Cathode repair material for electrolytic cell aluminum and preparation method thereof

Abstract

This invention discloses a cathode repair material for aluminum electrolytic cells and its preparation method. The repair material, on a dry basis, mainly consists of the following components by weight percentage: modified corundum 96.2%-98.0%, metallic silicon powder 0.5%-0.8%, passivated aluminum powder 0.5%-0.8%, nano-silica 0.6%-1.6%, rare earth mineral powder 0.4%-0.6%, and dispersant 0%-0.01%. The repair material of this invention can bond tightly with the graphite cathode, reducing crack formation and effectively improving erosion resistance, thereby extending the repair time.

Description

Technical Field

[0001] This invention relates to the field of electrolytic aluminum technology, specifically to a cathode repair material for aluminum electrolytic cells and its preparation method. Background Technology

[0002] In the electrolytic aluminum production process, the electrolytic cell, as the core production equipment, directly determines production efficiency, product quality, and production costs through the stable operation of its cathode system. Cathode damage, however, is a long-standing and common problem plaguing the industry, severely hindering its green and efficient development. Cathode damage is influenced by a combination of factors, with poor start-up quality during electrolytic cell roasting being the primary cause. Improper temperature gradient control or uneven heating rates during roasting can lead to thermal stress within the cathode carbon block, forming microcracks and creating a potential for subsequent damage. The long-term presence of defective cells (such as hot, cold, or pressurized cells) can cause uneven cathode current distribution, with excessively high current density in localized areas, leading to overheating and damage. As the electrolytic cell ages, the cathode carbon block is subjected to long-term effects such as sodium corrosion and scouring by molten aluminum, gradually reducing its structural strength and making it prone to cracks, erosion pits, and chipping. This can even lead to serious accidents such as electrolyte leakage and cathode steel rod melting, forcing premature shutdown and major repairs, resulting in significant economic losses.

[0003] In recent years, in response to the "dual carbon" target and energy conservation and emission reduction policies, all-graphite cathodes, due to their excellent electrical and thermal conductivity, have been widely used in electrolytic aluminum production. They can effectively reduce cell voltage, improve current efficiency, and reduce electricity consumption per ton of aluminum. However, the mechanical strength of all-graphite cathodes is significantly lower than that of semi-graphite cathodes, and their resistance to sodium corrosion and erosion is weaker, making cathode damage more prominent. Shortened cathode lifespan has become a fatal flaw restricting their large-scale application. Industry data shows that the average lifespan of all-graphite cathode electrolytic cells is 25%-35% shorter than that of semi-graphite cathode cells, with an average annual cathode wear height of 15-25mm. Cell shutdowns caused by damage result in huge additional losses and production risks for enterprises every year. Therefore, research on efficient and long-lasting cathode repair technologies has become an urgent need to solve industry pain points and promote the high-quality development of electrolytic aluminum.

[0004] Currently, the traditional online cathode repair method in the industry mainly uses magnesia or a mixture of magnesia and calcium fluoride for filling. Although this process is simple to operate and low in cost, and is widely used in the field, it has many unavoidable drawbacks. On the one hand, the particle size distribution and density of this type of repair material are unreasonable, resulting in loose filling, difficulty in achieving dense caking, and high porosity. The repair layer is prone to falling off and cracking, failing to form a stable protective structure, requiring repeated repairs. This not only increases the labor intensity of workers but also frequently interferes with the normal operation of the electrolytic cell, affecting the continuity of production. On the other hand, the repair material contains a high content of impurities such as iron. These impurities will integrate into the electrolyte system, lower the primary crystallization temperature of the electrolyte, disrupt the electrolyte balance, and lead to fluctuations in cell conditions and frequent occurrences of anode effects. At the same time, it will also contaminate the aluminum liquid, reduce the purity of the primary aluminum, and affect the added value of the product.

[0005] To improve repair effectiveness, corundum repair materials with α-alumina as the main component have emerged in the market in recent years. While their high-temperature resistance is superior to traditional magnesia-based repair materials, significant technical shortcomings remain. α-alumina has a melting point as high as 2050℃, making it difficult to melt and solidify at the electrolytic cell's operating temperature of 950℃-980℃. This prevents it from tightly bonding with the damaged cathode area, easily creating gaps between the repair material and the carbon block, thus failing to effectively prevent the penetration of molten aluminum and electrolyte. Furthermore, some products fail to meet impurity control standards, negatively impacting the electrolytic cell's operation and the quality of primary aluminum, thus failing to fundamentally solve the core problem of long-term cathode repair.

[0006] Based on the above industry status quo, this invention aims to trial a novel cathode repair material for electrolytic cells. Addressing the technical deficiencies of existing repair materials, it optimizes the raw material ratio and preparation process, focusing on solving the following key problems: 1. How to achieve high true density and high bulk density in the electrolytic cell repair material to ensure stable standing in flowing molten aluminum; 2. How to efficiently caking and densify the repair material in a 900℃ molten aluminum environment; 3. The problems of high impurities and short repair life; 4. The low success rate of dense caking in molten aluminum, increasing the labor intensity of workers and wasting repair material due to repeated repairs. This ensures that the repair material can quickly fill and tightly caking in the electrolytic cell working environment, forming a dense and stable protective layer that effectively resists sodium corrosion and molten aluminum erosion, achieving long-term repair of cathode damage. Simultaneously, it strictly controls the impurity content of the repair material to avoid negative impacts on the electrolyte system and the quality of primary aluminum, balancing repair effectiveness and production stability. This provides technical support for extending the service life of electrolytic cells, reducing production costs, and improving enterprise economic benefits, thus contributing to the green and low-carbon transformation of electrolytic aluminum. Summary of the Invention

[0007] To address the shortcomings of the existing technology, the purpose of this invention is to provide a cathode repair material for aluminum electrolytic cells and its preparation method.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A cathode repair material for aluminum electrolytic cells, on a dry basis, mainly consists of the following components in weight percentage: modified corundum 96.2%-98.0%, metallic silicon powder 0.5%-0.8%, passivated aluminum powder 0.5%-0.8%, nano-silica 0.6%-1.6%, rare earth mineral powder 0.4%-0.6%, and dispersant 0%-0.01%.

[0009] Preferably, the modified corundum is obtained by mechanically crushing and surface doping of rare earth oxides, alkaline earth metals or their oxides together with corundum, followed by sieving, magnetic separation to remove iron, resulting in 3-10 mm doped modified particles with a corundum weight content of 98.5%-99.5%.

[0010] Preferably, the duration of the aforementioned mechanical collision is 3-8 hours.

[0011] Preferably, the corundum material mentioned above has an α-alumina content of ≥98%, and can be selected from materials such as fused white corundum and sintered corundum.

[0012] The alkaline earth metals or their oxides can be selected from magnesium oxide or calcium oxide with a particle size of 1-5 μm; rare earth oxides can be selected from lanthanum oxide or cerium oxide with a particle size of 1-50 μm, etc.

[0013] Preferably, the modified corundum is graded during use, specifically, by volume ratio, large particles larger than 8mm account for 1%-5%, medium particles of 5-8mm account for 70%-90%, and fine powder of 3-5mm accounts for 9%-25%.

[0014] Preferably, the particle size of the silicon metal powder is 1-4.5 μm; the particle size of the passivated aluminum powder is 1.5-3.0 μm, and the passivated aluminum powder is passivated aluminum with an anti-oxidation treatment on the surface and has an active aluminum content of more than 90%; the particle size of the rare earth mineral powder is 20-50 μm; and the dispersant is PAA-Na or PEG400.

[0015] Preferably, the above-mentioned cathode erosion pit repair material for aluminum electrolytic cells is prepared by the following steps: Step 1), Preparation of modified corundum; Step 2): First, mix the nano-silica sol and dispersant evenly. Then, add metallic silicon powder, passivated aluminum powder, and rare earth mineral powder. After stirring evenly, turn on the ultrasonic generator at an ultrasonic frequency of 20kHz-40kHz. Add the modified corundum from Step 1). The ratio of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and modified corundum is 0.5-1:0.0005-0.001:0.05-0.1:0.05-0.1:0.05-0.1:1. Turn on the vacuum pump to evacuate the vacuum, controlling the vacuum level to -0.06MPa to -0.08MPa. Maintain the vacuum and ultrasonic environment for 15-45 minutes. Then, turn off the vacuum pump, open the air inlet valve, restore normal pressure, and turn off the ultrasonic generator. Finally, discharge the material and quickly dry it to obtain the finished product.

[0016] Preferably, the vacuuming rate should be controlled at 0.01 MPa / min for 8-15 minutes before the vacuuming process.

[0017] Preferably, the rare earth mineral powder is yttrium silica powder, and the nano silica sol is an acidic silica sol with a pH of 2-4, a particle size of 10-20 nm, and a silica content of 20%-30%.

[0018] Preferably, the rare earth mineral powder is epidote powder, and the nano silica sol is an alkaline sodium silica sol with a pH of 9.0-10.5, a particle size of 10-20 nm, and a silica content of 10%-20%.

[0019] Corundum's main component is α-alumina. While directly using it to repair cathodes is low-cost and highly corrosion-resistant, the physicochemical compatibility between corundum and carbon cathodes is extremely poor, causing the repair material to easily detach and making it difficult to maintain the repair effect. Therefore, this application modifies corundum to a certain extent, specifically through two processes: The first process involves mechanically colliding corundum with alkaline earth metals or their oxides and rare earth oxides, resulting in 0.5%-1.5% doping of the corundum surface with alkaline earth metal oxides and rare earth oxides, while maintaining the main corundum component at 98.5%-99.5%. Mechanical collision not only breaks down the corundum and rare earth oxides but also introduces a small amount of rare earth oxides and alkaline earth metal oxides onto the corundum particles. This ensures the stable repair effect of the corundum repair material and improves the compatibility and adhesion between the corundum-based repair material and the all-graphite cathode. The second process involves impregnating modified corundum with metallic silicon powder, passivated aluminum powder, rare earth mineral powder, nano-silica sol, and a dispersant using a vacuum ultrasonic treatment. The impregnation process, combined with ultrasound and vacuum treatment, allows the nano-silica sol, metallic silicon powder, and passivated aluminum powder to better penetrate the microscopic pores on the corundum surface. The addition of ultrasound and a dispersant helps to effectively disperse the denser metallic silicon powder, passivated aluminum powder, and rare earth mineral powder within the silica sol. The nano-silica, metallic silicon powder, passivated aluminum powder, and rare earth mineral powder coating the corundum surface also serve a special purpose: these repair material particles are added to an electrolytic cell. Essentially, an iron tool is used to probe the cell to identify areas with erosion pits and cracks, and then the corresponding repair particles are poured in. After being added, the repair particles first settle through an electrolyte layer (approximately 18 cm deep). Then it enters the aluminum liquid layer (about 25-30 cm deep). The cathode is below the aluminum liquid layer, so the cathode is immersed in the aluminum liquid. Because the electrolyte has a strong erosion ability, the nano-silica, silicon powder, passivated aluminum powder, and rare earth mineral powder adsorbed on the surface of the modified corundum particles have two major functions: First, they are used as surface modifiers, which promote the surface doping modification of the inner layer; second, they are used as protective agents. When the repair material particles settle through the electrolyte layer, the outer surface needs to contact the electrolyte. Part of the silica sol, metallic silicon powder, passivated aluminum powder, and rare earth mineral powder adsorbed on this surface will melt into the electrolytic cell, protecting the inner doped part (the doped part in the modified corundum preparation process) from being damaged.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses modified corundum. Compared to corundum itself, during the collision between alkaline earth metals or their oxides, rare earth oxides and corundum materials, the corundum lattice is distorted, generating a large number of defects such as dislocations and vacancies. Alkaline earth metals and other materials are intermittently embedded in the corundum lattice or attached to the fracture surface. This doping effect changes the local charge distribution of corundum (this doping mainly changes the melting point of the crystal particle interface in a short time, promoting the fusion between particles). In the subsequent thermal environment, it can achieve low-temperature rapid densification, and can significantly improve the interface wettability of corundum, and make the repair material tightly bonded to the all-graphite cathode, reducing crack generation, thereby effectively extending the repair time of the repair material. 2. The modified corundum of the present invention is graded, which can improve the density of the repair layer, block the penetration of aluminum liquid, and effectively inhibit the generation of cracks; 3. This invention involves impregnating modified corundum with nano-silica sol, metallic silicon powder, passivated aluminum powder, etc., under vacuum reduced pressure and ultrasonic treatment. This effectively promotes the adsorption of nano-silica sol and metallic silicon powder into the spaces between modified corundum particles and into the micropores and microcracks on the particle surface. This improves the coating effect of nano-silica sol and metallic silicon powder on modified corundum. This not only improves the bonding strength of the repair interface and reduces the risk of peeling, but the coating layer can also protect the doping effect on the surface of modified corundum, thereby effectively improving the erosion resistance of the repair material. Detailed Implementation

[0021] The present invention will now be further described with reference to specific embodiments. Any parts not detailed below are based on existing techniques in the art. Example 1

[0022] The cathode repair material for aluminum electrolytic cells in this embodiment, on a dry basis, mainly consists of the following components by weight percentage: 97.69% modified corundum, 0.5% metallic silicon powder, 0.5% passivated aluminum powder, 0.8% nano-silica, 0.5% rare earth mineral powder, and 0.01% dispersant.

[0023] The preferred corundum material is corundum with an α-alumina content of ≥98%, specifically fused white corundum; the alkaline earth metal oxide is lightly calcined magnesium oxide; the rare earth oxide is lanthanum oxide; the rare earth mineral is yttrium silicate; the nano silica sol is acidic silica sol with a pH of 2-4, a particle size of 10-20 nm, and a silica content of 20%-30%; the dispersant is PAA-Na; the metallic silicon powder has a particle size of 1-4.5 μm, and the passivating aluminum powder has a particle size of 1.5-3.0 μm; the rare earth mineral powder has a particle size of 20-50 μm.

[0024] The repair material preparation method in this embodiment is implemented through the following steps: Step 1) Preparation of modified corundum: All corundum is fed into a vertical impact crusher with an impact strength of 40-80 m / s. Rare earth oxides are continuously added to the vertical impact crusher through a quantitative feeding mechanism. At the same time, alkaline earth metal oxides (lightly calcined magnesium oxide) are added to the vertical impact crusher through a pneumatic conveying system. The airflow speed is controlled at 15-20 m / s. The ratio of rare earth oxides to alkaline earth metal oxides is 0.5-1:0.1-1 by weight. The continuous collision time is 3-8 hours. Then, the particles are screened and separated to select particles of 3-10 mm. The 3-10 mm particles are then magnetically separated to remove iron. These particles are used as modified corundum. The weight content of corundum in the modified corundum is 98.5%-99.5%, and the doping amount is 0.5%-1.5%. Step 2) Modified corundum gradation: Calculated by volume ratio, large particles larger than 8mm account for 5%, medium particles of 5-8mm account for 85%, and fine powder of 3-5mm account for 10%; Step 3) First, mix the nano-silica sol and dispersant evenly, then add metallic silicon powder, passivated aluminum powder, and rare earth mineral powder. After stirring evenly, turn on the ultrasonic generator with an ultrasonic frequency of 20kHz-40kHz, and add the modified corundum after gradation in Step 2). The ratio of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and modified corundum is 1:0.001:0.1:0.1:0.1:1. Turn on the vacuum pump to evacuate the vacuum, control the vacuum degree to -0.06MPa, and maintain the vacuum and ultrasonic environment for 30 minutes. Then, turn off the vacuum pump, open the air inlet valve, restore normal pressure, and turn off the ultrasonic generator. Finally, discharge the material and dry it quickly to obtain the finished product. Example 2

[0025] Except for the following differences, everything else is the same as in Example 1: The cathode repair material for aluminum electrolytic cells in this embodiment, on a dry basis, mainly consists of the following components by weight percentage: 98% modified corundum, 0.5% metallic silicon powder, 0.5% passivated aluminum powder, 0.6% nano-silica, and 0.4% rare earth mineral powder. Example 3

[0026] Except for the following differences, everything else is the same as in Example 1: The repair material preparation method in this embodiment is implemented through the following steps: Step 1), Preparation of modified corundum; Step 2): First, mix the nano-silica sol and dispersant evenly. Then, add metallic silicon powder, passivated aluminum powder, and rare earth mineral powder. After stirring evenly, turn on the ultrasonic generator at an ultrasonic frequency of 20kHz-40kHz. Add the modified corundum from Step 1). The ratio of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and modified corundum is 1:0.001:0.1:0.1:0.1:1. Turn on the vacuum pump to evacuate the vacuum to -0.06MPa and maintain the vacuum and ultrasonic environment for 30 minutes. Then, turn off the vacuum pump, open the air inlet valve, restore normal pressure, and turn off the ultrasonic generator. Finally, discharge the material and dry it quickly to obtain the finished product. Comparative Example 1

[0027] Except for the following differences, everything else is the same as in Example 1: The preparation method of the repair material in this comparative example is achieved through the following steps: Step 1), Preparation of modified corundum; Step 2), Modified corundum gradation; Step 3) First, mix the nano-silica sol and dispersant evenly, then add metallic silicon powder, passivated aluminum powder, and rare earth mineral powder. After stirring evenly, add the modified corundum from Step 1). The ratio of the amount of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and modified corundum is 1:0.001:0.1:0.1:0.1:1. After soaking for 3 hours, discharge and quickly dry to obtain the finished product. Comparative Example 2

[0028] Except for the following differences, everything else is the same as in Example 1: The cathode repair material for aluminum electrolytic cells in this comparative example, on a dry basis, mainly consists of the following components by weight percentage: 97.69% fused white corundum, 0.5% metallic silicon powder, 0.5% passivated aluminum powder, 0.8% nano-silica, 0.5% rare earth mineral powder, and 0.01% dispersant.

[0029] The preparation method of this comparative repair material is as follows: First, the nano-silica sol and dispersant are mixed evenly. Then, metallic silicon powder, passivated aluminum powder, and rare earth mineral powder are added and stirred evenly. After stirring, the ultrasonic generator is turned on with an ultrasonic frequency of 20kHz-40kHz. Then, fused white corundum is added. The ratio of the amount of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and fused corundum is 1:0.001:0.1:0.1:0.1:1. The vacuum pump is turned on to evacuate the vacuum, and the vacuum degree is controlled at -0.06MPa. Then, the vacuum and ultrasonic environment is maintained for 30 minutes. Afterward, the vacuum pump is turned off, the air inlet valve is opened, the atmospheric pressure is restored, and the ultrasonic generator is turned off. Finally, the material is discharged and quickly dried to obtain the finished product. Comparative Example 3

[0030] The cathode repair material for aluminum electrolytic cells in this comparative example has the following composition: 100% modified corundum.

[0031] Repair material preparation method: All fused white corundum is put into a vertical impact crusher with an impact strength of 40-80 m / s. Rare earth oxides and alkaline earth metal oxides (lightly calcined magnesium oxide) are then added to the vertical impact crusher through a pneumatic conveying system. The ratio of rare earth oxides to alkaline earth metal oxides is 0.5-1:0.1-1 by weight. The continuous collision time is 3-8 hours. Then, the material is screened and separated to select particles of 3-10 mm. The 3-10 mm particles are then magnetically separated to remove iron and used as modified corundum. The modified corundum has a corundum weight content of 98.5%-99.5% and a doping amount of 0.5%-1.5%.

[0032] The effects of the products from Examples 1, 2, and 3 and Comparative Examples 1, 2, and 3 were tested, and the specific results are shown in the table below: Table 1. Approximate time (in hours) required for pre-caking at different temperatures.

[0033] Table 2. Room temperature compressive strength achievable at 900℃ with different caking times (unit: MPa) .

Claims

1. A cathode repair material for aluminum electrolytic cells, characterized in that, Based on dry matter, it mainly consists of the following components by weight percentage: modified corundum 96.2%-98.0%, metallic silicon powder 0.5%-0.8%, passivated aluminum powder 0.5%-0.8%, nano silica 0.6%-1.6%, rare earth mineral powder 0.4%-0.6%, and dispersant 0%-0.01%.

2. The cathode repair material for aluminum electrolytic cells according to claim 1, characterized in that, The modified corundum mentioned above is obtained by mechanically crushing and surface doping of rare earth oxides, alkaline earth metals or their oxides together with corundum, followed by sieving, magnetic separation to remove iron, resulting in 3-10 mm doped modified particles with a corundum weight content of 98.5%-99.5%.

3. The cathode repair material for aluminum electrolytic cells according to claim 2, characterized in that, The duration of the aforementioned mechanical collision was 3-8 hours.

4. The cathode repair material for aluminum electrolytic cells according to claim 1, 2, or 3, characterized in that, The modified corundum is graded during use, specifically by volume ratio, with large particles larger than 8mm accounting for 1%-5%, medium particles of 5-8mm accounting for 70%-90%, and fine powder of 3-5mm accounting for 9%-25%.

5. The method for preparing the cathode repair material for aluminum electrolytic cells according to claim 4, characterized in that, It is prepared through the following steps: Step 1), Preparation of modified corundum; Step 2): First, mix the nano-silica sol and dispersant evenly. Then, add metallic silicon powder, passivated aluminum powder, and rare earth mineral powder. After stirring evenly, turn on the ultrasonic generator at an ultrasonic frequency of 20kHz-40kHz. Add the modified corundum from Step 1). The ratio of nano-silica sol, dispersant, metallic silicon powder, passivated aluminum powder, rare earth mineral powder, and modified corundum is 0.5-1:0.0005-0.001:0.05-0.1:0.05-0.1:0.05-0.1:

1. Turn on the vacuum pump to evacuate the vacuum, controlling the vacuum level to -0.06MPa to -0.08MPa. Maintain the vacuum and ultrasonic environment for 15-45 minutes. Then, turn off the vacuum pump, open the air inlet valve, restore normal pressure, and turn off the ultrasonic generator. Finally, discharge the material and quickly dry it to obtain the finished product.

6. The cathode repair material for aluminum electrolytic cells according to claim 5, characterized in that, For the first 8-15 minutes after the vacuuming begins, the pumping rate should be controlled at 0.01 MPa / min.

7. The cathode repair material for aluminum electrolytic cells according to claim 5, characterized in that, The particle size of the aforementioned silicon metal powder is 1-4.5 μm; the particle size of the passivated aluminum powder is 1.5-3.0 μm, and the passivated aluminum powder is passivated aluminum with an anti-oxidation treatment on the surface and has an active aluminum content of more than 90%; the particle size of the rare earth mineral powder is 20-50 μm; the aforementioned dispersant is PAA-Na or PEG400.

8. The cathode repair material for aluminum electrolytic cells according to claim 7, characterized in that, The rare earth mineral powder mentioned above is yttrium silica powder, and the nano silica sol is an acidic silica sol with a pH of 2-4, a particle size of 10-20 nm, and a silica content of 20%-30%.

9. The cathode repair material for aluminum electrolytic cells according to claim 7, characterized in that, Preferably, the rare earth mineral powder is epidote powder, and the nano silica sol is an alkaline sodium silica sol with a pH of 9.0-10.5, a particle size of 10-20 nm, and a silica content of 10%-20%.

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