A preparation method of waste carbon fiber toughened magnesium-aluminum spinel composite ceramic for aluminum electrolytic cell heat preservation cover plate

Abstract

This invention belongs to the field of advanced ceramic composite materials and refractory material preparation technology, and relates to a method for preparing waste carbon fiber toughened magnesium-aluminum spinel composite ceramics for use in insulation covers of aluminum electrolytic cells. MgO and α-Al₂O₃ are pre-dried, then mixed by wet ball milling in an organic alcohol medium. The resulting mixture is dried and sieved. The mixture is then subjected to two-stage calcination to obtain MgAl₂O₄ spinel powder. Waste carbon fibers are cleaned with organic solvents to remove oil and sizing, and dried to obtain pretreated waste carbon fibers. The pretreated waste carbon fibers undergo a light surface activation treatment to obtain activated waste carbon fibers. The activated waste carbon fibers are dispersed in an organic alcohol medium, ultrasonically dispersed, and then the MgAl₂O₄ spinel powder is added. The mixture is then mixed under low shear using mechanical stirring or drum mixing, dried, sieved, and granulated with a binder to obtain composite granulated powder. The composite granulated powder is molded and pressed to obtain a green body. The green body is first degummed at low temperature in an air atmosphere; then heated further and sintered at high temperature under an Ar protective atmosphere, followed by cooling to obtain waste carbon fiber toughened magnesium-aluminum spinel composite ceramics. This invention optimizes the material composition and preparation process to ensure that the material maintains good structural stability and thermal insulation performance in high-temperature environments.

Description

Technical Field

[0001] This invention belongs to the field of advanced ceramic composite materials and refractory material preparation technology, and relates to a method for preparing waste carbon fiber toughened magnesium aluminum spinel composite ceramic for use in aluminum electrolytic cell insulation cover plates. Background Technology

[0002] Aluminum electrolytic cells typically operate at temperatures around 950℃, accompanied by a highly corrosive atmosphere and thermal shock environment. As a critical component, the insulation cover plate needs to possess low thermal conductivity, good thermal shock resistance, and high structural stability. Traditional insulation materials suffer from problems such as easy cracking, short lifespan, and limited insulation effectiveness.

[0003] Magnesium aluminum spinel possesses excellent high-temperature stability and corrosion resistance, but its toughness is relatively low and its thermal shock resistance is limited. Carbon fiber exhibits excellent toughening effects, but it is easily damaged in high-temperature oxidizing environments.

[0004] Therefore, developing a ceramic composite material that combines low thermal conductivity, thermal shock resistance, and high-temperature stability is of great significance for improving the heat preservation efficiency of aluminum electrolytic cells. Summary of the Invention

[0005] To address the problems and shortcomings of the existing technology, this invention provides a waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells. This invention optimizes the material composition and preparation process, enabling it to maintain good structural stability and thermal insulation performance even in high-temperature environments.

[0006] This invention employs a process route of first synthesizing spinel and then introducing waste carbon fiber, combined with low-shear mixing and inert atmosphere sintering, to effectively preserve the carbon fiber structure, thereby improving the material's toughness and thermal shock resistance. This invention is achieved through the following technical solutions.

[0007] A method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramics for use in insulation covers of aluminum electrolytic cells, comprising the following steps: Spinel powder synthesis: (1) After pre-drying MgO and α-Al2O3, add organic alcohol medium for wet ball milling and mixing, and then dry and sieve the mixture; (2) The mixture was calcined in two stages to obtain MgAl2O4 spinel powder; Waste carbon fiber treatment: (3) The waste carbon fiber is cleaned with organic solvent to remove oil and sizing, and then dried to obtain pretreated waste carbon fiber; (4) The pretreated waste carbon fiber is subjected to light surface activation treatment to obtain activated waste carbon fiber; Preparation of composite powder: (5) Disperse the activated waste carbon fiber in an organic alcohol medium, add the MgAl2O4 spinel powder after ultrasonic dispersion, mix with low shear by mechanical stirring or drum mixing, dry, sieve and then add binder to granulate to obtain composite granulated powder. forming: (6) After loading the composite granulated powder into a mold, press it into shape to obtain a green body; Degumming and sintering: (7) Degumming the green body at low temperature in air atmosphere; (8) Continue heating and sinter at high temperature under Ar protective atmosphere to obtain waste carbon fiber toughened magnesium aluminum spinel composite ceramic.

[0008] The α-Al₂O₃ has a purity of ≥99.5% and a d₅₀ ≤1~2μm; the MgO has a purity of ≥98% and a d₅₀ ≤2~5μm; the mass ratio of MgO to α-Al₂O₃ is 0.395:1.

[0009] The wet ball milling uses ethanol or isopropanol as the medium, with a liquid-to-solid ratio of 1.5–2:1 mL / g, a ball-to-material ratio of 5–8:1, a ball milling speed of 250–300 rpm, and a ball milling time of 3–4 h; after drying, it is sieved through a 100-mesh sieve. The two-stage calcination includes: First, the temperature is increased from room temperature to 1100℃ at a rate of 5℃ / min and held for 2 hours; After cooling to room temperature and slightly crushing, the temperature is increased to 1500-1550℃ at 5℃ / min and held for 3-4 hours. After calcination, the mixture is ball-milled for 2 hours and then sieved through a 200-mesh sieve to obtain MgAl2O4 spinel powder.

[0010] The waste carbon fiber is cut into short sections to a length of 0.5-2 mm, then soaked in acetone for 30 min and ultrasonically cleaned for 10 min. This process is repeated twice and then dried at 80°C for 2 h. The mild surface activation treatment involves heating to 350°C at a rate of 5°C / min in air or N2 atmosphere and holding at that temperature for 20–30 min.

[0011] The activated waste carbon fiber is dispersed in an organic alcohol medium for 5-10 minutes, and then MgAl2O4 spinel powder is slowly added at a concentration of 0.5-2.0% of the composite granulation powder mass, and mechanically stirred or drum-mixed for 30-60 minutes. The binder is a 5wt% PVA aqueous solution, and its addition amount makes PVA account for 2wt% of the total powder mass. After granulation, it is sieved through a 40-60 mesh.

[0012] The pressing method is unidirectional pressing or unidirectional pressing followed by cold isostatic pressing; the unidirectional pressing pressure is 150-200 MPa, and the holding time is 30 s; the cold isostatic pressing pressure is 200 MPa, and the holding time is 1 min.

[0013] The low-temperature degumming process involves heating from room temperature to 200°C at a rate of 1°C / min and holding at that temperature for 1 hour; then heating from 200°C to 300°C at a rate of 1°C / min and holding at that temperature for 1 hour. The high-temperature sintering process involves: introducing Ar gas and purging for 20–30 min at an Ar flow rate of 200–500 mL / min; then heating from 300 °C to 600 °C at a rate of 1 °C / min and holding for 1 h; then heating from 600 °C to 1200 °C at a rate of 3 °C / min and holding for 1 h; finally heating from 1200 °C to 1500–1600 °C at a rate of 5 °C / min and holding for 2 h; and finally furnace cooling to room temperature at a rate not exceeding 5 °C / min.

[0014] A sintering aid is added during the degumming and sintering process. The sintering aid is Y2O3, and the amount added is 1 to 2 wt% of the total mass of the composite granulated powder; or the sintering aid is LiOH, and the amount added is 0.3 wt% of the total mass of the composite granulated powder.

[0015] A waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic is prepared by the above method. It has MgAl2O4 as the main crystalline phase and contains 0.5 to 2.0 wt% uniformly dispersed waste carbon fiber toughening phase. The fracture surface of the composite ceramic has fiber bridging, fiber pull-out pores and / or crack deflection morphology characteristics.

[0016] A type of waste carbon fiber toughened magnesium aluminum spinel composite ceramic is used for insulation cover plates or heat insulation covering structures of aluminum electrolytic cells.

[0017] The beneficial effects of this invention are: (1) The thermal insulation performance of waste carbon fiber toughened magnesium aluminum spinel composite ceramics is excellent: The material has a low thermal conductivity, which can significantly reduce heat loss in the electrolytic cell; (2) Waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramics have good thermal shock resistance: Carbon fiber toughening mechanism improves the material's crack resistance; (3) Waste carbon fiber toughened magnesium aluminum spinel composite ceramics have a long service life: Suitable for high-temperature and complex atmosphere environments; (4) The toughening mechanism is clear: The composite ceramic obtained by this invention can form typical toughening mechanisms such as fiber bridging, fiber pull-out, and crack deflection during the fracture process, which is beneficial to improving the problem of high brittleness of spinel ceramics.

[0018] (5) High value for resource utilization: This invention introduces waste carbon fiber into a high-temperature ceramic composite system, realizing the high-value utilization of waste carbon fiber, which has the dual significance of improving material performance and recycling resources. Attached Figure Description

[0019] Figure 1 This is the XRD pattern of MgAl2O4 spinel powder after two-stage calcination in Example 1 of the present invention; Figure 2 (a) is a physical image of the sintered body of waste carbon fiber toughened magnesium aluminum spinel composite ceramic prepared in Example 1 of the present invention; Figure 2 (b) is a SEM image of the fracture surface of the waste carbon fiber toughened magnesium aluminum spinel composite ceramic prepared in Example 1 of the present invention. Detailed Implementation

[0020] The preparation method of the waste carbon fiber toughened magnesium aluminum spinel composite ceramic for use in the heat insulation cover plate of aluminum electrolysis cell includes the following steps: Spinel powder synthesis: (1) After pre-drying MgO and α-Al2O3, add organic alcohol medium for wet ball milling and mixing, and then dry and sieve the mixture; (2) The mixture was calcined in two stages to obtain MgAl2O4 spinel powder; Waste carbon fiber treatment: (3) The waste carbon fiber is cleaned with organic solvent to remove oil and sizing, and then dried to obtain pretreated waste carbon fiber; (4) The pretreated waste carbon fiber is subjected to light surface activation treatment to obtain activated waste carbon fiber; Preparation of composite powder: (5) Disperse the activated waste carbon fiber in an organic alcohol medium, add the MgAl2O4 spinel powder after ultrasonic dispersion, mix with low shear by mechanical stirring or drum mixing, dry, sieve and then add binder to granulate to obtain composite granulated powder. forming: (6) After loading the composite granulated powder into a mold, press it into shape to obtain a green body; Degumming and sintering: (7) Degumming the green body at low temperature in air atmosphere; (8) Continue heating and sinter at high temperature under Ar protective atmosphere to obtain waste carbon fiber toughened magnesium aluminum spinel composite ceramic.

[0021] In some embodiments, the α-Al₂O₃ has a purity of ≥99.5% and a d₅₀ ≤1~2μm; the MgO has a purity of ≥98% and a d₅₀ ≤2~5μm; and the mass ratio of MgO to α-Al₂O₃ is 0.395:1.

[0022] In some embodiments, the wet ball milling uses ethanol or isopropanol as the medium, with a liquid-to-solid ratio of 1.5–2:1 mL / g. In some embodiments, the liquid-to-solid ratio is 1.5:1, 1.8:1, or 2:1 mL / g. The ball-to-material ratio is 5–8:1. In some embodiments, the ball-to-material ratio is 5:1, 6:1, 7:1, or 8:1. The ball milling speed is 250–300 rpm. In some embodiments, the ball milling speed is 250 rpm, 280 rpm, or 300 rpm. The ball milling time is 3–4 hours. In some embodiments, the ball milling time is 3 hours, 3.5 hours, or 4 hours. After drying, the material is sieved through a 100-mesh sieve. In some embodiments, the two-stage calcination includes: First, the temperature is increased from room temperature to 1100℃ at a rate of 5℃ / min and held for 2 hours; After cooling to room temperature and slightly crushing, the temperature is increased to 1500-1550℃ at 5℃ / min and held for 3-4 hours. In some specific embodiments, the temperature is increased to 1500℃, 1520℃ or 1550℃ and held for 3 hours, 3.5 hours or 4 hours. After calcination, the mixture is ball-milled for 2 hours and then sieved through a 200-mesh sieve to obtain MgAl2O4 spinel powder.

[0023] In some embodiments, the waste carbon fiber is cut to a length of 0.5-2 mm. In some specific embodiments, the waste carbon fiber is cut to a length of 0.5 mm, 1 mm or 2 mm. Then, it is soaked in acetone for 30 min and ultrasonically cleaned for 10 min. This process is repeated twice and then dried at 80°C for 2 h. In some embodiments, the mild surface activation treatment involves heating to 350°C at a rate of 5°C / min in air or N2 atmosphere and holding at that temperature for 20–30 min. In some specific embodiments, the holding time is 20 min, 25 min, or 30 min.

[0024] In some embodiments, the activated waste carbon fibers are dispersed in an organic alcohol medium for 5-10 minutes. In some specific embodiments, the dispersion time is 5 minutes, 8 minutes, or 10 minutes. Then, MgAl2O4 spinel powder is slowly added according to the amount of activated waste carbon fibers added, which is 0.5-2.0% of the composite granulation powder mass. In some specific embodiments, the amount of activated waste carbon fibers added is 0.5%, 1.0%, or 2.0% of the composite granulation powder mass. The mixture is mechanically stirred or drum-mixed for 30-60 minutes. In some specific embodiments, the mixing time is 30 minutes, 40 minutes, 50 minutes, or 60 minutes. The binder is a 5 wt% PVA aqueous solution, and the amount added is such that PVA accounts for 2 wt% of the total powder mass. After granulation, the mixture is sieved through a 40-60 mesh sieve.

[0025] In some embodiments, the pressing method is unidirectional pressing or unidirectional pressing followed by cold isostatic pressing; the unidirectional pressing pressure is 150-200 MPa, and in some specific embodiments, the unidirectional pressing pressure is 150 MPa, 180 MPa or 200 MPa, and the holding time is 30 s; the cold isostatic pressing pressure is 200 MPa, and the holding time is 1 min.

[0026] In some embodiments, the low-temperature degumming involves: raising the temperature from room temperature to 200°C at a rate of 1°C / min and holding it at that temperature for 1 hour; then continuing to raise the temperature from 200°C to 300°C at a rate of 1°C / min and holding it at that temperature for 1 hour. The high-temperature sintering process involves: introducing Ar gas and purging for 20–30 min (in some specific embodiments, purging for 20, 25, or 30 min), with an Ar flow rate of 200–500 mL / min (in some specific embodiments, 200, 300, 400, or 500 mL / min); then heating from 300 °C to 600 °C at a rate of 1 °C / min and holding for 1 h; then heating from 600 °C to 1200 °C at a rate of 3 °C / min and holding for 1 h; finally heating from 1200 °C to 1500–1600 °C at a rate of 5 °C / min (in some specific embodiments, heating to 1500, 1550, or 1600 °C and holding for 2 h); and finally furnace cooling to room temperature at a rate not exceeding 5 °C / min.

[0027] In some embodiments, a sintering aid is added during the degumming and sintering process. The sintering aid is Y2O3, and the amount added is 1 to 2 wt% of the total mass of the composite granulated powder. In some specific embodiments, the amount added is 1 wt%, 1.5 wt%, or 2 wt% of the total mass of the composite granulated powder; or the sintering aid is LiOH, and the amount added is 0.3 wt% of the total mass of the composite granulated powder.

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Example 1

[0029] The preparation method of the waste carbon fiber toughened magnesium aluminum spinel composite ceramic for use in the heat insulation cover plate of aluminum electrolysis cell includes the following steps: Spinel powder synthesis: (1) MgO (MgO purity ≥ 98%, d50 ≤ 2~5μm) and α-Al2O3 (α-Al2O3 purity ≥ 99.5%, d50 ≤ 1~2μm) were pre-dried at 100~120℃ for 2~4h, and then mixed by wet ball milling with ethanol organic alcohol medium at a mass ratio of MgO:Al2O3 = 0.395:1, wherein the liquid-solid ratio was 1.8:1mL / g, the ball-to-material ratio was 6:1, the ball milling speed was 280rpm, and the ball milling time was 4h; after drying, the mixture was sieved through a 100-mesh sieve. (2) The mixture was subjected to two-stage calcination to obtain MgAl2O4 spinel powder, specifically: The mixture was placed in a covered alumina crucible and heated from room temperature to 1100℃ at a rate of 5℃ / min and held for 2 hours. After cooling to room temperature and slightly crushing, the temperature is increased to 1500℃ at 5℃ / min and held for 3 hours; after calcination, it is ball-milled for 2 hours and then sieved through a 200-mesh sieve to obtain MgAl2O4 spinel powder. Waste carbon fiber treatment: (3) The waste carbon fiber is cleaned with organic solvent to remove oil and sizing, and then dried to obtain pretreated waste carbon fiber: Waste carbon fibers were cut into short sections of 0.5–2 mm, then soaked in acetone for 30 min and ultrasonically cleaned for 10 min. This process was repeated twice, and the carbon fibers were dried at 80°C for 2 h. (4) The pretreated waste carbon fiber is subjected to light surface activation treatment to obtain activated waste carbon fiber: Heat to 350℃ at a rate of 5℃ / min in air and hold for 20min. Preparation of composite powder: (5) Based on 100g of composite granulated powder, 1.0g of activated waste carbon fiber is dispersed in an ethanol organic alcohol medium, ultrasonically dispersed for 8min, and then the MgAl2O4 spinel powder is slowly added. The mixture is mechanically stirred for 45min, dried, and sieved through an 80-mesh sieve. Subsequently, 5wt% PVA aqueous solution is sprayed on, so that the amount of PVA added accounts for 2wt% of the total mass of the powder. After drying at 70℃, the mixture is granulated through a 40-60 mesh sieve to obtain composite granulated powder. forming: (6) After the composite granulated powder is loaded into the mold, it is pressed into shape. It is pressed in one direction at 180MPa for 30s and then further subjected to cold isostatic pressing at 200MPa for 1min to obtain the green body. Degumming and sintering: (7) Place the green blank in a covered alumina crucible, and spread a small amount of spinel powder in the crucible as a powder bed. First, debind it at low temperature in air atmosphere: first heat it to 200℃ at 1℃ / min in air and keep it at 1h, then heat it to 300℃ and keep it at 1h. (8) Continue heating and sinter at high temperature under Ar protective atmosphere. Introduce Ar gas and purge for 30 min. The Ar flow rate is 300 mL / min. Then, heat from 300℃ to 600℃ at a heating rate of 1℃ / min and hold for 1 h. Then, heat from 600℃ to 1200℃ at a heating rate of 3℃ / min and hold for 1 h. Finally, heat from 1200℃ to 1550℃ at a heating rate of 5℃ / min and hold for 2 h. Finally, cool the furnace to room temperature to obtain waste carbon fiber toughened magnesium aluminum spinel composite ceramic.

[0030] The XRD pattern of the MgAl2O4 spinel powder prepared in this embodiment is shown below. Figure 1 As shown in the image, the actual sintered body of the waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic is as follows. Figure 2 As shown in (a).

[0031] SEM images of the fracture surfaces of waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramics are shown below. Figure 2 As shown in (b), from Figure 2 As can be seen in (b), the fracture surface exhibits obvious fiber bridging, fiber pull-out, and crack deflection phenomena, indicating that the waste carbon fiber is relatively uniformly dispersed in the matrix and effectively absorbs energy during the fracture process, thereby significantly improving the fracture toughness and thermal shock resistance of the material. Example 2

[0032] Except for the following differences, the remaining steps are the same as in Example 1: the amount of waste carbon fiber added to the composite powder is 0.5wt%, that is, 99.5g of spinel powder and 0.5g of waste carbon fiber are added to every 100g of composite powder; the high-temperature sintering temperature is 1500℃, and the holding time is 2h. Waste carbon fiber toughened magnesium aluminum spinel composite ceramic is prepared. Example 3

[0033] Except for the following differences, the remaining steps are the same as in Example 1: the amount of waste carbon fiber added to the composite powder is 2.0 wt%, that is, 98.0 g of spinel powder and 2.0 g of waste carbon fiber are added to every 100 g of composite powder; after the waste carbon fiber is cleaned and dried, it is heated to 350 °C at 5 °C / min and held at that temperature for 30 min under N2 atmosphere for mild surface activation; the high-temperature sintering temperature is 1600 °C and held for 2 h. Waste carbon fiber toughened magnesium aluminum spinel composite ceramic is thus prepared. Example 4

[0034] Except for the following differences, the remaining steps are the same as in Example 1: During the low-temperature degumming and sintering process, the green body is placed in a covered alumina crucible, a small amount of spinel powder is spread in the crucible as a powder bed, and Y2O3 is added as a sintering aid, the amount of which is 1% of the total mass of the composite granulated powder, to prepare waste carbon fiber toughened magnesium aluminum spinel composite ceramic.

[0035] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramics for use in insulation covers of aluminum electrolytic cells, characterized in that... Includes the following steps: Spinel powder synthesis: (1) After pre-drying MgO and α-Al2O3, add organic alcohol medium for wet ball milling and mixing, and then dry and sieve the mixture; (2) The mixture was calcined in two stages to obtain MgAl2O4 spinel powder; Waste carbon fiber treatment: (3) The waste carbon fiber is cleaned with organic solvent to remove oil and sizing, and then dried to obtain pretreated waste carbon fiber; (4) The pretreated waste carbon fiber is subjected to light surface activation treatment to obtain activated waste carbon fiber; Preparation of composite powder: (5) Disperse the activated waste carbon fiber in an organic alcohol medium, add the MgAl2O4 spinel powder after ultrasonic dispersion, mix with low shear by mechanical stirring or drum mixing, dry, sieve and then add binder to granulate to obtain composite granulated powder. forming: (6) After loading the composite granulated powder into a mold, press it into shape to obtain a green body; Degumming and sintering: (7) Degumming the green body at low temperature in air atmosphere; (8) Continue to heat up and sinter at high temperature under Ar protective atmosphere to obtain waste carbon fiber toughened magnesium aluminum spinel composite ceramic.

2. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The α-Al₂O₃ has a purity of ≥99.5% and a d₅₀ ≤1~2μm; the MgO has a purity of ≥98% and a d₅₀ ≤2~5μm; the mass ratio of MgO to α-Al₂O₃ is 0.395:

1.

3. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The wet ball milling uses ethanol or isopropanol as the medium, with a liquid-to-solid ratio of 1.5–2:1 mL / g, a ball-to-material ratio of 5–8:1, a ball milling speed of 250–300 rpm, and a ball milling time of 3–4 h; after drying, it is sieved through a 100-mesh sieve. The two-stage calcination includes: First, the temperature is increased from room temperature to 1100℃ at a rate of 5℃ / min and held for 2 hours; After cooling to room temperature and slightly crushing, the temperature is increased to 1500-1550℃ at 5℃ / min and held for 3-4 hours. After calcination, the mixture is ball-milled for 2 hours and then sieved through a 200-mesh sieve to obtain MgAl2O4 spinel powder.

4. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The waste carbon fiber is cut into short sections to a length of 0.5-2 mm, then soaked in acetone for 30 min and ultrasonically cleaned for 10 min. This process is repeated twice and then dried at 80°C for 2 h. The mild surface activation treatment involves heating to 350°C at a rate of 5°C / min in air or N2 atmosphere and holding at that temperature for 20–30 min.

5. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The activated waste carbon fiber is dispersed in an organic alcohol medium for 5-10 minutes, and then MgAl2O4 spinel powder is slowly added at a concentration of 0.5-2.0% of the composite granulation powder mass, and mechanically stirred or drum-mixed for 30-60 minutes. The binder is a 5wt% PVA aqueous solution, and its addition amount makes PVA account for 2wt% of the total powder mass. After granulation, it is sieved through a 40-60 mesh.

6. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The pressing method is unidirectional pressing or unidirectional pressing followed by cold isostatic pressing; the unidirectional pressing pressure is 150-200 MPa, and the holding time is 30 s; the cold isostatic pressing pressure is 200 MPa, and the holding time is 1 min.

7. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in insulation covers of aluminum electrolytic cells according to claim 1, characterized in that: The low-temperature degumming process involves heating from room temperature to 200°C at a rate of 1°C / min and holding at that temperature for 1 hour; then heating from 200°C to 300°C at a rate of 1°C / min and holding at that temperature for 1 hour. The high-temperature sintering process involves: introducing Ar gas and purging for 20–30 min at an Ar flow rate of 200–500 mL / min; then heating from 300 °C to 600 °C at a rate of 1 °C / min and holding for 1 h; then heating from 600 °C to 1200 °C at a rate of 3 °C / min and holding for 1 h; finally heating from 1200 °C to 1500–1600 °C at a rate of 5 °C / min and holding for 2 h; and finally furnace cooling to room temperature at a rate not exceeding 5 °C / min.

8. The method for preparing waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic for use in aluminum electrolytic cell insulation covers according to claim 1, characterized in that: A sintering aid is added during the degumming and sintering process. The sintering aid is Y2O3, and the amount added is 1 to 2 wt% of the total mass of the composite granulated powder; or the sintering aid is LiOH, and the amount added is 0.3 wt% of the total mass of the composite granulated powder.

9. A type of waste carbon fiber-reinforced magnesium-aluminum spinel composite ceramic, characterized in that: Prepared by the method according to any one of claims 1 to 8, the composite ceramic has MgAl2O4 as the main crystalline phase and contains 0.5 to 2.0 wt% uniformly dispersed waste carbon fiber toughening phase inside, and the fracture surface of the composite ceramic has fiber bridging, fiber pull-out pores and / or crack deflection morphology characteristics.

10. A waste carbon fiber toughened magnesium aluminum spinel composite ceramic according to claim 9, used for an aluminum electrolytic cell insulation cover or heat insulation covering structure.

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