Preparation method of gradient pore ceramic membrane for oil-water emulsion separation

Abstract

The invention discloses a preparation method of a gradient pore ceramic membrane for oil-water emulsion separation, and belongs to the technical field of inorganic nonmetallic materials. According to the method, industrial solid waste red mud is used as a main raw material, bentonite is used as an auxiliary material, polyvinylpyrrolidone is added as a dispersing agent, polyvinyl alcohol is added as a binder, and the ceramic membrane with a directional gradient pore structure is prepared through the working procedures of ball-milling mixing, directional freeze-drying, solid-phase reaction sintering and the like, so that resource utilization of the red mud is effectively realized. Meanwhile, aiming at the technical bottleneck that the existing oil-water separation ceramic membrane is difficult to give consideration to high permeation flux and high oil interception rate, the directional freeze-drying process is systematically adopted for the first time, and a directional gradient pore structure suitable for oil-water emulsion separation is constructed by reasonably regulating and controlling each process parameter; the small-hole separation layer can accurately intercept oil drops, and the small-hole separation layer and the small-hole separation layer have a synergistic effect, so that an excellent oil interception effect is ensured while flux is improved. The method is environment-friendly, low in energy consumption and easy for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic materials technology, specifically a process for preparing gradient pore structure ceramic membranes using red mud as the main raw material via directional freeze-drying. This process is applicable to the field of oil-water emulsion separation. Background Technology

[0002] Red mud is a highly alkaline solid waste generated during alumina production, producing 0.6-2.5 tons of red mud for every ton of alumina produced. Currently, only a small amount of red mud is used in the production of building materials such as cement and bricks. Large quantities of red mud cannot be fully and effectively utilized and are instead disposed of by damming and stockpiling. Long-term stockpiling easily leads to soil alkalization, water pollution, and the risk of dam failure, making the resource utilization of red mud a challenge. However, red mud is rich in components such as alumina and silicon dioxide required for ceramic preparation, giving it a natural advantage in the preparation of ceramic membranes.

[0003] Every year, the world generates a huge amount of oily wastewater from industrial activities such as petroleum, metal processing, and food. This untreated oily wastewater not only seriously pollutes the environment but also threatens human health. Therefore, the efficient treatment of oily wastewater is of great environmental and health significance.

[0004] In recent years, freeze-drying molding technology has gained widespread attention in the field of ceramic membrane preparation due to its ability to construct porous structures through the sublimation process of a freezing medium, producing ceramic membranes with highly interconnected channels and adjustable porosity. Furthermore, the process is simple and environmentally friendly. However, several key technological bottlenecks remain in the preparation of gradient pore structures suitable for oil-water emulsion separation using freeze-drying technology. Precise control of ice crystal growth orientation is challenging, making it difficult to form interconnected directional channels. Establishing an effective matching relationship between freezing parameters and the solid-phase reaction regime is also difficult, hindering the achievement of a gradient pore size distribution. Furthermore, achieving a balance between the high-throughput transport characteristics of the macroporous support layer and the high retention accuracy of the microporous separation layer is challenging. These technological bottlenecks severely restrict the practical application of freeze-drying technology in oil-water emulsion separation.

[0005] Existing ceramic membranes suffer from problems such as expensive raw materials, complex preparation, and environmental pollution. The preparation of ceramic membranes suitable for oil-water emulsion separation using red mud combined with freeze-drying method has significant advantages in terms of process simplicity, environmental friendliness, and resource utilization of raw materials. Based on this, the present invention proposes the following technical solution. Summary of the Invention

[0006] To address the challenges of low utilization rates of existing red mud resources, urgent need for oily wastewater treatment, and technical issues such as high raw material costs, complex preparation processes, and difficulty in balancing high permeability and high oil rejection rates in existing oil-water emulsion separation ceramic membranes, this invention discloses a simple and environmentally friendly method for preparing gradient pore ceramic membranes for oil-water emulsion separation.

[0007] This invention is carried out according to the following steps: (1) Preparation of ceramic powder: Red mud and bentonite are mixed at a mass ratio of 27-29:2, and anhydrous ethanol is added at a mass-volume ratio of 29-31g:100mL. The mixture is then ball-milled in a ball mill jar to obtain uniform ceramic raw materials. After the slurry is sieved to separate the agate balls, it is heated and stirred in a water bath at 60℃ until the anhydrous ethanol is completely evaporated. Then it is dried in a forced-air drying oven and filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder.

[0008] (2) Preparation of organic mixture: Dispersant polyvinylpyrrolidone (PVP), binder polyvinyl alcohol (PVA) and deionized water are mixed in a mass ratio of 2:2-4:65 and stirred in an 80°C water bath until completely dissolved to obtain a stable organic solution.

[0009] (3) Preparation of ceramic slurry: The ceramic powder from step (1) and the organic mixture from step (2) are mixed at a mass ratio of 29-31:69-71, ball-milled, and then placed in a vacuum drying oven for vacuum degassing until no bubbles are present.

[0010] (4) Preparation of ceramic blank: Take the slurry from step (3) and place it in a polytetrafluoroethylene ring with brass at the bottom. Put it in a polystyrene foam box, add liquid nitrogen for directional freezing, and then transfer it to a freeze dryer to dry completely to obtain a cylindrical ceramic film blank.

[0011] (5) Solid-state reaction: The ceramic membrane preform was subjected to programmed calcination in air: the temperature was increased at a rate of 2℃ / min, first held at 400℃ for 180min, then increased to the range of 1050–1150℃ and held for 180min to obtain a porous ceramic membrane. Subsequently, the sample was cleaned by alternating ultrasonic oscillation with deionized water and anhydrous ethanol, repeated 3 times, with each cleaning lasting 5min. After cleaning, the sample was transferred to a constant temperature drying oven at 60℃ and dried. After drying, it was packaged and stored for later use.

[0012] In step (1), the ball milling speed is 300 r / min, the time is 5 h, the drying temperature is 60 ℃, and the drying time is more than 10 h; the combination of parameters can make the red mud and bentonite mixed evenly, and the resulting ceramic powder has a particle size of 200 mesh.

[0013] In step (3), the ball milling speed is 400 r / min and the ball milling time is 6 h.

[0014] In step (4), the thickness of the ceramic film blank is 1 cm, the directional freezing temperature is -150℃, the freezing time is 30 min, the temperature of the freeze dryer is -60℃, the pressure is 1 Pa, and the freeze drying time is 48 h.

[0015] In step (5), the cleaned ceramic film sample is dried in a constant temperature drying oven at 60℃ for more than 10 hours.

[0016] Compared with the prior art, this application has the following beneficial effects: This invention uses industrial solid waste red mud as the main raw material, achieving both high-value resource utilization of red mud and mitigating the environmental hazards caused by its stockpiling. The preparation process employs a combined process of ball milling-directional freeze-drying-solid-phase reaction, requiring no complex modification steps. The raw materials are all conventional chemical raw materials, making the process environmentally friendly, energy-efficient, and easy for industrial-scale production. Furthermore, this invention overcomes the technical bottleneck of existing oil-water emulsion separation ceramic membranes, which struggle to simultaneously achieve high permeability and high oil rejection rates, through systematic optimization of key parameters in each process. It can stably prepare ceramic membranes with a directional gradient pore structure, where the macroporous support layer and microporous separation layer are rationally matched, resulting in excellent oil-water separation performance. Therefore, the prepared ceramic membrane exhibits good repeatability and excellent overall performance, effectively meeting the needs of practical industrial applications. Attached Figure Description

[0017] Figure 1 The cross-sectional layered structure of the gradient pore ceramic membrane described in this invention and its SEM characterization are shown.

[0018] Figure 2 The graphs show the changes in permeation flux and oil rejection rate over time during the separation of oil-water emulsions using the gradient pore ceramic membrane described in this invention; where (a) represents the changes at different temperatures and (b) represents the changes at different binder contents.

[0019] Figure 3 This is a schematic diagram characterizing the water contact angle in air and the oil contact angle underwater of the gradient pore ceramic membrane described in this invention; where (a) is the curve of water contact angle changing with time, and (b) is the oil contact angle underwater. Detailed Implementation

[0020] Figure 1 As can be seen, this ceramic membrane exhibits a continuous directional gradient pore structure. The pores are arranged along the freezing direction and are uniformly interconnected. The pore size gradually increases from the bottom to the top surface along the membrane thickness, without obvious stratification. The bottom surface of the slurry is in contact with the low-temperature brass, resulting in the fastest cooling and finer ice crystals. After sintering, this forms a small-pore gradient section that acts as a separation zone to trap oil droplets. The upper surface cools more slowly, allowing ice crystals to grow fully and forming a large-pore gradient section that acts as a support mass transfer zone to reduce fluid resistance. The middle of the membrane is a pore size transition gradient section, achieving a smooth transition in pore size. This structure synergistically optimizes both retention accuracy and permeate flux, overcoming the technical bottleneck of traditional oil-water separation membranes where these two aspects are difficult to balance. Figure 2 This reflects the influence of different process parameters on the oil-water separation performance of ceramic membranes. Figure 3 It can be seen that the ceramic membrane is hydrophilic in air with a water contact angle of 0°, and oleophobic underwater with an oil contact angle of 144.4°.

[0021] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments. Example

[0022] 29g of red mud (waste residue from alumina industrial production, which can be used directly without pretreatment) was weighed and mixed evenly with 2g of bentonite. 100mL of anhydrous ethanol was added as the ball milling medium, and the mixture was ball-milled at 300r / min for 5 hours to obtain ceramic raw material. The raw material was sieved to separate agate balls, then stirred in a 60℃ water bath to evaporate the anhydrous ethanol, and then dried at 60℃ for 10 hours. The dried material was filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder. Polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and deionized water were weighed at a mass ratio of 2:2:65 and heated and stirred in an 80℃ water bath until completely dissolved to obtain a stable organic solution. The ceramic raw material was added to the above organic solution at a mass ratio of 31:69 and ball-milled at 400r / min for 6 hours to obtain a uniform ceramic slurry. The slurry was then subjected to… Vacuum degassing until no bubbles remain; place 4.5g of slurry into a polytetrafluoroethylene ring with brass at the bottom (inner diameter 2.8cm, outer diameter 3.8cm, height 4.0cm), controlling the thickness of the green body to be about 1cm, then place the whole thing into a polystyrene foam box, add liquid nitrogen to lower the temperature of the bottom brass to -150℃ and maintain it for 30min; after freezing, place the polytetrafluoroethylene ring together with the inner green body under -60℃ and 1Pa conditions for freeze drying for 48h until completely dry, to obtain a cylindrical ceramic film green body; calcine the green body in air atmosphere at a rate of 2℃ / min, hold it at 400℃ for 180min, then continue to raise the temperature to 1050℃ and hold it for 180min; after calcination, the ceramic film is cleaned by alternating ultrasonic vibration with deionized water and anhydrous ethanol 3 times, 5min each time, then dried at 60℃ for 10h, and packaged for later use. Weigh 0.5g of soybean oil and 0.005g of sodium dodecyl sulfonate, add them to 1L of deionized water, stir at 9000r / min for 30min, and then sonicate for 20min to obtain a stable and uniform oil-water emulsion. Assemble a self-made cross-flow circulation device using a pre-encapsulated ceramic membrane. The oil-water emulsion flows into the membrane module from the storage tank and contacts the surface of the ceramic membrane. Under a pressure of 0.2 bar, the feed liquid undergoes selective separation by the membrane to form permeate, which is collected in a glass bottle. In Example 1, after 60min of operation, the stable permeate flux was 455.6L / (m³). 2 ·h), with an oil interception rate of 91.97%. Example

[0023] 29g of red mud (waste residue from alumina industrial production, which can be used directly without pretreatment) was weighed and mixed evenly with 2g of bentonite. 100mL of anhydrous ethanol was added as the ball milling medium, and the mixture was ball-milled at 300r / min for 5 hours to obtain ceramic raw material. The raw material was sieved to separate agate balls, then stirred in a 60℃ water bath to evaporate the anhydrous ethanol, and then dried at 60℃ for 10 hours. The dried material was filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder. Polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and deionized water were weighed at a mass ratio of 2:2:65 and heated and stirred in an 80℃ water bath until completely dissolved to obtain a stable organic solution. The ceramic raw material was added to the above organic solution at a mass ratio of 31:69 and ball-milled at 400r / min for 6 hours to obtain a uniform ceramic slurry. The slurry was then subjected to… Vacuum degassing until no bubbles remain; place 4.5g of slurry into a polytetrafluoroethylene ring with brass at the bottom (inner diameter 2.8cm, outer diameter 3.8cm, height 4.0cm), controlling the thickness of the green body to about 1cm, then place the whole body into a polystyrene foam box, add liquid nitrogen to lower the temperature of the bottom brass to -150℃ and maintain it for 30min; after freezing, place the polytetrafluoroethylene ring together with the inner green body under -60℃ and 1Pa conditions for freeze drying for 48h until completely dry, obtaining a cylindrical ceramic film green body; calcine the green body in an air atmosphere at a rate of 2℃ / min, hold at 400℃ for 180min, then continue to raise the temperature to 1100℃ and hold for 180min; after calcination, the ceramic film is cleaned by alternating ultrasonic vibration with deionized water and anhydrous ethanol 3 times, 5min each time, then dried at 60℃ for 10h, and packaged for later use. Weigh 0.5g of soybean oil and 0.005g of sodium dodecyl sulfonate, add them to 1L of deionized water, stir at 9000r / min for 30min, and then sonicate for 20min to obtain a stable and uniform oil-water emulsion. Assemble a self-made cross-flow circulation device using a pre-encapsulated ceramic membrane. The oil-water emulsion flows into the membrane module from the storage tank and contacts the surface of the ceramic membrane. Under a pressure of 0.2 bar, the feed liquid undergoes selective separation by the membrane to form permeate, which is collected in a glass bottle. In Example 2, after 60min of operation, the stable permeate flux was 398.6L / (m³). 2 ·h), with an oil interception rate of 95.72%. Example

[0024] 29g of red mud (waste residue from alumina industrial production, which can be used directly without pretreatment) was weighed and mixed evenly with 2g of bentonite. 100mL of anhydrous ethanol was added as the ball milling medium, and the mixture was ball-milled at 300r / min for 5 hours to obtain ceramic raw material. The raw material was sieved to separate agate balls, then stirred in a 60℃ water bath to evaporate the anhydrous ethanol, and then dried at 60℃ for 10 hours. The dried material was filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder. Polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and deionized water were weighed at a mass ratio of 2:2:65 and heated and stirred in an 80℃ water bath until completely dissolved to obtain a stable organic solution. The ceramic raw material was added to the above organic solution at a mass ratio of 31:69 and ball-milled at 400r / min for 6 hours to obtain a uniform ceramic slurry. The slurry was then subjected to… Vacuum degassing until no bubbles remain; place 4.5g of slurry into a polytetrafluoroethylene ring with brass at the bottom (inner diameter 2.8cm, outer diameter 3.8cm, height 4.0cm), controlling the thickness of the green body to be about 1cm, then place the whole thing into a polystyrene foam box, add liquid nitrogen to lower the temperature of the bottom brass to -150℃ and maintain it for 30min; after freezing, place the polytetrafluoroethylene ring together with the inner green body under -60℃ and 1Pa conditions for freeze drying for 48h until completely dry, to obtain a cylindrical ceramic film green body; calcine the green body in air atmosphere at a rate of 2℃ / min, hold at 400℃ for 180min, then continue to raise the temperature to 1150℃ and hold for 180min; after calcination, the ceramic film is cleaned by alternating ultrasonic vibration with deionized water and anhydrous ethanol 3 times, 5min each time, then dried at 60℃ for 10h, and packaged for later use. Weigh 0.5g of soybean oil and 0.005g of sodium dodecyl sulfonate, add them to 1L of deionized water, stir at 9000r / min for 30min, and then sonicate for 20min to obtain a stable and uniform oil-water emulsion. Assemble a self-made cross-flow circulation device using a pre-encapsulated ceramic membrane. The oil-water emulsion flows from the storage tank into the membrane module and contacts the ceramic membrane surface. Under a pressure of 0.2 bar, the feed liquid undergoes selective separation by the membrane to form permeate, which is collected in a glass bottle. In Example 3, after 60min of operation, the stable permeate flux was 286.0L / (m³). 2 ·h), with an oil interception rate of 91.31%. Example

[0025] 27g of red mud (waste residue from alumina industrial production, which can be used directly without pretreatment) was weighed and mixed evenly with 2g of bentonite. 100mL of anhydrous ethanol was added as the ball milling medium, and the mixture was ball-milled at 300r / min for 5 hours to obtain ceramic raw material. The raw material was sieved to separate agate balls, then stirred in a 60℃ water bath to evaporate the anhydrous ethanol, and then dried at 60℃ for 10 hours. The dried material was filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder. Polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and deionized water were weighed at a mass ratio of 2:4:65 and heated and stirred in an 80℃ water bath until completely dissolved to obtain a stable organic solution. The ceramic raw material was added to the above organic solution at a mass ratio of 29:71 and ball-milled at 400r / min for 6 hours to obtain a uniform ceramic slurry. The slurry was then subjected to… Vacuum degassing until no bubbles remain; place 4.5g of slurry into a polytetrafluoroethylene ring with brass at the bottom (inner diameter 2.8cm, outer diameter 3.8cm, height 4.0cm), controlling the thickness of the green body to about 1cm, then place the whole body into a polystyrene foam box, add liquid nitrogen to lower the temperature of the bottom brass to -150℃ and maintain it for 30min; after freezing, place the polytetrafluoroethylene ring together with the inner green body under -60℃ and 1Pa conditions for freeze drying for 48h until completely dry, obtaining a cylindrical ceramic film green body; calcine the green body in an air atmosphere at a rate of 2℃ / min, hold at 400℃ for 180min, then continue to raise the temperature to 1100℃ and hold for 180min; after calcination, the ceramic film is cleaned by alternating ultrasonic vibration with deionized water and anhydrous ethanol 3 times, 5min each time, then dried at 60℃ for 10h, and packaged for later use. 0.5g of soybean oil and 0.005g of sodium dodecyl sulfonate were weighed and added to 1L of deionized water. The mixture was stirred at 9000 rpm for 30 minutes, followed by ultrasonic treatment for 20 minutes to obtain a stable and homogeneous oil-water emulsion. An encapsulated ceramic membrane was assembled into a self-made cross-flow circulation device. The oil-water emulsion flowed from the storage tank into the membrane module and contacted the surface of the ceramic membrane. Under a pressure of 0.2 bar, the feed liquid underwent selective separation by the membrane to form permeate, which was collected in a glass bottle. In Example 4, the stable permeate flux was 147.9 L / (m³) after 60 minutes of operation. 2 ·h), with an oil interception rate of 92.26%.

Claims

1. A method for preparing a gradient-pore ceramic membrane for oil-water emulsion separation, characterized in that, Includes the following steps: (1) Preparation of ceramic powder: Red mud and bentonite were mixed evenly in a ball mill jar, and anhydrous ethanol was added as the ball milling medium at a mass-volume ratio of 29-31g:100mL. The mixture was ball milled to obtain uniform ceramic raw material. After the raw material was sieved to separate the agate balls, the filtered raw material was placed in a beaker and heated and stirred in a water bath at 60℃ until the anhydrous ethanol was completely evaporated. Then it was dried in a forced-air drying oven and filtered through a 200-mesh sieve to obtain red mud-based ceramic raw material powder. The mass ratio of red mud to bentonite was 27-29:

2. (2) Preparation of organic mixture: Dispersant polyvinylpyrrolidone (PVP) and binder polyvinyl alcohol (PVA) are added to deionized water and stirred until completely dissolved under water bath heating at 80°C to obtain a uniformly mixed and stable organic solution; the mass ratio of polyvinylpyrrolidone:polyvinyl alcohol:deionized water is 2:2-4:

65. (3) Preparation of ceramic slurry: The ceramic powder obtained in step (1) is added to the above organic mixture and ball-milled to obtain a stable and uniform organic-inorganic ceramic slurry; the prepared ceramic slurry is placed in a vacuum drying oven for vacuum degassing until the slurry is free of bubbles; the mass ratio of ceramic powder to organic mixture is 29-31:69-71. (4) Preparation of ceramic blank: Take the slurry obtained in step (3), place it in a polytetrafluoroethylene ring with brass at the bottom, and then put the whole thing into a polystyrene foam box. Add liquid nitrogen to lower the bottom brass to the set temperature and perform directional freezing. Then transfer it to a freeze dryer to dry and obtain a cylindrical ceramic film blank. (5) Solid-state reaction: The ceramic membrane preform was calcined in air atmosphere: the temperature was programmed to rise at a rate of 2℃ / min, first held at 400℃ for 180min, then raised to the range of 1050-1150℃ and held for 180min to obtain a porous ceramic membrane; then it was cleaned by alternating ultrasonic oscillation with deionized water and anhydrous ethanol, repeated 3 times, each cleaning time was 5min; after cleaning, the sample was placed in a constant temperature drying oven at 60℃ for constant temperature drying treatment, and after drying, it was packaged for later use.

2. The preparation method according to claim 1, characterized in that... In step (1), the ball milling speed is 300 r / min, the time is 5 h, the drying temperature is 60 ℃, and the drying time is more than 10 h; the combination of parameters can make the red mud and bentonite mix evenly, and the resulting ceramic powder has a particle size of 200 mesh.

3. The preparation method according to claim 1, characterized in that... In step (3), the ball milling speed is 400 r / min and the ball milling time is 6 h.

4. The preparation method according to claim 1, characterized in that... In step (4), the thickness of the ceramic film blank is 1 cm, the directional freezing temperature is -150℃, the freezing time is 30 min, the temperature of the freeze dryer is -60℃, the pressure is 1 Pa, and the freeze drying time is 48 h.

5. The preparation method according to claim 1, characterized in that... In step (5), the ceramic film sample after cleaning is dried in a constant temperature drying oven at 60℃ for more than 10 hours.

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