Small-aperture silicon carbide ultrafiltration membrane and preparation method thereof

Abstract

The invention provides a small-aperture silicon carbide ultrafiltration membrane and a preparation method thereof, and belongs to the technical field of silicon carbide ceramic membranes, and the preparation method comprises the following steps: preparing a support raw material by using silicon carbide powder and carbon black as raw materials; preparing suspension slurry containing a support body raw material, polycarbosilane and attapulgite powder; sintering a support raw material into a silicon carbide membrane support, coating the silicon carbide membrane support with the suspension slurry, drying, and sintering again to obtain the silicon carbide membrane. According to the invention, the attapulgite powder is doped in the separation membrane layer material, and then high-temperature sintering is carried out, so that silicon carbide and attapulgite nanofibers in the separation membrane layer are mutually bonded to form a firm interwoven network structure, thereby obtaining the silicon carbide membrane material which is more uniform in pore size distribution and is more compact.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide ceramic membranes, and particularly to a small-aperture silicon carbide ultrafiltration membrane and a preparation method thereof. Background Art

[0002] Membrane separation is a highly efficient and precise separation form. The performance of the membrane material is related to the separation accuracy and cost of the separation process, and will affect the purity and cost of the separated product. Currently, the membrane materials that can be industrially applied are mainly polymer membrane materials and ceramic membrane materials. Compared with polymer membrane materials, ceramic membranes show their unique advantages in many harsh application systems due to their excellent material properties, and have become one of the fastest-developing and most promising varieties in the membrane field, and are ideal membrane materials for applications in the chemical and petrochemical industries.

[0003] Silicon carbide ceramic membranes have the advantages of acid and alkali resistance, high hydrophilicity and oleophobicity, and high mechanical strength, and are key component materials for membrane separation technology. At present, great progress has been made in the research and application of silicon carbide ceramic separation membrane materials, but the design of the membrane aperture still cannot meet the actual production requirements. SiC has strong covalent bond characteristics, high hardness, and great processing difficulty. It is difficult to batch prepare high-purity, ultrafine, spherical, and narrow particle size distribution SiC nanometer powder. During the sintering process of SiC, agglomeration or recrystallization is easy to occur. Stacking layers to form pores layer by layer according to the asymmetric structure design principle will make the membrane layer too thick, which makes it difficult to prepare small-aperture silicon carbide ceramic filter membranes. Summary of the Invention

[0004] In view of the technical problems existing in the background art, the present invention provides a small-aperture silicon carbide ultrafiltration membrane, a preparation method and an application thereof, aiming to solve the problem of difficult preparation of small-aperture silicon carbide ceramic filter membranes.

[0005] In a first aspect, the present invention provides a preparation method of a silicon carbide ultrafiltration membrane, comprising the following steps: Preparing a support green body using silicon carbide powder and carbon black as raw materials; Preparing a suspension slurry containing the support green body, polycarbosilane and attapulgite powder; First sintering the support green body into a silicon carbide membrane support, and then coating the suspension slurry on the silicon carbide membrane support, and sintering again after drying to obtain a small-aperture silicon carbide ultrafiltration membrane.

[0006] The present invention prepares a separation membrane layer by sintering a green body of a support doped with attapulgite powder on the surface of the support, and through the limitation of the sintering process, the separation membrane layer has a dense structure and a more uniform small pore size distribution. Specifically, the sintering process of the separation membrane layer in an argon atmosphere is as follows: the sintering temperature is 1400 - 1600 °C, and after 1000 °C, it is sintered at 1 h for every 200 °C and held for 1 h. It can be understood that when the last time is less than 200 °C, it can be directly heated to the target temperature, sintered for 1 h and held for 1 h. Experimental data shows that when the sintering temperature of the separation membrane is 1000 °C, attapulgite begins to undergo the transformation from quartz to cristobalite, and gradually forms a dense structure with SiO 2 as the matrix and MgO·SiO 2 as the second phase; at 1100 °C, attapulgite binds with the silicon matrix to form a complex network structure; as the sintering progresses, when the temperature is greater than 1200 °C, some closed micropores fuse with each other to form larger open pores, the porosity slightly increases, but the pore size distribution becomes more uniform and the density of the sintered body increases. In addition, if the sintering temperature is lower than 1000 °C, there is no crystallization reaction process for the powder mixture such as attapulgite, that is, the structure and properties of attapulgite will change significantly at high temperatures, but the structure transformation may not be achieved below 1000 °C; if the sintering temperature is too high, although the pore size changes little, the hardness of the membrane layer decreases significantly. In addition, the argon protection atmosphere helps to maintain the integrity of the microstructure of the silicon carbide material.

[0007] Preferably, in the above preparation method, the mass ratio of carbon black to silicon carbide powder is 1:(4 - 9). The present invention uses silicon carbide powder and carbon black doped sintering to prepare the support. The carbon black participates in the sintering to adjust the performance of the sintered body. On the one hand, it prevents the occurrence of sintering cracks and "undercooking", which affect the bulk density and strength. On the other hand, an appropriate amount of carbon black is beneficial to the growth of silicon carbide crystals during the sintering process, making the support structure complete, the membrane pore distribution uniform, and improving the synthesis efficiency and quality.

[0008] In some embodiments, the green body of the support is prepared by the following steps: silicon carbide powder, carbon black and a solvent are ball-milled to obtain a mixed slurry, and the mixed slurry can be obtained as the green body of the support after drying; wherein, the solvent is a mixed solution of anhydrous ethanol and ultrapure water. For example, in some embodiments, the mass ratio of anhydrous ethanol to ultrapure water is 5:1.

[0009] Preferably, in the above preparation method, the mass ratio of attapulgite powder to the green body of the support is 1:(1.5 - 2.5). During the optimization of attapulgite powder doping, while ensuring the membrane filtration efficiency and accuracy requirements, it is also necessary to reduce the thickness of the transition layer and lower the sintering temperature of the separation membrane layer. For example, in some embodiments of the present invention, when the mass ratio of attapulgite to the green body of the support is 0.4:1 and the sintering temperature is 1400 °C, the average pore diameter of the obtained separation membrane layer can reach 0.03 μm, and the overall porosity of the ceramic membrane is greater than 48%.

[0010] Preferably, in the above preparation method, the mass ratio of polycarbosilane to the green body of the support is 1:(1.5 - 4). Adding an appropriate amount of polycarbosilane (PCS) to the suspension slurry has at least the following effects: contributing to the chemical stability and thermal stability during the sintering process, improving the strength and acid and alkali resistance of the silicon carbide material, and enhancing the coating strength.

[0011] Preferably, in the above preparation method, absolute ethanol and polyethylene glycol 6000 are also included in the suspension slurry, which act as a dispersant and a binder respectively, facilitating granulation.

[0012] In some embodiments, the preparation method of the suspension slurry includes the following operations: adding polycarbosilane and water to the green body of the support, and stirring under heating conditions; ball-milling the prepared slurry with absolute ethanol, attapulgite powder and polyethylene glycol 6000 to obtain the suspension slurry. Preferably, the stirring temperature is 50 - 60 °C.

[0013] In some embodiments, the preparation of the silicon carbide membrane support includes the following operations: Mixing the green body of the support with sodium hydroxypropyl methylcellulose (HPMC), glycerol, oleic acid, polyethylene glycol 6000 and water, and obtaining the silicon carbide membrane support after aging, debinding and sintering in sequence. Preferably, the mass ratio of the green body of the support to sodium hydroxypropyl methylcellulose, glycerol, oleic acid, polyethylene glycol 6000, water is (0.9 - 1):(0.15 - 0.2):(0.05 - 0.2):(0.05 - 0.1):(0.1 - 0.15):(0.3 - 0.5). Preferably, the aging time is 40 - 48 h. Preferably, the sintering procedure is: in an argon atmosphere, the sintering temperature is 1600 - 1800 °C, sintering for 1 h and holding for 1 h every 200 °C from 1000 - 1600 °C, and sintering for 1 h and holding for 45 min every 100 °C above 1600 °C.

[0014] In some embodiments, dip-coating is used to coat the silicon carbide membrane support, and it is dried by air blowing after coating. Preferably, the number of coating times is 3 - 5 times.

[0015] In a second aspect, the present invention provides a small-aperture silicon carbide ultrafiltration membrane prepared according to the above method. Specifically, the silicon carbide ultrafiltration membrane has an asymmetric structure and can be sequentially divided into a support layer, an intermediate transition layer, and a separation membrane layer; it should be noted that the intermediate transition layer is not a distinct and homogeneous membrane layer, but rather refers to the interactive structure between the support layer and the separation membrane layer.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: The separation membrane layer material of the silicon carbide ceramic membrane provided by the present invention is doped with attapulgite powder. Through sintering, silicon carbide and attapulgite nanofibers are bonded to each other to form a firm intertwined network structure, thereby obtaining a silicon carbide ceramic membrane with a more uniform pore size distribution and an increased sintered body density of the separation membrane layer, providing a new method for manufacturing a dense separation membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the present invention, the drawings used in the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 is a schematic structural diagram of the small-aperture silicon carbide ultrafiltration membrane in an embodiment of the present invention; Figure 2 is a flowchart for preparing the small-aperture silicon carbide ultrafiltration membrane in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The embodiments of the technical solutions of the present invention will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and thus are only examples and should not be used to limit the protection scope of the present invention.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention; the terms "including" and "having" and any variations thereof in this document are intended to cover non-exclusive inclusion.

[0021] Reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present invention. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0022] Although significant progress has been made in the research and application of silicon carbide ceramic separation membrane materials, due to the strong covalent bond characteristics, high hardness, and difficult processing of SiC, the pore size of current silicon carbide ceramic membranes still cannot meet the actual production requirements. To solve the technical problem of the difficult preparation of small-pore silicon carbide ceramic filter membranes, this application provides a method for preparing small-pore silicon carbide ultrafiltration membranes. By adding attapulgite powder to the ceramic membrane separation layer material and making the attapulgite nanofibers and silicon carbide adhere to each other to form a strong intertwined network structure under a specific sintering process, the pore densification of the membrane is effectively improved while reducing the stacking thickness of the intermediate transition layer.

[0023] Please refer to Figure 1 , in the first aspect, an embodiment of the present invention provides a small-pore silicon carbide ultrafiltration membrane, which is composed of a support layer, a separation membrane layer, and an intermediate transition layer, forming an asymmetric membrane structure; wherein, attapulgite is doped in the material used for the separation membrane layer, and the pore size of the separation membrane layer can be effectively controlled by limiting the doping ratio of attapulgite powder and the sintering temperature of the separation membrane layer.

[0024] Please refer to Figure 2 , in the second aspect, an embodiment of the present invention provides a method for preparing a small-pore silicon carbide ultrafiltration membrane, including the following steps: The slurry obtained by ball-milling silicon carbide powder, carbon black, and a solvent is dried to obtain a green body of the support. The green body of the support is mixed with HPMC, glycerol, oleic acid, polyethylene glycol, and water, and after aging, debinding, and sintering in sequence, a silicon carbide membrane support is obtained. Prepare a suspension slurry containing the green body of the support, polycarbosilane, and attapulgite powder, and use the dip-coating method to coat the suspension slurry on the silicon carbide membrane support multiple times. After drying, sintering is carried out again to obtain the product.

[0025] When the sintering temperature is greater than 1000 °C, attapulgite undergoes a transformation from quartz to cristobalite, and gradually forms a dense structure with SiO 2 as the matrix, and part of MgO·SiO 2 will form a dense structure. After doping, the attapulgite nanofibers in the silicon carbide membrane adhere to each other to form a strong intertwined network structure, the pore size distribution becomes more uniform, and the density of the sintered body of the separation layer increases.

[0026] The following lists some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as a limitation to the present invention. For those not specified in the embodiments regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in the field or according to the product specifications. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0027] Example 1 This example provides a small-aperture silicon carbide ultrafiltration membrane and its preparation method. The specific preparation process is as follows: (1) Preparation of the support green body.

[0028] Take 100 g of silicon carbide powder, 40 g of carbon black and a solvent (composed of 100 g of absolute ethanol and 20 g of ultrapure water), ball mill for 4 h, and then dry in an oven at 100 °C for 24 h to obtain the support green body.

[0029] (2) Preparation of the silicon carbide membrane support.

[0030] Take 100 g of the support green body, mix it with 20 g of HPMC, 20 g of glycerol, 10 g of oleic acid, 15 g of polyethylene glycol 6000 and 30 g of water for 20 min, let it age for 24 h, and degrease for 24 h; finally, sinter in a sintering furnace under an argon atmosphere at 1000 - 1600 °C, sinter for 1 h and hold for 1 h every 200 °C, above 1600 °C, sinter for 1 h and hold for 45 min every 100 °C, start to cool down after holding at 1800 °C for 1 h, and obtain the silicon carbide membrane support after cooling is completed.

[0031] (3) Preparation of the silicon carbide ultrafiltration membrane.

[0032] Take 20 g of the support green body, add 8 g of polycarbosilane and 40 g of pure water, stir in a water bath at 60 °C for 12 h. Ball mill the prepared slurry together with 50 g of absolute ethanol, 8 g of attapulgite powder and 3 g of polyethylene glycol 6000 for 12 h to obtain a suspension slurry.

[0033] Use the dipping method to coat the suspension slurry on the silicon carbide membrane support 4 times repeatedly, dry (dry in a blast dryer at 100 °C for 2 h), and then sinter in a sintering furnace under an argon atmosphere to obtain the silicon carbide ultrafiltration membrane; among them, the sintering temperature is 1500 °C, sinter for 1 h and hold for 1 h every 200 °C within 1000 - 1400 °C, and finally directly raise the temperature to 1500 °C, sinter for 1 h and hold for 1 h.

[0034] After testing, the average pore diameter of the separation membrane layer is 0.03 μm, the average porosity of the silicon carbide ultrafiltration membrane is 48.35%, and the average Vickers hardness is 2650.

[0035] Example 2 This example provides a small-aperture silicon carbide ultrafiltration membrane and its preparation method. Different from Example 1, in this example, the aging time in step (2) is set to 12 h or 48 h.

[0036] After testing, compared with Example 1, when the aging time is 12 h, the average porosity of the silicon carbide ultrafiltration membrane is 45.12%, and when the aging time is 48 h, the average porosity of the membrane has no obvious change.

[0037] Example 3 This example provides a small-aperture silicon carbide ultrafiltration membrane and a preparation method thereof. Different from Example 1, in this example, the coating times of the suspension slurry in step (3) are set to 2, 4, 6, and 8 times respectively.

[0038] After testing, when the coating times are less (2 times), the strength of the silicon carbide ultrafiltration membrane becomes poor, and the average Vickers hardness is lower than 2300. When the coating times are more than 4 times, the average pore diameter decreases slightly but the porosity is low, and the average porosity is less than 40%.

[0039] Comparative Example 1 This example provides a small-aperture silicon carbide ultrafiltration membrane and a preparation method thereof. Different from Example 1, in this example, the sintering temperature in step (3) is reduced to 1350 °C, and it is sintered for 1 h and held for 1 h every 200 °C above 1000 °C (only the last heating to 1350 °C is required).

[0040] After testing, compared with Example 1, the average pore diameter of the obtained silicon carbide ultrafiltration membrane remains basically unchanged, the average porosity is greater than 48, but the hardness of the membrane layer decreases significantly, and the average Vickers hardness is lower than 2000. The above results show that when the sintering temperature of the separation membrane layer is insufficient, it will lead to poor material hardness, and the reason may be that the internal stress of the green body is not fully released, and deformation or microcracks may occur due to uneven shrinkage during cooling.

[0041] In summary, the present invention uses a silicon carbide ceramic membrane as a substrate, and through adding attapulgite in the separation membrane layer material and controlling the sintering process, an interwoven network structure is formed in the separation membrane layer, thereby obtaining a dense separation membrane, providing a new method for the preparation of small-aperture silicon carbide ceramic membranes.

[0042] It should be noted that the present invention is not limited to the above embodiments. The above embodiments are only examples, and embodiments having the same constitution and the same function and effect as the technical idea within the technical solution scope of the present invention are all included in the technical scope of the present invention. In addition, within the scope not departing from the gist of the present invention, various deformations that those skilled in the art can think of are applied to the embodiments, and other ways constructed by combining some constituent elements in the embodiments are also included in the scope of the present invention.

Claims

1. A method for preparing a silicon carbide ultrafiltration membrane, characterized in that: The following steps are involved: The support raw material is prepared by using silicon carbide powder and carbon black as raw materials; preparing a suspension slurry comprising the support raw material, polycarbosilane and attapulgite powder; After the support raw material is sintered into a silicon carbide membrane support, the suspension slurry is coated on the silicon carbide membrane support, dried and then sintered twice to obtain a small-pore silicon carbide ultrafiltration membrane.

2. The preparation method according to claim 1, characterized in that: The re-sintering procedure is: Ar atmosphere, sintering temperature is 1400-1600°C, and after 1000°C, sintering at 200°C for 1 hour and keeping at this temperature for 1 hour.

3. The preparation method according to claim 1, characterized in that: The mass ratio of the carbon black to the silicon carbide powder is 1:(1.5-4).

4. The preparation method according to claim 1, characterized in that: The method for preparing the raw material of the support body comprises: mixing silicon carbide powder, carbon black and solvent by ball milling, and drying to obtain the raw material of the support body; The solvent is a mixture of ethanol and water.

5. The preparation method according to claim 1, characterized in that: The mass ratio of the polycarbosilane to the support raw material is 1:(1.5-4).

6. The preparation method according to claim 1, characterized in that: The mass ratio of the attapulgite powder to the support raw material is 1:(1.5-2.5).

7. The preparation method according to claim 1, characterized in that: The suspension slurry also contains anhydrous ethanol and polyethylene glycol.

8. The preparation method according to claim 1, characterized in that: The preparation of the silicon carbide film support comprises the following steps: The support raw material is mixed with sodium hydroxypropyl methylcellulose, glycerol, oleic acid, polyethylene glycol and water, and then aged, debonded and sintered in sequence to obtain a silicon carbide membrane support.

9. The preparation method according to claim 1, characterized in that: The coating is carried out by slurry dipping method for 3 to 5 times.

10. A small-pore silicon carbide ultrafiltration membrane prepared according to the preparation method according to any one of claims 1 to 9, wherein the average pore size is ≤32.5 nm.

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