Non-burned cement separation membrane with connected gradient pore structure and preparation method and application thereof

Abstract

The application provides a baking-free cement separation membrane with a connected gradient pore structure and a preparation method and application, and belongs to the technical field of separation membrane materials. The preparation method comprises the following steps: mixing a cement-based cementitious material, a mineral admixture, a frozen pore-forming medium, a binder and a dispersant, ball milling to obtain a cement slurry, and then performing directional freezing treatment on the cement slurry to form frozen medium crystals arranged along the freezing direction in the interior of the cement slurry, performing freeze-drying to obtain a porous cement membrane green body, and then performing curing treatment on the porous cement membrane green body, so that the baking-free cement separation membrane can be obtained after the curing treatment is completed. The cement separation membrane with the connected gradient pore structure can be prepared without high-temperature sintering, the introduction of the mineral admixture further improves the permeability and separation efficiency of the separation membrane material, the good structural stability and mechanical properties are maintained, and the problems of the existing cement separation membrane, such as difficult adjustment of the pore structure and insufficient filtration performance, are solved.

Description

Technical Field

[0001] This invention relates to the field of separation membrane materials technology, and in particular to a non-fired cement separation membrane with a connected gradient pore structure, its preparation method, and its application. Background Technology

[0002] Membrane-based separation technology is widely used in industrial processes for material separation, purification, and concentration due to its high separation efficiency, good stability, and low operating costs. Among various separation membrane materials, ceramic filter membranes are used in separation processes under high-temperature and corrosive media conditions because of their high mechanical strength, good chemical stability, and strong environmental adaptability. However, ceramic filter membranes are usually prepared by high-temperature sintering, typically above 900℃, resulting in high energy consumption during the preparation process. Furthermore, the high cost of raw materials and complex preparation processes limit their further widespread application.

[0003] To reduce the manufacturing cost of separation membranes, existing research has used cement-based materials to replace traditional ceramic materials in the preparation of porous separation membranes. Cement materials have the advantages of wide availability of raw materials, low cost, and no need for high-temperature sintering. In existing technologies, some studies have used silicate cement and quartz as raw materials to prepare porous cement-based supports through dry pressing and curing, verifying the feasibility of cement materials as membrane materials. However, in this type of method, the pore structure is mainly determined by the compaction voids formed during the molding process and the pores generated by hydration shrinkage. The pore size distribution and pore connectivity are difficult to adjust, and a long curing time is required to achieve the strength required for use. To improve the pore structure of cement membranes, some studies have used cryogenic casting to prepare porous cement membranes. However, existing silicate cement membranes usually require a long curing time, and the hydration products generated during the continuous hydration of silicate cement tend to gradually fill the pores, reducing the pore size of the membrane material and leading to a decrease in pure water permeation flux. In addition, the method of directional freezing using a cooling bath has high requirements for temperature control and equipment conditions, while when using a heat-drying curing method, rapid moisture migration can easily cause pore wall shrinkage, local structural damage, and drying cracking, which in turn affects the pore structure and filtration performance of the membrane material.

[0004] Compared to silicate cement, calcium aluminate cement exhibits rapid hardening and early strength characteristics, achieving sufficient strength within a shorter curing time. Furthermore, the composition of its hydration products and pore structure are influenced by curing conditions, providing a new approach for adjusting the pore structure of cement-based separation membranes. However, current research on calcium aluminate cement-based separation membranes is relatively lacking, especially in the area of ​​directly preparing cement separation membranes using methods such as low-temperature freezing, vacuum freeze-drying, and mineral admixture adjustment without the use of cooling bath equipment. Research on optimizing pore structure, improving pore connectivity, and achieving high permeability and suitable mechanical strength remains insufficient.

[0005] Therefore, it is necessary to propose a method for preparing a non-fired cement separation membrane with a connected gradient pore structure to solve the technical problems existing in the prior art. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a non-fired cement separation membrane with a connected gradient pore structure, its preparation method, and its application. This invention enables the preparation of cement separation membranes with gradient pore structures without the need for high-temperature sintering. By optimizing the pore structure, the permeability and separation efficiency of the membrane material are improved, while maintaining good structural stability and mechanical properties. This solves the technical problems existing in the preparation of existing cement separation membranes, such as limited pore structure formation methods, difficulty in adjusting pore size distribution, and insufficient filtration performance.

[0007] To achieve the above objectives, the present invention provides the following solution: In a first aspect, the present invention provides a method for preparing a non-fired cement separation membrane with a connected gradient pore structure, comprising the following steps: S1. Cement slurry is prepared by mixing cement-based cementitious materials, mineral admixtures, cryogenic pore-forming media, binders and dispersants, and then ball milling the mixture. S2. The cement slurry is subjected to directional freezing treatment to form freezing medium crystals arranged along the freezing direction inside the cement slurry. After freeze-drying treatment, a porous cement membrane green body is obtained. S3. The porous cement membrane green body is cured to allow the cement-based cementitious material to undergo hydration and solidification. Through the hydration reaction of the cement-based cementitious material, a cement-based skeleton with a connected gradient pore structure is formed, thereby obtaining a non-fired cement separation membrane with a connected gradient pore structure.

[0008] Preferably, in step S1, the cement-based cementitious material includes calcium aluminate cement, and the mineral admixture is used to partially replace the calcium aluminate cement. The mineral admixture is selected from one or two of silica fume, fly ash, slag, or quartz powder.

[0009] Preferably, the amount of mineral admixture added accounts for 5% to 40% of the total mass of the mixed powder composed of calcium aluminate cement and mineral admixture, and the total solid content of calcium aluminate cement and mineral admixture accounts for 30% to 60% of the total mass of cement slurry; when the mineral admixture includes two components, the mass ratio between the two mineral admixtures is 1:9 to 9:1.

[0010] Preferably, in step S1, the binder is one of sodium carboxymethyl cellulose, polyvinyl alcohol, or hydroxypropyl methyl cellulose, and the amount of binder added accounts for 0.5% to 1.5% of the total mass of calcium aluminate cement and mineral admixtures.

[0011] Preferably, in step S1, the dispersant is one of polycarboxylate-based high-efficiency water-reducing agent, naphthalene-based high-efficiency water-reducing agent, or lignin sulfonate-based water-reducing agent, and the amount of the dispersant added accounts for 0.5% to 2% of the total mass of calcium aluminate cement and mineral admixtures.

[0012] Preferably, in step S1, the cryogenic pore-forming medium is one or two of water, tert-butanol, or camphene, and the cryogenic pore-forming medium accounts for 40% to 70% of the total mass of the cement slurry; when the cryogenic pore-forming medium includes two components, the mass ratio between the two cryogenic pore-forming media is 1:9 to 9:1.

[0013] Preferably, in step S2, the temperature of the directional freezing treatment is -20 to -80°C, and the freezing time is 0.5 to 2 hours.

[0014] Preferably, in step S3, the curing treatment includes initial curing and immersion curing. The temperature for both initial curing and immersion curing is 10~60℃, the humidity of the curing environment is not less than 95%, and the initial curing time is 12~24h; the immersion curing time is 1~7d.

[0015] In a second aspect, the present invention provides a non-fired cement separation membrane with a connected gradient pore structure prepared according to the above preparation method, wherein the non-fired cement separation membrane comprises a cement-based continuous skeleton and a connected gradient pore structure distributed in the cement-based continuous skeleton. The pore size distribution of the non-fired cement separation membrane exhibits a bimodal characteristic, with the characteristic peak of small pores located at 0.1~0.35μm and the characteristic peak of large pores located at 8.0~17.52μm. The compressive strength of the non-fired cement separation membrane is 3~8MPa, and the porosity is 40%~55%. Under pure water test conditions of 25℃ and a transmembrane pressure difference of 1 bar, the permeation flux of the non-fired cement separation membrane is 350~4000 L·m. -2 ·h -1 After filtration for 30 minutes, the retention rate of oil-in-water emulsion was 90.8%~95%.

[0016] Thirdly, the present invention also provides an application of the above-mentioned non-fired cement separation membrane with gradient pore structure in fluid filtration and separation.

[0017] Compared with the prior art, the present invention discloses at least the following technical effects: (1) This invention uses a combination of directional freezing and freeze-drying to prepare cement separation membranes. During freezing, the freezing medium crystals grow along the freezing direction to form a porous structure template. After freeze-drying to remove the freezing medium crystals, a porous cement membrane green body is obtained. Then, a continuous cement-based skeleton is formed by the hydration and solidification of a cement-based cementitious material, resulting in a cement separation membrane with an interconnected gradient pore structure. This preparation method does not require high-temperature sintering, reducing energy consumption during the preparation process and giving the resulting membrane material good compressive strength.

[0018] (2) The present invention uses calcium aluminate cement as cement-based cementitious material and introduces mineral admixtures to adjust the composition and structure of the membrane material. By changing the composition of cement slurry and curing conditions, the cement hydration products and mineral admixtures form a stable skeleton together, improve the internal pore structure and pore connectivity of the membrane material, and make the obtained cement separation membrane have high permeation flux and good retention performance.

[0019] (3) By selecting freezing pore-forming medium, binder and dispersant, the present invention adjusts the dispersion state of cement slurry so that the slurry maintains good stability during freezing, which is conducive to the formation of freezing medium crystals and the maintenance of pore structure. At the same time, the preparation method of combining low temperature freezing, freeze drying and curing reduces the high temperature sintering process, simplifies the preparation process, reduces the input of materials and equipment, and makes the preparation process more energy-saving and environmentally friendly, with good prospects for engineering application. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart illustrating the preparation method of a non-fired cement separation membrane with a connected gradient pore structure according to the present invention.

[0022] Figure 2 The graph shows the change in pure water permeation flux of the non-fired cement separation membrane with a connected gradient pore structure prepared in Example 2 of the present invention under different water curing times.

[0023] Figure 3 The image shows a scanning electron microscope image of the surface morphology of the non-fired cement separation membrane with a connected gradient pore structure prepared in Example 5 of the present invention, where a and b are the microstructures of different regions on the surface of the cement separation membrane.

[0024] Figure 4The image shows a scanning electron microscope image of the cross-sectional morphology of the non-fired cement separation membrane with a connected gradient pore structure prepared in Example 5 of the present invention, where a, b and c are the microstructures of different regions of the cement separation membrane cross-section.

[0025] Figure 5 This is a pore size distribution diagram of the non-fired cement separation membrane with a connected gradient pore structure prepared in Example 5 of the present invention.

[0026] Figure 6 The figure shows the test results of the oil-water separation performance of the non-fired cement separation membrane with a connected gradient pore structure prepared in Example 5 of the present invention. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 As shown, the present invention provides a method for preparing a non-fired cement separation membrane with a connected gradient pore structure, comprising the following steps: S1. Cement slurry is prepared by mixing cement-based cementitious materials, mineral admixtures, cryogenic pore-forming media, binders and dispersants, and then ball milling the mixture.

[0030] Specifically, cement-based cementitious materials, mineral admixtures, cryogenic pore-forming media, binders, and dispersants are added and mixed in a set ratio to initially disperse the components evenly, and then ball milling is performed to prepare cement slurry.

[0031] The cement-based binder includes calcium aluminate cement, and mineral admixtures are used to partially replace calcium aluminate cement. The mineral admixtures are selected from one or two of silica fume, fly ash, slag, or quartz powder.

[0032] The amount of mineral admixture added accounts for 5% to 40% of the total mass of the mixed powder composed of calcium aluminate cement and mineral admixture, and the total solid content of calcium aluminate cement and mineral admixture accounts for 30% to 60% of the total mass of cement slurry. When the mineral admixture includes two components, the mass ratio between the two mineral admixtures is 1:9 to 9:1.

[0033] By adjusting the composition ratio of calcium aluminate cement to mineral admixtures, the composition state of solid particles in cement slurry can be changed, allowing the mineral admixtures to participate in the hydration reaction of cement-based cementitious materials and affecting the formation state of the cement-based skeleton.

[0034] Furthermore, the cryogenic pore-forming medium is one or two of water, tert-butanol, or camphene, and accounts for 40% to 70% of the total mass of the cement slurry. When the cryogenic pore-forming medium includes two components, the mass ratio between the two cryogenic pore-forming media is 1:9 to 9:1.

[0035] During the freezing process, the freezing medium undergoes a phase transition to form freezing medium crystals. These crystals act as pore-forming templates, influencing the distribution of cement particles and providing a basis for the formation of gradient pore structures.

[0036] The binder is one of sodium carboxymethyl cellulose, polyvinyl alcohol, or hydroxypropyl methyl cellulose, and the amount of binder added accounts for 0.5% to 1.5% of the total mass of calcium aluminate cement and mineral admixtures.

[0037] The dispersant is one of polycarboxylate-based high-efficiency water-reducing agents, naphthalene-based high-efficiency water-reducing agents, or lignin sulfonate-based water-reducing agents. The amount of dispersant added accounts for 0.5% to 2% of the total mass of calcium aluminate cement and mineral admixtures.

[0038] By adding binders and dispersants, the dispersion state of solid particles in cement slurry can be improved, the uniformity of the slurry can be increased, the slurry stratification during freezing can be reduced, and the crystals of the freezing medium can be stably formed.

[0039] In addition, the ball milling time is 0.5 to 3 hours. By controlling the ball milling time, cement particles and mineral admixtures are fully dispersed, avoiding insufficient particle dispersion due to too short a ball milling time, while also avoiding the ball milling time being too long and affecting the freezing and pore-forming state of the slurry.

[0040] S2. The cement slurry is subjected to directional freezing treatment to form freezing medium crystals arranged along the freezing direction inside the cement slurry. After freeze-drying treatment, a porous cement membrane green body is obtained.

[0041] Specifically, the cement slurry prepared in step S1 is subjected to directional freezing treatment, which causes the freezing pore-forming medium to freeze and form freezing medium crystals arranged along the freezing direction.

[0042] The directional freezing treatment involves temperatures ranging from -20°C to -80°C and freezing times from 0.5 to 2 hours. By controlling the freezing temperature and time, the freezing pore-forming medium freezes in a predetermined direction. The freezing medium crystals gradually grow and redistribute the surrounding cement particles, resulting in a directional pore structure within the slurry. After freezing, the samples undergo freeze-drying, which can be completed in 24 hours, or even longer (24–48 hours). During freeze-drying, the freezing medium crystals sublimate and are removed, transforming the original crystal-occupied areas into pore structures, thus obtaining a porous cement film green body.

[0043] S3. The porous cement membrane green body is cured to allow the cement-based cementitious material to undergo hydration and solidification. Through the hydration reaction of the cement-based cementitious material, a cement-based skeleton with a connected gradient pore structure is formed, thereby obtaining a non-fired cement separation membrane with a connected gradient pore structure.

[0044] Specifically, the porous cement membrane green body obtained in step S2 is cured to allow the cement-based cementitious material to undergo a hydration reaction, forming a continuous cement-based skeleton, and fixing the pore structure formed by freeze-drying.

[0045] The curing process includes initial curing and immersion curing. Both initial and immersion curing are performed at temperatures ranging from 10 to 60°C, with an ambient humidity of at least 95%, for a duration of 12 to 24 hours. Initial curing induces preliminary hydration of the cementitious materials, increasing the strength of the cement membrane green body and preparing it for immersion curing. After initial curing, the samples are immersed in water for 1 to 7 days. Immersion curing further hydrates the cementitious materials, and the hydration products gradually form a continuous skeletal structure, improving the structural stability of the membrane material. After immersion curing, the membrane can be soaked in anhydrous ethanol for displacement treatment or directly dried using a vacuum freeze dryer to obtain a non-fired cement separation membrane with a connected gradient pore structure.

[0046] Furthermore, the non-fired cement separation membrane prepared by the above method includes a cement-based continuous framework and a connected gradient pore structure distributed within the cement-based continuous framework. This non-fired cement separation membrane with a connected gradient pore structure can be applied in the field of fluid filtration and separation, specifically in the treatment of oily wastewater, removal and pretreatment clarification of suspended particles in industrial and municipal wastewater treatment, pretreatment filtration for seawater and brackish water desalination, liquid-solid separation in chemical processes, purification of flue gas condensate and reuse of circulating water, retention of microorganisms and colloids in biopharmaceutical and food and beverage processing, reuse of pulp and paper white water, and purification of circulating water and prefiltration of catalyst carriers in fuel cells.

[0047] The above content will be further described below through specific implementation methods.

[0048] Example 1 This embodiment provides a method for preparing a non-fired cement separation membrane with a connected gradient pore structure, specifically including the following steps: First, prepare the cement slurry. Select an average particle size d. 50 Calcium aluminate cement with a thickness of approximately 15 μm was used as a cement-based cementitious material. 57 g of calcium aluminate cement and 3 g of slag were added as solid components to a mixture consisting of 28 g of tert-butanol, 82 g of deionized water, 30 g of 1% hydroxypropyl methylcellulose solution, and 0.75 g of 40% polycarboxylate superplasticizer solution. After thorough mixing, the mixture was ball-milled for 2 hours to ensure uniform dispersion of the components, thus obtaining a calcium aluminate cement-based slurry.

[0049] Subsequently, the obtained calcium aluminate cement-based slurry was subjected to directional freezing treatment. Specifically, the slurry was placed in a directional freezing mold and frozen at -30°C for 1.5 hours, causing the formation of cryogenic medium crystals arranged along the freezing direction inside the slurry. After freezing, the frozen sample was demolded and transferred to a freeze dryer for vacuum freeze-drying for 24 hours to remove the cryogenic medium crystals and obtain a porous calcium aluminate cement membrane green body.

[0050] The obtained porous calcium aluminate cement membrane green body was cured and solidified. First, it was cured in a constant temperature and humidity environment of 35℃ and 95% for 12 hours to give the green body a certain strength; then it was immersed in deionized water at 35℃ for 5 days. After the immersion curing, it was dried by anhydrous ethanol replacement to obtain a non-fired cement separation membrane with an interconnected gradient pore structure.

[0051] The cement separation membrane prepared in this embodiment has a compressive strength of 4.3 MPa and a porosity of 53%. Its pore size distribution exhibits a bimodal characteristic, with a characteristic peak for small pores of approximately 0.25 μm and a characteristic peak for large pores of approximately 10.8 μm. Under pure water filtration conditions of 25°C and a transmembrane pressure difference of 1 bar, the pure water permeate flux of the membrane material is approximately 800 L·m³. -2 ·h -1 After filtration testing with an oil-in-water emulsion for 30 minutes, the oil phase retention rate reached 91.2%.

[0052] Example 2 The difference between this embodiment and Embodiment 1 is that mineral admixtures are added to the cement slurry to adjust the composition and pore structure of the cement-based materials.

[0053] Specifically, 50g of calcium aluminate cement and 5g of quartz powder are added as solid components to 50g of 1% hydroxypropyl methylcellulose solution and 2.5g of 40% polycarboxylate superplasticizer solution. After mixing and ball milling for 2 hours, calcium aluminate cement-based slurry containing mineral admixtures is obtained.

[0054] The obtained calcium aluminate cement-based slurry was subjected to directional freezing treatment at a temperature of -20℃ for 2 hours. After freezing, it was demolded and then subjected to vacuum freeze-drying for 48 hours to obtain a porous cement membrane green body.

[0055] Subsequently, the porous cement membrane preform was initially cured for 12 hours in an environment of 60°C and 95% relative humidity, and then transferred to 60°C deionized water for 1 day of immersion curing. After being dried by replacement with anhydrous ethanol, the non-fired cement separation membrane was obtained.

[0056] like Figure 2 As shown, the pure water permeation flux of the cement separation membrane prepared in this embodiment gradually decreases with increasing immersion curing time. Specifically, the membrane material exhibits a relatively high pure water permeation flux of approximately 4000 L·m⁻¹ after 1 day of immersion curing. -2 ·h -1 As the water conditioning time was extended to 3 days, 5 days, and 7 days, the pure water permeation flux decreased to approximately 3300 L·m⁻¹. -2 ·h -1 2600L·m -2 ·h -1 and 2400 L·m -2 ·h -1 .

[0057] The results above show that as the immersion curing time is further extended, the decrease in pure water permeation flux of the membrane material gradually decreases. The decrease in flux is more obvious when the curing time is extended from 1 day to 3 days and from 3 days to 5 days, while the decrease in flux is significantly reduced when the curing time is extended from 5 days to 7 days. This indicates that as the hydration reaction of the cement-based cementitious material continues, the internal pore structure of the membrane material gradually becomes stable, and the pure water permeation flux also gradually becomes stable.

[0058] Furthermore, similar trends were observed in previous experiments involving prolonged immersion curing of samples without mineral admixtures. Specifically, as the curing time increased from 14 days to 28 days and then to 56 days, the variation in pure water permeation flux of the membrane material decreased further, especially after 14 days of curing, when the pure water permeation flux generally stabilized. This result indicates that cement-based cementitious materials continuously hydrate and generate hydration products in the early stages of curing, which fill some pores and channels, thus causing a significant decrease in pure water permeation flux. As the curing time increases, the hydration reaction gradually slows down, the impact of newly added hydration products on the pore structure weakens accordingly, and the internal pore structure of the membrane material gradually stabilizes. Therefore, the decrease in pure water permeation flux gradually decreases and eventually stabilizes.

[0059] The membrane material obtained in this embodiment has a compressive strength of 3.5 MPa and a porosity of 55%. The characteristic peak for small pores is approximately 0.2 μm, and the characteristic peak for large pores is approximately 17.52 μm. Under conditions of 25°C and a transmembrane pressure difference of 1 bar, the pure water permeate flux after 1 day of curing is approximately 4000 L·m³. -2 ·h -1 The retention rate of the oil-in-water emulsion after filtration for 30 minutes reached 90.8%.

[0060] Example 3 This embodiment further employs composite mineral admixtures to adjust the membrane material structure.

[0061] Specifically, 54.7g of calcium aluminate cement, 6g of silica fume, and 6g of quartz powder are added to a mixture consisting of 62.5g of deionized water, 20g of a 5% polyvinyl alcohol solution, and 1.33g of lignin sulfonate water-reducing agent. After mixing, the mixture is ball-milled for 1.5 hours to obtain a cement slurry containing composite mineral admixtures.

[0062] The obtained cement slurry was frozen at -30℃ for 1.5 hours. After freezing to form cryogenic medium crystals, it was vacuum freeze-dried for 36 hours to obtain a porous cement membrane green body.

[0063] Subsequently, the membrane was initially cured for 18 hours at 25°C and 95% relative humidity, then soaked in deionized water at 25°C for 3 days, and finally dried by replacement with anhydrous ethanol to obtain the cement separation membrane.

[0064] Test results show that the membrane material has a compressive strength of 3.0 MPa, a porosity of 52%, a micropore characteristic peak of approximately 0.31 μm, and a macropore characteristic peak of approximately 10.5 μm. In pure water filtration tests, the permeate flux is approximately 820 L·m⁻². -2 ·h -1 The water-in-oil emulsion retention rate reached 92.6%.

[0065] Example 4 In this embodiment, fly ash is used as a mineral admixture.

[0066] Specifically, 54g of calcium aluminate cement and 36g of fly ash were added to a mixture consisting of 5g of deionized water, 56g of sodium carboxymethyl cellulose solution with a mass fraction of 1%, and 2.6g of polycarboxylate superplasticizer solution with a mass fraction of 40%. After mixing, the mixture was ball-milled for 0.5 hours to obtain cement slurry.

[0067] The obtained cement slurry was frozen at -20℃ for 2 hours and then subjected to vacuum freeze-drying for 48 hours. Subsequently, it was initially cured at 10℃ and 95% relative humidity for 24 hours, then immersed in deionized water at 10℃ for 7 days, and finally dried by replacement with anhydrous ethanol to obtain a cement separation membrane.

[0068] Testing revealed that the membrane material has a compressive strength of 5.1 MPa, a porosity of 40%, a micropore characteristic peak of approximately 0.1 μm, and a macropore characteristic peak of approximately 8.5 μm. Its pure water permeation flux is approximately 350 L·m⁻³. -2 ·h -1 The retention rate of the oil-in-water emulsion after filtration for 30 minutes reached 95.0%.

[0069] Example 5 This embodiment uses a composite cryogenic pore-forming medium to prepare a cement separation membrane.

[0070] Specifically, 69g of calcium aluminate cement and 11g of slag were added to a mixture consisting of 61.6g of deionized water, 8.67g of tert-butanol, 16g of a 5% polyvinyl alcohol solution, and 2g of a 40% polycarboxylate superplasticizer solution. After mixing, the mixture was ball-milled for 0.5h to obtain cement slurry.

[0071] The obtained cement slurry was frozen at -80℃ for 0.5h, and then vacuum freeze-dried for 36h to obtain a porous cement membrane green body.

[0072] The obtained porous cement membrane green body was initially cured for 18 hours in an environment of 35℃ and 95% relative humidity, and then immersed in deionized water at 35℃ for 5 days. After being dried by replacement with anhydrous ethanol, the non-fired cement separation membrane was obtained.

[0073] like Figure 3 As shown, the surface of the non-fired cement separation membrane prepared in this embodiment exhibits a continuous skeleton structure composed of hydration products of cement-based cementitious materials, with pores of different sizes distributed on the surface, and the pores having a certain degree of connectivity. Figure 3As can be seen from a and b in the figure, no obvious large-area structural collapse phenomenon appeared on the surface of the membrane material, indicating that the directional freezing and freeze-drying process can maintain the porous structure of the membrane material; at the same time, there is a distribution of pore structures with different sizes in different regions, which corresponds to the characteristic of the membrane material obtained in this embodiment having a gradient pore structure.

[0074] like Figure 4 As shown, the cross-section of the non-fired cement separation membrane prepared in this embodiment exhibits a continuous skeleton structure composed of hydration products of cement-based cementitious materials. The cross-section contains pores of different sizes, and shows certain differences in pore structure along the membrane thickness direction. Figure 4 a, b, and c in the figure show the microstructure at different locations on the membrane cross section. It can be observed that an interconnected pore structure is formed between the cement-based skeleton, indicating that the cryogenic medium crystals formed during the directional freezing process are retained after freeze-drying and cured to form a stable porous structure.

[0075] like Figure 5 As shown, the non-fired cement separation membrane prepared in this embodiment has obvious bimodal pore size distribution characteristics. Its pore size distribution curves show peaks at approximately 0.14 μm and 15.72 μm, respectively, indicating that there are both micron-scale pore structures and larger-scale channel structures inside the membrane material.

[0076] Among these features, the smaller surface pore size structure enhances the membrane material's ability to retain microparticles and oil droplets, while the larger internal pore size structure facilitates fluid transport within the membrane, improving the membrane material's permeability. This bimodal pore size distribution structure, combined with the formation of a crystal template by the freezing medium during directional freezing and the hydration and solidification process of the cement-based cementitious material, results in a cement-based framework with a multi-scale pore structure in the obtained membrane material.

[0077] like Figure 6 As shown, the non-fired cement separation membrane prepared in this embodiment exhibits stable filtration performance during oil-water separation. With increasing filtration time, the permeate flux of the membrane material initially decreases but then gradually stabilizes. Even after prolonged filtration, it maintains a high permeate capacity, indicating that the interconnected channels formed inside the membrane can maintain fluid transport.

[0078] Meanwhile, the oil phase rejection rate remained at a high level during the filtration process and gradually stabilized at about 92% as the filtration time increased, indicating that the cement separation membrane has a good oil-water separation effect.

[0079] Based on the pore size distribution test results of this embodiment, it can be seen that the membrane material has a bimodal pore size structure. The larger internal pore size structure is beneficial to reducing fluid transmission resistance and increasing permeation flux, while the smaller surface pore size structure can enhance the retention of oil droplets, thereby enabling the membrane material to maintain good permeation performance and separation effect during oil-water separation.

[0080] The test results are summarized as follows: The membrane material obtained in this embodiment has a compressive strength of 8.0 MPa and a porosity of 55%, with a characteristic peak for small pores of approximately 0.20 μm and a characteristic peak for large pores of approximately 15.72 μm. Under pure water conditions of 25℃ and a transmembrane pressure difference of 1 bar, the pure water permeate flux after 5 days of curing is 2250 L·m⁻². -2 ·h -1 The retention rate of the oil-in-water emulsion after filtration for 30 minutes reached 92.5%.

[0081] Based on the test results of Examples 1 to 5 above, the results are summarized in Table 1.

[0082] Table 1. Performance test results of the non-fired cement separation membranes prepared in different embodiments.

[0083] As shown in Table 1, the present invention employs a method combining directional freezing, freeze-drying, and curing with cement-based cementitious materials to prepare a non-fired cement separation membrane with a gradient pore structure. The membrane materials obtained in each embodiment all exhibit a bimodal pore size distribution, with the characteristic peak for small pores located at 0.10–0.31 μm and the characteristic peak for large pores located at 8.5–17.52 μm. This indicates that the pore structure of the membrane material can be adjusted by modifying the composition of the cement slurry, freezing conditions, and curing conditions.

[0084] In Example 2, a composite cementitious material composed of calcium aluminate cement and quartz powder was used to prepare a membrane material under low freezing temperature conditions. The resulting membrane material had a porosity of 55% and a macropore characteristic peak of 17.52 μm, thus exhibiting high pore connectivity. Its pure water permeation flux reached 4000 L·m⁻¹. -2 ·h -1 .

[0085] Example 5 uses a mixture of calcium aluminate cement and slag, combined with a relatively low freezing temperature, to prepare a membrane material. The resulting membrane material achieved a compressive strength of 8.0 MPa, indicating that the introduction of mineral admixtures can improve the cement-based skeleton structure, enhance the mechanical properties of the membrane material, and maintain good pore structure characteristics. Its pure water permeation flux reached 2250 L·m³. -2 ·h -1 The water-in-oil emulsion retention rate reached 92.5%.

[0086] Example 4 uses fly ash as a mineral admixture and forms a fine pore structure through a lower curing temperature. The resulting membrane material has a pore characteristic peak of 0.10 μm and has a high retention capacity, with a water-in-oil emulsion retention rate of 95.0%.

[0087] Based on the above test results, the non-fired cement separation membrane prepared by this invention has good pore structure adjustment capability, with a porosity of 40%~55%, a compressive strength of 3.0~8.0 MPa, and a pure water permeation flux of 350~4000 L·m⁻¹. -2 ·h -1 The water-in-oil emulsion retention rate reaches 90.8%~95.0%, which can meet the needs of fluid filtration and separation applications.

[0088] Therefore, by adopting the above-mentioned non-fired cement separation membrane with a connected gradient pore structure, its preparation method, and its application, cement separation membranes with gradient pore structures can be prepared without high-temperature sintering. By optimizing the pore structure, the permeability and separation efficiency of the membrane material can be improved, while maintaining good structural stability and mechanical properties. This solves the technical problems existing in the preparation of cement separation membranes, such as limited pore structure formation methods, difficulty in adjusting pore size distribution, and insufficient filtration performance.

[0089] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0090] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing a non-fired cement separation membrane with a connected gradient pore structure, characterized in that, Includes the following steps: S1. Cement slurry is prepared by mixing cement-based cementitious materials, mineral admixtures, cryogenic pore-forming media, binders and dispersants, and then ball milling the mixture. S2. The cement slurry is subjected to directional freezing treatment to form freezing medium crystals arranged along the freezing direction inside the cement slurry. After freeze-drying treatment, a porous cement membrane green body is obtained. S3. The porous cement membrane green body is cured to allow the cement-based cementitious material to undergo hydration and solidification. Through the hydration reaction of the cement-based cementitious material, a cement-based skeleton with a connected gradient pore structure is formed, thereby obtaining a non-fired cement separation membrane with a connected gradient pore structure.

2. The preparation method according to claim 1, characterized in that, In step S1, the cement-based cementitious material includes calcium aluminate cement, and the mineral admixture is used to partially replace the calcium aluminate cement. The mineral admixture is selected from one or two of silica fume, fly ash, slag, or quartz powder.

3. The preparation method according to claim 2, characterized in that, The amount of mineral admixture added accounts for 5% to 40% of the total mass of the mixed powder composed of calcium aluminate cement and mineral admixture, and the total solid content of the calcium aluminate cement and mineral admixture accounts for 30% to 60% of the total mass of cement slurry; when the mineral admixture includes two components, the mass ratio between the two mineral admixtures is 1:9 to 9:

1.

4. The preparation method according to claim 3, characterized in that, In step S1, the binder is one of sodium carboxymethyl cellulose, polyvinyl alcohol, or hydroxypropyl methyl cellulose, and the amount of binder added accounts for 0.5% to 1.5% of the total mass of calcium aluminate cement and mineral admixtures.

5. The preparation method according to claim 3, characterized in that, In step S1, the dispersant is one of polycarboxylate-based high-efficiency water-reducing agent, naphthalene-based high-efficiency water-reducing agent, or lignin sulfonate-based water-reducing agent, and the amount of the dispersant added accounts for 0.5% to 2% of the total mass of calcium aluminate cement and mineral admixtures.

6. The preparation method according to claim 1, characterized in that, In step S1, the cryogenic pore-forming medium is one or two of water, tert-butanol, or camphene, and the cryogenic pore-forming medium accounts for 40% to 70% of the total mass of the cement slurry; when the cryogenic pore-forming medium includes two components, the mass ratio between the two cryogenic pore-forming media is 1:9 to 9:

1.

7. The preparation method according to claim 1, characterized in that, In step S2, the temperature of the directional freezing treatment is -20 to -80°C, and the freezing time is 0.5 to 2 hours.

8. The preparation method according to claim 1, characterized in that, In step S3, the curing treatment includes initial curing and immersion curing. The temperature for both initial curing and immersion curing is 10~60℃, the humidity of the curing environment is not less than 95%, and the initial curing time is 12~24h; the immersion curing time is 1~7d.

9. A non-fired cement separation membrane with a connected gradient pore structure prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The non-fired cement separation membrane includes a cement-based continuous skeleton and a connected gradient pore structure distributed in the cement-based continuous skeleton; The pore size distribution of the non-fired cement separation membrane exhibits a bimodal characteristic, with the characteristic peak of small pores located at 0.1~0.35μm and the characteristic peak of large pores located at 8.0~17.52μm. The compressive strength of the non-fired cement separation membrane is 3~8MPa, and the porosity is 40%~55%. Under pure water test conditions of 25℃ and a transmembrane pressure difference of 1 bar, the permeation flux of the non-fired cement separation membrane is 350~4000 L·m. -2 ·h -1 After filtration for 30 minutes, the retention rate of oil-in-water emulsion was 90.8%~95%.

10. The application of a non-fired cement separation membrane with a connected gradient pore structure as described in claim 9 in fluid filtration and separation.

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