Preparation method of high-purity tubular silicon carbide outer coating film

Abstract

This invention relates to a method for preparing a high-purity tubular silicon carbide external coating film. The external coating film is applied to the outside of single-channel or multi-channel tubular silicon carbide supports of various sizes using a negative / positive pressure coating process. The film layer consists of one or two layers. The preparation method of the film layer is as follows: First, silicon carbide powder, silicon source, pH adjuster, dispersant, and solvent are mixed to obtain a uniform and stable slurry with high solids content. Then, using a special negative / positive pressure process, the slurry is uniformly coated onto the outside of the single-channel or multi-channel tubular silicon carbide support. After drying, it has a certain strength. Finally, it is sintered under a protective atmosphere without pressure to obtain a high-purity silicon carbide external coating film that is tightly bonded to the support. This invention innovatively coats a silicon carbide membrane separation layer onto the outer wall of a support body, enabling it to process liquids with no upper limit on solid content. Furthermore, the membrane layer is tightly bonded to the support body, exhibits good erosion resistance, high filtration accuracy, good backwashing effect, and is not prone to clogging. It has broad application prospects in fields such as coal mine wastewater separation, high-viscosity industrial wastewater filtration, recovery of acid solutions containing precious metals, and purification or concentration of high-concentration slurries under harsh working conditions.

Description

Technical Field

[0001] This invention belongs to the field of ceramic membrane technology, specifically relating to a method for preparing a high-purity tubular silicon carbide external coating film. Background Technology

[0002] Membrane separation technology, as a highly efficient, environmentally friendly, and low-cost separation process, has been widely applied in water treatment, energy, chemical industry, and other fields. Among membrane separation technologies, inorganic membranes possess higher mechanical strength, high-temperature resistance, and corrosion resistance compared to organic membranes, making them more promising for high-efficiency separation in harsh environments. Silicon carbide ceramic membranes, as high-performance non-oxide ceramic membranes, offer advantages such as good hydrophilicity, high mechanical strength, high temperature resistance, corrosion resistance, oxidation resistance, and high water flux, making them ideal for liquid separation processes in high-temperature, highly corrosive, and high-concentration environments.

[0003] Currently, the most widely used silicon carbide ceramic membrane is the tubular silicon carbide ceramic membrane, in which the separation membrane layer is coated inside the support channel. Compared with plate membranes, it has the advantages of larger membrane area per unit volume, longer service life, higher separation efficiency, and lower backwash frequency. However, even for such high-performance tubular silicon carbide ceramic membranes, there are strict upper limits on the solid content of the treated liquid to avoid complete blockage of the membrane channel leading to failure. Existing common filtration processes include dead-end filtration and cross-flow filtration: dead-end filtration has lower energy consumption but is prone to fouling, and the ideal suspended solids concentration of the treated liquid needs to be controlled at <1000ppm (1‰); cross-flow filtration has higher energy consumption, although it can use the high-speed flow of the concentrate to flush out fouling on the membrane surface, but it can only treat liquids with a solids concentration of 10,000-20,000ppm (1-2%).

[0004] However, industrial production processes often generate liquids with solid content as high as 20-30% or even higher, far exceeding the processing limits of existing membrane treatment technologies. This leads to the membrane channels being easily and completely fouled, and even backwashing cannot restore membrane flux. Therefore, a pre-precipitation process is often required, which not only increases treatment costs but also reduces separation efficiency. To broaden the application of silicon carbide ceramic membranes in the separation of high-solid-content liquids such as coal mine wastewater separation, high-viscosity industrial wastewater filtration, recovery of precious metal-containing acid solutions, and purification or concentration of high-concentration slurries under harsh operating conditions, increasing the upper limit of the solid content of suspended solids that can be processed has become an urgent technical problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing silicon carbide ceramic membranes, such as their inability to handle high solid content liquids, easy clogging of membrane channels, and inability to be restored by backwashing. This invention provides a method for preparing a high-purity tubular silicon carbide external coating membrane, which achieves efficient separation of high solid content liquids through innovative structural design and process optimization.

[0006] To achieve the above objectives, the present invention provides a method for preparing a high-purity tubular silicon carbide external coating film, comprising the following steps: 1) Preparation of silicon carbide support: Silicon carbide powder I, silicon source, plasticizer, binder, sintering aid and dispersant are mixed in proportion, and after kneading, extrusion molding, drying, debinding and sintering, a high-purity silicon carbide support is obtained; 2) Coating of silicon carbide film: Silicon carbide powder II, silicon source, pH adjuster, activator and dispersant are mixed in proportion to form a slurry. The slurry is coated on the outside of the silicon carbide support obtained in step 1) using negative pressure / positive pressure coating technology. After drying, the silicon carbide ceramic support with external film is obtained. 3) Sintering of silicon carbide outer coating: The silicon carbide support obtained in step 2) is placed in a high-temperature sintering furnace and sintered at high temperature under a protective atmosphere to obtain a high-purity silicon carbide porous ceramic outer coating.

[0007] Furthermore, the silicon carbide powder I has a particle size of 5-60 μm and a purity greater than 99%; the silicon source is an organosilicon source or an inorganic silicon source, wherein the inorganic silicon source is selected from one or more of silicon, silicon monoxide, silicon dioxide, and silicon nitride, and the organosilicon source is selected from one or more of tetraethyl orthosilicate, tetrabutyl orthosilicate, and silane coupling agent.

[0008] Furthermore, the plasticizer is selected from one or more of polyethylene glycol, starch, and various cellulose ethers; the binder is selected from one or more of polyvinyl alcohol, acrylic acid, and silica sol; the sintering aid is selected from one or more of boron carbide, silicon nitride, and silicon oxide; the dispersant is water or ethanol; and the mass ratio of silicon carbide powder I, silicon source, plasticizer, binder, sintering aid, and dispersant is 100:5-10:3-6:5-10:0.1-1:25-30.

[0009] Furthermore, the silicon carbide ceramic support obtained by extrusion molding in step 1) has a single-channel or multi-channel structure, a support diameter of 8-100 mm, and a purity greater than 99%.

[0010] Furthermore, the sintering temperature in step 1) is 1850-2300℃. The specific sintering steps are as follows: heat up to 1400-1500℃ at a heating rate of 10-15℃ / min, hold for 1-2 hours, then heat up to 1850-2300℃ at a heating rate of 3-5℃ / min, hold for 2-4 hours, and finally cool to room temperature with the furnace.

[0011] Furthermore, the average particle size of the silicon carbide powder II is controlled by the gradation of one or more silicon carbide powders, with a particle size range of 0.1-10 μm and a purity greater than 99%. Powders of a single particle size will produce large voids when stacked. By scientifically grading two or more silicon carbide powders of different particle sizes, fine particles can fill the voids between coarse particles. The application of this "closest packing" theory results in a higher density and more uniform structure in the dried and sintered film preform, thereby obtaining a final film with narrower pore distribution, more controllable pore size, and higher mechanical strength. The lower limit of 0.1 μm ensures that a sufficient proportion of fine particles are present to achieve effective gradation filling and form a fine film surface. At the same time, powders with excessively small particle sizes have a huge specific surface area and extremely high surface energy, making them prone to agglomeration in slurry, difficult to disperse and stabilize, and excessive activity during sintering may lead to over-densification or abnormal grain growth, which is not conducive to the formation of a stable, porous structure. The 10μm maximum particle size limit is designed to ensure the smoothness and uniformity of the membrane surface. Excessively large particles can lead to a rough membrane surface and even defects, directly affecting the membrane's separation accuracy and antifouling capabilities. Furthermore, overly coarse particles are more prone to settling during coating due to gravity and other factors, impacting membrane thickness uniformity.

[0012] The pH adjuster is selected from one or more of sodium hydroxide, potassium hydroxide, ammonia, and tetramethylammonium hydroxide; the activator is selected from polyvinylpyrrolidone and polyethylene glycol; the mass ratio of silicon carbide powder II, silicon source, pH adjuster, activator, and dispersant is 1:0.05-0.1:0.01-0.05:0.03-0.8:4-100.

[0013] Furthermore, the slurry has a solid content of 1-30 vol%, the slurry flow time during negative pressure coating is 10-30 s, and the coating pressure is 0.01-0.3 MPa.

[0014] Furthermore, the sintering temperature in step 3) is 1450-1700℃. The specific sintering steps are as follows: heat up to 1100-1350℃ at a heating rate of 5-10℃ / min, hold for 0.5-1.5h, then heat up to 1450-1700℃ at a heating rate of 1-5℃ / min, hold for 1-3h, and finally cool to room temperature with the furnace.

[0015] Furthermore, steps 2) and 3) can be repeated 1-2 times. The outer coating film applied to the support is composed of two film layers with different formulations and thicknesses, and the purity of the outer coating film is greater than 99%.

[0016] Furthermore, the protective atmosphere in step 3) is an inert gas, selected from one or more of argon, nitrogen, and helium.

[0017] The beneficial effects of this invention are: (1) The present invention sets the separation membrane layer of the silicon carbide ceramic outer coating membrane outside the support body, which is not limited by the narrow space inside the channel. Solid particles in the liquid will not block the membrane channel, and it is not easy to cause fouling. The backwashing effect is better. It can handle almost all high solid content liquids commonly used in industrial production, which greatly expands the application scenarios of silicon carbide ceramic membranes, improves separation efficiency, eliminates the need for additional pre-precipitation process, and reduces processing costs.

[0018] (2) The silicon carbide ceramic outer coating film prepared by the present invention has high bonding strength with the support, high mechanical strength of the film layer itself, and excellent erosion resistance. It can withstand the erosion of high flow rate liquid without external support protection, achieve precise separation, is not prone to defects due to film layer detachment, and has a long service life.

[0019] (3) Since the separation membrane is coated on the outside of the support, the surface condition of the membrane can be directly observed by the naked eye, making it convenient to detect whether there are defects such as cracks and holes in the membrane, which can efficiently identify unqualified products and reduce the risk of subsequent use.

[0020] (4) The coating method of the present invention can adapt to various specifications of single-channel and multi-channel silicon carbide supports, has strong adaptability to industrial mass production, and the prepared external coating membrane has good compatibility with existing membrane modules and supporting filtration equipment. It does not require large-scale modification of existing equipment and is easy to promote and apply. Attached Figure Description

[0021] Figure 1 These are comparison images of the external coating film of the present invention and the internal coating film of the prior art. Detailed Implementation

[0022] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0023] In the following embodiments, the silicon carbide support is prepared by the following method: Silicon carbide powder with a particle size of 45 μm, silicon nitride with a particle size of 4 μm, carboxymethyl cellulose, polyvinyl alcohol, alumina and yttrium oxide (3:2), and pure water were mixed in a mass ratio of 100:8:5:8:1:25. After kneading and extrusion molding, single-channel or multi-channel silicon carbide support green bodies were obtained. After drying, the green bodies were debonded at 600℃ for 4 hours, and then placed in a sintering furnace for pressureless sintering under an argon atmosphere. The sintering process was as follows: the temperature was increased to 1400℃ at a rate of 10℃ / min and held for 1 hour; then the temperature was increased to 1900℃ at a rate of 5℃ / min and held for 3 hours; after the holding period, the temperature was cooled to room temperature with the furnace to obtain the silicon carbide support. The support had an average pore size of 3.8 μm, a porosity of 46.5%, and a flexural strength of 31.2 MPa.

[0024] Example 1 A method for preparing a high-purity tubular silicon carbide external coating film includes the following steps: (1) Silicon carbide powder with a particle size of 1 μm, silica sol, ammonia, polyethylene glycol and ethanol are mixed in a mass ratio of 1:0.08:0.03:0.05:50 and ball-milled for 3 hours to obtain a uniform separation layer slurry.

[0025] (2) The separation layer slurry is coated on the outside of the support by negative pressure coating. The coating is carried out for 10 seconds under a pressure of 0.05 MPa, dried at 40°C for 2 hours in a drying oven, and then heated to 70°C for 3 hours to obtain a silicon carbide support with an external coating.

[0026] (3) The silicon carbide support obtained in step (2) is placed in a sintering furnace and sintered under an argon atmosphere. The temperature is increased to 1200℃ at a rate of 10℃ / min and held for 1h. Then the temperature is increased to 1550℃ at a rate of 5℃ / min and held for 1.5h. After the holding period, the temperature is cooled to room temperature with the furnace to obtain a high-purity silicon carbide ceramic outer coating.

[0027] Example 2 A method for preparing a high-purity tubular silicon carbide external coating film includes the following steps: (1) A mixture of silicon carbide powder with a particle size of 1.5 μm and silicon carbide powder with a particle size of 4 μm was obtained by grading. Silica sol, tetramethylammonium hydroxide, ammonium citrate and pure water were mixed and ball-milled for 3 hours at a mass ratio of 1:0.08:0.05:0.05:80 to obtain a uniform separation layer slurry.

[0028] (2) The separation layer slurry is coated on the outside of the support by positive pressure coating. The coating is carried out for 20 seconds under a pressure of 0.2 MPa, dried at 35°C for 3 hours in a drying oven, and then heated to 60°C for 6 hours to obtain a silicon carbide support with an external coating.

[0029] (3) The silicon carbide support obtained in step (2) is placed in a sintering furnace and sintered under an argon atmosphere. The temperature is increased to 1350℃ at a rate of 10℃ / min and held for 0.5h. Then the temperature is increased to 1650℃ at a rate of 3℃ / min and held for 2h. After the holding period, the temperature is cooled to room temperature with the furnace to obtain a high-purity silicon carbide ceramic outer coating.

[0030] Example 3 A method for preparing a high-purity tubular silicon carbide external coating film includes the following steps: (1) Silicon carbide powder with a particle size of 1 μm, silica sol, ammonia, polyethylene glycol and ethanol are mixed in a mass ratio of 1:0.08:0.04:0.08:100 and ball-milled for 2 hours to obtain a uniform separation layer slurry.

[0031] (2) The separation layer slurry is coated on the outside of the support by negative pressure coating. The coating is carried out for 15 seconds under a pressure of 0.1 MPa, dried at 40°C for 2 hours in a drying oven, and then heated to 70°C for 3 hours to obtain a silicon carbide support with an external coating.

[0032] (3) The silicon carbide support obtained in step (2) is placed in a sintering furnace and sintered under an argon atmosphere. The temperature is increased to 1350℃ at a rate of 10℃ / min and held for 0.5h. Then the temperature is increased to 1650℃ at a rate of 3℃ / min and held for 2h. After the holding period, the temperature is cooled to room temperature with the furnace to obtain the first outer coating separation film layer.

[0033] (4) Repeat step (1), except that the particle size of the silicon carbide powder used is changed to 0.5 μm, and the ratio of silicon carbide powder, ethanol and polyvinylpyrrolidone is 100:25:5.

[0034] (5) Repeat steps (2) and (3), wherein the casting time in step (2) is 10s, and in step (3) the temperature is raised to 1100℃ at a rate of 10℃ / min and held for 0.5h, and then raised to 1450℃ at a rate of 3℃ / min and held for 3h. After the holding is completed, the temperature is cooled to room temperature with the furnace to obtain a high-purity silicon carbide ceramic outer coating film.

[0035] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a high-purity tubular silicon carbide external coating film, characterized in that, Includes the following steps: 1) Preparation of silicon carbide support: Silicon carbide powder I, silicon source, plasticizer, binder, sintering aid and dispersant are mixed in proportion, and after kneading, extrusion molding, drying, debinding and sintering, a high-purity silicon carbide support is obtained; 2) Coating of silicon carbide film: Silicon carbide powder II, silicon source, pH adjuster, activator and dispersant are mixed in proportion to form a slurry. The slurry is coated on the outside of the silicon carbide support obtained in step 1) using negative pressure / positive pressure coating technology. After drying, the silicon carbide ceramic support with external film is obtained. 3) Sintering of silicon carbide outer coating: The silicon carbide support obtained in step 2) is placed in a high-temperature sintering furnace and sintered at high temperature under a protective atmosphere to obtain a high-purity silicon carbide porous ceramic outer coating.

2. The preparation method according to claim 1, characterized in that, The silicon carbide powder I has a particle size of 5-60 μm and a purity greater than 99%; the silicon source is an organosilicon source or an inorganic silicon source, wherein the inorganic silicon source is selected from one or more of silicon, silicon monoxide, silicon dioxide, and silicon nitride, and the organosilicon source is selected from one or more of tetraethyl orthosilicate, tetrabutyl orthosilicate, and silane coupling agent.

3. The preparation method according to claim 1, characterized in that, The plasticizer is selected from one or more of polyethylene glycol, starch, and various cellulose ethers; the binder is selected from one or more of polyvinyl alcohol, acrylic acid, and silica sol; the sintering aid is selected from one or more of boron carbide, silicon nitride, and silicon oxide; the dispersant is water or ethanol; the mass ratio of silicon carbide powder I, silicon source, plasticizer, binder, sintering aid, and dispersant is 100:5-10:3-6:5-10:0.1-1:25-30.

4. The preparation method according to claim 1, characterized in that, The silicon carbide ceramic support obtained by extrusion molding in step 1) has a single-channel or multi-channel structure, a support diameter of 8-100 mm, and a purity greater than 99%.

5. The preparation method according to claim 1, characterized in that, Step 1) The sintering temperature is 1850-2300℃. The specific sintering steps are as follows: heat up to 1400-1500℃ at a heating rate of 10-15℃ / min, hold for 1-2 hours, then heat up to 1850-2300℃ at a heating rate of 3-5℃ / min, hold for 2-4 hours, and finally cool to room temperature with the furnace.

6. The preparation method according to claim 1, characterized in that, The average particle size of the silicon carbide powder II is controlled by one or more silicon carbide powder gradations, with a particle size range of 0.1-10 μm and a purity greater than 99%. The pH adjuster is selected from one or more of sodium hydroxide, potassium hydroxide, ammonia, and tetramethylammonium hydroxide. The activator is selected from polyvinylpyrrolidone and polyethylene glycol. The mass ratio of silicon carbide powder II, silicon source, pH adjuster, activator, and dispersant is 1:0.05-0.1:0.01-0.05:0.03-0.8:4-100.

7. The preparation method according to claim 1, characterized in that, The slurry has a solid content of 1-30 vol%, a flow time of 10-30 s during negative pressure coating, and a coating pressure of 0.01-0.3 MPa.

8. The preparation method according to claim 1, characterized in that, Step 3) The sintering temperature is 1450-1700℃. The specific sintering steps are as follows: heat up to 1100-1350℃ at a heating rate of 5-10℃ / min, hold for 0.5-1.5h, then heat up to 1450-1700℃ at a heating rate of 1-5℃ / min, hold for 1-3h, and finally cool to room temperature with the furnace.

9. The preparation method according to claim 1, characterized in that, Steps 2) and 3) can be repeated 1-2 times. The outer coating film applied to the support is composed of two layers with different formulations and thicknesses, and the purity of the outer coating film is greater than 99%.

10. The preparation method according to claim 1, characterized in that, The protective atmosphere in step 3) is an inert gas, selected from one or more of argon, nitrogen, and helium.

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