Magnesium aluminate spinel cover plate with compact surface and porous interior for aluminum electrolysis as well as preparation method and application of magnesium aluminate spinel cover plate

Abstract

The invention belongs to the technical field of advanced refractory ceramic material preparation, and relates to a magnesium aluminate spinel cover plate with a compact surface and a porous interior for aluminum electrolysis as well as a preparation method and application of the magnesium aluminate spinel cover plate. The preparation method comprises the following steps: dispersing MgAl2O4 nano powder in distilled water, adding a polyacrylic acid dispersant, adjusting the pH value to 10, carrying out ball milling, and carrying out micro-fluidic deagglomeration to obtain uniform slurry; carrying out slip casting on the uniform slurry, and drying to obtain a pure MgAl2O4 green body; immersing the pure MgAl2O4 green body into a Mg (NO3) 2 solution for standing, and carrying out vacuum drying to obtain a magnesium gradient green body; and sintering the magnesium gradient green body, and then cooling to room temperature to obtain the magnesium aluminate spinel with compact surface and porous interior. A segmented sintering mechanism is realized through magnesium concentration gradient regulation and control, the problem that surface compactness and internal porosity cannot be considered in a traditional process is solved, the green special refractory ceramic matrix composite material of magnesium aluminate spinel with compact surface and porous internal is prepared, and the product is suitable for the fields of electrolytic aluminum, high-temperature catalytic carriers, gas filtration and the like and has wide application prospects. The process is simple and low in energy consumption.

Description

Technical Field

[0001] This invention relates to a dense-surfaced, porous-internal magnesium-aluminum spinel cover plate for aluminum electrolysis, its preparation method, and its application, belonging to the field of advanced refractory ceramic material preparation technology. Background Technology

[0002] Aluminum electrolytic cells are the core equipment in the aluminum industry for producing aluminum through the electrolytic reduction of alumina. Their operation is characterized by high temperatures, high energy consumption, and high continuity. To ensure the long-term stable operation of the electrolytic cells, effective insulation of the cell structure is essential to reduce heat loss, lower energy consumption, and extend the cell's service life.

[0003] Currently, aluminum electrolysis cells typically use loose covering materials for insulation, which involves filling the top or surrounding area of ​​the cell with high-molecular-weight waste electrolyte and alumina for insulation. This method is simple in structure and easy to construct, and was widely used in early production. However, as aluminum electrolysis technology has developed towards larger scale, higher current density, and longer lifespan, the traditional loose covering material insulation method has gradually revealed many drawbacks, such as: 1) The loose covering material used in the existing aluminum electrolysis cell is prone to loosening during use, and the thickness and uniformity of the covering are difficult to control stably, resulting in insufficient heat insulation effect on the upper part of the electrolysis cell and low overall heat preservation efficiency. 2) Loose covering material is prone to cracking, collapse or partial exposure, causing the top of the anode or the anode-air interface to be exposed to an oxidizing atmosphere, which can easily lead to adverse effects such as anodizing. 3) During the pole replacement operation, the spreading, cleaning and recycling of loose covering material mostly rely on manual labor, which is labor-intensive and has poor consistency and controllability. 4) It is difficult to effectively coordinate with systems such as automation, online monitoring and robotic operations, which in turn restricts the intelligent upgrading of the electrolysis production process.

[0004] To address the aforementioned issues, a technical solution of thermal insulation cover plates for aluminum electrolytic cells has been proposed in recent years. By installing a structured thermal insulation cover plate on the upper part of the electrolytic cell, effective heat insulation and sealing of the cell are achieved. This method is beneficial for improving insulation efficiency, improving the thermal field distribution within the cell, and to a certain extent enhancing the safety and stability of the electrolytic cell operation, thus showing promising application prospects.

[0005] In the field of refractory and functional materials for aluminum electrolysis cells, magnesium aluminum spinel has been widely used due to its excellent high-temperature stability, corrosion resistance, thermal shock resistance, and good compatibility with the aluminum electrolysis environment. However, key technical challenges remain when applying magnesium aluminum spinel to the insulation cover plates of aluminum electrolysis cells.

[0006] The insulation cover plate for aluminum electrolytic cells requires both density and porosity in its material properties: on the one hand, the material needs to have a certain degree of density to ensure its mechanical strength, structural stability, and corrosion resistance; on the other hand, the material needs to have high porosity to reduce thermal conductivity and improve thermal insulation performance. However, existing magnesium-aluminum spinel materials often struggle to simultaneously achieve both dense and porous structures during preparation and application, resulting in a mutually restrictive relationship that limits their further application in the field of aluminum electrolytic cell insulation cover plates.

[0007] Therefore, there is an urgent need to provide a new technical solution to address the defects of loose covering materials in aluminum electrolytic cells and overcome the technical bottleneck of magnesium aluminum spinel materials in achieving both density and porosity, thereby enabling aluminum electrolytic cells to operate efficiently, stably, and for a long time. Summary of the Invention

[0008] To address the problems and shortcomings of existing technologies, this invention provides a surface-dense, internally porous magnesium-aluminum spinel, its preparation method, and its applications. The core of this invention is the creation of a magnesium concentration gradient through Mg(NO3)2 impregnation and drying, combined with pressureless sintering at 1600℃ to achieve a segmented mechanism of "surface liquid-phase sintering for densification - internal solid-state sintering for pore preservation," ultimately obtaining the target gradient structure. This invention is achieved through the following technical solutions.

[0009] A method for preparing a dense-surfaced, porous-internal magnesium-aluminum spinel cover plate for aluminum electrolysis, comprising the following steps: Slurry preparation: MgAl2O4 nanoparticles were dispersed in distilled water, polyacrylic acid dispersant was added, pH was adjusted to 10, and the mixture was ball-milled and then deagglomerated by microfluidic control to obtain a uniform slurry; Preparation of magnesium gradient green body: The uniform slurry was poured into molds and dried to obtain pure MgAl2O4 green body; Pure MgAl2O4 green body was immersed in Mg(NO3)2 aqueous solution and allowed to stand, then vacuum dried to obtain magnesium gradient green body; Segmented pressureless sintering: Magnesium gradient green bodies are sintered and then cooled to room temperature to obtain magnesium aluminum spinel with a dense surface and porous interior.

[0010] The MgAl2O4 nanopowder has a purity of ≥99.9%, D50=55nm, and BET specific surface area of ​​28-31m² / g. The MgAl2O4 nanopowder is dispersed in distilled water to control the solid content at 20vol.

[0011] The amount of polyacrylic acid dispersant added is 0.5% of the mass of MgAl2O4 nanoparticles, and the pH is adjusted to 10 with ammonia.

[0012] The ball milling was performed 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 was performed once under a pressure of 1000 bar.

[0013] The uniform slurry was injected into a porous Al2O3 mold for slurry casting; it was first dried at 80°C under normal pressure for 2 hours, and then dried under vacuum at 60°C for 4 hours.

[0014] The concentration of the Mg(NO3)2 aqueous solution is 0.012-0.015 mol / L, and the liquid-solid ratio of the green body to the solution is 5:1 mL / g.

[0015] The magnesium gradient green blank is heated to 1600℃ in an air atmosphere at a rate of 5℃ / min, held at that temperature for 6 hours, and then cooled to room temperature at a rate of 3℃ / min. During the sintering of the magnesium gradient green blank, the stacking spacing of the magnesium gradient green blank is ≥5mm.

[0016] A dense-surfaced, porous-internal magnesium-aluminum spinel cover plate, prepared by the above method, has a surface relative density ≥99% and an internal relative density 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.

[0017] The aluminum electrolysis magnesium aluminum spinel cover plate with a dense surface and porous interior is subjected to surface polishing treatment, with a polishing accuracy Ra≤0.1μm.

[0018] A surface-dense, internally porous magnesium aluminum spinel cover plate prepared by the above method is used as a heat-insulating cover plate in the aluminum electrolysis process.

[0019] The beneficial effects of this invention are: (1) The structural advantages of simultaneously achieving both density and porosity: This invention uses structural design to create a composite structure in the same thermal insulation cover plate where the magnesium aluminum spinel material has a relatively dense surface and a relatively porous interior. This ensures the mechanical strength and impermeability required by the cover plate in the high temperature and strong corrosion environment of the aluminum electrolysis cell, while effectively reducing the overall thermal conductivity of the material. This solves the technical problem that existing magnesium aluminum spinel materials cannot simultaneously meet the requirements of density and thermal insulation performance. (2) Significantly improves the heat preservation effect of aluminum electrolysis cells, replacing the traditional landfill method: Compared with existing landfill insulation methods, the aluminum electrolytic cell insulation cover plate provided by this invention has a stable structure, controllable thickness and performance, which can effectively reduce heat loss from the electrolytic cell, improve the thermal field distribution of the cell, reduce energy consumption, and avoid problems such as easy settling and difficult maintenance of landfill materials. It is suitable for the operation requirements of modern large-scale, high current density aluminum electrolytic cells. (3) The material has good compatibility with the aluminum electrolysis environment and a long service life: This invention uses magnesium aluminum spinel material as the main component. Compared with traditional insulation materials, it has better high-temperature stability, corrosion resistance and thermal shock resistance. It can adapt to the long-term continuous operation of aluminum electrolysis cells, reduce the damage and replacement frequency of insulation structure, and improve the overall operation stability and safety of electrolysis cells. (4) The process is highly controllable and adaptable to different electrolytic cell operating conditions: By rationally controlling the material structure and preparation process, the density and pore structure of the insulation cover plate can be adjusted within a certain range, thereby achieving a balance between insulation performance and structural strength. This meets the differentiated performance requirements of insulation cover plates for aluminum electrolytic cells of different models and operating conditions, and has good engineering application value.

[0020] (5) The aluminum electrolysis cover plate with dense surface and porous interior 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 a) XRD pattern after sintering; b) Actual image of the product after sintering in Example 1; c) Actual image of the break (internal) in Example 1. Detailed Implementation

[0022] The method for preparing the dense-surfaced, porous-internal magnesium-aluminum spinel cover plate for aluminum electrolysis includes the following steps: (1) Slurry preparation: MgAl2O4 nanoparticles were dispersed in distilled water, polyacrylic acid dispersant was added, pH was adjusted to 10, and after ball milling, microfluidic deagglomeration was carried out to obtain a uniform slurry; (2) Preparation of magnesium gradient green body: The uniform slurry is poured into molds and dried to obtain pure MgAl2O4 green body; Pure MgAl2O4 green body was immersed in Mg(NO3)2 aqueous solution and allowed to stand, then vacuum dried to obtain magnesium gradient green body; (3) Segmented pressureless sintering: Magnesium gradient green blanks are sintered and then cooled to room temperature to obtain magnesium aluminum spinel with dense surface and porous interior.

[0023] In some embodiments, in step (1), 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 distilled water to control the solid content to 20vol.

[0024] In some embodiments, the amount of polyacrylic acid dispersant added in step (1) is 0.5% of the mass of MgAl2O4 nanoparticles, and the pH in step (1) is adjusted to 10 with ammonia.

[0025] In some embodiments, step (1) ball milling is ball milling at 60-100 rpm for 12 hours, for example 60 rpm, 80 rpm or 100 rpm, using high-purity alumina balls with a diameter of 5 mm and a ball-to-material ratio of 3:1 g / g.

[0026] In some implementations, step (1) microfluidic deagglomeration is performed once at a pressure of 1000 bar.

[0027] In some embodiments, the uniform slurry is injected into a porous Al2O3 mold for slurry casting; it is first dried at 80°C under normal pressure for 2 hours, and then dried under vacuum at 60°C for 4 hours.

[0028] In some embodiments, the concentration of the Mg(NO3)2 aqueous solution is 0.012-0.015 mol / L, for example 0.012 mol / L, 0.014 mol / L, or 0.015 mol / L, and the liquid-solid ratio of the green body to the solution is 5:1 mL / g.

[0029] In some embodiments, the magnesium gradient green blank is heated to 1600°C at 5°C / min in an air atmosphere, held at that temperature for 6 hours, and then cooled to room temperature at 3°C / min; during the sintering of the magnesium gradient green blank, the stacking spacing of the magnesium gradient green blank is ≥5mm.

[0030] A dense-surfaced, porous-internal magnesium-aluminum spinel cover plate, prepared by the above method, has a surface relative density ≥99% and an internal relative density 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.

[0031] The aluminum electrolysis magnesium aluminum spinel cover plate with a dense surface and porous interior is subjected to surface polishing treatment, with a polishing accuracy Ra≤0.1μm.

[0032] Another aspect of the present invention provides a surface-dense, internally porous magnesium aluminum spinel cover plate for use as a heat-insulating cover plate in the aluminum electrolysis process.

[0033] This method is used in conjunction with an integrated "slurry casting-magnesium impregnation-pressureless sintering" equipment, which includes: Slurry preparation unit: ball mill (with speed control), microfluidic deagglomeration device, pH meter; Green body forming unit: porous Al2O3 slurry injection mold, vacuum drying oven (with temperature / vacuum control), Mg concentration detector; Sintering unit: box furnace (with programmed temperature rise function), temperature sensor (accuracy ±1℃); Characterization units: Archimedes density analyzer, electron probe microanalyzer (EPMA), scanning electron microscope (SEM).

[0034] The vacuum drying oven has a built-in wind speed adjustment module (wind speed 0.5-1m / s) to ensure uniform migration of Mg²⁺; the box furnace is equipped with a green billet fixing rack to ensure that the green billet stacking distance is ≥5mm and to avoid sintering adhesion.

[0035] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0036] Example 1 MgAl2O4 nanoparticles (purity ≥99.9%, D50≈55nm, BET specific surface area 28–31m² / g) were added to distilled water to prepare a slurry, with the solid content controlled at 20 vol%. Polyacrylic acid dispersant was added at 0.5% of the mass of MgAl2O4 nanoparticles, and the pH of the slurry was adjusted to 10 with ammonia. The resulting slurry was ball-milled at 80 rpm for 12 h using high-purity alumina balls (5 mm in diameter) as the milling medium, with a ball-to-particle ratio of 3:1 g / g. Subsequently, the ball-milled slurry was subjected to microfluidic deagglomeration once at 1000 bar to obtain a homogeneous slurry.

[0037] The above uniform slurry was injected into a porous Al2O3 mold for slurry casting. After demolding, it was first dried at 80°C under normal pressure for 2 hours, and then dried under vacuum at 60°C for 4 hours to obtain pure MgAl2O4 green body.

[0038] Pure MgAl2O4 green blanks were immersed in an aqueous solution of Mg(NO3)2 with a concentration of 0.014 mol / L and a liquid-solid ratio of 5:1 mL / g. Subsequently, vacuum drying was performed to form a concentration gradient of Mg²⁺ in the green blank from the surface to the interior, resulting in a magnesium gradient green blank.

[0039] Magnesium gradient green blanks were placed in an air atmosphere for pressureless sintering. The temperature was increased to 1600℃ at a rate of 5℃ / min and held for 6 hours, and then cooled to room temperature at a rate of 3℃ / min. During sintering, the green blanks were stacked with a spacing of ≥5mm to avoid sticking, resulting in a magnesium aluminum spinel cover plate with a dense surface and a porous interior.

[0040] The XRD pattern of the sintered magnesium-aluminum spinel cover plate in this embodiment is shown below. Figure 1 As shown in (a), this embodiment yields a relatively pure magnesium-aluminum spinel cover plate. A photograph of the sintered magnesium-aluminum spinel cover plate is shown below. Figure 1 As shown in (b), the internal structure of the fracture surface of the magnesium aluminum spinel cover plate is as follows. Figure 1 As shown in (c).

[0041] from Figure 1As shown in (c), the obtained cover plate exhibits a gradient structure of "dense surface / porous interior," with a surface relative density ≥99%, an interior relative density ≤91%, and a surface shell thickness of 0.1–0.3 mm; the surface grain size is >15 μm, the interior grain size is 290–320 nm, and the interior pore size is 50–200 nm. The cover plate surface is polished to a polishing precision Ra ≤0.1 μm.

[0042] Example 2 Except for the following differences, the remaining steps are the same as in Example 1: In the slurry preparation stage, the ball milling speed is 60 rpm and the ball milling is 12 h; in the magnesium impregnation stage, the concentration of Mg(NO3)2 aqueous solution is 0.012 mol / L and the liquid-solid ratio is still 5:1 mL / g; the amount of other dispersant added (0.5 wt%), pH (10), microfluidic deagglomeration conditions (1000 bar × 1), slurry casting and drying regime (80℃ atmospheric pressure 2 h → 60℃ vacuum 4 h) and air atmosphere pressureless sintering regime (5℃ / min to 1600℃ holding for 6 h → 3℃ / min cooling, stacking spacing ≥ 5 mm) remain unchanged.

[0043] The resulting product also forms a gradient structure of dense surface and porous interior, meeting the following specifications: surface relative density ≥99%, internal relative density ≤91%, surface shell thickness 0.1–0.3 mm, and internal pore diameter 50–200 nm; and can be polished to Ra≤0.1 μm as needed.

[0044] Example 3 Except for the following differences, the remaining steps are the same as in Example 1: In the slurry preparation stage, the ball milling speed is 100 rpm for 12 h; in the magnesium impregnation stage, the concentration of Mg(NO3)2 aqueous solution is 0.015 mol / L, and the liquid-solid ratio is still 5:1 mL / g; the other conditions (solid content 20 vol%, PAA 0.5 wt%, pH=10, microfluidic 1000 bar×1, slurry casting and drying regime, and air atmosphere pressureless sintering: heating to 1600℃ at 5℃ / min and holding for 6 h, cooling at 3℃ / min, stacking spacing ≥5 mm) remain unchanged.

[0045] The resulting product has a stable surface-dense and internally porous gradient structure, meeting structural indicators such as surface relative density ≥99%, internal relative density ≤91%, shell thickness 0.1–0.3 mm, and internal pore diameter 50–200 nm. The surface can be polished to Ra≤0.1 μm to meet assembly or sealing requirements.

[0046] 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 magnesium aluminum spinel cover plate with a dense surface and porous interior for aluminum electrolysis, characterized in that... Includes the following steps: Slurry preparation: MgAl2O4 nanoparticles were dispersed in distilled water, polyacrylic acid dispersant was added, the pH was adjusted to 9-12, and the mixture was ball-milled and then deagglomerated by microfluidic control to obtain a uniform slurry; Magnesium gradient green body preparation: The uniform slurry was poured into molds and dried to obtain pure MgAl2O4 green bodies; the pure MgAl2O4 green bodies were immersed in Mg(NO3)2 aqueous solution and allowed to stand, and then vacuum dried to obtain magnesium gradient green bodies. Segmented pressureless sintering: Magnesium gradient green bodies are sintered and then cooled to room temperature to obtain magnesium aluminum spinel with a dense surface and porous interior.

2. The method for preparing a dense-surfaced, porous-internal magnesium-aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The MgAl2O4 nanopowder has a purity of ≥99.9%, D50=55nm, and BET specific surface area of ​​28-31m² / g. The MgAl2O4 nanopowder is dispersed in distilled water to control the solid content to 10-30 vol.

3. The method for preparing a dense-surfaced, porous magnesium aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The amount of polyacrylic acid dispersant added is 0.5% of the mass of MgAl2O4 nanoparticles, and the pH is adjusted to 9-12 with ammonia.

4. The method for preparing a dense-surfaced, porous magnesium aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The ball milling was performed 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 was performed once under a pressure of 1000 bar.

5. The method for preparing a dense-surfaced, porous magnesium-aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The uniform slurry was injected into a porous Al2O3 mold for slurry casting; it was first dried at 80°C under normal pressure for 2 hours, and then dried under vacuum at 60°C for 4 hours.

6. The method for preparing a dense-surfaced, porous-internal magnesium-aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The concentration of the Mg(NO3)2 aqueous solution is 0.012-0.015 mol / L, and the liquid-solid ratio of the green body to the solution is 5:1 mL / g.

7. The method for preparing a dense-surfaced, porous-internal magnesium aluminum spinel cover plate for aluminum electrolysis according to claim 1, characterized in that: The Mg gradient green body is heated to 1600℃ in air atmosphere at a rate of 5℃ / min, held at that temperature for 6 hours, and then cooled to room temperature at a rate of 3℃ / min. During the sintering of the magnesium gradient green body, the stacking spacing of the magnesium gradient green body is ≥5mm.

8. A dense-surfaced, porous-internal magnesium-aluminum spinel cover plate, characterized in that: It includes the preparation of any one of claims 1 to 7, with a surface relative density ≥99% and an internal relative density within 91%; a surface shell thickness of 0.1-0.3 mm, a surface grain size >15 μm, an internal grain size of 290-320 nm, and an internal pore diameter of 50-200 nm.

9. The dense-surfaced, porous-internal magnesium aluminum spinel cover plate according to claim 8, characterized in that: The aluminum electrolysis magnesium aluminum spinel cover plate with a dense surface and porous interior is subjected to surface polishing treatment, with a polishing accuracy Ra≤0.1μm.

10. A surface-dense, internally porous magnesium aluminum spinel cover plate according to claim 8 or 9, used as an insulation cover plate in aluminum electrolysis processes.

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