from the membrane module.

[0063] Transmittance (T) = C p / Cf × 100%.

[0064] Water recovery rate (Y) = Q p / Q f × 100%, where Q p Q represents the permeate flow rate of the membrane module. f The influent flow rate of the membrane module is measured using a rotor flow meter.

[0065] Total water yield Y of the system total = Q 产水总 / Q 原水 × 100%, where Q 产水总 Q is the permeate flow rate for the second-stage reverse osmosis system. 原水 This refers to the system's raw water inflow rate.

[0066] Standardized flux (Jstd) is calculated using the membrane element manufacturer's standard calibration method, correcting the actual operating flux to standard conditions (25°C, standard operating pressure, standard feed water concentration). The calibration formula is Jstd. std = J actual × (Δπ std / Δπ actual ), where Δπ is the osmotic pressure difference, and the standardized flux is triggered when chemical cleaning is below 80% of the initial value.

[0067] Flux recovery rate = J 清洗后 / J 初始 × 100%.

[0068] The amount of lime added was recorded by the cumulative reading of the metering pump, in units of kg / m³ wastewater.

[0069] The amount of sludge generated is measured by weighing the sludge after filter pressing, with the unit being kg / m³ of wastewater.

[0070] The phosphorus content in the sludge was determined by measuring the mass fraction of phosphorus pentoxide using the quinoline phosphomolybdate gravimetric method after the sludge was dried and ground.

[0071] Heat transfer coefficient (K) = Q / (A × ΔT), where Q is the heat exchange power of the evaporator (W), A is the heat exchange area (m²), ΔT is the logarithmic mean temperature difference (°C), and Q is calculated based on the steam condensate volume and latent heat of vaporization.

[0072] Phosphorus resource utilization rate η = (Q 产水 × C p,产水 ) / (Q 原水 × C f,原水 ) × 100%, where Q 产水 Q 原水 C represents the production water flow rate and the raw water flow rate, respectively. p,产水 Cf,原水 These represent the phosphate concentrations in the produced water and the raw water, respectively.

[0073] Example 1 This embodiment provides a multi-stage membrane separation method for high-phosphorus and high-fluoride wastewater under acidic conditions. The raw water is taken from the production wastewater of a wet-process phosphoric acid production enterprise, and the water quality conditions are phosphate (PO4³⁺). - : 8500 mg / L; fluoride ions (F - : 2200 mg / L; sulfate (SO4²) - : 4500 mg / L; calcium ions (Ca²⁺) + : 350 mg / L; magnesium ions (Mg²⁺) + ): 180 mg / L; iron ions (Fe³⁺) + ): 65 mg / L; aluminum ions (Al³⁺) + ): 42 mg / L; pH value: 2.3; conductivity: 18500 μS / cm.

[0074] The specific processing steps are as follows: Figure 1 As shown in the diagram, the process flow diagram is as follows: Figure 2 As shown, a multi-stage membrane separation method for high-phosphorus and high-fluoride wastewater under acidic conditions includes the following steps: (1) Adjustment of acidic raw water After large particles of impurities are removed by the screen, the raw water enters the equalization tank for homogenization. The retention time is 8 hours, and the pH value is measured to be 2.3, which is within the range of 1.0 to 4.0. No additional adjustment is required. The pH of the raw water after adjustment is 2.3, which meets the pH requirements of the first-stage reverse osmosis feed water.

[0075] (2) Pretreatment The adjusted acidic raw water was fed into a security filter with a filtration accuracy of 5μm. The turbidity of the effluent was 0.8NTU and the SDI value was 4.2, which met the membrane feed water requirements.

[0076] (3) First-stage reverse osmosis separation An organic phosphonate-based acid-resistant antiscalant, hydroxyethylidene diphosphonic acid (HEDP), at a dosage of 8 mg / L, was added to the precision filtered water. This water was then fed into a primary acid-resistant reverse osmosis membrane module. The primary acid-resistant reverse osmosis membrane module was a polyamide composite acid-resistant reverse osmosis membrane with a membrane area of ​​30 m². The operating conditions were: pressure 4.0 MPa, temperature 28 °C, membrane surface flow velocity 0.12 m / s, and water recovery rate 65%. The conductivity of the primary product water was 420 μS / cm, with phosphate concentrations of 85 mg / L and fluoride concentrations of 28 mg / L. The primary concentrate contained phosphate concentrations of 18200 mg / L, fluoride concentrations of 5100 mg / L, sulfate concentrations of 11100 mg / L, calcium concentrations of 890 mg / L, magnesium concentrations of 450 mg / L, and a pH of 2.1.

[0077] (4) Nanofiltration separation The primary concentrate is fed into an acid-resistant nanofiltration membrane module, which is a polyamide composite acid-resistant nanofiltration membrane with a molecular weight cutoff of 400 Da and a membrane area of ​​20 m². The operating conditions are a pressure of 2.0 MPa and a temperature of 28 °C.

[0078] The nanofiltration membrane has a sulfate rejection rate of 97.5%, a calcium ion rejection rate of 96.8%, a magnesium ion rejection rate of 97.1%, a ferric ion rejection rate of 99.2%, a aluminum ion rejection rate of 99.0%, and a phosphate permeability of 42%.

[0079] The resulting nanofiltration permeate contained 7560 mg / L phosphate, 1950 mg / L fluoride, 110 mg / L sulfate, 21 mg / L calcium, 12 mg / L magnesium, 0.5 mg / L iron, 0.4 mg / L aluminum, and approximately 144 mg / L of impurity ions, meeting the quality requirements for phosphate resource recovery solution. The resulting nanofiltration concentrate contained 28700 mg / L phosphate, 8250 mg / L fluoride, 18600 mg / L sulfate, and had a pH of 1.9.

[0080] (5) Secondary reverse osmosis separation The primary permeate is fed into the secondary acid-resistant reverse osmosis membrane module, which is a polyamide composite acid-resistant reverse osmosis membrane with a membrane area of ​​15 m². The operating conditions are a pressure of 2.5 MPa, a temperature of 28 °C, and a water recovery rate of 80%.

[0081] The conductivity of the secondary permeate is 8 μS / cm; the phosphate content in the secondary permeate is 0.8 mg / L and the fluoride content is 0.2 mg / L, which meets the process water reuse standard; the conductivity of the secondary concentrate is 2100 μS / cm.

[0082] (6) Resource recycling Nanofiltration concentrate and secondary concentrate are discharged from the concentrated brine system and treated separately; the total water recovery rate of the system reaches 69.0%.

[0083] Nanofiltration permeate was mechanically steam-compressed and concentrated to a phosphoric acid mass fraction of 18%. The total impurities of calcium, magnesium, iron, and aluminum were tested and found to be 75 mg / L. It can be directly used as a raw material for the production of iron phosphate. The phosphoric acid solution was reacted with ferrous sulfate heptahydrate at 75°C and pH 1.5. After filtration, washing, and drying, iron phosphate product was obtained with a purity of 99.3% and an iron-to-phosphorus molar ratio of 0.998, which meets the battery-grade iron phosphate standard.

[0084] After 90 days of continuous operation in this embodiment, the standardized flux of the first-stage reverse osmosis membrane was 87% of the initial value, and the standardized flux of the nanofiltration membrane was 91% of the initial value. The chemical cleaning cycle was 45 days, the cleaning solution was 1.0% hydrochloric acid solution, the cleaning temperature was 30°C, and the cleaning time was 60 minutes. After cleaning, the flux basically recovered to more than 98% of the initial value.

[0085] Example 2 In this embodiment, the raw water was taken from a phosphate fertilizer production company, and the water quality conditions were: phosphate (PO4³⁺) - : 4200 mg / L; fluoride ions (F - : 3800 mg / L; sulfate (SO4²) - ): 8200 mg / L; calcium ions (Ca²⁺) + : 520 mg / L; magnesium ions (Mg²⁺) + ): 210 mg / L; iron ions (Fe³⁺) + ): 38 mg / L; pH value: 1.8; conductivity: 24800 μS / cm.

[0086] The processing steps are as follows: (1) Adjustment of acidic raw water After large particles of impurities are removed by the screen, the raw water enters the equalization tank for homogenization. The retention time is 8 hours, and the pH of the raw water is measured to be 1.8, which meets the requirements and no additional adjustment is needed.

[0087] (2) Pretreatment The acidic raw water was filtered using a bag filter with a filtration accuracy of 10μm, resulting in an effluent turbidity of 0.6 NTU and an SDI of 3.8.

[0088] (3) First-stage reverse osmosis separation Add 10 mg / L of polycarboxylic acid-resistant scale inhibitor to the effluent after pretreatment in step (1) and send it into the first-stage acid-resistant reverse osmosis membrane module. The first-stage acid-resistant reverse osmosis membrane module is a sulfonated polyethersulfone acid-resistant reverse osmosis membrane with a membrane area of ​​30 m². The operating conditions are: pressure 4.5 MPa, temperature 25 ℃, membrane surface flow velocity 0.10 m / s, and water recovery rate 60%.

[0089] The conductivity of the primary product water is 580 μS / cm; the phosphate concentration in the primary product water is 105 mg / L, and the fluoride concentration is 52 mg / L; the phosphate concentration in the primary concentrate water is 9800 mg / L, the fluoride concentration is 8600 mg / L, the sulfate concentration is 18500 mg / L, the calcium concentration is 1180 mg / L, and the magnesium concentration is 475 mg / L.

[0090] (4) Nanofiltration separation The primary concentrate is fed into an acid-resistant nanofiltration membrane module, which is a sulfonated polyethersulfone acid-resistant nanofiltration membrane with a molecular weight cutoff of 350 Da. The operating conditions are a pressure of 2.2 MPa and a temperature of 25 °C.

[0091] The nanofiltration permeate contained 4350 mg / L of phosphate, 3120 mg / L of fluoride, 82 mg / L of sulfate, 32 mg / L of calcium, 13 mg / L of magnesium, and a total of approximately 127 mg / L of impurities. The nanofiltration concentrate contained 15200 mg / L of phosphate and 34800 mg / L of sulfate.

[0092] (5) Secondary reverse osmosis separation The primary permeate is fed into the secondary acid-resistant reverse osmosis membrane module, which is a polyamide composite acid-resistant reverse osmosis membrane with a membrane area of ​​15 m², a pressure of 2.8 MPa, a temperature of 25 °C, and a water recovery rate of 82%.

[0093] The conductivity of the resulting secondary permeate is 12 μS / cm; the phosphate concentration in the secondary permeate is 1.2 mg / L and the fluoride concentration is 0.4 mg / L, which meets the standards for reused water.

[0094] (6) Resource recycling Nanofiltration concentrate and secondary concentrate are discharged from the concentrated brine system and treated separately; the total water recovery rate of the system reaches 65.0%.

[0095] The pH of the nanofiltration permeate is adjusted to 4.0 with liquid ammonia, then heated and concentrated. After crystallization and filtration, monoammonium phosphate is obtained with a purity of 98.5%, a nitrogen content of 12.2%, and a phosphorus pentoxide content of 61.8%, meeting the industrial-grade monoammonium phosphate standard.

[0096] Example 3 In this example, the raw water was taken from a lithium iron phosphate precursor production plant, and the wastewater originated from phosphorus-containing washing wastewater from the iron phosphate production section. The water quality conditions were: phosphate (PO4³⁺) - ): 12800 mg / L; fluoride ions (F - : 5600 mg / L; sulfate (SO4²) - ): 3200 mg / L; iron ions (Fe³⁺) + ): 680 mg / L; aluminum ions (Al³⁺) + ): 120 mg / L; calcium ions (Ca²⁺) + : 95 mg / L; magnesium ions (Mg²⁺) + ): 48 mg / L; pH value: 1.5; conductivity: 31200 μS / cm.

[0097] The processing steps are as follows: (1) Adjustment of acidic raw water After large particles of impurities are removed by the screen, the raw water enters the equalization tank for homogenization. The retention time is 8 hours, and the pH of the raw water is measured to be 1.5, which is within the range of 1.0 to 4.0, so no adjustment is required.

[0098] (2) Pretreatment Acidic raw water is fed into a filter with a filtration precision of 5μm, resulting in an effluent turbidity of 0.5 NTU and an SDI of 3.2.

[0099] (3) First-stage reverse osmosis separation Sulfonated copolymer acid-resistant scale inhibitor was added to the effluent after pretreatment in step (1) at a dosage of 12 mg / L and sent to the first-stage acid-resistant reverse osmosis membrane module. The first-stage acid-resistant reverse osmosis membrane module was a polypiperazine amide acid-resistant reverse osmosis membrane with a membrane area of ​​40 m². The operating conditions were: pressure 5.0 MPa, temperature 30 °C, membrane surface flow velocity 0.15 m / s, and water recovery rate 65%.

[0100] The resulting primary permeate had a conductivity of 650 μS / cm, phosphate concentration of 125 mg / L, and fluoride concentration of 78 mg / L; the resulting primary concentrate had phosphate concentration of 32800 mg / L, fluoride concentration of 14800 mg / L, sulfate concentration of 8200 mg / L, iron concentration of 1740 mg / L, aluminum concentration of 310 mg / L, and a pH of 1.3.

[0101] (4) Nanofiltration separation The primary concentrate is fed into an acid-resistant nanofiltration membrane module. The acid-resistant nanofiltration membrane module is a polypiperazine amide acid-resistant nanofiltration membrane with a molecular weight cutoff of 450 Da. The operating conditions are: pressure 2.5 MPa, temperature 30 °C. The nanofiltration membrane has a 99.5% rejection rate for iron ions, a 99.3% rejection rate for aluminum ions, a 98.2% rejection rate for sulfate ions, a 97.0% rejection rate for calcium ions, and a 38% permeability for phosphate ions.

[0102] The nanofiltration permeate contained 12460 mg / L of phosphate, 5170 mg / L of fluoride, 8.7 mg / L of iron, 2.2 mg / L of aluminum, 148 mg / L of sulfate, and a total of approximately 159 mg / L of impurities; the nanofiltration concentrate contained 52100 mg / L of phosphate.

[0103] (5) Secondary reverse osmosis separation The primary permeate is fed into the secondary acid-resistant reverse osmosis membrane module, which is a polyamide composite acid-resistant reverse osmosis membrane with a membrane area of ​​15 m², a pressure of 3.5 MPa, a temperature of 30 °C, and a water recovery rate of 78%.

[0104] The conductivity of the resulting secondary permeate is 15 μS / cm; the phosphate content in the secondary permeate is 1.5 mg / L and the fluoride content is 0.6 mg / L, which meets the standards for reused water.

[0105] (6) Resource recycling Nanofiltration concentrate and secondary concentrate are discharged from the concentrated brine system and treated separately; the total water recovery rate of the system reaches 67.0%.

[0106] Nanofiltration permeate is mechanically steam-compressed and concentrated to a phosphoric acid mass fraction of 25%. The resulting phosphoric acid solution contains 22 mg / L of iron ions and 5.6 mg / L of aluminum ions, which can be directly reused in the iron phosphate production system. Based on the total amount of phosphate in the influent, the phosphorus resource utilization rate is over 92.5%.

[0107] Comparative Example 1 This comparative example uses the same raw water as in Example 1, and is treated by the traditional lime neutralization and precipitation method. Lime slurry is added to the wastewater to raise the pH to 7.5, the mixture is stirred and reacted for 30 min, and after sedimentation for 1 h, it is separated by pressure filtration.

[0108] Tests showed that the lime dosage in this comparative example was 4.8 kg / m³; 24.5 kg / m³ of phosphogypsum sludge was generated in the wastewater; the phosphorus pentoxide content in the sludge was 8.2%, which could not be recycled and reused, and had to be disposed of as solid waste in landfill; the supernatant contained 2.2 mg / L of residual phosphate ions, 1.8 mg / L of residual fluoride ions, and a conductivity as high as 4850 μS / cm, and contained a large number of calcium ions and sulfate ions, which could not be used directly as process water; the actual water recovery rate of the system was only 47%; the phosphorus resource recovery rate was 0%, and all of it entered the sludge.

[0109] The effluent from Comparative Example 1 did not meet the requirements for process reuse water, with excessively high conductivity and high concentrations of calcium ions and sulfate ions.

[0110] Comparative Example 2 This comparative example uses the same raw water as in Example 1, but removes the nanofiltration separation step from the process flow and replaces it with a direct series connection of primary and secondary reverse osmosis separation; the operating conditions of the primary reverse osmosis separation are the same as in Example 1; the primary concentrate is used directly as the resource recovery liquid without nanofiltration separation.

[0111] Testing revealed that the total impurity ion concentration in the primary concentrate of this comparative example reached 12580 mg / L, far exceeding the impurity requirements for ferric phosphate or ammonium phosphate feedstock solutions, making it unsuitable for direct use in the production of high-value chemicals. Evaporation and concentration of the primary concentrate resulted in severe scaling due to its extremely high calcium and magnesium sulfate content. After 10 days of operation, the evaporator heat exchange tubes showed significant scaling, with the heat transfer coefficient dropping to 60% of its initial value, necessitating a shutdown for cleaning. The total water recovery rate was only 55%, failing to achieve the high recovery rate target. Furthermore, the purity of the recovered phosphoric acid did not meet the production requirements of downstream high-value chemicals.

[0112] Compared with Example 1, this comparative example did not perform a nanofiltration step, which resulted in the inability to achieve high-value utilization of phosphoric acid resources, and the overall water recovery rate was reduced by 14.0%.

[0113] Comparative Example 3 This comparative example uses the same raw water as in Example 1. The pH of the wastewater is first adjusted to 5.5-6.5, and then sent to the same type of first-stage acid-resistant reverse osmosis membrane module for treatment under the same conditions. The remaining steps are the same as in Example 1.

[0114] Tests showed that, under pH conditions of 5.5–6.5, the solubility product of calcium ions, phosphate ions, and fluoride ions in the wastewater was significantly reduced. Calcium phosphate and calcium fluoride began to precipitate on the membrane surface. After 72 hours of operation, the standardized flux of the primary reverse osmosis membrane dropped to 48% of its initial value. The chemical cleaning cycle was shortened to 5 days, and after cleaning with 1.0% hydrochloric acid, the flux only recovered to 82% of its initial value, indicating significant irreversible membrane fouling. After 15 days of operation, the system was forced to shut down for large-scale chemical cleaning.

[0115] Compared with Example 1, the membrane fouling rate increased by about 9 times and the cleaning cycle was shortened by about 9 times under non-acidic conditions in this comparative example, indicating that operation under acidic conditions of pH 1.0 to 4.0 is one of the key control factors for the stable operation of this method.

[0116] The processing effects of each embodiment are shown in Table 1.

[0117] Table 1 The test results of the above embodiments and comparative examples show that the multi-stage membrane separation method for high phosphorus and fluoride wastewater under acidic conditions provided in this application achieves the synergistic goals of efficient desalination, high water recovery rate and high-value recovery of phosphoric acid resources through a combination of first-stage reverse osmosis, nanofiltration and second-stage reverse osmosis under acidic conditions. All indicators are significantly better than the traditional lime neutralization method and the reverse osmosis process without nanofiltration step.

[0118] The present application has been described in detail above. Specific embodiments have been used to illustrate the principles and implementation methods of the present application. The descriptions of the embodiments above are only for the purpose of helping to understand the present application and its core ideas. It should be noted that those skilled in the art can make several improvements and modifications to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A multi-stage membrane separation method for high-phosphorus and high-fluoride wastewater under acidic conditions, characterized in that, Includes the following steps: (1) Adjustment of acidic raw water Acidic industrial wastewater containing phosphorus and fluoride is used as raw water, and the pH value of the raw water is controlled within the range of 1.0 to 4.0 to obtain adjusted acidic raw water; (2) First-stage reverse osmosis separation The acidic raw water adjusted in step (1) is sent into the first-stage acid-resistant reverse osmosis membrane module for separation treatment to obtain first-stage permeate and first-stage concentrate. (3) Nanofiltration separation The primary concentrate is fed into an acid-resistant nanofiltration membrane module for separation treatment to obtain nanofiltration permeate and nanofiltration concentrate. (4) Secondary reverse osmosis separation The primary permeate is fed into a secondary acid-resistant reverse osmosis membrane module for deep desalination to obtain secondary permeate and secondary concentrate.

2. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, The pH value of the acidic raw water after adjustment in step (1) is 1.0 to 3.5; The raw water in step (1) has a phosphate concentration of 1000-15000 mg / L, a fluoride concentration of 500-8000 mg / L, a sulfate concentration of 2000-25000 mg / L, and a conductivity of 5000-35000 μS / cm.

3. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, Step (2) The operating pressure of the first-stage reverse osmosis separation is 1.0 to 8.0 MPa, the temperature is 10 to 45℃, and the membrane surface flow velocity is 0.08 to 0.20 m / s; The first-stage acid-resistant reverse osmosis membrane module described in step (2) intercepts dissolved salts, including phosphate, sulfate, fluoride, calcium, magnesium, iron, and aluminum ions, thereby significantly reducing the salt content of the first-stage permeate and increasing the salt content of the first-stage concentrate. Step (2) has a water recovery rate of 55% to 75%.

4. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, Step (3) The molecular weight cutoff of the acid-resistant nanofiltration membrane module is 200-800 Da; The operating pressure for nanofiltration separation in step (3) is 0.5–4.0 MPa and the temperature is 10–40 °C.

5. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 4, characterized in that, Step (3) Nanofiltration separation has a rejection rate of no less than 95% for sulfate ions, calcium ions, magnesium ions, iron ions, and aluminum ions; Step (3) Nanofiltration separation has a permeability of 30%–50% for phosphate and dihydrogen phosphate ions; The nanofiltration permeate obtained in step (3) meets the following conditions: the phosphate concentration is not less than 5000 mg / L, the fluoride concentration is not less than 2000 mg / L, and the total concentration of impurity ions such as calcium, magnesium, iron, aluminum and sulfate does not exceed 500 mg / L.

6. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, Step (4) The operating pressure for the secondary reverse osmosis separation is 1.0–6.0 MPa, and the temperature is 10–40 °C; The water recovery rate of the secondary reverse osmosis separation in step (4) is 70% to 85%.

7. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, Before step (2), an acid-resistant scale inhibitor is added to the adjusted acidic raw water at a dosage of 2-15 mg / L; the acid-resistant scale inhibitor is selected from at least one of organophosphonate scale inhibitors, polycarboxylic acid scale inhibitors, or sulfonated copolymer scale inhibitors.

8. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, The membrane material of the primary acid-resistant reverse osmosis membrane module and / or the secondary acid-resistant reverse osmosis membrane module is selected from at least one of polyamide composite acid-resistant reverse osmosis membrane, sulfonated polyethersulfone acid-resistant reverse osmosis membrane, or polypiperazine amide acid-resistant reverse osmosis membrane; the membrane material of the acid-resistant nanofiltration membrane module is selected from at least one of polyamide composite acid-resistant nanofiltration membrane, sulfonated polyethersulfone acid-resistant nanofiltration membrane, or polypiperazine amide acid-resistant nanofiltration membrane.

9. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, The nanofiltration permeate was concentrated by evaporation to recover a phosphoric acid solution with a mass fraction of 10% to 30%. The recovered phosphoric acid solution was reacted with iron salts to produce iron phosphate.

10. The multi-stage membrane separation method for high-phosphorus and fluoride wastewater under acidic conditions as described in claim 1, characterized in that, The nanofiltration permeate was reacted with ammonium salts to produce monoammonium phosphate and diammonium phosphate products.

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