Low-temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and preparation method thereof

Abstract

The invention discloses a low-temperature sintering asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof, the preparation method comprises the following steps: a) preparation of a coating liquid: firstly dispersing a binder in a solution, adding silicon carbide particle powder, carrying out magnetic stirring dispersion for a period of time, then adding a dispersant, and stirring; then putting the stirred and mixed film coating liquid and the grinding balls into a ball milling tank for ball milling; and adjusting the pH value of the ball-milled silicon carbide coating liquid with ammonia water, and carrying out vacuum defoaming to finally obtain the high-stability and uniformly-dispersed silicon carbide coating liquid. Wherein the addition amount of the silicon carbide coarse particles is 0.5-50 wt% of the mass of silicon carbide; b) coating; c) drying; and d) sintering. The asymmetric porous silicon carbide ceramic membrane prepared by the method is low in sintering temperature, and the prepared separation layer membrane is controllable in pore size and uniform in pore size distribution; the method has excellent application prospects in the field of membrane separation.

Description

Technical Field

[0001] The invention relates to a low-temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof, belonging to the technical field of silicon carbide ceramic membrane separation layers. Background Art

[0002] With the rapid development of my country's economy, the amount of wastewater treated in industrial production and daily life has continued to increase. Membrane separation technology has become an important way to treat and recycle wastewater. Compared with organic membranes, inorganic ceramic membranes have many advantages such as high mechanical strength, resistance to organic solvents, resistance to chemical erosion, non-deformation of membrane pores, strong anti-pollution ability, and easy cleaning. They have been successfully applied in the fields of chemistry, petrochemicals, food, biology, medicine, and environmental protection. However, in terms of industrial wastewater, its wastewater often has the characteristics of large discharge volume, high temperature, high alkalinity, and high acidity, which puts higher requirements on the filtration performance of inorganic ceramic membranes. The widely used oxide ceramic membranes have a shortened service life in strong acid and alkali environments, which accelerates the replacement cycle of membrane materials and greatly increases the cost of sewage treatment. In addition, the hydrophilic properties of oxide membrane materials are generally low, resulting in low flux and low sewage treatment efficiency, which also increases the cost of sewage treatment to a certain extent. Silicon carbide ceramic membranes have attracted people's attention due to their advantages such as high temperature resistance, pollution resistance, large flux, good mechanical properties, and long service life. Ceramic membranes are usually asymmetric structures, and their filtration accuracy is determined by the pore size of the separation layer and the integrity and flatness of the membrane layer, while the support layer plays a mechanical support role. At present, the initial particles used to prepare the separation layer of silicon carbide ceramic membranes are composed of fine silicon carbide particles. However, since the pore size of the support body is mostly between 5 and 15 μm, the coating liquid is more likely to leak during the preparation of the separation layer, which in turn leads to membrane defects such as uneven thickness and cracking of the prepared membrane layer, which causes a sharp change in the filtration pressure drop during the application of the membrane, resulting in poor filtration effect. Therefore, how to prevent the leakage of the membrane coating liquid and apply it evenly and controllably has become a major difficulty and challenge in membrane construction.

[0003] At present, the sintering methods for preparing the separation layer of porous silicon carbide ceramic membranes mainly include recrystallization sintering, liquid phase sintering and oxidation sintering. Recrystallization sintering is based on the principle of evaporation and condensation. Under the protection of high-temperature inert gas, fine silicon carbide particles evaporate and condense to form a sintering neck between coarse particles. For example, patent CN115430297A discloses a production method for silicon carbide ceramic membranes for heavy metal wastewater treatment, which sequentially prepares a support body, a transition layer, and a silicon carbide ceramic membrane layer; the mixed material is extruded into a blank, the blank is dried by hot air, recrystallized and sintered to obtain a support body, and the transition layer is mixed and ball milled to obtain a fiber layer slurry, which is sprayed to the outer surface of one side of the support membrane layer by spraying, mainly playing a transition role between the separation layer and the support layer; finally, the separation layer is prepared by adding polycarbosilane. The ingredients in this method are relatively large and the proportions are complex. At the same time, the sintering process needs to go through three times, and high-temperature sintering (sintering temperature 1500-1700 ℃) is required under a protective atmosphere. The sintering energy consumption is high, the cost is increased, and mass production is difficult. Liquid phase sintering is a sintering process that produces a liquid phase in a binary system or a multi-component oxide composition when the temperature reaches the melting point of a certain component. For example, patent CN113999046B discloses a method for preparing a silicon carbide ceramic membrane by low-temperature reaction sintering. This method generates a new β-SiC phase and a NaCl-KCl molten salt system by liquid phase reaction sintering at 1350-1600°C. The silicon carbide particles are connected together under the flow of liquid phase to form a porous ceramic. The ingredients are complex and contain a variety of raw materials such as silicon powder, carbon source, molten salt and kaolin. After sintering, problems such as residual silicon or residual carbon are very likely to occur, resulting in the final prepared silicon carbide ceramic membrane having insufficient purity and limited application scenarios.

[0004] Oxidative sintering is based on the oxidative bonding mechanism, accompanied by the Ostwald ripening process and the volume expansion of SiC particles. During the sintering process, the carbon residues in the silicon carbide ceramic membrane will be gradually oxidized and removed, the silicon carbide surface will be exposed, and more silicon carbide will be oxidized to form silicon dioxide, so that the silicon carbide surface has a large number of hydroxyl groups and is hydrophilic. Compared with recrystallization sintering, it has high requirements for raw materials, high sintering temperature, and high energy consumption; liquid phase sintering adds sintering aids, and there are sintering aid residues or free silicon, etc., which will reduce the performance of silicon carbide itself. Therefore, oxidative sintering has become an important direction for research and development. For example, patent CN115745618A discloses a method for improving the mechanical strength of porous silicon carbide membranes. The method uses an oxidative sintering method to prepare silicon carbide ceramic membranes. The oxidative sintering temperature is 1000~1300 ℃. The prepared ceramic membrane has high mechanical strength and improves the service life of the silicon carbide ceramic membrane.

[0005] In summary, the purpose of the present invention is to reduce the sintering cost and energy consumption, and an oxidation sintering method is adopted; secondly, in order to reduce the number of coating times and increase the flux, a separation layer is directly coated on the support body after oxidation sintering, which reduces the presence of the intermediate layer and realizes cross-layer preparation. In addition, the pore size difference between the support body, the intermediate layer and the separation layer of the traditional ceramic membrane is small, and the coating is easy to form a film; after removing the intermediate layer, the pore size difference between the separation layer particles and the support body is large, and leakage is easy to occur during coating. For this reason, the present invention adds a small amount of coarse silicon carbide particles to the silicon carbide coating liquid, in which the large particles play a supporting role, improve the coating quality of the membrane coating liquid, reduce defects and leakage, so as to improve and strengthen the technical advantages of silicon carbide ceramic separation membranes and expand their market scale and application areas. Summary of the invention

[0006] In view of the above problems existing in the prior art, the object of the present invention is to provide a low-temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof.

[0007] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows: The present invention provides a low-temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof, comprising the following steps: a) Preparation of coating liquid: firstly, the binder is dispersed in the solution, and the silicon carbide particle powder is added, and the solution is initially dispersed by magnetic stirring for a period of time, and then the dispersant is added and stirred again; then, the coating liquid after simple mixing is put into a ball mill together with the grinding balls for ball milling, and the silicon carbide coating liquid after ball milling is adjusted with ammonia water and then vacuum filtered to remove bubbles, and finally a highly stable and uniformly dispersed silicon carbide coating liquid is obtained; b) coating: coating the coating solution prepared in step a) by an immersion coating method to obtain a ceramic film green body; c) Drying: The ceramic membrane green body prepared in step b) is first dried naturally, and then placed in a forced air drying oven for drying; d) Sintering: The silicon carbide ceramic membrane green body dried in step c) is subjected to oxidation sintering in an air atmosphere.

[0008] In one embodiment, in step a), the silicon carbide particles are at least two of 5 μm, 1 μm, 0.5 μm, and 0.1 μm.

[0009] In one embodiment, in step a), the amount of silicon carbide coarse particles added is 0.5 to 50 wt % of the mass of silicon carbide.

[0010] In one implementation scheme, in step a), the magnetic stirring is performed at 500 rpm for 20 to 60 min; and the ball milling is performed at 200 to 400 rpm for 2 to 18 h.

[0011] In one embodiment, in step b), when the film is coated by dipping, the dipping time is 20 to 80 s, and the dipping and pulling speed is 1000 to 5000 μm / s.

[0012] In one implementation scheme, in step c), the obtained silicon carbide ceramic membrane green body is naturally dried for 2 to 4 hours, and the drying temperature in a forced air drying oven is 40 to 100° C., and the drying time is 8 to 16 hours.

[0013] In one embodiment, in step d), the temperature is increased from room temperature to 500-700°C at a heating rate of 1-10°C / min, kept at that temperature for 60-120 min, then increased to 800-1100°C at a heating rate of 1-5°C / min, kept at that temperature for 60-150 min, and finally cooled naturally to room temperature.

[0014] The present invention provides a low-temperature sintered asymmetric porous silicon carbide ceramic membrane prepared by the above preparation method, the separation layer membrane thickness is 20-50 μm, the average pore size is 0.1-0.5 μm Compared with the prior art, the present invention has the following significant beneficial effects: 1. The raw materials used in the preparation method of the present invention are simple, and silicon carbide particles of different coarseness and fineness are used. An appropriate amount of coarse silicon carbide particles act as a stacking skeleton, effectively reducing the penetration of fine particles into the support layer. At the same time, the silicon carbide support body used is made by low-temperature oxidation sintering, and the silicon carbide separation layer green body after coating is also oxidized and sintered at low temperature, so that the silicon carbide ceramic membrane finally obtained is made of pure silicon carbide phase, which effectively maintains the firmness of the bonding between the grains of the silicon carbide separation layer and the support layer, thereby effectively improving the mechanical strength and acid and alkali corrosion resistance of the ceramic membrane. The preparation method of the present invention is simple, has low sintering energy consumption, mild conditions, simple and easy-to-obtain raw materials, low cost, and short production cycle, and is very easy to realize large-scale industrial production; 2. The method for preparing an asymmetric porous silicon carbide ceramic membrane separation layer by low-temperature sintering provided by the present invention effectively reduces the particle leakage of fine particles into the large-pore support layer. The prepared separation layer has good film-forming effect, clear boundary between the support layer and the separation layer, and uniform membrane thickness; the pore size distribution is narrow and the pore size is uniform, which can be applied to separation and purification in the field of microfiltration (for example, the pore size of the membrane layer in Example 2 is 310 nm), and the pure water flux is greater than 2000 L·m -2 ·h -1 bar -1 The integrated silicon carbide ceramic membrane module equipment can provide an ideal solution to the "bottleneck" problem of membrane technology under extreme working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a SEM image of the surface of the porous silicon carbide ceramic membrane prepared in Example 1 of the present invention; Figure 2 This is a SEM image of a cross section of a porous silicon carbide ceramic membrane prepared in Example 1 of the present invention; Figure 3 This is a SEM image of 500 nm silicon carbide particles used as raw material in Example 1 of the present invention; Figure 4 This is a pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in Example 1 of the present invention; Figure 5 This is a SEM image of a cross section of a porous silicon carbide ceramic membrane prepared in Example 2 of the present invention; Figure 6 This is a pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in Example 3 of the present invention; Figure 7 This is a SEM image of a cross section of a porous silicon carbide ceramic membrane prepared in Comparative Example 1 of the present invention; Figure 8 This is a pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0016] The technical scheme of the present invention is further described in detail and completely below in conjunction with the examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples without specifying specific conditions are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. Example

[0017] a) Preparation of coating liquid: First, weigh 0.1 g of dispersant in water, stir magnetically at 500 r / min for 60 min; add silicon carbide particle powder, 2 g of 1 μm silicon carbide particles, stir for 5 min, add 9 g of 0.5 μm silicon carbide particles, stir for 30 min; add 0.5 g of dispersant and stir for 30 min; move the coating liquid and grinding balls into a ball mill for ball milling at a speed of 350 r / min for 4 h. After ball milling, adjust the pH of the silicon carbide coating liquid to 9-10 with ammonia water, and then perform vacuum filtration to remove bubbles, and finally obtain a highly stable and evenly dispersed silicon carbide coating liquid; b) Coating: The coating solution prepared in step a) is coated by a dip-pull method, the dip-pull speed is 3000 μm / s, the dip time is 30 s, and the dip number is once.

[0018] c) Drying: The ceramic membrane green body prepared in step b) is dried in a natural state for 4 h, and then placed in a forced air drying oven at 60° C. for 12 h; d) Sintering: The dried silicon carbide ceramic membrane green body in step c) is subjected to oxidation sintering in an air atmosphere, with the temperature increased from room temperature to 500°C at a heating rate of 1°C / min, kept at that temperature for 120 min, then increased to 900°C at a heating rate of 1°C / min, kept at that temperature for 120 min, and finally naturally cooled to room temperature.

[0019] Figure 1 : is a SEM image of the surface of the porous silicon carbide ceramic membrane prepared in this embodiment. From the microstructure, it can be seen that the porous silicon carbide ceramic membrane prepared in this embodiment has a good sintering effect between particles and a uniform pore distribution; Figure 2 This is a SEM image of the cross section of the porous silicon carbide ceramic membrane prepared in this embodiment. It can be found that the separation layer and the support layer have a clear boundary, and there is no particle leakage of fine particles; Figure 4 This is the pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in Example 1 of the present invention. The pore size is concentrated at 392 nm, and the pure water flux tested is 2233 L·m -2 ·h -1 bar -1 . Example

[0020] a) Preparation of coating liquid: First, weigh 0.1 g of dispersant in water, stir magnetically at 500 r / min for 60 min; add silicon carbide particle powder, 2 g of 1 μm silicon carbide particles, stir for 5 min, add 9 g of 0.5 μm silicon carbide particles, stir for 30 min; add 0.5 g of dispersant and stir for 30 min; move the coating liquid and grinding balls into a ball mill for ball milling at a speed of 350 r / min for 4 h. After ball milling, adjust the pH of the silicon carbide coating liquid to 9-10 with ammonia water, and then perform vacuum filtration to remove bubbles, and finally obtain a highly stable and evenly dispersed silicon carbide coating liquid; b) Coating: The coating solution prepared in step a) is coated by a dip-pull method, the dip-pull speed is 3000 μm / s, the dip time is 30 s, and the dip number is once.

[0021] c) Drying: The ceramic membrane green body prepared in step b) is dried in a natural state for 4 h, and then placed in a forced air drying oven at 60° C. for 12 h; d) Sintering: The dried silicon carbide ceramic membrane green body in step c) is subjected to oxidation sintering in an air atmosphere, with the temperature rising from room temperature to 600°C at a heating rate of 1°C / min, kept at that temperature for 90 min, then raised to 900°C at a heating rate of 1°C / min, kept at that temperature for 150 min, and finally naturally cooled to room temperature.

[0022] Figure 5This is a SEM image of the cross section of the porous silicon carbide ceramic membrane prepared in Example 2. It can be found that the separation layer and the support layer have a clear boundary, and there is no particle leakage of fine particles. Example

[0023] a) Preparation of coating liquid: First, weigh 0.1 g of dispersant in water, stir magnetically at 500 r / min for 60 min; add silicon carbide particle powder, 2 g of 1 μm silicon carbide particles, stir for 5 min, add 9 g of 0.5 μm silicon carbide particles, stir for 30 min; add 0.5 g of dispersant and stir for 30 min; move the coating liquid and grinding balls into a ball mill for ball milling at a speed of 350 r / min for 4 h. After ball milling, adjust the pH of the silicon carbide coating liquid to 9-10 with ammonia water, and then perform vacuum filtration to remove bubbles, and finally obtain a highly stable and evenly dispersed silicon carbide coating liquid; b) Coating: The coating solution prepared in step a) is coated by a dip-pull method, the dip-pull speed is 3000 μm / s, the dip time is 30 s, and the dip number is once.

[0024] c) Drying: The ceramic membrane green body prepared in step b) is dried in a natural state for 4 h, and then placed in a forced air drying oven at 60° C. for 12 h; d) Sintering: The dried silicon carbide ceramic membrane green body in step c) is subjected to oxidation sintering in an air atmosphere, with the temperature rising from room temperature to 700°C at a heating rate of 1°C / min, kept at that temperature for 60 min, then raised to 900°C at a heating rate of 1°C / min, kept at that temperature for 120 min, and finally naturally cooled to room temperature.

[0025] Figure 6 This is a pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in Example 3 of the present invention, and the pore size is concentrated at 310 nm.

[0026] The steps of Comparative Example 1 are the same as those of Example 1, except that no 1 μm silicon carbide particles are added to the raw materials, and the particles of the coating liquid are single silicon carbide particles with a particle size of 0.5 μm.

[0027] Figure 7 The surface cross-sectional view of the porous silicon carbide ceramic membrane prepared in this comparative example shows that the porous silicon carbide ceramic membrane prepared in this comparative example has an uneven film thickness and cracks appear in the film layer. Figure 2 With Example 2 Figure 5 Compared with the thickness of medium-porous silicon carbide ceramic membrane, the addition of a small amount of coarse particles has a significant effect on the integrity of the membrane layer. The membrane layer integrity of the sample without adding coarse particles is poor, and cracks are prone to appear in the membrane layer after sintering.

[0028] Figure 8The pore size distribution diagram of the porous silicon carbide ceramic membrane prepared in this comparative example is shown in the figure. It can be seen that the porous silicon carbide ceramic membrane prepared in this comparative example has a large pore size and a wide pore size distribution, and the pore size distribution range is 600-700 nm; Figure 4 In comparison, the addition of a small amount of coarse particles plays a skeleton role in the particle stacking. Small particles can fill the gaps between large particles, increasing the packing density between particles, thereby making the pore size after sintering smaller and more uniformly distributed.

[0029] In addition, the pure water flux of the porous silicon carbide ceramic membrane prepared in this comparative example was tested. The results showed that the pure water flux of the porous silicon carbide ceramic membrane in this comparative example was 1904 L·m -2 ·h -1 bar -1 The pore size of the membrane layer in this comparative example is large but the pure water flux is small, which is also due to the decrease in pure water flux caused by the infiltration of fine particles.

[0030] Combined with Examples 1 to 3, it can be seen that the addition of a small amount of coarse-grained silicon carbide particles has a significant effect on the film-forming effect, pore structure and pure water flux of the asymmetric porous silicon carbide ceramic membrane. The addition of coarse-grained silicon carbide particles can enhance the stacking between silicon carbide particles, improve the film-forming effect, pore structure and pure water flux of the membrane layer, significantly improve and strengthen the technical advantages of silicon carbide ceramic separation membranes, and provide a solution for expanding its market scale and application areas.

[0031] Finally, it should be pointed out that the above are only some preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above contents of the present invention all fall within the scope of protection of the present invention.

Claims

1. A low-temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof, characterized in that: The steps include: a) Preparation of coating solution: First, the binder is dispersed in the solution, and the silicon carbide particle powder is added, and the solution is initially dispersed by magnetic stirring for a period of time, and then the dispersant is added and stirred again; next, the simply mixed coating liquid and the grinding balls are put into a ball mill for ball milling, and the silicon carbide coating liquid after ball milling is adjusted with ammonia water and then vacuum filtered to remove bubbles, and finally a highly stable and evenly dispersed silicon carbide coating liquid is obtained; b) coating: coating the film on the support by the dip-coating method in step a) to obtain a ceramic film green body; c) Drying: The ceramic membrane green body prepared in step b) is first dried naturally, and then placed in a forced air drying oven for drying; d) Sintering: The silicon carbide ceramic membrane green body dried in step c) is subjected to oxidation sintering in an air atmosphere.

2. The preparation method according to claim 1, characterized in that: In step a), the silicon carbide particles are at least two of 5 μm, 1 μm, 0.5 μm, and 0.1 μm.

3. The preparation method according to claim 1, characterized in that: In step a), the amount of silicon carbide coarse particles added is 0.5 to 50 wt% of the mass of silicon carbide.

4. The preparation method according to claim 1, characterized in that: In step a), the magnetic stirring is performed at 500 rpm for 20 to 60 min; the ball milling speed is 200 to 400 rpm for 2 to 18 h.

5. The preparation method according to claim 1, characterized in that: In step b), when the film is coated by dipping method, the dipping time is 20 to 80 s, and the dipping and pulling speed is 1000 to 5000 μm / s.

6. The preparation method according to claim 1, characterized in that: In step c), the obtained silicon carbide ceramic membrane green body is naturally dried for 2 to 4 hours, and dried in a blast drying oven at a temperature of 40 to 100° C. for 8 to 16 hours.

7. The preparation method according to claim 1, characterized in that: In step d), the temperature is increased from room temperature to 500-700°C at a heating rate of 1-10°C / min, kept at that temperature for 60-120 min, then increased to 800-1100°C at a heating rate of 1-5°C / min, kept at that temperature for 60-150 min, and finally cooled naturally to room temperature.

8. A low temperature sintered asymmetric porous silicon carbide ceramic membrane separation layer and a preparation method thereof, characterized in that: The preparation method is described in any one of claims 1 to 8.

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