Boiler continuous drainage high-temperature ion interception method based on gradient aperture ceramic membrane

Abstract

The invention relates to the technical field of ion interception of wastewater treatment, in particular to a boiler continuous drainage high-temperature ion interception method based on a gradient aperture ceramic membrane, which comprises the following steps: treating at 120-180 DEG C and 0.2-0.6 MPa by adopting a gradient membrane assembly containing a support layer, a transition layer and a separation layer; the support layer contains alpha-AlO and the like, the transition layer contains TiO and the like, and the separation layer contains cordierite, nano CeO, modified ZnO, graphene quantum dots and MXene nanosheets. The modified ZnO is prepared by reacting ZnO with KH-550; the method comprises the following steps: S1, pretreating to remove particles, and adjusting pH; s2, preheating; s3, carrying out membrane separation on intercepted ions; s4, backwashing; and S5, regenerating. The device improves the high-temperature ion rejection rate, is high in flux, is anti-pollution, prolongs the service life of the membrane, reduces the maintenance cost, and is suitable for efficient treatment of boiler continuous drainage water.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ion interception technology for wastewater treatment, in particular to a high-temperature ion interception method for boiler continuous drainage based on gradient-pore-size ceramic membranes. BACKGROUND

[0002] As an important source of wastewater in industrial production, boiler continuous drainage is rich in hardness ions such as Ca²⁺, Mg²⁺ and SiO3²⁻. Direct discharge not only wastes water resources, but also affects the operating efficiency of subsequent treatment equipment due to scaling problems. Currently, the main methods for treating boiler continuous drainage include ion exchange, reverse osmosis and electrodialysis, but these methods have obvious limitations under high-temperature conditions.

[0003] Ion exchange relies on the adsorption capacity of resins for ions. However, at temperatures above 100°C, resins are prone to thermal degradation, exchange capacity drops by more than 50%, and frequent regeneration leads to high operating costs. Reverse osmosis can achieve high ion interception rates, but organic membrane materials will have damaged pore structures and reduced pressure resistance at temperatures above 120°C, making them unable to adapt to the high-temperature and high-pressure characteristics of boiler continuous drainage, with a membrane life typically less than 3 months. Electrodialysis drives ion migration through an electric field, but electrodes are prone to corrosion at high temperatures, and the concentration polarization phenomenon is severe, resulting in a 30-40% increase in energy consumption, making it difficult to meet the needs of large-scale industrial applications.

[0004] Ceramic membranes have gradually gained attention in high-temperature water treatment due to their excellent high-temperature resistance and chemical stability, but existing ceramic membranes are mostly single-pore-size structures, which have the problem of balancing water permeability and interception rate: large-pore-size membranes have good water permeability but insufficient interception rate (Ca²⁺ interception rate is only 60-70%), and small-pore-size membranes have high interception rates but are prone to clogging, with a rapid flux decay rate (flux decreases by more than 50% after 24 hours of operation). In addition, the separation layer of traditional ceramic membranes mostly uses single oxide materials (such as Al2O3 and ZrO2), which lack functional groups on the surface, limiting the selective adsorption capacity for ions and making it impossible to achieve efficient interception of specific ions (such as SiO3²⁻) in boiler continuous drainage.

[0005] In terms of membrane preparation technology, the interlayer bonding strength of existing gradient ceramic membranes is generally low (≤10 MPa), and delamination is prone to occur under alternating high-temperature and high-pressure conditions. In addition, the sintering process is prone to cause membrane cracking due to thermal stress. Meanwhile, the cleaning and regeneration technology of the membrane is not perfect. Traditional chemical cleaning methods (such as hydrochloric acid solution) can remove some scale, but will corrode the active layer of the membrane, reducing the service life of the membrane. Physical backwashing has poor cleaning effect on stubborn scale, resulting in a shortened operation cycle of the membrane to 1-2 weeks.

[0006] With the development of industrial boilers to high temperature and high pressure, higher requirements for the high temperature resistance, high efficiency and stability of continuous drainage treatment technology are put forward. Therefore, the development of a gradient pore size ceramic membrane with high retention rate, large flux and long service life and its supporting treatment method has become the key to solving the problem of high temperature ion retention of boiler continuous drainage. SUMMARY

[0007] (I) Technical problems solved

[0008] In view of the deficiencies of the prior art, the present application provides a boiler continuous drainage high temperature ion retention method based on gradient pore size ceramic membrane.

[0009] (II) Technical solutions

[0010] A boiler continuous drainage high temperature ion retention method based on gradient pore size ceramic membrane adopts a gradient pore size ceramic membrane assembly to retain ions from boiler continuous drainage with a temperature of 120-180℃ and a pressure of 0.2-0.6MPa. The gradient pore size ceramic membrane has a support layer, a transition layer and a separation layer from bottom to surface, and each layer is composed of the following components in mass fraction:

[0011] Support layer: alpha-Al2O3 60-70 parts, ZrO2 10-15 parts, SiC 5-10 parts, sintering aid La2O3 2-5 parts, average pore size 8-15μm, thickness 2-4mm; transition layer: TiO2 50-60 parts, ZrO2 20-30 parts, SiO2 10-15 parts, average pore size 1-3μm, thickness 50-100μm; separation layer: cordierite 40-50 parts, nano CeO2 5-10 parts, modified nano ZnO 3-8 parts, graphene quantum dots 1-3 parts, two-dimensional MXene nanosheet 0.5-2 parts, average pore size 50-200nm, thickness 5-15μm;

[0012] Among them, the modified nano ZnO is prepared by reaction of ZnO with 3-aminopropyl triethoxysilane (KH-550), and the amino group density on the surface of the modified ZnO is 1.2-2.0mmol / g; the MXene nanosheet is Ti3C2T x , and the surface hydroxyl group density after etching with hydrofluoric acid is 2.5-3.5mmol / g, and the sheet diameter is 500-1000nm;

[0013] The method comprises the following steps:

[0014] S1: pretreatment, removing particles with a particle size of ≥10μm from the boiler continuous drainage by mechanical filtration, adding 0.01-0.05wt% of hydroxyethylidene diphosphonic acid (HEDP) to chelate free metal ions, and adjusting the pH to 6.5-8.0;

[0015] S2: preheating, heating the pretreated water to 120-180℃, maintaining the pressure at 0.2-0.6MPa;

[0016] S3: membrane separation, passing the preheated water into the gradient pore size ceramic membrane module at a flow rate of 1.5-3.0m / s, filtering at an operating temperature of 120-180℃ and an operating pressure of 0.2-0.6MPa, and retaining Ca²⁺, Mg²⁺, SiO3²⁻ and other ions in the water, with a retention rate of ≥95%;

[0017] S4: backwashing, every 2-4h of operation, using deionized water with a temperature of 130-190℃ and a pressure of 0.7-1.0MPa to backwash the membrane module, with a washing time of 5-10min;

[0018] S5: regeneration, every month, using a 0.5-2.0wt% citric acid solution to soak the membrane module at 60-80℃ for 2-4h, and then washing with deionized water until neutral.

[0019] Preferably, 2-5 parts of yttrium-stabilized zirconium oxide (YSZ) is also added to the separation layer, the YSZ has a particle size of 50-100nm, forms a solid solution with cordierite, is nanoscale dispersed by high-energy ball milling, and the Y2O3 doping amount in the YSZ is 3-8mol%.

[0020] Preferably, the graphene quantum dots are surface functionalized and modified, the graphene quantum dots are nitrogen and sulfur co-doped, the nitrogen content is 5-8at%, the sulfur content is 2-3at%, the graphene quantum dots are prepared by a hydrothermal method, the particle size is 3-8nm, and the surface carboxyl content is 0.8-1.5mmol / g.

[0021] Preferably, the sintering of the support layer adopts a two-step method: first, pre-sintering at 1200-1300℃ for 1-2h to remove the organic binder, and then sintering at 1500-1650℃ for 2-4h; the sintering temperature of the transition layer is 1200-1350℃, and the holding time is 1-2h; the sintering temperature of the separation layer is 900-1050℃, and the holding time is 0.5-1h; the sintering of each layer adopts an air atmosphere, the heating rate is 5-10℃ / min, and the difference between the sintering temperatures of adjacent layers is controlled to be 200-300℃ to avoid interlayer cracking.

[0022] Preferably, in step S3, the membrane module adopts a cross-flow filtration mode, the concentrated water reflux ratio is 3:1-5:1, and the flow rate is adjusted by online monitoring to maintain the shear force on the membrane surface at 15-25Pa under the conditions of an operating temperature of 140-160℃ and an operating pressure of 0.3-0.5MPa.

[0023] Preferably, in step S4, 0.1-0.5wt% of a nano-SiO2 dispersion and 0.05-0.1wt% of a zwitterionic surfactant cocamidopropyl betaine are added to the backwashing water.

[0024] Preferably, the porosity of the gradient-pore ceramic membrane is continuously distributed along the thickness direction, the porosity of the support layer is 40-50%, the porosity of the transition layer is 30-40%, the porosity of the separation layer is 20-30%, and the difference in porosity between each layer is controlled within 10-15%, which is realized by adding pore-forming agents, the pure water flux of the membrane is 800-1200 L / (m²·h·MPa) at 25°C; and the pore-forming agent for the support layer is activated carbon, the pore-forming agent for the transition layer is starch, and the pore-forming agent for the separation layer is PEG2000.

[0025] Preferably, the mechanical filtration in step S1 adopts three-stage series filtration: 50μm stainless steel filter screen→20μm polypropylene filter membrane→5μm ceramic filter membrane, the filtration precision is ≤5μm, and 0.005-0.01wt% of nano-TiO2 photocatalyst is added after filtration, the nano-TiO2 photocatalyst is activated by ultraviolet light irradiation for 30min.

[0026] Preferably, the separation layer is prepared by layer-by-layer self-assembly process: the surface of the transition layer is first subjected to hydroxylation treatment, then modified ZnO sol with positive electricity and MXene dispersion liquid with negative electricity are alternately coated, the coating is repeated for 3-5 times, then cordierite-based sol is coated, after each coating, drying is performed at 60-80°C for 10-15min, and finally sintering is performed to form a separation layer with interlayer bonding strength ≥20MPa.

[0027] Preferably, the citric acid solution in step S5 contains 0.1-0.3wt% of sodium fluoride and 0.05-0.1wt% of thiourea, the fluoride ions form a soluble complex with calcium and magnesium scales, and the thiourea inhibits the oxidation of metal ions.

[0028] (Three) Beneficial technical effects

[0029] Compared with the existing technology, the beneficial effects of the present application are:

[0030] 1. The separation layer introduces modified nano-ZnO, MXene nanosheets and functionalized graphene quantum dots, and through chemical adsorption and sieving synergistic effect, the retention rate of Ca²⁺, Mg²⁺ and SiO3²⁻ is improved, solving the problem of insufficient retention of specific ions in traditional ceramic membranes. The gradient-pore design makes the porosity of the support layer, the transition layer and the separation layer continuously distributed, which improves the pure water flux while ensuring high retention rate.

[0031] 2. Yttrium-stabilized zirconia and cordierite form a solid solution, which is dispersed by high-energy ball milling, thereby improving the high-temperature impact resistance of the membrane, and the retention rate is still ≥90% after 50 times of cold and hot cycle at 100-200°C; the interlayer bonding strength is ≥20MPa, which avoids the delamination phenomenon under high temperature and high pressure, prolongs the service life of the membrane, and improves the service life compared with traditional ceramic membranes.

[0032] 3. Cross-flow filtration combined with online shear force regulation reduces membrane fouling rate; backwashing with nano-SiO2 and amphoteric surfactant, flux recovery rate ≥95%; regeneration liquid introduces sodium fluoride and thiourea complex system, retention rate is restored to more than 98% of the initial value, reducing downtime maintenance time and cost. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a process flow chart of a boiler continuous drainage high-temperature ion interception method based on gradient pore size ceramic membrane proposed by the present application;

[0034] Figure 2 is a columnar comparison chart of backwashing and regeneration effects of examples and comparative examples;

[0035] Figure 3 is a line comparison chart of ion retention rates of examples and comparative examples;

[0036] Figure 4 is a columnar line comparison chart of flux retention rates after 300h operation and retention rate retention rates after 50 times of cold and hot cycles of examples and comparative examples. DETAILED DESCRIPTION

[0037] According to Figures 1 to 4 , the specific embodiments of the present application are as follows:

[0038] Example 1

[0039] Preparation of gradient pore size ceramic membrane

[0040] Support layer: take α-Al2O3 65 parts, ZrO2 12 parts, SiC 8 parts, La2O3 3 parts by mass fraction, add 5wt% polyvinyl alcohol binder, ball mill (rotation speed 400 rpm) for 2h to particle size ≤2μm. Pressed into a circular sheet with a diameter of 50mm and a thickness of 3mm, first pre-burned at 1250℃ for 1.5h (to remove the binder), then heated to 1550℃ for sintering (air atmosphere, heating rate 8℃ / min), to obtain a support layer with an average pore size of 12μm and a porosity of 45%.

[0041] Transition layer: take TiO2 55 parts, ZrO2 25 parts, SiO2 12 parts, add deionized water to prepare a slurry with a solid content of 30wt%, ball mill for 1h to a particle size ≤1μm. Coated on the surface of the support layer by dip coating method, dried at 60℃ for 30min, then sintered at 1280℃ for 1.5h (heating rate 5℃ / min), to form a transition layer with an average pore size of 2μm, a thickness of 80μm and a porosity of 35%.

[0042] Separation layer: Modified ZnO preparation: ZnO and KH-550 were reacted in ethanol at a mass ratio of 10:1 at 60℃ for 2 h, and dried to obtain modified ZnO with an amino density of 1.6 mmol / g; MXene dispersion: Ti3C2T x After hydrofluoric acid etching, the material was ultrasonically dispersed (400W) in water at a concentration of 1 wt% (hydroxyl density 3.0 mmol / g). Layer-by-layer self-assembly was then performed: the transition layer was hydroxylated and then alternately coated with modified ZnO sol (positively charged) and MXene dispersion (negatively charged) four times. Finally, a mixed sol (20 wt% solid content) of 45 parts cordierite, 28 parts nano CeO, and 2 parts graphene quantum dots was coated. After drying at 60℃ for 15 min, the material was sintered at 980℃ for 1 h (heating rate 5℃ / min) to form a separation layer with an average pore size of 120 nm, a thickness of 10 μm, a porosity of 25%, and an interlayer bonding strength of 22 MPa.

[0043] High-temperature ion retention process

[0044] S1 Pretreatment: Boiler wastewater (Ca²⁺ 250mg / L, Mg²⁺ 120mg / L, SiO3²⁻ 80mg / L) was filtered through three stages (50μm→20μm→5μm), 0.03wt%HEDP was added, and the pH was adjusted to 7.0, with a colloid removal rate of 99.2%.

[0045] S2 preheating: Heat to 150℃, maintain pressure at 0.4MPa.

[0046] S3 membrane separation: cross-flow filtration, flow rate 2.0 m / s, concentrate reflux ratio 4:1, membrane surface shear force 20 Pa, operation time 3 h.

[0047] S4 backwash: Backwash for 8 minutes every 3 hours with 160℃, 0.8MPa deionized water (containing 0.3wt% nano SiO2 + 0.08wt% cocamidopropyl betaine).

[0048] S5 regeneration: Soak in 1.0wt% citric acid (containing 0.2wt% sodium fluoride + 0.08wt% thiourea) at 70℃ for 3 hours monthly, then rinse until neutral.

[0049] Example 2

[0050] Gradient pore size ceramic membrane preparation

[0051] Support layer: Weigh out 60 parts by weight of α-Al₂O₃, 15 parts by weight of ZrO₂, 8 parts by weight of SiC, and 3 parts by weight of La₂O₃. Add 5 wt% polyvinyl alcohol binder and ball mill (400 rpm) for 2 hours until the particle size is ≤2 μm. Press into discs with a diameter of 50 mm and a thickness of 3 mm. Pre-fire at 1250℃ for 1.5 hours (to remove the binder), then sinter at 1600℃ for 3 hours (in air atmosphere, heating rate 8℃ / min) to obtain a support layer with an average pore size of 12 μm and a porosity of 42%.

[0052] Transition layer: Weigh 50 parts TiO2, 30 parts ZrO2, and 12 parts SiO2, add deionized water to prepare a slurry with a solid content of 30wt%, and ball mill for 1 hour until the particle size is ≤1μm. Apply the slurry to the surface of the support layer by dip coating, dry at 60℃ for 30 min, and then sinter at 1320℃ for 1.5 h (heating rate 5℃ / min) to form a transition layer with an average pore size of 2μm, a thickness of 100μm, and a porosity of 35%.

[0053] Separation layer: Modified ZnO preparation: ZnO and KH-550 were reacted in ethanol at a mass ratio of 10:1 at 60℃ for 2 h, and dried to obtain modified ZnO with an amino density of 1.2 mmol / g; MXene dispersion: Ti3C2T x After hydrofluoric acid etching, the material was ultrasonically dispersed (400W) in water at a concentration of 1 wt% (hydroxyl density 3.0 mmol / g). Layer-by-layer self-assembly was then performed: the transition layer was hydroxylated and then alternately coated with modified ZnO sol (positively charged) and MXene dispersion (negatively charged) three times. Finally, a mixed sol (20 wt% solid content) of 50 parts cordierite, 25 parts nano CeO, and 2 parts graphene quantum dots was coated. After drying at 60℃ for 15 min, the material was sintered at 980℃ for 1 h (heating rate 5℃ / min) to form a separation layer with an average pore size of 180 nm, a thickness of 10 μm, a porosity of 25%, and an interlayer bonding strength of 21 MPa.

[0054] High-temperature ion retention process

[0055] S1 Pretreatment: Boiler wastewater (Ca²⁺ 250mg / L, Mg²⁺ 120mg / L, SiO3²⁻ 80mg / L) was filtered through three stages (50μm→20μm→5μm), 0.05wt%HEDP was added, and the pH was adjusted to 7.5, with a colloid removal rate of 99.2%.

[0056] S2 preheating: Heat to 160℃, maintain pressure at 0.5MPa.

[0057] S3 membrane separation: cross-flow filtration, flow rate 2.5 m / s, concentrate reflux ratio 4:1, membrane surface shear force 25 Pa, operation time 3 h.

[0058] S4 backwash: Backwash for 10 minutes every 3 hours with 160℃, 0.8MPa deionized water (containing 0.5wt% nano SiO2 + 0.08wt% cocamidopropyl betaine).

[0059] S5 regeneration: Soak in 1.0wt% citric acid (containing 0.2wt% sodium fluoride + 0.08wt% thiourea) at 70℃ for 3 hours monthly, then rinse until neutral.

[0060] Example 3

[0061] Gradient pore size ceramic membrane preparation

[0062] Support layer: Weigh out 70 parts by weight of α-Al₂O₃, 12 parts by weight of ZrO₂, 5 parts by weight of SiC, and 3 parts by weight of La₂O₃. Add 5 wt% polyvinyl alcohol binder and ball mill (400 rpm) for 2 hours until the particle size is ≤2 μm. Press into discs with a diameter of 50 mm and a thickness of 3 mm. Pre-fire at 1250℃ for 1.5 hours (to remove the binder), then sinter at 1500℃ for 3 hours (in air atmosphere, heating rate 8℃ / min) to obtain a support layer with an average pore size of 12 μm and a porosity of 48%.

[0063] Transition layer: Weigh 55 parts TiO2, 25 parts ZrO2, and 12 parts SiO2, add deionized water to prepare a slurry with a solid content of 30 wt%, and ball mill for 1 h until the particle size is ≤1 μm. Apply the slurry to the surface of the support layer using a dip-coating method, dry at 60℃ for 30 min, and then sinter at 1280℃ for 1.5 h (heating rate 5℃ / min) to form a transition layer with an average pore size of 2 μm, a thickness of 50 μm, and a porosity of 32%.

[0064] Separation layer: Modified ZnO preparation: ZnO and KH-550 were reacted in ethanol at a mass ratio of 10:1 at 60℃ for 2 h, and dried to obtain modified ZnO with an amino density of 1.6 mmol / g; MXene dispersion: Ti3C2T x After hydrofluoric acid etching, the material was ultrasonically dispersed (400W) in water at a concentration of 1wt% (hydroxyl density 3.0mmol / g). Layer-by-layer self-assembly was then performed: the transition layer was hydroxylated and then alternately coated with modified ZnO sol (positively charged) and MXene dispersion (negatively charged) for four cycles. Finally, a mixed sol (solid content 20wt%) consisting of 45 parts cordierite, 8 parts nano CeO2, 2 parts graphene quantum dots, and 3 parts yttrium-stabilized zirconium oxide (YSZ, Y2O3 5mol%) was coated. The YSZ was dispersed by ball milling (400rpm, 3h). After drying at 60℃ for 15min, the material was sintered at 980℃ for 1h (heating rate 5℃ / min) to form a separation layer with an average pore size of 80nm, a thickness of 10μm, a porosity of 25%, and an interlayer bonding strength of 22MPa.

[0065] High-temperature ion retention process

[0066] S1 Pretreatment: Boiler wastewater (Ca²⁺ 250mg / L, Mg²⁺ 120mg / L, SiO3²⁻ 80mg / L) was filtered through three stages (50μm→20μm→5μm), 0.01wt%HEDP was added, and the pH was adjusted to 6.5, with a colloid removal rate of 99.2%.

[0067] S2 preheating: Heat to 130℃, maintain pressure at 0.3MPa.

[0068] S3 membrane separation: cross-flow filtration, flow rate 1.8 m / s, concentrate reflux ratio 4:1, membrane surface shear force 18 Pa, operation time 3 h.

[0069] S4 backwash: Backwash for 8 minutes every 3 hours with 160℃, 0.8MPa deionized water (containing 0.3wt% nano SiO2 + 0.08wt% cocamidopropyl betaine).

[0070] S5 regeneration: Soak in 0.8wt% citric acid (containing 0.2wt% sodium fluoride + 0.08wt% thiourea) at 70℃ for 2.5h per month, then rinse until neutral.

[0071] Example 4

[0072] Gradient pore size ceramic membrane preparation

[0073] Support layer: Weigh out 65 parts by weight of α-Al₂O₃, 12 parts by weight of ZrO₂, 8 parts by weight of SiC, and 3 parts by weight of La₂O₃. Add 5 wt% polyvinyl alcohol binder and ball mill (400 rpm) for 2 hours until the particle size is ≤2 μm. Press into discs with a diameter of 50 mm and a thickness of 3 mm. Pre-fire at 1250℃ for 1.5 hours (to remove the binder), then sinter at 1550℃ for 3 hours (in air atmosphere, heating rate 8℃ / min) to obtain a support layer with an average pore size of 12 μm and a porosity of 45%.

[0074] Transition layer: Weigh 55 parts TiO2, 25 parts ZrO2, and 12 parts SiO2, add deionized water to prepare a slurry with a solid content of 30 wt%, and ball mill for 1 h until the particle size is ≤1 μm. Apply the slurry to the surface of the support layer using a dip-coating method, dry at 60℃ for 30 min, and then sinter at 1280℃ for 1.5 h (heating rate 5℃ / min) to form a transition layer with an average pore size of 2 μm, a thickness of 80 μm, and a porosity of 35%.

[0075] Separation layer: Modified ZnO preparation: ZnO and KH-550 were reacted in ethanol at a mass ratio of 10:1 at 60℃ for 2 h, and dried to obtain modified ZnO with an amino density of 1.6 mmol / g; MXene dispersion: Ti3C2T xAfter etching with hydrofluoric acid, the material was ultrasonically dispersed (400W) in water at a concentration of 1wt% (hydroxyl density 3.0mmol / g). Graphene quantum dots were prepared by hydrothermal reaction at 190℃ for 11h with glucose, thiourea, and urea in a mass ratio of 10:3:2, with a nitrogen content of 6at%, a sulfur content of 2.5at%, and a particle size of 3-8nm. Layer-by-layer self-assembly: After hydroxylation treatment, the transition layer was alternately coated with modified ZnO sol (positively charged) and MXene dispersion (negatively charged) for 4 times. Finally, a mixed sol (solid content 20wt%) of 45 parts cordierite, 8 parts nano CeO2, 2 parts of the above-mentioned graphene quantum dots, and 2 parts MXene was coated. After drying at 60℃ for 15min, the mixture was sintered at 980℃ for 1h (heating rate 5℃ / min) to form a separation layer with an average pore size of 200nm, a thickness of 10μm, a porosity of 25%, and an interlayer bonding strength of 23MPa.

[0076] High-temperature ion retention process

[0077] S1 Pretreatment: Boiler wastewater (Ca²⁺ 250mg / L, Mg²⁺ 120mg / L, SiO3²⁻ 80mg / L) was filtered through three stages (50μm→20μm→5μm), 0.03wt%HEDP was added, and the pH was adjusted to 7.0, with a colloid removal rate of 99.2%.

[0078] S2 preheating: Heat to 170℃, maintain pressure at 0.6MPa.

[0079] S3 membrane separation: cross-flow filtration, flow rate 3.0 m / s, concentrate reflux ratio 5:1, membrane surface shear force 20 Pa, operation time 3 h.

[0080] S4 backwash: Backwash for 8 minutes every 3 hours with 160℃, 0.8MPa deionized water (containing 0.3wt% nano SiO2 + 0.08wt% cocamidopropyl betaine).

[0081] S5 regeneration: Soak in 1.0wt% citric acid (containing 0.2wt% sodium fluoride + 0.1wt% thiourea) at 70℃ for 3 hours monthly, then rinse until neutral.

[0082] Comparative Example

[0083] Ceramic membrane preparation: A single ZrO2 ceramic membrane was prepared by weighing 95 parts ZrO2 and 5 parts La2O3 by mass, ball milling, pressing and molding, and sintering at 1500℃ for 3h to obtain a membrane with an average pore size of 200nm and a thickness of 3mm. The membrane had no gradient structure and the separation layer was unmodified with ZnO, MXene and graphene quantum dots.

[0084] Ion retention process

[0085] S1 Pretreatment: Boiler wastewater is filtered only through a 50μm filter screen, without the addition of HEDP, and the pH is natural (approximately 8.5).

[0086] S2 preheating: Heat to 150℃, maintain pressure at 0.4MPa.

[0087] S3 membrane separation: dead-end filtration, flow rate 1.0m / s, no concentrate recirculation.

[0088] S4 backwash: Backwash for 8 minutes every 3 hours with 25℃, 0.8MPa deionized water, without additives.

[0089] S5 regeneration: Soak in 1wt% hydrochloric acid at 60℃ for 3 hours every month, then rinse until neutral.

[0090] The backwashing and regeneration effects of the examples and comparative examples are shown in the table below:

[0091] Table 1

[0092] Item Example 1 Example 2 Example 3 Example 4 Comparative Example Flux recovery rate (%) after backwash 96.5 95.8 97.0 96.2 72.3 Recovery rate (%) of rejection after regeneration 98.3 98.0 98.5 98.6 85.2

[0093] The performance test results of the examples and comparative examples are shown in the table below:

[0094] Table 2

[0095] Test item Example 1 Example 2 Example 3 Example 4 Comparative Example Ca2+ rejection rate (%) 98.6 98.2 98.0 98.8 72.5 Mg2+ rejection rate (%) 97.5 97.0 96.8 97.8 68.3 SiO32" rejection (%) 98.2 97.9 97.5 98.5 65.1 Pure water flux (L / (m2-h-MPa)) 1000 950 1100 980 550 Flux retention rate (%) after 300 h operation 88 86 89 87 42 Rejection retention rate (%) after 50 times of cold-hot cycle 91 90 92 91 58 Membrane life (month) 22 20 23 21 6

[0096] As can be seen from the two tables above, Examples 1-4, due to their gradient pore size design and the synergistic effect of the support layer, transition layer, and separation layer, ensure both high rejection rate and improved permeability, with pure water flux being 1.7-2 times that of the comparative examples. In the separation layer, the amino groups of modified ZnO, the hydroxyl groups of MXene, and the functionalized graphene quantum dots form an adsorption network, increasing the ion rejection rate. The addition of YSZ enhances resistance to temperature shock, maintaining a rejection rate of over 90% after thermal cycling. Backwashing with nano-SiO2 and surfactants, and the use of a compound solution for regeneration, significantly improve flux and rejection rate recovery, extend membrane life, and fully verify the superiority of this invention.

[0097] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A boiler continuous drainage high-temperature ion interception method based on gradient pore ceramic membrane, which adopts a gradient pore ceramic membrane assembly to perform ion interception on boiler continuous drainage with a temperature of 120-180 ℃ and a pressure of 0.2-0.6 MPa, characterized in that, The gradient pore size ceramic membrane comprises a support layer, a transition layer and a separation layer from bottom to surface, and each layer is composed of the following components in mass fraction: Support layer: α-Al2O3 60-70 parts, ZrO2 10-15 parts, SiC 5-10 parts, sintering aid La2O3 2-5 parts, average pore size 8-15 μm, thickness 2-4 mm; Transition layer: TiO2 50-60 parts, ZrO2 20-30 parts, SiO2 10-15 parts, average pore size 1-3 μm, thickness 50-100 μm; Separation layer: cordierite 40-50 parts, nano CeO2 5-10 parts, modified nano ZnO 3-8 parts, graphene quantum dots 1-3 parts, two-dimensional MXene nanosheet 0.5-2 parts, average pore size 50-200 nm, thickness 5-15 μm; The modified nano-ZnO is prepared by the reaction of ZnO with 3-aminopropyl triethoxysilane (KH-550), and the amino density on the surface of the modified ZnO is 1.2-2.0 mmol / g; the MXene nanosheet is Ti3C2T x , and the hydroxyl density on the surface of the MXene nanosheet after etching by hydrofluoric acid is 2.5-3.5 mmol / g, and the sheet diameter is 500-1000 nm. The method comprises the following steps: S1: pretreatment, removing particles with a particle size of ≥10 μm from the boiler continuous drainage by mechanical filtration, adding 0.01-0.05 wt% of hydroxyethylidene diphosphonic acid (HEDP) to chelate free metal ions, and adjusting the pH to 6.5-8.0; S2: preheating, heating the pretreated water to 120-180℃, and maintaining the pressure at 0.2-0.6 MPa; S3: membrane separation, passing the preheated water into the gradient pore size ceramic membrane assembly at a flow rate of 1.5-3.0 m / s, and filtering at an operating temperature of 120-180℃ and an operating pressure of 0.2-0.6 MPa to remove Ca²⁺, Mg²⁺, SiO3²⁻ and other ions in the water, with a removal rate of ≥95%; S4: backwashing, using deionized water with a temperature of 130-190℃ and a pressure of 0.7-1.0 MPa to backwash the membrane assembly every 2-4 h, and the washing time is 5-10 min; S5: regeneration, soaking the membrane assembly in a 0.5-2.0 wt% citric acid solution at 60-80℃ for 2-4 h every month, and then washing with deionized water until neutral.

2. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes according to claim 1, characterized in that, It also includes adding 2-5 parts of yttrium stabilized zirconia (YSZ) to the separation layer, the YSZ has a particle size of 50-100 nm, forms a solid solution with cordierite, is nano-sized by high-energy ball milling, and the Y2O3 doping amount in the YSZ is 3-8 mol%.

3. The boiler blowdown high temperature ion rejection process based on gradient- pore ceramic membranes according to claim 1, characterized in that, It also includes surface functionalization modification of the graphene quantum dots, the graphene quantum dots are nitrogen and sulfur co-doped, the nitrogen content is 5-8 at%, the sulfur content is 2-3 at%, the graphene quantum dots are prepared by a hydrothermal method, the particle size is 3-8 nm, and the surface carboxyl content is 0.8-1.5 mmol / g.

4. The boiler blowdown high temperature ion rejection process based on gradient- pore ceramic membranes according to claim 1, characterized in that, The sintering of the support layer adopts a two-step method: first pre-sintering at 1200-1300℃ for 1-2 h to remove the organic binder, and then sintering at 1500-1650℃ for 2-4 h; The sintering temperature of the transition layer is 1200-1350℃, and the holding time is 1-2 h; the sintering temperature of the separation layer is 900-1050℃, and the holding time is 0.5-1 h, the sintering of each layer adopts an air atmosphere, the heating rate is 5-10℃ / min, and the sintering temperature difference between adjacent layers is controlled within 200-300℃ to avoid interlayer cracking.

5. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes according to claim 1, characterized in that, The membrane module in step S3 adopts a cross-flow filtration mode, the concentrated water backflow ratio is 3:1-5:1, the operation temperature is 140-160℃, the operation pressure is 0.3-0.5MPa, and the membrane surface shear force is maintained at 15-25Pa by adjusting the flow rate through online monitoring.

6. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes according to claim 1, characterized in that, In step S4, the backwashing water is added with 0.1-0.5wt% of a nano-SiO2 dispersion and 0.05-0.1wt% of a zwitterionic surfactant, cocamidopropyl betaine.

7. The boiler blowdown high temperature ion rejection process based on gradient- pore ceramic membranes according to claim 1, characterized in that, The gradient-pore-size ceramic membrane has a continuous gradient distribution of porosity along the thickness direction, the support layer has a porosity of 40-50%, the transition layer has a porosity of 30-40%, the separation layer has a porosity of 20-30%, and the difference in porosity between the layers is controlled to be 10-15%, which is achieved by adding a pore-forming agent, the pure water flux of the membrane is 800-1200L / (m²・h・MPa) at 25℃, the pore-forming agent for the support layer is activated carbon, the pore-forming agent for the transition layer is starch, and the pore-forming agent for the separation layer is PEG2000.

8. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes according to claim 1, characterized in that, In step S1, the mechanical filtration adopts a three-stage series filtration: 50μm stainless steel filter screen→20μm polypropylene filter membrane→5μm ceramic filter membrane, the filtration precision is ≤5μm, and 0.005-0.01wt% of a nano-TiO2 photocatalyst is added after filtration, the nano-TiO2 photocatalyst is activated by ultraviolet light irradiation for 30min.

9. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes according to claim 1, characterized in that, The separation layer is prepared by a layer-by-layer self-assembly process: the surface of the transition layer is first subjected to hydroxylation treatment, then a positively charged modified ZnO sol is alternately coated with a negatively charged MXene dispersion, the process is repeated for 3-5 times, then a cordierite-based sol is coated, after each coating, drying is performed at 60-80℃ for 10-15min, and finally sintering is performed to form a separation layer with an interlayer bonding strength of ≥20MPa.

10. The boiler blowdown high temperature ion rejection process based on gradient pore ceramic membranes as claimed in claim 1, wherein, In step S5, the citric acid solution contains 0.1-0.3wt% of sodium fluoride and 0.05-0.1wt% of thiourea, the fluoride ions form a soluble complex with calcium and magnesium scales, and the thiourea inhibits the oxidation of metal ions.

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