CN102701778B - Preparation method for ceramic film with hierarchical pore structure - Google Patents

Abstract

The invention relates to a method for preparing a porous ceramic membrane, which belongs to the technical field of ceramic membrane preparation. By optimizing the particle size of the ceramic aggregate, raw material powders with different particle sizes and pore-forming agents with different acidity, alkalinity and particle size are selected respectively, and a uniformly dispersed coating film suspension is prepared by ball milling. The dipping-coating process is used to coat the film sequentially according to the particle size of the pore-forming agent and its acidity and alkalinity. The preparation of each layer of film requires the film-making solution prepared by acidic and alkaline pore-forming agents of the same particle size in turn. In dipping-coating, the purpose of pore formation is achieved by the subsequent acid-base neutralization reaction, and according to the change of the film layer, the particle size of the selected pore-forming agent is gradually reduced, and then it is cured, dried and sintered. The preparation of a ceramic membrane with controllable pore size and a hierarchical pore structure with gradient distribution is realized by changing the particle size of the pore-forming agent. By reasonably controlling the thickness, porosity and pore size of the separation membrane, the material has high mechanical properties, low cost and high permeation and separation efficiency.

Description

A kind of preparation method of ceramic membrane with hierarchical porous structure

technical field

The invention relates to a method for preparing a porous ceramic membrane, in particular to a method for preparing a ceramic membrane with a controllable pore diameter and a hierarchical porous structure with gradient distribution.

Background technique

Due to the advantages of excellent high separation efficiency, high temperature resistance, solvent resistance, anti-microbial, acid and alkali resistance, high mechanical strength, easy cleaning and regeneration, porous inorganic ceramic membranes have been applied in food, beverage, plant (drug) It can be used for separation, clarification, purification, concentration, sterilization, desalination, etc. in the process. It is mainly composed of a support, an intermediate layer and a separation layer. The three layers generally have a pore gradient, and the pore diameter changes regularly with the thickness, so as to not only achieve the purpose of separation, but also meet the requirements of saving power consumption. The more continuous the pore size of the gradient distribution, the smaller the pressure loss during use; the narrower the pore size distribution, the higher the separation efficiency. Due to the limitation of raw materials, the current preparation of porous ceramic supports mainly uses powders with a particle size distribution of 1-20 μm as raw materials, and is prepared by dry pressing, grouting and plastic extrusion, etc. Apply uniformly dispersed film-forming solution for the first time. The disadvantage of this method is that the pore size distribution is not easy to control, resulting in low separation efficiency, and the pore size distribution is too wide, which will easily cause problems such as infiltration of the membrane-forming solution and incomplete membrane layer during the subsequent further coating process, affecting the porosity. The quality of the ceramic membrane.

In order to obtain a ceramic membrane with a multi-level gradient pore structure, CN101670244A uses gravity sedimentation and vacuum suction to deposit particles of different particle sizes in the suspension in order from large to small, thereby obtaining a ceramic membrane with a gradient pore structure. CN101182233A discloses a method for preparing a porous ceramic membrane with an open directional pore structure, a high porosity, and a gradient change in the pore diameter in the membrane thickness direction by adopting a tape casting process and a freeze-drying process. Although the above method can simplify the preparation process of ceramic membranes to a certain extent, it is only applicable to flat ceramic membranes, and is not applicable to the currently commonly used tubular multi-channel ceramic membrane tubes. CN101323528A discloses a method for preparing a continuous-pore gradient ceramic tube, which combines the advantages of centrifugal molding and gel injection molding, and continuously Pore Gradient Ceramic Tube. Although this method can omit the vacuum degassing procedure of traditional gel casting, and is easy to operate and has high efficiency, on the one hand, the centrifugal process requires high speed; Large-diameter multi-channel ceramic membrane tubes used in engineering fields. In addition, pore structures of different shapes and pore sizes can be obtained by using pore-forming agents of different shapes and particle sizes, but due to the limitation of the particle size of the pore-forming agents, there are fewer pore-forming agents with a particle size below 50nm, which limits the porous ceramic ultrafiltration membrane. , Preparation of nanofiltration membrane.

In view of the above problems, the present invention proposes to select raw material powders with different particle sizes and pore-forming agents with different acidity and alkalinity and particle sizes by optimizing the particle size of the ceramic aggregate, and prepare a uniformly dispersed coating film suspension by ball milling. The dipping-coating process is used to coat the film sequentially according to the particle size of the pore-forming agent and its acidity and alkalinity. The preparation of each layer of film requires the film-making solution prepared by acidic and alkaline pore-forming agents of the same particle size in turn. In dipping-coating, the purpose of pore formation is achieved by the subsequent acid-base neutralization reaction, and according to the change of the film layer, the particle size of the selected pore-forming agent is gradually reduced, and then it is cured, dried and sintered. The preparation of a ceramic membrane with controllable pore size and a hierarchical pore structure with gradient distribution is realized by changing the particle size of the pore-forming agent. By reasonably controlling the parameters of various process conditions and controlling the thickness, porosity and pore size of the separation membrane, the material has high mechanical properties, low cost and high permeation and separation efficiency.

The purpose of the present invention is to provide a method for preparing a ceramic membrane with a controllable pore size and a gradient distribution multi-level pore structure, which overcomes the low separation efficiency of the ceramic membrane, large pressure loss and less than 50 nm due to the pore structure and pore size distribution of the ceramic membrane. The problem that the hole is not easy to prepare. By optimizing the raw material powder with suitable particle size and pore-forming agents with different acidity, alkalinity and particle size, and reasonably controlling the process parameters, uniformly dispersed suspensions with different particle sizes are prepared, and the dipping-coating process is applied on the ceramic membrane support. The surface is coated with films in order of particle size from large to small, and each layer of film needs to be immersed in the coating solution prepared by the same particle size and different acid-base pore-forming agents for a certain period of time. The acid-base neutralization reaction between the pore-forming agents realizes a porous ceramic membrane with controllable pore size, hierarchical pore structure with gradient distribution, high mechanical properties and separation and permeation efficiency. The invention has simple preparation process, low requirements on material system and low production cost, and is mainly used in the field of water treatment, and can also be used in other various solid-liquid separation and filtration processes, which is beneficial to popularization and application.

Contents of the invention

Aiming at the problems of low separation efficiency, large pressure loss and difficult preparation of pores below 50nm in ceramic membranes due to the pore structure and pore size distribution of ceramic membranes, the present invention proposes to use the acid-base neutralization reaction between pore-forming agents to realize the purpose of pore formation , and the pore size can be controlled and the gradient distribution is realized through the particle size of the pore-forming agent. The main contents are as follows:

①Raw material

The ceramic aggregate of the porous ceramic membrane can be one or more of alumina, zirconia, silicon dioxide, silicon carbide, titanium oxide, mullite, and cordierite, and the average particle size is between 0.02-10 μm.

Acidic pore-forming agent: ammonium chloride, aspartic acid, glutamic acid, polyvinyl alcohol, etc., the average particle size is between 0.02-1 μm, and it is formulated into an aqueous solution with a content of 5-30% (mass ratio);

Alkaline pore-forming agent: NaHCO ₃ , Na ₂ CO ₃ , ammonium bicarbonate, corn starch, urea, lysine, arginine, histidine, etc., with an average particle size of 0.02-1 μm, which can be formulated into Content is the aqueous solution of 5-30% (mass ratio);

②Preparation of impregnation-coating slurry

Mixed powders (ceramic aggregate + pore-forming agent) of different particle sizes are used respectively to prepare coating slurry. In the mixed powder, ceramic aggregate accounts for (70-90% by mass ratio), and pore-forming agent accounts for (10-30% mass ratio), the volume ratio of the mixed powder to water is 1.5-2:1, and the ball milling time is 4-12 hours;

a Use ceramic aggregate with a particle size of d _c1 , acid pore-forming agent with a particle size of d _k1 , and water to prepare a uniformly dispersed coating slurry A ₁ , and use HCl, glutamic acid, nitric acid, etc. to adjust the pH of the slurry The value is 3-6;

b Use ceramic aggregate with a particle size of d _c1 , alkaline pore-forming agent with a particle size of d _k1 , and water to prepare a uniformly dispersed coating slurry B ₁ , and use NaOH, caustic soda, etc. to adjust the pH value of the slurry to 8 -12;

c Use ceramic aggregate with a particle size of d _c2 , acidic pore-forming agent with a particle size of d _k2 , and water to prepare a uniformly dispersed coating slurry A ₂ , and use HCl, glutamic acid, nitric acid, etc. to adjust the pH of the slurry value 3-6;

d Use ceramic aggregate with a particle size of d _c2 , alkaline pore-forming agent with a particle size of d _k2 , and water to prepare a uniformly dispersed coating slurry B ₂ , and use NaOH, caustic soda, etc. to adjust the pH value of the slurry to 8 -12;

where d _c1 >d _c2 , d _k1 >d _k2

To prepare a separation layer with a smaller pore size distribution, ceramic aggregates with smaller particle sizes, acidic and alkaline pore formers can be successively selected to prepare coating slurries A ₃ , B ₃ , A ₄ , B ₄ . . .

③ Membrane coating

At least two layers of membranes are coated inside the channel of the porous ceramic membrane support body, which is prepared by a dipping-coating process. The thickness of the top layer membrane is 5-20 μm, the porosity is 40-80%, and the pore diameter is between 0.002-0.05 μm.

In order to prevent the slurry from penetrating into the pores, the porous ceramic membrane support is first immersed in ethanol for 2-5 hours, so that the air in the pores is replaced by ethanol. If a tubular support is used, the outer surface of the ceramic membrane support is covered with aluminum foil; if a flat support is used, both ends of the support are covered with aluminum foil. Then, according to the order of particle size from large to small, the porous ceramic membrane support body was immersed in the coating slurry _A1 and the preheated _B1 to prepare the intermediate layer, and then dried for 12-60h, and the acid was passed between the pore-forming agents. The base neutralizes the reaction to form pores. Then, the above-mentioned ceramic membrane support is immersed in the coating slurry A ₂ and the preheated B ₂ in order to prepare a separation layer, then dried for 12-60 hours, and finally sintered at 1200-1400 ° C and kept for 2 hours to obtain a ceramic membrane. Among them, the action time of the support body and the slurry is 10-90s, and the thickness and pore size of the membrane are controlled by adjusting the action time of the two, the particle size of the raw material and the pore-forming agent. To prepare a separation layer with a smaller pore size distribution, it is necessary to use a smaller particle size ceramic aggregate and a pore-forming agent to prepare a coating slurry and repeat the above dipping-coating steps.

Through the above process, a ceramic membrane with controllable pore size and a multi-level pore structure with gradient distribution can be obtained. According to the size of the pore size of the required separation layer, the number of membrane layers is between 2 and 5.

The present invention improves the traditional preparation process of porous ceramic membranes, by optimizing raw material powders with suitable particle sizes and pore-forming agents with different acidity and alkalinity and particle sizes, and by rationally controlling process parameters, uniformly dispersed and different particle sizes are prepared. Suspension, using the dipping-coating process to coat the membrane on the surface of the ceramic membrane support in order of particle size from large to small, and each layer of membrane needs to be sequentially coated with the same particle size and different acid-base pore-forming agents The prepared coating solution is soaked for a certain period of time, and the acid-base neutralization reaction between the pore-forming agents in the subsequent process is used to realize the porous ceramic membrane with controllable pore size and a hierarchical pore structure with gradient distribution. The pores of the prepared porous ceramic membrane are uniform honeycomb, the porosity can reach 30-60%, and the pore diameter is between 0.002-0.05 μm. Using the above method, the pore diameter of the support can be 10-30 μm, the pore diameter of the first layer of membrane is 0.8-1 μm, the pore diameter of the second layer of membrane is 0.3-0.5 μm, the pore diameter of the third layer of membrane is 0.05-0.1 μm, the The pore diameter of the four-layer membrane is 0.01-0.05 μm, and the pore diameter of the fifth-layer membrane is 0.002-0.005 μm. The ceramic membrane prepared by the invention has great potential applications in seawater desalination, sewage treatment, fine chemical industry and many other fields.

Detailed ways

The present invention will be further described below by embodiment.

Embodiment one:

Dry-mix ceramic aggregate ZrO ₂ (1 μm, 70%) and 5wt% acidic pore-forming agent ammonium chloride (1 μm, 30%) for 4 hours, then mix powder and water at a volume ratio of 1.5:1 and mix by ball milling 4h to prepare coating slurry A ₁ , according to the same process as above, ceramic aggregate ZrO ₂ (1μm, 70%), 5wt% alkaline pore-forming agent sodium carbonate (1μm, 30%), and water ball mill were mixed for 4h to prepare coating Film slurry B ₁ .

Ceramic aggregate ZrO ₂ (0.5 μm, 70%), 5wt% acidic pore-forming agent ammonium chloride (0.5 μm, 30%), and water ball mill were mixed for 4 hours to prepare coating slurry A ₂ , and ceramic aggregate ZrO ₂ (0.5 μm, 70%), 5wt% alkaline pore-forming agent sodium carbonate (0.5 μm, 30%), and water ball milling for 4 hours to prepare coating slurry B ₂ .

First soak the porous ceramic membrane tube in ethanol for 3 hours, then wrap the outside of the ceramic membrane tube with aluminum foil and immerse it in slurry A ₁ for 60 seconds, and then immerse it in slurry B ₁ preheated to 80°C for 60 seconds to make the pore-forming agent Acid-base neutralization reaction occurs between them to form holes, and dry to form an intermediate layer. Then immerse the ceramic membrane tube in the slurry A ₂ for 60s, then immerse it in the slurry B ₂ preheated to 80°C for 60s to make acid-base neutralization reaction between the pore-forming agents to form holes, and dry to form a separation layer. Sintering to obtain a ceramic membrane. When the sintering temperature is 1400°C and the holding time is 2 hours, the thickness of the porous ceramic membrane obtained is 10 μm, the average pore diameter of the separation layer is 0.5 μm, the porosity is 50%, and the pure water flux is 1000 L/m under the pressure of 0.1 MPa. ² h.

Embodiment two:

Dry-mix ceramic aggregate Al ₂ O ₃ (1 μm, 90%), 25wt% acidic pore-forming agent glutamic acid (1 μm, 10%) for 6 hours, then mix powder and water at a volume ratio of 2:1 and mix The coating slurry A ₁ was prepared by ball milling and mixing for 10 hours. Ceramic aggregate Al ₂ O ₃ (1 μm, 90%), 25wt% alkaline pore-forming agent sodium bicarbonate (1 μm, 10%), water The above process was ball milled and mixed for 10 hours to prepare coating slurry B ₁ ;

Dry-mix ceramic aggregate Al ₂ O ₃ (0.5 μm, 90%), 25wt% acidic pore-forming agent glutamic acid (0.5 μm, 10%) for 6 hours, and then press the volume ratio of mixed powder to water to be 2:1 The ingredients were mixed by ball milling for 10 hours to prepare coating slurry A ₂ , and ceramic aggregate Al ₂ O ₃ (0.5 μm, 90%), 25wt% alkaline pore-forming agent sodium bicarbonate (0.5 μm, 10% ), water was mixed by ball milling for 10h according to the above-mentioned process to prepare coating film slurry B ₂ ;

Ceramic aggregate Al ₂ O ₃ (0.25 μm, 90%), 25wt% acidic pore-forming agent glutamic acid (0.25 μm, 10%), and water ball mill were mixed for 10 hours to prepare coating slurry A ₃ , and ceramic aggregate Al ₂ O ₃ (0.25 μm, 90%), 25wt% alkaline pore-forming agent sodium bicarbonate (0.25 μm, 10%), and water ball mill were mixed for 10 h to prepare coating slurry B ₃ .

First, soak the ceramic membrane tube in ethanol for 4 hours, then wrap the outside of the ceramic membrane tube with aluminum foil and immerse it in slurry A ₁ for 90 seconds, and then immerse it in slurry B ₁ preheated to 60°C for 90 seconds to make the pore-forming agent Acid-base neutralization reaction occurs between holes to form holes, followed by drying for 48h. Then immerse the ceramic membrane tube in the slurry A ₂ for 90s, then immerse it in the slurry B ₂ preheated to 80°C for 90s to make the acid-base neutralization reaction between the pore-forming agents to form pores and dry for 48h. Finally, the ceramic membrane tube was immersed in slurry _A3 for 90s, and then immersed in slurry _B3 preheated to 80°C for 90s to cause acid-base neutralization reaction between the pore-forming agents to form pores and dry for 48 hours to form a separation layer. When the sintering temperature is 1380°C and the holding time is 2 hours, the thickness of the porous ceramic membrane obtained is 15 μm, the average pore diameter of the separation layer is 0.05 μm, the porosity is 30%, and the pure water flux is 700 L/m under the pressure of 0.1 MPa. ² h.

Embodiment three:

The ceramic aggregate mullite (1 μm, 75%) and 15wt% acidic pore-forming agent aspartic acid (1 μm, 25%) were dry mixed for 12 hours, and then the volume ratio of the mixed powder to water was 1.5:1. Ball milling and mixing for 12 hours to prepare coating slurry A ₁ , ceramic aggregate mullite (1 μm, 75%), 15wt% alkaline pore-forming agent corn starch (1 μm, 25%), and water were ball milled and mixed for 12 hours according to the above process Prepare coating slurry B ₁ .

Dry mix ceramic aggregate mullite (0.5 μm, 75%), 15wt% acidic pore-forming agent aspartic acid (0.5 μm, 25%) for 12 hours, and then press the volume ratio of mixed powder to water to be 1.5:1 The ingredients were mixed by ball milling for 12 hours to prepare coating slurry A ₂ , and the ceramic aggregate mullite (0.5 μm, 75%), 15wt% alkaline pore-forming agent corn starch (0.5 μm, 25%), Water ball milling and mixing for 12 hours to prepare coating slurry B ₂ .

Ceramic aggregate mullite (0.25 μm, 75%), 15wt% acidic pore-forming agent aspartic acid (0.25 μm, 25%), and water ball mill were mixed for 12 hours to prepare coating slurry A ₃ , and ceramic aggregate Mullite (0.25 μm, 75%), 15wt% alkaline pore-forming agent corn starch (0.25 μm, 25%), and water ball mill were mixed for 12 hours to prepare coating slurry B ₃ .

Ceramic aggregate mullite (0.1 μm, 75%), 15wt% acidic pore-forming agent aspartic acid (0.1 μm, 25%), and water ball mill were mixed for 12 hours to prepare coating slurry A ₄ , and the ceramic aggregate Mullite (0.1 μm, 75%), 15wt% alkaline pore-forming agent corn starch (0.1 μm, 25%), and a water ball mill were mixed for 12 hours to prepare coating slurry B ₄ .

First, soak the ceramic membrane tube in ethanol for 5 hours, then wrap the outside of the ceramic membrane tube with aluminum foil and immerse it in slurry A ₁ for 80 seconds, then immerse it in slurry B ₁ preheated to 80°C for 80 seconds, and then dry it for 24 hours. Acid-base neutralization reaction occurs between pore-forming agents to form pores. Then immerse the ceramic membrane tube in the slurry A ₂ for 80s, then immerse it in the slurry B ₂ preheated to 80℃ for 80s and then dry it for 24h, then repeat the above process in turn until A ₃ , B ₃ and A ₄ , B ₄ Coating completed. When the sintering temperature is 1300°C and the holding time is 2 hours, the thickness of the porous ceramic membrane obtained is 15 μm, the average pore diameter of the separation layer is 0.01 μm, the porosity is 50%, and the pure water flux is 500 L/m under the pressure of 0.1 MPa. ² h.

Embodiment four:

The ceramic aggregate cordierite (1 μm, 85%) and 10wt% acidic pore-forming agent polyvinyl alcohol (1 μm, 15%) were dry mixed for 6 hours, and then the volume ratio of the mixed powder to water was 2:1 and mixed by ball milling Prepare the coating film slurry _A1 in 6 hours, and mix the ceramic aggregate cordierite (1 μm, 85%), 10wt% alkaline pore-forming agent ammonium bicarbonate (1 μm, 15%), and water for 6 hours to prepare the coating film according to the same process as above Slurry B ₁ .

The ceramic aggregate cordierite (0.5 μm, 85%) and 10wt% acidic pore-forming agent polyvinyl alcohol (0.5 μm, 15%) were dry mixed for 6 hours, and then the volume ratio of the mixed powder to water was 2:1 and mixed. Ball milling and mixing for 6 hours to prepare coating slurry _A2 , ceramic aggregate cordierite (0.5 μm, 85%), 10 wt% alkaline pore-forming agent ammonium bicarbonate (0.5 μm, 15%), and water were mixed according to the same process as above 6h Prepare coating slurry B ₂ .

Ceramic aggregate cordierite (0.25 μm, 85%), 10wt% acidic pore-forming agent polyvinyl alcohol (0.25 μm, 15%), and water mill were mixed for 6 hours to prepare coating slurry A ₃ , and ceramic aggregate cordierite ( 0.25 μm, 85%), 10wt% alkaline pore-forming agent corn starch (0.25 μm, 15%), and water ball milling for 6 hours to prepare coating slurry B ₃ .

The ceramic aggregate cordierite (0.1 μm, 85%), 10wt% acidic pore-forming agent polyvinyl alcohol (0.1 μm, 15%), and a water mill were mixed for 6 hours to prepare coating film slurry A ₄ , and the ceramic aggregate cordierite ( 0.1 μm, 85%), 10wt% basic pore-forming agent ammonium bicarbonate (0.1 μm, 15%), and water ball milling for 6 hours to prepare coating slurry B ₄ .

The ceramic aggregate cordierite (0.05 μm, 85%), 10wt% acidic pore-forming agent polyvinyl alcohol (0.05 μm, 15%), and a water mill were mixed for 6 hours to prepare coating film slurry A ₅ , and the ceramic aggregate cordierite ( 0.05 μm, 85%), 10wt% basic pore-forming agent ammonium bicarbonate (0.05 μm, 15%), and water ball milling for 6 hours to prepare coating slurry B ₅ .

First, soak the ceramic membrane tube in ethanol for 3 hours, then wrap the outside of the ceramic membrane tube with aluminum foil and immerse it in slurry A ₁ for 90 seconds, then immerse it in slurry B ₁ preheated to 80°C for 90 seconds, and then dry it for 12 hours. Acid-base neutralization reaction occurs between pore-forming agents to form pores. Subsequently, the ceramic membrane tube was immersed in slurry A ₂ for 90 seconds, then immersed in slurry B ₂ preheated to 80°C for 90 seconds, and then dried for 12 hours. Then repeat the above process in turn until the coating of A ₃ , B ₃ , A ₄ , B ₄ and A ₅ , B ₅ is completed. When the sintering temperature is 1250°C, the obtained porous ceramic membrane has a thickness of 15 μm, an average pore diameter of the separation layer of 0.005 μm, a porosity of 40%, and a pure water flux of 300 L/m ² h at a pressure of 0.1 MPa.

Claims (3)

1. the preparation method of a hierarchical porous structure ceramic membrane is characterized in that, described preparation method carries out successively as follows:

1. raw material

The ceramic aggregate of ceramic membrane: adopt one or more in aluminum oxide, zirconium white, silicon-dioxide, silicon carbide, titanium oxide, mullite, the trichroite, median size is between 0.02-10 μ m;

Acid pore-forming material: ammonium chloride, aspartic acid, L-glutamic acid, polyvinyl alcohol, median size are made into the aqueous solution that content is the 5-30% mass ratio with it between 0.02-1 μ m;

Alkalescence pore-forming material: NaHCO ₃, Na ₂CO ₃, bicarbonate of ammonia, W-Gum, urea, Methionin, arginine, Histidine, median size is made into the aqueous solution that content is the 5-30% mass ratio with it between 0.02-1 μ m;

2. flood-film the preparation of slurry

Adopt respectively the mixed powder of different-grain diameter, be that ceramic aggregate+pore-forming material prepares the slurry of filming, ceramic aggregate accounts for the 70-90% mass ratio in the described mixed powder, and pore-forming material accounts for the 10-30% mass ratio, the volume ratio of mixed powder and water is 1.5-2: 1, and Ball-milling Time 4-12 hour;

It is d that a adopts particle diameter _C1Ceramic aggregate, particle diameter be d _K1Acid pore-forming material, water for ball milling are mixed with homodisperse coating slurry A ₁, regulating slurry pH value with HCl, L-glutamic acid, nitric acid is 3-6;

It is d that b adopts particle diameter _C1Ceramic aggregate, particle diameter be d _K1Alkalescence pore-forming material, water for ball milling are mixed with homodisperse coating slurry B ₁, regulating slurry pH value with NaOH is 8-12;

It is d that c adopts particle diameter _C2Ceramic aggregate, particle diameter be d _K2Acid pore-forming material, water for ball milling are mixed with homodisperse coating slurry A ₂, regulate slurry pH value 3-6 with HCl, L-glutamic acid, nitric acid;

It is d that d adopts particle diameter _C2Ceramic aggregate, particle diameter be d _K2Alkalescence pore-forming material, water for ball milling are mixed with homodisperse coating slurry B ₂, regulating slurry pH value with NaOH is 8-12;

D wherein _C1＞d _C2, d _K1＞d _K2

3. the coating of film

At porous ceramic film support channel interior applied in two coats film, adopt the preparation of dipping-coating processes, the thickness of top layer film is 5-20 μ m, and porosity is 40-80%, and the aperture is between 0.002-0.05 μ m.

2. preparation method according to claim 1 is characterized in that: in step 3. in the coating processes of film, porous ceramic film support was immersed first in the ethanol 2-5 hour, the air in the hole is substituted by ethanol.

3. preparation method according to claim 1 and 2, it is characterized in that: when porous ceramic film support adopts the tubular type supporter, the outside surface of ceramic film support is covered with aluminium foil, perhaps when porous ceramic film support adopts flat supporter, the two ends of ceramic film support are covered with aluminium foil, then successively porous ceramic film support is immersed the slurry A that films according to the descending order of particle diameter ₁With the B through preheating ₁The slurry A that films subsequent drying 12-60 hour, is immersed with above-mentioned ceramic film support more successively in middle preparation middle layer ₂With the B through preheating ₂Middle preparation separating layer dry 12-60 hour, at last at 1200-1400 ℃ of sintering and be incubated 2 hours, obtains the hierarchical porous structure ceramic membrane.