A surface densification porous magnesium-aluminum spinel composite material for an aluminum electrolytic cell heat preservation cover plate, a preparation method and application thereof
By preparing a surface-densified porous magnesium-aluminum spinel composite material, the problem of balancing the density and porosity of magnesium-aluminum spinel materials in aluminum electrolytic cell insulation covers was solved. This enabled the resource utilization of secondary aluminum ash and improved material performance. The material is suitable for aluminum electrolytic cell insulation covers, reducing heat loss and improving operational stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- KUNMING UNIV OF SCI & TECH
- Filing Date
- 2026-04-03
- Publication Date
- 2026-07-14
AI Technical Summary
Existing magnesium-aluminum spinel materials are difficult to balance in terms of density and porosity during preparation, which limits their application in the field of aluminum electrolytic cell insulation covers. At the same time, secondary aluminum ash resources are not effectively utilized, resulting in resource waste and environmental problems.
High-purity alumina was prepared by secondary aluminum ash, and a porous MgAl2O4 spinel matrix was prepared by combining MgO and a pore-forming agent. The surface was then densified with nano-MgAl2O4 powder and rare earth sintering aid to form a composite structure with a dense surface and porous interior.
It realizes the high-value resource utilization of secondary aluminum ash. The material has the properties of surface anti-corrosion and anti-permeability and internal low thermal conductivity. It is suitable for the insulation cover of aluminum electrolysis cell, reducing heat loss and improving operational stability.
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Figure CN122380888A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of advanced refractory ceramic material preparation and secondary aluminum ash resource utilization, and relates to a surface-densified porous magnesium-aluminum spinel composite material for aluminum electrolytic cell insulation cover plate, its preparation method and application. Background Technology
[0002] Aluminum electrolytic cells are core equipment in the aluminum industry for producing metallic aluminum through the electrolysis of alumina molten salt. They operate at high temperatures, bear heavy heat loads, and have long continuous working cycles. During long-term operation, significant heat loss occurs in the upper and surrounding areas of the electrolytic cell, increasing energy consumption and affecting the thermal field distribution and operational stability. Therefore, developing thermal insulation components for aluminum electrolytic cells that are structurally stable, have excellent thermal insulation performance, and are suitable for complex operating conditions is of great significance for energy conservation and emission reduction in the aluminum electrolysis industry.
[0003] Currently, aluminum electrolytic cells often use loose covering material for insulation. Although this method is convenient, it has problems in practical applications, such as difficulty in controlling the covering thickness, local collapse, easy cracking, high labor intensity, and difficulty in automation. It is difficult to meet the requirements of modern large-scale prebaked aluminum electrolytic cells for efficient insulation, structured installation, and long-term stable operation.
[0004] Magnesium aluminum spinel (MgA) materials possess excellent high-temperature stability, thermal shock resistance, corrosion resistance, and good compatibility with the aluminum electrolysis environment, making them a promising functional material for aluminum electrolysis cells. However, the insulation covers for aluminum electrolysis cells not only require materials with high mechanical strength and corrosion resistance, but also low thermal conductivity and good thermal insulation performance; that is, the material should simultaneously possess a relatively dense surface and a relatively porous internal structure. Existing MgA spinel materials often struggle to achieve both density and porosity during preparation, thus limiting their application in the field of aluminum electrolysis cell insulation covers.
[0005] Existing technologies often improve the performance of magnesium-aluminum spinel materials by doping them with other substances. For example, patent application number 2017107321789 describes a method for preparing rare-earth aluminosilicate rod-shaped reinforced magnesium-aluminum spinel materials. This method uses industrial magnesium oxide and alumina powders as raw materials, which are ball-milled, mixed, and dried before adding rare-earth compounds. After molding, the mixture is sintered and cooled in a furnace to obtain the product. In other words, magnesium-aluminum spinel materials are conventionally prepared by adding rare-earth oxides and then sintering. However, this invention prepares rare-earth oxides into a slurry, which is then applied to the substrate via impregnation, vacuum impregnation, brushing, scraping, or a combination thereof. Currently, there is no existing method for preparing surface-densified porous magnesium-aluminum spinel in this manner.
[0006] On the other hand, secondary aluminum ash is a significant solid waste generated in the aluminum and recycled aluminum industries, containing a high proportion of alumina, aluminum nitride, salts, and some impurities. Improper handling not only wastes resources but may also cause environmental problems. If the aluminum resources in secondary aluminum ash can be converted into high-purity alumina, and then further processed into high-value-added magnesium-aluminum spinel insulation components, it would be possible to combine solid waste resource utilization with the preparation of advanced ceramic materials, possessing significant economic and environmental value.
[0007] Therefore, there is an urgent need to provide a new technical solution that organically combines the resource utilization of secondary aluminum ash to prepare high-purity alumina, the construction of a porous magnesium-aluminum spinel matrix, and surface densification treatment to prepare a surface-densified porous magnesium-aluminum spinel composite material suitable for the insulation cover of aluminum electrolytic cells. Summary of the Invention
[0008] To address the problems and shortcomings of existing technologies, this invention provides a method for preparing a porous magnesium-aluminum spinel insulation cover plate doped with rare earth oxides for aluminum electrolytic cells. The core of this invention lies in: firstly, obtaining high-purity α-Al₂O₃ from secondary aluminum ash through reverse flotation, alkali leaching, precipitation, and calcination; then, using the obtained high-purity alumina as the aluminum source, preparing a porous MgAl₂O₄ spinel matrix in combination with MgO and a pore-forming agent; finally, surface densifying the matrix using a surface slurry of nano-MgAl₂O₄ powder and rare earth sintering aid, followed by high-temperature sintering to obtain an integrated magnesium-aluminum spinel composite structure with a dense surface and porous interior. This invention is achieved through the following technical solutions.
[0009] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a surface-densified porous magnesium-aluminum spinel composite material for use in insulation covers of aluminum electrolytic cells includes the following steps: Preparation of high-purity alumina: After drying and sieving the secondary aluminum ash, reverse flotation is performed to remove silicon and soluble salts. The resulting concentrate is then subjected to alkali leaching, precipitation and calcination to obtain high-purity α-Al2O3 powder. Preparation of porous spinel matrix: The high-purity α-Al2O3 powder and MgO are mixed according to the stoichiometric ratio of MgAl2O4, and a pore-forming agent and binder are added. After ball milling, molding, debinding and pre-reaction sintering, a porous MgAl2O4 spinel matrix is obtained. Preparation of surface densification slurry: Nano MgAl2O4 powder, Y2O3 and / or Sc2O3 rare earth sintering aid are mixed with solvent, dispersant and binder to prepare surface densification slurry; Surface densification treatment: After surface pretreatment of the porous MgAl2O4 spinel matrix, the surface densification slurry is applied by impregnation, vacuum impregnation, brushing, scraping or a combination thereof, and after drying, it is sintered at high temperature to obtain a magnesium aluminum spinel composite material with a dense surface and porous interior.
[0010] The secondary aluminum ash is first dried at 105℃ for 8-12 hours, then cooled, lightly ground, and passed through a 100-300 mesh sieve.
[0011] The reverse flotation desilication and soluble salt removal process is as follows: the secondary aluminum ash after screening is adjusted to a pulp concentration of 10-30 wt% and a pH of 7.5-9.5. Water glass is used as a modifier, sodium oleate as a collector, and terpineol as a frother. The amount of water glass added is 1-4 g / kg of secondary aluminum ash, the amount of sodium oleate added is 1-5 g / kg of secondary aluminum ash, and the amount of terpineol added is 0.5-2 g / kg of secondary aluminum ash. The flotation time is 3-8 min to obtain concentrate.
[0012] The alkaline leaching process involves using a NaOH solution with a concentration of 1-5 mol / L, a liquid-to-solid ratio of 5-15:1 mL / g, a leaching temperature of 70-95℃, and a leaching time of 1-4 h to obtain an alkaline leaching solution. Precipitation is achieved by adjusting the pH of the alkaline leaching solution to 8.5-10.0 and adding Al(OH)3 seed crystals for aging precipitation to obtain a precipitate. Calcination is performed by calcining the precipitate at a temperature of 1000-1200℃ for a holding time of 3-5 h.
[0013] The high-purity α-Al2O3 powder is mixed with MgO in a molar ratio of 1:1. The pore-forming agent is one or a mixture of polypropylene microspheres, polystyrene microspheres, starch, and resin spheres in any proportion, and the amount of pore-forming agent added is 1-10 wt% of the total solid mass. The binder is one or a mixture of PVA, PEG, and methylcellulose in any proportion, and the amount of binder added is 1-5 wt% of the total solid mass.
[0014] The ball milling mixing was carried out at 60-100 rpm for 12 hours. High-purity alumina balls with a diameter of 5 mm were used for ball milling, and the ball-to-material ratio was 3:1 g / g. The microfluidic deagglomeration was carried out once under a pressure of 1000 bar. The debinding and pre-reaction sintering process includes: heating to 250-350℃ at room temperature at a rate of 0.5-2℃ / min and holding for 0.5-2h; then heating to 500-700℃ and holding for 1-3h; finally heating to 1250-1450℃ and holding for 2-5h, followed by furnace cooling.
[0015] In the surface densification slurry, the purity of MgAl2O4 nanopowder is ≥99.9%, D50=55nm, and BET specific surface area is 28-31m² / g. The MgAl2O4 nanopowder is dispersed in a solvent with a solid content controlled at 10-30 vol. The amount of rare earth sintering aid added is 1-5 wt% of the total mass of the surface densification slurry dry powder. The dispersant is ammonium polyacrylate, and the amount added is 1-5 wt% of the total mass of the surface densification slurry dry powder. The binder is PVA, and the amount added is 3-8 wt% of the total mass of the surface densification slurry dry powder.
[0016] The vacuum impregnation process involves a vacuum degree of -0.06 to -0.09 MPa, a pressure holding time of 3-10 min, followed by immersion for 5-20 min after restoring to normal pressure, and 1-4 coating cycles. After each coating, the product is dried at 60-100℃ for 10-60 min.
[0017] A surface-densified porous magnesium-aluminum spinel composite material for use in insulation covers of aluminum electrolytic cells, with a surface relative density ≥99% and an internal relative density within 91%; the surface shell thickness is 0.1-0.3mm, the surface grain size is >15μm, the internal grain size is 290-320nm, and the internal pore diameter is 50-200nm.
[0018] An application of a surface-densified porous magnesium-aluminum spinel composite material for thermal insulation cover plates of aluminum electrolysis cells, serving as a thermal insulation cover plate, heat insulation cover plate, or high-temperature refractory heat insulation component in the aluminum electrolysis process.
[0019] The beneficial effects of this invention are: (1) Realize the high-value resource utilization of secondary aluminum ash: This invention uses secondary aluminum ash as the main aluminum source and obtains high-purity alumina through reverse flotation, alkaline leaching, precipitation and calcination processes. It then further prepares magnesium aluminum spinel composite material, realizing the transformation of industrial solid waste into high-value-added refractory ceramic materials, which has both resource utilization and environmental benefits. (2) It combines the structural advantages of a dense surface and a porous interior: This invention achieves a composite structure with high surface density and high internal porosity in the same component through the process design of "porous spinel skeleton + surface densification slurry + high temperature re-sintering", so that the material has the characteristics of surface erosion resistance and impermeability and internal low thermal conductivity and good thermal insulation. (3) Applicable to the working conditions of aluminum electrolytic cell insulation cover plate: The material obtained by this invention has good high-temperature stability, thermal shock resistance and structural integrity, and is suitable for use as an insulation cover for aluminum electrolysis cells. It can reduce heat loss, improve the thermal field distribution of the cell, and improve operational stability. (4) The process route is controllable and the structural parameters are adjustable: By adjusting the reverse flotation conditions, alkali leaching regime, pore-forming agent addition amount, surface slurry composition, and final sintering regime, the material purity, matrix porosity, surface thickness, and density can be controlled within a certain range to meet the performance requirements of insulation components for different types of electrolytic cells.
[0020] (5) The rare earth oxide-doped porous magnesium aluminum spinel heat insulation cover material for aluminum electrolytic cells prepared by the present invention is also a green special refractory ceramic matrix composite material with very good fire resistance. Attached Figure Description
[0021] Figure 1 This is the XRD pattern of the surface-densified porous magnesium-aluminum spinel composite material prepared in Example 1 of this invention; Figure 2 This is a SEM image of the cross section of the surface-densified porous magnesium-aluminum spinel composite material prepared in Example 1 of the present invention.
[0022] Figure 3 This is a schematic diagram of the porous structure inside the surface-densified porous magnesium-aluminum spinel composite material prepared in Example 1 of the present invention. Detailed Implementation
[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.
[0024] The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for use in the insulation cover of aluminum electrolysis cells includes the following steps: Preparation of high-purity alumina: After drying and sieving the secondary aluminum ash, reverse flotation is performed to remove silicon and soluble salts. The resulting concentrate is then subjected to alkali leaching, precipitation and calcination to obtain high-purity α-Al2O3 powder. Preparation of porous spinel matrix: The high-purity α-Al2O3 powder and MgO are mixed according to the stoichiometric ratio of MgAl2O4, and a pore-forming agent and binder are added. After ball milling, molding, debinding and pre-reaction sintering, a porous MgAl2O4 spinel matrix is obtained. Preparation of surface densification slurry: Nano MgAl2O4 powder, Y2O3 and / or Sc2O3 rare earth sintering aid are mixed with solvent, dispersant and binder to prepare surface densification slurry; Surface densification treatment: After surface pretreatment of the porous MgAl2O4 spinel matrix, the surface densification slurry is applied by impregnation, vacuum impregnation, brushing, scraping or a combination thereof, and after drying, it is sintered at high temperature to obtain a magnesium aluminum spinel composite material with a dense surface and porous interior.
[0025] In some implementations, the secondary aluminum ash is first dried at 105°C for 8-12 hours, then cooled, lightly ground, and passed through a 100-300 mesh sieve. In some embodiments, for example, the secondary aluminum ash is first dried at 105°C for 8 hours, 10 hours, or 12 hours; or, after cooling, it is lightly ground and passed through a 100-mesh, 150-mesh, 200-mesh, or 300-mesh sieve.
[0026] In some embodiments, the reverse flotation desilication and soluble salt removal process involves: adjusting the slurry of the screened secondary alumina ash to a slurry concentration of 10-30 wt% and a pH of 7.5-9.5. In some embodiments, for example, the secondary alumina ash slurry is adjusted to a slurry concentration of 10 wt%, 20 wt%, or 30 wt% and a pH of 7.5, 8, 8.5, or 9.5, using water glass as a modifier, sodium oleate as a collector, and terpineol as a foaming agent; the amount of water glass added is 1-4 g / kg of secondary alumina ash, and the amount of sodium oleate added is 1-5 g / kg of secondary alumina ash. The amount of terpineol added is 0.5-2 g / kg of secondary aluminum ash, and the flotation time is 3-8 min to obtain concentrate; in some embodiments, for example: the amount of water glass added is 1 g / kg, 2 g / kg or 4 g / kg of secondary aluminum ash, the amount of sodium oleate added is 1 g / kg, 2 g / kg or 5 g / kg of secondary aluminum ash, the amount of terpineol added is 0.5 g / kg, 1 g / kg or 2 g / kg of secondary aluminum ash, and the flotation time is 3 min, 4 min, 5 min or 8 min.
[0027] In some embodiments, the alkaline leaching uses a NaOH solution with a concentration of 1-5 mol / L, a liquid-to-solid ratio of 5-15:1 mL / g, a leaching temperature of 70-95℃, and a leaching time of 1-4 hours to obtain the alkaline leaching solution. In some embodiments, for example, the alkaline leaching uses a NaOH solution with a concentration of 1 mol / L, 2 mol / L, 3 mol / L, or 5 mol / L, a liquid-to-solid ratio of 5:1 mL / g, 10:1 mL / g, or 15:1 mL / g, a leaching temperature of 70℃, 80℃, 85℃, or 95℃, and a leaching time of 1 hour, 2 hours, or 4 hours. h is used to obtain an alkaline leaching solution; precipitation is carried out by adjusting the pH of the alkaline leaching solution to 8.5-10.0 and adding Al(OH)3 seed crystals for aging precipitation to obtain a precipitate; calcination is carried out by calcining the precipitate at a temperature of 1000-1200℃ for a holding time of 3-5h. In some embodiments, for example, precipitation is carried out by adjusting the pH of the alkaline leaching solution to 8.5, 9.0 or 10.0 and adding Al(OH)3 seed crystals for aging precipitation to obtain a precipitate; calcination is carried out by calcining the precipitate at a temperature of 1000℃, 1100℃ or 1200℃ for a holding time of 3h, 4h or 5h.
[0028] In some embodiments, the high-purity α-Al2O3 powder is mixed with MgO in a molar ratio of 1:1. The pore-forming agent is one or a mixture of several of polypropylene microspheres, polystyrene microspheres, starch, and resin spheres in any proportion. The amount of pore-forming agent added is 1-10 wt% of the total solid mass. In some embodiments, for example, the amount of pore-forming agent added is 1 wt%, 6 wt%, 8 wt%, or 10 wt% of the total solid mass. The binder is one or a mixture of several of PVA, PEG, and methylcellulose in any proportion. The amount of binder added is 3-8 wt% of the total solid mass. In some embodiments, for example, the amount of binder added is 3 wt%, 5 wt%, or 8 wt% of the total solid mass.
[0029] In some embodiments, the ball milling mixing involves ball milling at 60-100 rpm for 12 hours using high-purity alumina balls with a diameter of 5 mm and a ball-to-material ratio of 3:1 g / g; the microfluidic deagglomeration involves microfluidic deagglomeration once at a pressure of 1000 bar; in some embodiments, for example, the ball milling mixing involves ball milling at 60 rpm, 80 rpm, 90 rpm, or 100 rpm for 12 hours using high-purity alumina balls; The debinding and pre-reaction sintering process includes: heating at room temperature at a rate of 0.5-2℃ / min to 250-350℃ and holding for 0.5-2 hours; then heating to 500-700℃ and holding for 1-3 hours; finally heating to 1250-1450℃ and holding for 2-5 hours, followed by furnace cooling. In some embodiments, for example, heating at room temperature at a rate of 0.5℃ / min, 1℃ / min, or 2℃ / min to 250℃, 300℃, 320℃, or 350℃ and holding for 0.5 hours, 1 hour, or 2 hours; then heating to 500℃, 600℃, or 700℃ and holding for 1 hour, 2 hours, or 3 hours; finally heating to 1250℃, 1300℃, or 1450℃ and holding for 2 hours, 3 hours, 4 hours, or 5 hours, followed by furnace cooling.
[0030] In some embodiments, the MgAl2O4 nanopowder in the surface densification slurry has a purity ≥99.9%, D50=55nm, and BET specific surface area of 28-31m² / g. The MgAl2O4 nanopowder is dispersed in a solvent to control the solid content at 10-30 vol. The amount of rare earth sintering aid added is 1-5 wt% of the total mass of the surface densification slurry dry powder. In some embodiments, the amount of rare earth sintering aid added is 1 wt%, 2 wt%, or 5 wt% of the total mass of the surface densification slurry dry powder, for example. The dispersant is ammonium polyacrylate, and the amount added is 1 wt%, 3 wt%, or 5 wt% of the total mass of the surface densification slurry dry powder. The binder is PVA, and the amount added is 3 wt%, 5 wt%, or 8 wt% of the total mass of the surface densification slurry dry powder.
[0031] In some embodiments, the vacuum impregnation is performed under a vacuum of -0.06 to -0.09 MPa, with a holding time of 3-10 min, followed by immersion for 5-20 min after returning to normal pressure, and the number of coatings is 1-4, with each coating followed by drying at 60-100°C for 10-60 min. In some embodiments, the vacuum impregnation is performed under a vacuum of -0.06 MPa, -0.07 MPa, or -0.09 MPa, with a holding time of 3 min, 5 min, 8 min, or 10 min, followed by immersion for 5 min, 10 min, 15 min, or 20 min after returning to normal pressure, and the number of coatings is 1, 2, 3, or 4, with each coating followed by drying at 60°C, 70°C, or 100°C for 10 min, 20 min, or 60 min. Example 1
[0032] The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for use in the insulation cover of aluminum electrolysis cells includes the following steps: (1) Preparation of high-purity alumina: 1.1. Dry the secondary aluminum ash at 105℃ for 12 hours, cool it, and then lightly grind and sieve it through a 200-mesh sieve. 1.2. Add water to 200g of sieved secondary alumina ash to adjust the slurry concentration to 20wt%, place it in a flotation device, first add 2g / kg water glass of secondary alumina ash and stir for 2min, then adjust the pH of the slurry to 8.5; then add 3g / kg sodium oleate of secondary alumina ash, then add 1g / kg terpineol of secondary alumina ash, aerate and float for 5min, and perform reverse flotation for desilication and removal of soluble salts. After flotation, retain the sediment in the tank as concentrate, wash twice with 60℃ deionized water at a liquid-to-solid ratio of 5:1, and dry at 105℃ for 12h. 1.3. Take 100g of concentrate and leach it with 1.0L of 3mol / L NaOH solution at 90℃ and 400rpm for 2 hours. After leaching, filter while hot and retain the leaching solution; 1.4. Cool the alkaline leaching solution to 60℃, and add 2mol / L HCl dropwise while stirring to adjust the pH to 9.2±0.2; add 1wt% Al(OH)3 seed crystals according to the theoretical Al(OH)3 yield, age at 60℃ for 2h, let stand at room temperature for 12h, filter and collect the precipitate; 1.5 Wash the precipitate with deionized water until the filtrate is nearly neutral, dry at 110℃ for 12h; calcine by increasing the temperature from room temperature to 600℃ at 5℃ / min and holding for 1h, then increasing the temperature to 1150℃ at 3℃ / min and holding for 4.5h to obtain high-purity α-Al2O3 powder. (2) Preparation of porous spinel matrix: 2.1 100g of high-purity α-Al2O3 powder and MgO were mixed at a molar ratio of 1:1, 4.2g of polypropylene microspheres were added as a pore-forming agent and 22g of 5wt% PVA solution was added as a binder. The mixture was ball-milled at 100rpm for 12h using high-purity alumina balls with a diameter of 5mm and a ball-to-material ratio of 3:1g / g. Microfluidic deagglomeration was performed in a single step under a pressure of 1000bar. 2.2 Debinding and pre-reaction sintering include: heating to 300℃ at room temperature at 1 / min and holding for 1h; then heating to 600℃ and holding for 2h; finally heating to 1350℃ and holding for 3h, followed by furnace cooling to obtain a porous MgAl2O4 spinel matrix; Preparation of surface densification slurry: 25g of nano MgAl2O4 powder, 1g of Y2O3 and 0.4g of Sc2O3 rare earth sintering aid were added to 42g of deionized water and 18g of anhydrous ethanol, mixed with 15g of 10wt% PVA solution and 0.8g of ammonium polyacrylate dispersant, and ball-milled for 2h to obtain surface densification slurry; (3) Surface densification treatment: The porous MgAl2O4 spinel matrix was pretreated by lightly grinding with 800-grit sandpaper, then ultrasonically cleaned with alcohol for 10 min, and dried at 60℃. Subsequently, it was immersed in a surface densification slurry, vacuumed to -0.08 MPa and maintained for 5 min, then immersed for another 10 min after returning to normal pressure. After removal, excess slurry was scraped off, and the wet film thickness was controlled at 150-200 μm. It was then dried at 80℃ for 30 min. This immersion coating process was repeated twice. Finally, the treated sample was sintered by heating from room temperature to 600℃ at 2℃ / min and holding for 1 h, then heating to 1550℃ at 5℃ / min and holding for 3 h. After furnace cooling, a surface-densified porous magnesium-aluminum spinel composite material was obtained.
[0033] This surface-densified porous magnesium-aluminum spinel composite material for use in the insulation cover of aluminum electrolysis cells has a surface relative density ≥99% and an internal relative density within 91%; the surface shell thickness is 0.1-0.3mm, the surface grain size is >15μm, the internal grain size is 290-320nm, and the internal pore diameter is 50-200nm.
[0034] The XRD pattern of the surface-densified porous magnesium-aluminum spinel composite material prepared in this embodiment is shown below. Figure 1 As shown, from Figure 1 The diffraction peaks of the sample are basically consistent with the characteristic diffraction peaks of MgAl2O4 spinel, indicating that the main crystalline phase is MgAl2O4. No obvious impurity phase peaks such as MgO and α-Al2O3 were observed, suggesting that the precursor reaction was relatively complete and the resulting material has high phase purity. The SEM image of the cross-section of the surface-densified porous MgAl2O4 spinel composite material is shown below. Figure 2 It shows that, from Figure 2 As can be seen, the sample cross-section exhibits a distinct gradient structure of "dense surface - porous interior," with a continuous dense shell layer on the surface that is tightly bonded to the porous spinel matrix inside. No obvious cracks or delamination defects were observed at the interface. A schematic diagram of the porous internal structure of the surface-dense porous magnesium-aluminum spinel composite material prepared in this embodiment is shown below. Figure 3 As shown. From Figure 3 As can be seen, the material retains a relatively uniform porous skeleton structure, which is beneficial to reduce the thermal conductivity of the material and improve its thermal insulation performance, thus forming an integrated composite structure together with the dense surface layer.
[0035] The material obtained by this invention forms a continuous dense layer on the surface and maintains a porous structure inside, making it suitable for use as an insulation cover for aluminum electrolytic cells. Example 2
[0036] Except for the following differences, the remaining steps are the same as in Example 1: In the reverse flotation stage, the pulp pH is adjusted to 8.0; In the alkaline leaching stage, the NaOH concentration was adjusted to 2 mol / L, and the leaching time was 3 hours. In the porous spinel matrix preparation stage, the amount of polypropylene microspheres added was adjusted to 4 wt% of the total solid mass. In the surface densification slurry, only 3 wt% Y₂O₃ was added, without adding Sc₂O₃. The final sintering process was adjusted to 1600℃ for 2 hours. The resulting product also forms a composite structure with a relatively dense surface and a relatively porous interior, which can be used as a heat insulation cover material for aluminum electrolytic cells. Example 3
[0037] Except for the following differences, the remaining steps are the same as in Example 1: In the reverse flotation stage, the amount of water glass added is 3 g / kg of secondary aluminum ash; in the porous spinel matrix preparation stage, the amount of polypropylene microspheres added is adjusted to 2.5 wt% of the total solid mass; in the surface densification treatment, three vacuum impregnation coatings are used; and the final sintering regime is adjusted to 1550℃ for 2 hours. The resulting product has a good balance between surface densification and internal porosity, which can meet the comprehensive requirements of corrosion resistance and thermal insulation for insulation cover plates of different structural sizes.
[0038] 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 a surface-densified porous magnesium-aluminum spinel composite material for use in insulation covers of aluminum electrolytic cells, characterized in that... Includes the following steps: Preparation of high-purity alumina: After drying and sieving the secondary aluminum ash, reverse flotation is performed to remove silicon and soluble salts. The resulting concentrate is then subjected to alkali leaching, precipitation and calcination to obtain high-purity α-Al2O3 powder. Preparation of porous spinel matrix: The high-purity α-Al2O3 powder and MgO are mixed according to the stoichiometric ratio of MgAl2O4, and a pore-forming agent and binder are added. After ball milling, molding, debinding and pre-reaction sintering, a porous MgAl2O4 spinel matrix is obtained. Preparation of surface densification slurry: Nano MgAl2O4 powder, Y2O3 and / or Sc2O3 rare earth sintering aid are mixed with solvent, dispersant and binder to prepare surface densification slurry; Surface densification treatment: After surface pretreatment of the porous MgAl2O4 spinel matrix, the surface densification slurry is applied by impregnation, vacuum impregnation, brushing, scraping or a combination thereof, and after drying, it is sintered at high temperature to obtain a magnesium aluminum spinel composite material with a dense surface and porous interior.
2. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of the aluminum electrolytic cell according to claim 1, characterized in that: The secondary aluminum ash is first dried at 105℃ for 8-12 hours, then cooled, lightly ground, and passed through a 100-300 mesh sieve.
3. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of aluminum electrolytic cell according to claim 1, characterized in that: The reverse flotation desilication and soluble salt removal process is as follows: the secondary aluminum ash after screening is adjusted to a pulp concentration of 10-30 wt% and a pH of 7.5-9.
5. Water glass is used as a modifier, sodium oleate as a collector, and terpineol as a frother. The amount of water glass added is 1-4 g / kg of secondary aluminum ash, the amount of sodium oleate added is 1-5 g / kg of secondary aluminum ash, and the amount of terpineol added is 0.5-2 g / kg of secondary aluminum ash. The flotation time is 3-8 min to obtain concentrate.
4. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of the aluminum electrolytic cell according to claim 1, characterized in that: The alkaline leaching process involves using a NaOH solution with a concentration of 1-5 mol / L, a liquid-to-solid ratio of 5-15:1 mL / g, a leaching temperature of 70-95℃, and a leaching time of 1-4 h to obtain an alkaline leaching solution. Precipitation is achieved by adjusting the pH of the alkaline leaching solution to 8.5-10.0 and adding Al(OH)3 seed crystals for aging precipitation to obtain a precipitate. Calcination is performed by calcining the precipitate at a temperature of 1000-1200℃ for a holding time of 3-5 h.
5. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the insulation cover plate of aluminum electrolytic cells according to claim 1, characterized in that: The high-purity α-Al2O3 powder is mixed with MgO in a molar ratio of 1:
1. The pore-forming agent is one or a mixture of polypropylene microspheres, polystyrene microspheres, starch, and resin spheres in any proportion, and the amount of pore-forming agent added is 1-10 wt% of the total solid mass. The binder is one or a mixture of PVA, PEG, and methylcellulose in any proportion, and the amount of binder added is 1-5 wt% of the total solid mass.
6. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of the aluminum electrolytic cell according to claim 1, characterized in that: The ball milling mixing was carried out at 60-100 rpm for 12 hours. High-purity alumina balls with a diameter of 5 mm were used for ball milling, and the ball-to-material ratio was 3:1 g / g. The microfluidic deagglomeration was carried out once under a pressure of 1000 bar. The debinding and pre-reaction sintering process includes: heating to 250-350℃ at room temperature at a rate of 0.5-2℃ / min and holding for 0.5-2h; then heating to 500-700℃ and holding for 1-3h; finally heating to 1250-1450℃ and holding for 2-5h, followed by furnace cooling.
7. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of the aluminum electrolytic cell according to claim 1, characterized in that: In the surface densification slurry, the purity of MgAl2O4 nanopowder is ≥99.9%, D50=55nm, and BET specific surface area is 28-31m² / g. The MgAl2O4 nanopowder is dispersed in a solvent with a solid content controlled at 10-30 vol. The amount of rare earth sintering aid added is 1-5 wt% of the total mass of the surface densification slurry dry powder. The dispersant is ammonium polyacrylate, and the amount added is 1-5 wt% of the total mass of the surface densification slurry dry powder. The binder is PVA, and the amount added is 3-8 wt% of the total mass of the surface densification slurry dry powder.
8. The method for preparing the surface-densified porous magnesium-aluminum spinel composite material for the heat insulation cover plate of the aluminum electrolytic cell according to claim 1, characterized in that: The vacuum impregnation process involves a vacuum degree of -0.06 to -0.09 MPa, a pressure holding time of 3-10 min, followed by immersion for 5-20 min after restoring to normal pressure, and 1-4 coating cycles. After each coating, the product is dried at 60-100℃ for 10-60 min.
9. A surface-densified porous magnesium-aluminum spinel composite material for use in insulation covers of aluminum electrolytic cells, characterized in that: Prepared by any one of the preparation methods described in claims 1 to 8, the surface relative density is ≥99%, the internal relative density is within 91%; the surface shell thickness is 0.1-0.3 mm, the surface grain size is >15 μm, the internal grain size is 290-320 nm, and the internal pore diameter is 50-200 nm.
10. The application of the surface-densified porous magnesium-aluminum spinel composite material for aluminum electrolysis cell insulation cover plate according to claim 9, as an insulation cover plate, heat insulation cover plate or high-temperature refractory heat insulation component in the aluminum electrolysis process.