Fabrication of ceramic membranes using a natural gelling agent by the centrifugal gel casting method.

Abstract

A new preparation method for fabricating ceramic membranes using centrifugal gel casting technique using egg white as a natural gelling agent is disclosed. Initially, egg white is mixed with an appropriate amount of silicone or non-silicone antifoam agents using a mechanical stirrer. Ceramic powders are then added to this blend. The slurry is transferred to a centrifugal casting machine to form the raw green body. The mold containing this green body is then placed in a warm water bath or an electric oven for a specified duration. Finally, the green body is extracted from the mold and subjected to a sintering process in an electric furnace. Depending on the particle size and sintering temperature, the sintered ceramic membrane is well-suited as a support, microfilter membrane, or ultrafilter membrane. The resulting ceramic membrane exhibits suitable strength, porosity, and permeability, making it appropriate for a wide range of membrane filtration applications.

Description

Fabrication of ceramic membranes using a natural gelling agent by the centrifugal gel casting method.

[0001] The present invention pertains to a novel method for manufacturing ceramic membranes with a controlled pore size distribution throughout the membrane thickness. Ceramic membranes produced by this method can be applied in water and wastewater treatment as well as in the food, pharmaceutical, chemical, oil and gas, electronics and environmental sectors. The improved fabrication process enhances membrane performance and efficiency by ensuring controlled pore size distribution and relatively high porosities, thereby reducing costs and extending the lifespan of the membranes.Technical Problem and Statement of Objectives:

[0002] In recent years, there has been a growing interest in the production of inorganic (mineral) membranes for processes where the conditions of temperature, pressure, and chemical environment preclude the use of polymer membranes. These membranes have become an important segment of the market due to their superior properties compared to polymer alternatives. Polymer membranes can generally withstand temperatures between 100 to 300 degrees Celsius, whereas inorganic materials such as ceramics (e.g. silicon carbide, zirconium oxide, titanium oxide, aluminum oxide) can endure much higher temperatures and are ideal for use in extremely alkaline or highly acidic conditions. The development of inorganic membranes for microfiltration and ultrafiltration began in the early 1980s, and these membranes are now available on a commercial scale. Currently, the manufacture of asymmetric ceramic membranes typically involves multiple stages. Essentially, porous ceramic membranes usually consist of a composite structure, including a porous support, one or more porous intermediate layers, and an effective membrane layer. In many instances, the production of the support (support / intermediate layers) can be accomplished in a single stage. One method that enables the production of a composite structural membrane (support / intermediate layers) with a continuous pore size distribution (functionally graded) in a single ceramic process step is centrifugal casting. This holds tremendous industrial significance. Typically, the cost of producing composite (multi-layer) ceramic membranes is high. Thus, a method that can establish a desired distribution of pore sizes throughout the membrane thickness in a single step could substantially reduce the costs of ceramic membranes mass-production. Researchers have employed this method to fabricate ceramic membranes, but a recurring element in all articles and patents is the use of powders smaller than 5 micrometers. Consequently, the resulting membrane typically experiences a high pressure drop during the separation process. Moreover, the suspension used for producing graded ceramic membranes often incorporates polymers like polyvinyl alcohol, ammonium polymethacrylate, and similar binders, requiring a very high centrifugal force for shaping the desired membrane due to the increased viscosity of the solution, which practically raises the production costs. Therefore, we need the slurry used in this method to allow particles larger than 8 microns to easily remain suspended and not settle down. To achieve this, the gel casting method can be employed. Gel casting technology was first utilized by a group of researchers at the Massachusetts Technology Center in 1987 for shaping ceramics using the tape casting process. They managed to obtain silicone tapes with excellent properties in the raw state through simultaneous shaping and polymerization. The monomer used was butyl methacrylate, which formed cross-links with the co-monomer methylene bisacrylamide, offering controlled particle distribution in the produced tapes and eliminating the need for solvent removal from the part, which was one of the advantages of using this method. The US 4894194 patent first used the term gel casting in 1990. By using monomers soluble in organic solvents and creating a fluid mixture of ceramic powder and monomer solution, transferring them to a mold, and applying heat, the ceramic powder was stabilized in a network of cross-linked polymers. The raw part obtained from this method was machinable in its raw state. Additionally, Omata and his colleagues in the same year (10.1002 / 9780470313831.ch26), due to problems in drying and removing organic solvents from the cast part, employed aqueous gel casting. Thus, gel casting technology can be broadly applied in manufacturing parts from a single type of ceramic powder to complex composite parts from various ceramic powders, and since the operations necessary for filling the mold—namely, the preparation of the slurry and its flow—are separate from the stabilization mechanism—the transformation of the ceramic slurry into a raw part—gel casting does not possess the major disadvantages and fundamental flaws of previous methods. In this method, a high volumetric load of ceramics in the slurry plays a significant role in increasing the density of the raw part, which aids in firing and achieving the maximum final density—close to the theoretical density—in the fired part.

[0003] Therefore, the advantages of the gel casting method generally include a high loading rate of ceramic powder, very high strength of the raw part, machinability of the raw piece, and the ability to create complex shapes in various sizes. However, as is evident, the conventional gel casting method requires a monomer, cross-linker, initiator, and often a reaction accelerator to form a gel. Moreover, the entire process is influenced by factors such as the concentration of the components and the temperature at which the reaction occurs. Thus, combining the traditional gel casting method with the centrifugal casting process results in a more complex method that significantly complicates the control and reproducibility of the ceramic membrane production process. Therefore, our goal is to use gel casting without the typical materials used in this process. In summary, the aim of this invention is to use natural egg white as a natural gel, which consistently exhibits stable properties. For instance, egg white typically maintains a constant viscosity, and the primary factor affecting its gelation process is temperature. Therefore, it is influenced by far fewer parameters compared to using amide monomers or similar substances for gelation. Additionally, this method is highly cost-effective and easy to control in the manufacturing process of ceramic membranes.

[0004] Background Knowledge and Previous Advances Related to the Claimed Invention:

[0005] The Chinese patent number ZL95102519.8 describes a method and device for producing high-purity ceramic tubular membranes using a dual-layer mold with a rigid outer layer and a meltable inner layer. The process begins by introducing the slurry into the mold and subjecting it to centrifugation. As the rotation speed increases gradually, alumina particles in the slurry adhere to the inner wall of the mold, starting with larger particles and progressively getting smaller. Subsequently, the raw molded membrane is subjected to high-temperature sintering, resulting in an asymmetric alumina base with high purity. However, it is essential to note that this method requires the removal of the meltable inner layer mold at temperatures between 50 to 100 degrees Celsius, thereby increasing preparation costs.

[0006] The patent document WO 9821164 introduces a graded ceramic membrane. This invention utilizes colloidal instability techniques to create gradient structures using a suspension containing multiple particle size distributions. Creating a controllable colloidal suspension with a broad range of particle sizes is crucial. With this method, during the settling process, particles of different sizes settle at varying rates due to gravity, resulting in a porous ceramic material with a distributed pore size. However, this method is limited to producing flat membranes and cannot be used to make tubular membranes.

[0007] The Chinese patent ZL02144131.6 introduces an innovative method for producing thin-walled ceramic tubes using a centrifugal molding device. However, this process involves the use of acrylamide monomer, a highly toxic neurotoxin, which may pose limitations for industrial applications. The method comprises several steps: first, alpha alumina is ground into a fine powder. Then, this powder is mixed in a gel injection molding solution and stirred under vacuum to create a uniform suspension slurry. This slurry is then injected into the tank of the centrifugal molding device. The molded sheet is dried and becomes porous, and finally, a thin-walled ceramic tube is obtained. The resulting ceramic tube has a density in the range of 3.89-3.92 grams per cubic centimeter.

[0008] The method for preparing a ceramic with a gradient pore size, disclosed in Chinese patent ZL200610011551.3, includes the following steps: initially, deionized water is added to ceramic powder to prepare a slurry via ball milling. Then, deionized water is added to baked organic materials, and the slurry is prepared using ultrasonic dispersion or ball milling. In the next step, the slurry is rapidly stirred in a static vacuum mixer equipped with a pre-designed mold. This mixture undergoes processes of baking, molding, drying, and sintering to produce ceramic materials with a distribution of pore sizes that have continuous pores.

[0009] Proposed Solution for Existing Technical Problems with Detailed Description:

[0010] The aim of this invention is to introduce a novel method for producing ceramic membranes by combining centrifugal casting and gel casting techniques, in a manner that avoids the complexities and high costs associated with conventional centrifugal methods and the use of expensive raw materials for preparing the initial solution. A major limitation of the centrifugal process is its reliance on finely granulated powders, which complicates the production of ceramic membranes with high permeability. Furthermore, this method often requires high centrifugal forces, making it economically inefficient. On the other hand, traditional gel casting not only necessitates the use of carcinogenic substances such as acrylamide but also involves materials like cross-linking agents, initiators, and accelerators, and is highly temperature-sensitive, presenting significant challenges in controlling the membrane fabrication process. In this invention, we propose to use raw egg white as a natural gel and combine these two methods at low speeds to produce ceramic membranes. This approach aims to simplify the manufacturing process while avoiding the use of hazardous chemicals and reducing dependency on costly materials and high-energy inputs.Technical Solution for This Invention:

[0011] The technical solution proposed in this invention involves the following steps for producing ceramic membranes using egg white:

[0012] 1. Slurry Preparation:

[0013] Initially, egg white (used as a binder), ceramic powder, and an antifoam agent are mixed using a mechanical mixer to create a homogeneous ceramic slurry.

[0014] 2. Centrifugal Gel Casting Method:

[0015] The prepared slurry is transferred into a cylinder, which is then placed in a centrifuge with adjustable speeds. After a set period, the cylinder is moved to a temperature-controlled bath to complete the gelation process.

[0016] 3. Membrane Removal:

[0017] The raw ceramic membrane is taken out of the cylinder. The cylinder containing the ceramic membrane is positioned between two clamps, and using a mandrel, whose outer diameter matches the inner diameter of the cylinder, the membrane is pressed out.

[0018] 4. Drying:

[0019] The fabricated ceramic membrane is air-dried or placed at a temperature of 40 degrees Celsius for 24 hours.

[0020] 5. Sintering:

[0021] The dried raw ceramic membrane is then sintered to the desired temperature in a furnace and allowed to naturally cool to room temperature, resulting in a sintered ceramic membrane.

[0022] The ceramic powder used can be one of the following: silicon carbide, aluminum oxide, zirconium oxide, titanium oxide, silicon oxide, kaolin, or a combination thereof. Egg white acts as both a gelling agent and a porogen. The egg whites are obtained from standard eggs used daily. To extract the egg white, the eggs are cracked, and the whites are separated without any filtration and directly used in the solution preparation. At this stage, commercially available natural egg white can also be utilized. To prevent foaming during mechanical mixing, a very small amount of water-based silicone antifoam is used. The cylinder used in this invention can be made of steel, glass, or plastic with a highly polished internal surface, which is lubricated using a cloth soaked in silicone oil or other silicone or non-silicone-based sprays to facilitate easy membrane removal. For gelation, either a water bath at a specific temperature or an oven can be used.

[0023] The slurry composition by weight percentage is as follows:

[0024] - Ceramic powder: 30-95%

[0025] - Egg white: 1-50%

[0026] - Silicone antifoam: 0-10%

[0027] - The ceramic powder can include alumina powder, silica powder, zirconia powder, titanium dioxide powder, silicon carbide powder, kaolin powder, or any mixture of these powders with an average particle size of 0.1-100 micrometers.

[0028] During initial mixing, the antifoam is added to the slurry to prevent foam formation. These antifoams can be silicone-based or non-silicone. After the desired slurry is prepared, it is transferred into the cylinder. The centrifugal force used can range from 500 to 9500 revolutions per minute, and the centrifugation process can last from 1 to 45 minutes. After centrifugation, any remaining solution is drained, and the cylinder, sealed at both ends, is placed in an oven or water bath at temperatures ranging from 40 to 85 degrees Celsius for 1 to 60 minutes. The amount of slurry used in the centrifugal gel casting process directly correlates with the cylinder volume and subsequently with the required thickness of the ceramic membrane. The larger the cylinder, the more slurry is needed and vice versa. If the cylinder is completely filled, the resulting membrane thickness will be substantial. Typically, depending on the initial powder loading, 50 to 85 percent of the cylinder's volume is filled.

[0029] In the third step, the cylinder containing the ceramic membrane is placed between two clamps, and using a mandrel (made of plastic, metal, or metal alloy) with an outer diameter equal to the inner diameter of the cylinder, the membrane is pressed and taken out. Since the inner surface of the cylinder has been previously coated with silicone or non-silicone lubricants, the membrane is removed without any difficulty.

[0030] In the fourth step, after removing the ceramic membrane, it is air-dried for 1 to 24 hours, during which 20-25% of its moisture content is naturally lost, and shrinkage is limited to 0.5-3%. Alternatively, the membrane can be placed in an oven at a constant temperature of 30-85 degrees Celsius for 1 to 72 hours.

[0031] In the fifth step, the membrane is initially heated in an electric furnace at a rate of 1-10 degrees per minute to temperatures ranging from 120 to 550 degrees Celsius and is sintered for 0.5 to 10 hours, then heated further at a rate of 1-10 degrees per minute to temperatures up to 2000 degrees Celsius and sintered for an additional 1 to 10 hours. During this phase, any binder in the ceramic membrane structure burns off, creating porosity and forming pores.

[0032] Generally, the ceramic membrane produced may exhibit a pore size gradient along its thickness, with the outer surface featuring the largest pores and the inner surface having the smallest pores. This gradient is dependent on the average particle size of the powder used in the membrane, among others.

[0033] Providing more detailed information using practical examples:

[0034] To better understand the functionality of the invention, we will elaborate it using practical examples. The materials used in the following examples are:

[0035] - Alpha alumina and silicon carbide ceramic powders, either laboratory or industrial grade, with particle sizes ranging from 0.1 to 100 microns and purity levels from 90 to 99 percent.

[0036] - Natural egg white: obtained directly from everyday eggs without any purification.

[0037] - Industrial-grade water-based silicone antifoam.Example 1

[0038] The fabrication of an ceramic membrane using the centrifugal gel casting method involves five main steps: slurry preparation, centrifugal gel casting, demolding, drying, and sintering. The primary and secondary steps are illustrated in

[0039] Slurry Preparation:

[0040] First, antifoam is added to the raw egg white, and finally, ceramic powder is gradually added. The mass percentage composition of the solution is as follows: 47.7% silicon carbide powder with an average diameter of 10 microns, 52% raw egg white, and 0.3% water-based silicone antifoam.

[0041] Centrifugal Gel Casting:

[0042] 40 milliliters of the slurry are transferred into a cylindrical steel mold (18.5 cm in length and 17 mm in inner diameter). The mold is placed horizontally in the centrifugal casting device () and rotated for 20 minutes at a speed of 3200 rpm. The remaining solution is then removed, and the cylindrical mold containing the raw ceramic membrane is placed in a water bath at 80 degrees Celsius for 10 minutes.

[0043] Demolding:

[0044] After the egg white gelation process within the raw membrane is complete, the membrane is removed using a mandrel () with an outer diameter equal to the inner diameter of the cylinder containing the raw ceramic membrane.

[0045] Drying:

[0046] The extracted raw ceramic membrane is left in open air for 24 hours, during which it loses approximately 20-25% of its weight without any longitudinal or radial shrinkage.

[0047] Sintering:

[0048] The dried raw ceramic membrane is placed in a temperature-controlled furnace. The heating process starts from room temperature and gradually increases at a rate of 2 degrees Celsius per minute until it reaches 120 degrees Celsius, where it is held for 30 minutes. The temperature then increases at the same rate to 350 degrees Celsius and is held for another 30 minutes. The heating process continues, raising the temperature to 550 degrees Celsius for one hour. In the next stage, the temperature rises at a rate of 3 degrees Celsius per minute to 1450 degrees Celsius and is maintained at this temperature for 3 hours. Finally, the membrane is naturally cooled down to room temperature.

[0049] The produced ceramic membrane has a length of 18 cm, an outer diameter of 17 mm, and a thickness of 2 mm. Additionally, Figures 4 and 5 show SEM images of the membrane's inner and outer surfaces, respectively. There is no significant difference in the particle size distribution between Figures 4 and 5.presents the mercury porosimetry results of the silicon carbide ceramic membrane, showing a pore size distribution ranging from 5 micrometers to 10 nanometers. Furthermore, the three-point bending test results for this membrane indicate a flexural strength of 12.54 MPa, demonstrating its excellent strength for use as a ceramic membrane. The average pore diameter in this membrane is 2.8 microns with an open porosity of 41%, making it suitable as a support for manufacturing microfiltration and ultrafiltration ceramic membranes.Example 2

[0050] First, the raw egg white and water-based silicone antifoam are mixed using a mechanical stirrer. Then, alpha alumina powder with an average particle size of less than 2 microns is gradually added to the slurry. Following this, alpha alumina powder with an average particle size of 20 microns is gradually added to the slurry. The weight percentages of the slurry components are as follows: 49.1% alumina powder (weight ratio of fine to coarse alumina is 1:10), 48.4% natural egg white, and 2.5% silicone antifoam.

[0051] Centrifugal Gel Casting:

[0052] 25 milliliters of the slurry are transferred into a cylindrical steel mold and placed horizontally in the centrifugal casting device () and the mold is rotated for 20 minutes at 3200 rpm. The remaining solution is then removed, and the cylindrical mold containing the raw ceramic membrane is placed in a water bath at 80 degrees Celsius for 10 minutes.

[0053] Demolding:

[0054] After the egg white gelation process within the raw membrane is complete, the membrane is removed using a mandrel with an outer diameter equal to the inner diameter of the cylinder containing the raw ceramic membrane.

[0055] Drying:

[0056] The extracted raw ceramic membrane is left in open air for 24 hours, during which it loses approximately 20-25% of its weight without any noticeable longitudinal or radial shrinkage.

[0057] Sintering:

[0058] The dried raw ceramic membrane is placed in a temperature-controlled furnace. The heating process starts from room temperature and gradually increases at a rate of 2 degrees Celsius per minute until it reaches 120 degrees Celsius, where it is held for 30 minutes. The temperature then increases at the same rate to 350 degrees Celsius and is held for another 30 minutes. The heating process continues, raising the temperature to 550 degrees Celsius for one hour. In the next stage, the temperature rises at a rate of 3 degrees Celsius per minute to 1450 degrees Celsius and is maintained at this temperature for 3 hours. The resulting ceramic membrane has a length of 8 cm, an outer diameter of 17 mm, and a thickness of 2.5 mm, and it features a fully asymmetric structure.

[0059] As shown in, the inner surface has much finer particles compared to the outer surface, as seen inAdditionally, Figures 9a and 9b show SEM images of the cross-section of the fabricated membrane. In Figure 9b (higher magnification), a distinct layer of finer particles on the inner wall (top part of the image) is evident.illustrates the pore size distribution of the fabricated ceramic membrane. Based on Figures 9 and 10, it is clear that the ceramic membrane not only has a pore size gradient but is also constructed from two completely separate layers that have been sintered simultaneously. Depending on the particle sizes used in the membrane, it can be utilized as a microfilter or ultrafilter.Example 3:

[0060] First, the raw egg white is mixed with silicone antifoam using a mechanical stirrer. Then, beta-silicon carbide powder with an average particle size of 2 microns is gradually added, followed by beta-silicon carbide powder with an average particle size of 45 microns. The weight percentages of the slurry components are as follows: 43.6% silicon carbide powder (with a weight ratio of fine to coarse silicon carbide being 1:10), 54.23% natural egg white, and 2.17% silicone antifoam.

[0061] Centrifugal Gel Casting:

[0062] 22 milliliters of the slurry are transferred into a cylindrical steel mold and placed horizontally in the centrifugal casting device () and the mold is rotated for 20 minutes at a speed of 3200 rpm. The remaining solution is then removed, and the cylindrical mold containing the raw ceramic membrane is placed in a water bath at 80 degrees Celsius for 10 minutes.

[0063] Demolding:

[0064] After the egg white gelation process within the raw membrane is complete, the membrane is removed using a mandrel with an outer diameter equal to the inner diameter of the cylinder containing the raw ceramic membrane.

[0065] Drying:

[0066] The extracted raw ceramic membrane is left in open air for 24 hours, during which it loses approximately 20-25% of its weight without any noticeable longitudinal or radial shrinkage.

[0067] Sintering:

[0068] The dried raw ceramic membrane is placed in a temperature-controlled furnace. The heating process starts from room temperature and gradually increases at a rate of 2 degrees Celsius per minute until it reaches 120 degrees Celsius, where it is held for 30 minutes. The temperature then increases at the same rate to 350 degrees Celsius and is held for another 30 minutes. The heating process continues, raising the temperature to 550 degrees Celsius for one hour. In the next stage, the temperature rises at a rate of 3 degrees Celsius per minute to 1450 degrees Celsius and is maintained at this temperature for 3 hours.

[0069] The resulting ceramic membrane has a length of 8 cm, an outer diameter of 16.7 mm, and a thickness of 2.2 mm. According to the electron microscope analysis, the fabricated membrane has an asymmetric structure. Additionally, the membrane has an average pore size of 9.47 microns, a porosity of 31.4%, and a deionized water permeability of 28000 liters per square meter per hour per bar.Example 4

[0070] First, the raw egg white is mixed with silicone antifoam using a mechanical stirrer. Then, zirconia oxide powder with an average particle size of 1 micron and beta-silicon carbide powder with an average particle size of 10 microns are gradually added. The weight percentages of the slurry components are as follows: 46.3% silicon carbide powder (with a weight ratio of zirconia oxide to silicon carbide being 1:20), 51.2% natural egg white, and 2.5% antifoam.

[0071] Centrifugal Gel Casting:

[0072] 25 milliliters of the slurry are transferred into a cylindrical steel mold and placed horizontally in the centrifugal casting device () and the mold is rotated for 20 minutes at a speed of 3200 rpm. The remaining solution is then removed, and the cylindrical mold containing the raw ceramic membrane is placed in a water bath at 80 degrees Celsius for 10 minutes.

[0073] Demolding:

[0074] After the egg white gelation process within the raw membrane is complete, the membrane is removed using a mandrel with an outer diameter equal to the inner diameter of the cylinder containing the raw ceramic membrane.

[0075] Drying:

[0076] The extracted raw ceramic membrane is left in open air for 24 hours, during which it loses approximately 20-25% of its weight without any noticeable longitudinal or radial shrinkage.

[0077] Sintering:

[0078] The dried raw ceramic membrane is placed in a temperature-controlled furnace. The heating process starts from room temperature and gradually increases at a rate of 2 degrees Celsius per minute until it reaches 250 degrees Celsius, where it is held for one hour. The temperature then increases at the same rate to 600 degrees Celsius and is held for another one and a half hours. In the next stage, the temperature rises at a rate of 3 degrees Celsius per minute to 1150 degrees Celsius and is maintained at this temperature for 2 hours.

[0079] The resulting ceramic membrane has a length of 8 cm, an outer diameter of 16.7 mm, and a thickness of 2.7 mm. According to the electron microscope analysis, the fabricated membrane has an asymmetric structure. Additionally, the composite membrane has an open porosity of 48.5%, an average pore size of 2.2 microns, and a pore size distribution ranging from 8 microns to 50 nanometers.Explanation of Figures, Diagrams, and Schematics

[0080] : A schematic overview of the process for fabricating asymmetric ceramic membranes using the centrifugal gel casting method.

[0081] : The device constructed for the centrifugal casting process, consisting of 12 main components, with each component's function described below:

[0082] - Component (1): The chassis that holds the entire system to prevent vibrations and ensure it is perfectly leveled.

[0083] - Component (2): The driver or inverter, used to turn the device on and off and to adjust the speed of the electric motor.

[0084] - Component (3): The electric motor that converts electrical energy into mechanical energy and transmits the required rotational force via the shaft to the belt (5) and pulleys (4) and (6).

[0085] - Components (4) and (6): Pulleys used for transmitting rotation with variable axial distances between the driving and driven components.

[0086] - Component (5): A V-belt resistant to vibrations, used for high-speed power transmission.

[0087] - Component (7): Holds the cylindrical mold containing the slurry.

[0088] - Components (8) and (9): Designed to prevent potential deviations of the cylinder during rotation.

[0089] - Component (10): The cylindrical mold containing the slurry, placed between components (7) and (11).

[0090] - Component (11): Holds the cylindrical mold containing the slurry.

[0091] - Component (12): Used to secure the cylindrical mold during rotation.

[0092] : The diagram of the mandrel used for taking out the raw ceramic membrane from the metal cylinder.

[0093] : SEM image of the inner surface of the silicon carbide ceramic membrane sintered at 1450°C in an air atmosphere (related to Example 1).

[0094] : SEM image of the outer surface of the ceramic membrane sintered at 1450°C in an air atmosphere (related to Example 1).

[0095] : This image shows the pore size distribution using mercury porosimetry in the silicon carbide membrane fabricated at 1450°C (related to Example 1).

[0096] : SEM image of the inner surface of the asymmetric alpha-alumina ceramic membrane sintered at 1450°C in an air atmosphere (related to Example 2).

[0097] : SEM image of the outer surface of the asymmetric alpha-alumina ceramic membrane sintered at 1450°C in an air atmosphere (related to Example 2).

[0098] : SEM image of the cross-section of the asymmetric alpha-alumina ceramic membrane sintered at 1450°C in an air atmosphere (related to Example 2).

[0099] : This image shows the pore size distribution using mercury porosimetry in the silicon carbide membrane fabricated at 1450°C (related to Example 2).

[0100] Clear and Precise Statement of the Advantages of the Invention:

[0101] The advantages of this invention include:

[0102] - Natural Gelation Agent: Egg white acts as a natural gelation agent, requiring only heat for activation. This makes it an environmentally friendly and economically viable option compared to other materials used in gel casting processes, such as amides, acrylic acid, and other binders.

[0103] - Consistent Viscosity: The viscosity of egg white generally remains within a narrow range, making it easy to work with. This means that the process control parameters for its gelation are much fewer compared to using amides for gel casting, for instance.

[0104] - Binder and Porogen: Besides being an excellent binder that provides high mechanical strength to the raw ceramic membranes, egg white also acts as a porogen, creating porosity within the membrane structure.

[0105] - No Need for Ultrasonic Defoaming: The slurry does not require ultrasonic defoaming because the centrifugal force used in the process effectively removes any bubbles formed in the slurry.

[0106] - Particle Size Flexibility: A significant limitation of the centrifugal casting method has been the use of powders with sizes less than 7 micrometers. Larger particles become difficult to stabilize, even with large amounts of dispersants. However, egg white can overcome this limitation. Its viscosity can trap particles and prevent them from settling during stirring, allowing the use of powders with particle sizes from 7 micrometers up to 100 micrometers without the need for dispersants.

[0107] - Simplified Drying Process: One of the critical stages in ceramic part fabrication using gel casting is the initial drying process. Typically, this requires desiccant solutions or special conditions to reduce shrinkage and prevent longitudinal and radial cracks, making it complex. Using egg white as the gelation agent simplifies this, allowing the drying process to be done effortlessly in open air. The ceramic membrane produced by this method loses 20-25% of its weight with minimal shrinkage (1-3%), the lowest among all ceramic membrane fabrication methods.

[0108] - Smooth Surface: The inner and outer surfaces of the ceramic membrane produced using this method are very smooth. Depending on the particle sizes used, it can be employed in microfiltration, ultrafiltration processes, or as a base for ceramic membranes.

Claims

Claim 1: A novel method for fabricating ceramic membranes comprising the following steps:- Step 1: Mixing raw egg white, silicone antifoam, and ceramic powder in a beaker using a mechanical stirrer.- Step 2: Coating the inner surface of a cylindrical mold with silicone or non-silicone lubricants.- Step 3: Pouring the prepared slurry into the cylinder and placing it horizontally in a centrifugal casting device.- Step 4: After the centrifugal casting process, removing the remaining solution from the cylinder and placing the cylinder containing the raw ceramic membrane in a water bath to complete the gelation process.- Step 5: Extracting the raw ceramic membrane using a mandrel and drying it in open air or an electric oven.- Step 6: Heating the dried ceramic membrane in an electric furnace to the desired temperature and then allowing it to cool naturally to room temperature.Claim 2: In the method described in Claim 1, the egg white can be obtained from freshly broken eggs or pre-prepared egg white. The weight percentage used can range from 1-50% based on the ceramic powder weight.Claim 3: In the method described in Claim 1, the antifoam used can be silicone or non-silicone, water-based or oil-based, with an amount ranging from 0.1-10% by weight of the ceramic powder.Claim 4: In the method described in Claim 1, the ceramic powder can be aluminum oxide, silicon carbide, silicon oxide, zirconia oxide, kaolin, titanium oxide, or a mixture of these powders, with an amount ranging from 20-90% by weight of the solution.Claim 5: In the method described in Claim 1, the lubricants can be applied as a spray or liquid.Claim 6: In the method described in Claim 1, the centrifuge speed can range from 100 to 9500 rpm, and the centrifuge time can vary from 1 to 45 minutes.Claim 7: In the method described in Claim 1, the water bath temperature can range from 35 to 100 degrees Celsius, and the cylinder containing the raw ceramic membrane can remain in the bath for 1 to 65 minutes.Claim 8: In the method described in Claim 1, the cylinder containing the ceramic membrane is placed between two clamps and the membrane is extracted by pressing a mandrel, which has been pre-coated with a lubricant spray, into the cylinder. The mandrel 's outer diameter is equal to the inner diameter of the cylinder containing the raw ceramic membrane.Claim 9: In the method described in Claim 1, the drying process for the membrane can be done in open air or in an oven. The drying temperature can range from 25 to 100 degrees Celsius, and the drying time can vary from 1 to 50 hours.Claim 10: In the method described in Claim 1, the shrinkage of the dried membrane ranges from 0.5 to 3 percent.Claim 11: In the method described in Claim 1, the cylindrical mold can be made of steel, glass, or polymer.Claim 12: In the method described in Claim 1, the dried ceramic membrane undergoes thermal treatment for sintering as follows: it is heated at a rate of 1-10 degrees Celsius per minute to a temperature of 100-200 degrees Celsius and held for 1-120 minutes. It is then heated at the same rate to a temperature of 400-600 degrees Celsius and held for 1-180 minutes. Finally, it is heated at a rate of 1-10 degrees Celsius per minute to a temperature of 1200-2100 degrees Celsius and held for 0-20 hours. The membrane is then allowed to cool naturally to room temperature.

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