Anti-oxidation surface treating agent for prebaked anode and application of anti-oxidation surface treating agent

Abstract

The invention discloses an anti-oxidation surface treating agent for a prebaked anode and application of the anti-oxidation surface treating agent. The surface treating agent comprises the following components: a base material (aluminum sol or silica sol), an antioxidant (zirconium boride), a reinforcing phase (silicon carbide), a binder (aluminum dihydrogen phosphate), an auxiliary additive (rare earth oxide and the like) and water. Through cross-scale collaborative design of an aluminum / silica sosol-ZrB2-SiC composite system and in combination with an Al-O-C chemical bonding interface, a ZrO2-B2O3 oxygen diffusion barrier and a rare earth grain boundary passivation mechanism, the problems of oxidation resistance, interface stability splitting, coating thermal stress stripping, fluorinion permeation and the like in the traditional technology are solved, and oxidation resistance and interface stability are achieved at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon materials for aluminum electrolysis, and particularly relates to an antioxidant surface treatment agent for pre-baked anodes and its application. Background Art

[0002] As the core conductive material of an aluminum electrolysis cell, the pre-baked anode undertakes the dual functions of conducting electricity and supporting the electrolysis reaction in a high-temperature (950 °C) molten salt electrolyte environment. However, the intense reactions of its surface with oxygen, CO2, and fluorides (such as Na3AlF6) (such as C + O2 → CO2↑, C + CO2 → 2CO↑) lead to oxidation consumption, directly driving up the production cost of electrolytic aluminum and carbon emissions. Although existing technologies attempt to improve performance through means such as coating, composite material modification, and antioxidant doping, they all have significant limitations. For example, inorganic ceramic coatings (such as SiC, Al2O3) can improve oxidation resistance, but the difference in thermal expansion coefficients between them and the carbon matrix (4.5×10 -6 / °C for SiC and 2.8×10 -6 / °C for the carbon matrix) is likely to cause microcracks, and at high temperatures, they react with CO2 to form gaseous products (SiC + 3CO2 → SiO2 + 4CO↑), resulting in structural failure; carbon-based composite materials (such as graphite reinforcements) can increase the density, but the poor wettability of graphite with the electrolyte (contact angle > 110°) will increase the anode overvoltage, and the reinforcing phase is preferentially oxidized to form pits, accelerating corrosion; while antioxidant doping (such as metallic silicon) can reduce the oxidation rate, but the introduced low-melting-phase (such as Si-Fe eutectic) will weaken the high-temperature strength, and at the same time, the ash content increases (0.5% → 1.2%), polluting the purity of the aluminum liquid.

[0003] The core contradiction of the existing technology lies in the disconnection between the functional singularity and the complexity of the working conditions. For example, although ceramic coatings are resistant to high-temperature oxidation, they are difficult to resist the pulverization caused by fluoride ion penetration (F- diffusion coefficient reaches 10 -7 cm 2 / s); while carbon-based modified materials increase the density, but their reaction with the electrolyte (3C + 4Na3AlF6 → 4Al + 12NaF + 3CF4↑) exacerbates the consumption. In addition, most solutions ignore the collaborative protection requirements under the coupled action of thermal-chemical multi-fields: the interface between the coating and the matrix lacks chemical bonding and is easily peeled off under thermal cycling; the multi-layer structure (such as gradient coating) has complex processes and high energy consumption (150 kWh / t anode); although solutions such as rare earth modification are effective, the cost surges by 40% - 60%, making it difficult to be industrially promoted.

[0004] Deeper technical bottlenecks are reflected in the insufficient adaptability of the material system to the electrolysis environment. Taking the phosphate-bonded silicon carbide coating as an example (J.Mater.Sci., 2021), the P2O5 gas released by its high-temperature decomposition reaction (2AlPO4→Al2O3+P2O5↑) pollutes the electrolyte, while the local corrosion and increased resistance caused by metal additives (such as a 15% increase in resistance when the coating thickness > 200μm) directly affect the current efficiency. These problems essentially stem from the fragmented optimization of multiple objectives such as "anti-oxidation - interface stability - economy" in traditional technologies. There is an urgent need to develop a new type of surface treatment agent with chemical inertness, thermal matching, and process feasibility to achieve long-term synergistic protection of pre-baked anodes in extreme environments and promote the upgrading of the electrolytic aluminum industry towards low-carbon and high-efficiency directions. Summary of the Invention

[0005] In view of the above problems, the present invention provides an antioxidant surface treatment agent for pre-baked anodes and its application. Through the cross-scale collaborative design of the aluminum / silica sol-ZrB2-SiC composite system, combined with the Al-O-C chemical bonding interface, the ZrO2-B2O3 oxygen diffusion barrier, and the rare earth grain boundary passivation mechanism, it solves problems such as the fragmentation of anti-oxidation and interface stability, coating thermal stress peeling, and fluoride ion penetration in traditional technologies, and has both anti-oxidation and interface stability.

[0006] The technical solution of the present invention is: an antioxidant surface treatment agent for pre-baked anodes, characterized in that the components of the surface treatment agent are in a weight ratio of: 35%-50% matrix material, 20%-30% antioxidant, 10%-20% reinforcing phase, 5%-10% binder, 2%-10% auxiliary additive, 0-10% deionized water.

[0007] The matrix material is aluminum sol (Al2O3·nH2O) or silica sol (SiO2·nH2O), the solid content of the sol is 20%-40%, the pH value is 3.5-5.0, the sol particle size is 10-50nm, and the specific surface area > 200m 2 / g.

[0008] The antioxidant is zirconium boride (ZrB2).

[0009] The reinforcing phase is silicon carbide (SiC).

[0010] The binder is aluminum dihydrogen phosphate (Al(H2PO4)3).

[0011] The components and weight ratio of the auxiliary additive are: 30wt%-100wt% rare earth oxide, 0wt%-50wt% bentonite, 0wt%-30wt% nano-TiO2, 0-50wt% silicon carbide fiber, 0-20wt% ethanol.

[0012] The rare earth oxide is one or both of La2O3 and Y2O3.

[0013] Preparation method:

[0014] (1) Premixing and dispersion: Mix the matrix sol (aluminum sol / silica sol) with the solvent (water / ethanol) and stir until homogeneous.

[0015] (2) Powder addition: Sequentially add the main fillers (such as antioxidant ZrB2, reinforcing phase SiC) and auxiliary powders (such as La2O3 / Y2O3, etc.) and disperse until there is no agglomeration.

[0016] (3) Binder introduction: Dropwise add the binder Al(H2PO4)3 solution and stir to form a stable slurry.

[0017] Furthermore, the stable slurry can be further sieved (150 - 200 mesh) and degassed under vacuum to improve the slurry uniformity.

[0018] Furthermore, if the auxiliary additive uses nano - TiO2, it needs to be ultrasonicated with water and then dispersed.

[0019] Furthermore, if the auxiliary additive uses silicon carbide fibers, they are added after adding the binder, and stirred and dispersed evenly to form a stable slurry.

[0020] The present invention also provides the application of the above antioxidant surface treatment agent in the antioxidant surface treatment of pre - baked anodes.

[0021] Usage method of the antioxidant surface treatment agent: (1) Coating: Apply the stable slurry to the surface of the pre - baked anode workpiece by spraying, roll - coating or scraping, and control the wet film thickness to be 100 - 200 μm; (2) Curing: Gradually increase the temperature or dry at a constant temperature to form a dense protective layer.

[0022] Each raw material and its function:

[0023] Matrix material (sol): Forms a continuous network structure through dehydration and polycondensation, serving as the framework basis of the surface treatment agent. After sintering, the aluminum sol generates α - Al2O3, adjusts the CTE of the overall surface treatment agent through nano - pore design to match the CTE of the pre - baked anode, avoids thermal stress cracking, and generates amorphous aluminosilicate (Si - O - Al bond) at high temperatures to inhibit the intercalation reaction of fluorides.

[0024] Zirconium boride: Plays a role in in - situ oxidation protection and fluorine barrier in the surface treatment agent, that is, it reacts with oxygen at 950 °C to form a ZrO2 - B2O3 composite layer, where the liquid phase of B2O3 fills the grain boundary pores of ZrO2, and the oxygen diffusion coefficient is reduced to 10 -14 cm 2 / s, ZrO2 reacts with Na3AlF6 in the electrolyte to form a dense ZrF4 layer, inhibiting the penetration of fluoride ions.

[0025] Reinforcing phase: In the present invention, high-hardness particles are uniformly dispersed in the surface treatment agent to resist the erosion of electrolyte flow. Silicon carbide forms a SiO2 passivation layer in a CO2 atmosphere, delaying the oxidation rate.

[0026] Aluminum dihydrogen phosphate: Reacts with the carbon matrix at high temperature to form Al-O-C bonds, enhancing the interfacial bonding strength. AlPO4 can fill the microcracks of the surface treatment agent, reducing the porosity, and forming an AlF3-PO4 - composite film in an F-containing 3- environment to inhibit acid etching.

[0027] Rare earth oxides: Can pin the grain boundaries of the surface treatment agent, inhibiting grain growth at high temperature.

[0028] Bentonite: The montmorillonite content is ≥85%. The special layered structure can adsorb sol particles, improving the compactness of the surface treatment agent.

[0029] Nano-TiO2: It is of rutile type. TiO2 forms Ti-O-La bonds with La2O3, enhancing the F-chemisorption ability and synergistically blocking fluorine.

[0030] Silicon carbide fiber: Selectively absorbs energy in the microwave field to achieve internal gradient sintering of the surface treatment agent. In addition, the fibers can be interspersed therein to improve the flexural strength.

[0031] The technical effects of the present invention are as follows:

[0032] 1. Through the cross-scale collaborative design of the aluminum / silica sol-ZrB2-SiC composite system, combined with the Al-O-C chemical bonding interface, ZrO2-B2O3 oxygen diffusion barrier and rare earth grain boundary passivation mechanism, the present invention can be used to improve the oxidation resistance and interfacial stability of pre-baked anodes during aluminum electrolysis, extend the anode service life, and solve problems such as oxidation resistance, coating thermal stress peeling and fluoride ion penetration in traditional technologies.

[0033] 2. The present invention finally realizes that the oxidation weight loss rate of the pre-baked anode in a high-temperature (950 °C) aluminum electrolysis cell is less than 11.6% (a decrease of more than 20%), and the net anode consumption drops from the usual 470 kg / t-Al to 445 kg / t-Al.

[0034] 3. The present invention solves the pain points such as coating peeling, single function and high cost, provides a reliable material basis for industrial application, provides a material-level solution for the high-efficiency energy-saving and low-carbon production of the aluminum electrolysis industry, and has both oxidation resistance and interfacial stability. Specific embodiments

[0035] The following describes its effects through examples.

[0036] Aluminum sol / silica sol: pH value 3.5 - 5.0, sol particle size 10 - 50 nm, specific surface area > 200 m 2 / g, normal temperature storage period ≥ 6 months, no gel precipitation.

[0037] Zirconium boride: purity ≥ 99.5%, oxygen content < 0.3%, D 50 = 2 - 5 μm, spherical or quasi-spherical particles, hexagonal crystal system, lattice integrity > 95%.

[0038] Silicon carbide: α-SiC phase content ≥ 98%, Mohs hardness 9.5, particle size 1 - 5 μm, hardness ≈ 28 GPa.

[0039] La2O3 / Y2O3: purity ≥ 99.9%, particle size 50 - 200 nm, specific surface area > 20 m 2 / g. Rare earth oxides can pin the grain boundaries of the surface treatment agent and inhibit grain growth at high temperatures.

[0040] Bentonite: montmorillonite content ≥ 85%. The special layered structure can adsorb sol particles and improve the compactness of the surface treatment agent.

[0041] Nano-TiO2: rutile type, particle size 20 - 50 nm, surface hydroxyl density ≥ 3 per nm 2 . TiO2 forms Ti - O - La bonds with La2O3, enhancing the F-chemisorption ability and synergistically blocking fluorine.

[0042] Silicon carbide fiber: fiber diameter 5 - 10 μm, length 100 - 200 μm, dielectric loss tangent > 0.1.

[0043] Example 1:

[0044] The formula and ratio are: 42 wt% aluminum sol (40% solid content), 28 wt% ZrB2, 12 wt% SiC, 10 wt% Al(H2PO4)3, 5 wt% La2O3, 3 wt% deionized water.

[0045] Preparation and usage steps:

[0046] (1) Premixing and dispersion: Mix the aluminum sol and deionized water, and perform ultrasonic treatment at 40 kHz for 30 min until uniform;

[0047] (2) Adding powders: Add ZrB2, SiC, and La2O3 in sequence, and ball mill at 300 rpm for 2 h until there is no agglomeration;

[0048] (3) Introducing the binder: Slowly drip the Al(H2PO4)3 solution and stir at 600 rpm for 1 h to form a stable slurry;

[0049] (4) Filtration: Pass through a 200-mesh sieve, let it stand for 30 min under a vacuum of -0.1 MPa to defoam, and make a surface treatment agent;

[0050] (5) Spraying: Use airless spraying, with a pressure of 0.6 MPa and a coating thickness of 120 ± 10 μm;

[0051] (6) Curing: Dry at 150 °C for 2 h.

[0052] Calcination oxidation test: High-temperature oxidation calcination in an air atmosphere at 900 °C for 200 h.

[0053] Result: The average burn-off rate is 0.37%. After calcination, the surface is grayish-white, without visible cracks, oxidation points, the interface is tightly bonded, and the protection effect is excellent.

[0054] Example 2:

[0055] The formula and ratio are: 38 wt% aluminum sol (40% solid content), 25 wt% ZrB2, 10 wt% SiC, 9 wt% Al(H2PO4)3, 5 wt% La2O3, 3 wt% nano-TiO2, 10 wt% deionized water.

[0056] Preparation and usage steps:

[0057] (1) Nano-dispersion: Mix nano-TiO2 with water, ultrasonically treat at 100 W for 1 h, and then add aluminum sol and ultrasonically treat until evenly dispersed;

[0058] (2) Powder addition: Add La2O3, ZrB2, and SiC in sequence, ball mill at 250 rpm for 4 h until there is no agglomeration;

[0059] (3) Binder introduction: Slowly drop the Al(H2PO4)3 solution, stir at 600 rpm for 1 h to form a stable slurry;

[0060] (3) Filtration: Pass through a 150-mesh sieve, let it stand for 30 min under a vacuum of -0.1 MPa to defoam, and make a surface treatment agent;

[0061] (4) Spraying: Use airless spraying, with a pressure of 0.5 MPa and a thickness of about 100 μm;

[0062] (5) Curing: Gradually increase the temperature, dry at 150 °C for 1 h → dry at 200 °C for 1 h.

[0063] Calcination oxidation test: Calcinate in an air atmosphere at 900 °C for 200 h.

[0064] Result: The average burn-off rate is 0.30%. The surface is dense and white, without cracks, oxidation points, the interface is firmly bonded, and the protection effect is remarkable.

[0065] Example 3:

[0066] The formula and proportion are: 35wt% silica sol (30% solid content), 30wt% ZrB2, 15wt% SiC, 10wt% Al(H2PO4)3, 5wt% Y2O3, 5wt% silicon carbide fiber (aspect ratio 20:1).

[0067] Preparation and usage steps:

[0068] (1) Premixing and dispersion: Mix the silica sol with deionized water and perform ultrasonic treatment at 40 kHz for 30 minutes until uniform;

[0069] (2) Powder addition: Add ZrB2, SiC and Y2O3 in sequence and ball mill at 300 rpm for 2 hours until there is no agglomeration;

[0070] (3) Binder introduction: Slowly drop the Al(H2PO4)3 solution and stir at 600 rpm for 1 hour to form a stable slurry;

[0071] (4) Fiber dispersion: Add silicon carbide fiber and stir at 550 rpm for 1 h to prepare a surface treatment agent;

[0072] (5) Coating: Use a brush to scrape and apply, with a wet film thickness of 200 μm;

[0073] (6) Curing: Dry at 200 °C for 2 h to form a dense coating.

[0074] Calcination oxidation test: Calcinate in air atmosphere at 900 °C for 200 h.

[0075] Result: The average burn-off rate is 0.43%. The surface shows uniform silver-gray color. The silicon carbide fibers penetrate inside the surface treatment agent and there are no thermal stress cracks.

[0076] Example 4:

[0077] The formula and proportion are: 35wt% aluminum sol (40% solid content), 20wt% ZrB2, 20wt% SiC, 10wt% Al(H2PO4)3, 5wt% La2O3, 5wt% silicon carbide fiber (aspect ratio 20:1), 5wt% deionized water.

[0078] Preparation and usage steps:

[0079] (1) Premixing and dispersion: Mix the aluminum sol with deionized water and perform ultrasonic treatment at 40 kHz for 30 minutes until uniform;

[0080] (2) Powder addition: Add ZrB2, SiC and La2O3 in sequence and ball mill at 300 rpm for 2 hours until there is no agglomeration;

[0081] (3) Binder introduction: Slowly dropwise add the Al(H2PO4)3 solution and stir at 600 rpm for 1 hour to form a stable slurry;

[0082] (4) Fiber dispersion: Add silicon carbide fibers and stir at 550 rpm for 1 h to prepare a surface treatment agent;

[0083] (5) Coating: Use a spatula to evenly coat on the surface of the pre-baked anode, and control the wet film thickness to be 200 ± 20 μm;

[0084] (6) Curing: Gradually increase the temperature, dry at 150 °C for 1 h → dry at 200 °C for 1 h to form a dense coating.

[0085] Calcination oxidation test: Calcinate in air atmosphere at 900 °C for 200 h.

[0086] Results: The average burn-off rate is 0.23%. The surface shows a uniform silver-gray color. The silicon carbide fibers penetrate inside the surface treatment agent, and there are no thermal stress cracks.

[0087] Through component optimization (such as the ZrB2 content of 20 wt% - 30 wt%) and process innovation (water-based formula), the present invention meets the core requirements of antioxidant, interface stability, low cost, and rapid manufacturing. All data are based on laboratory verification and industrial pilot test results, and can be directly adapted to the upgrade of the electrolytic aluminum production line.

Claims

1. An antioxidant surface treatment agent for prebaked anodes, characterized in that: The components of the surface treatment agent are as follows: 35%-50% matrix material, 20%-30% antioxidant, 10%-20% reinforcing phase, 5%-10% binder, 2%-10% auxiliary additive and 0-10% water in weight ratio; The matrix material is aluminum sol or silica sol; the antioxidant is zirconium boride, the reinforcing phase is silicon carbide; and the binder is aluminum dihydrogen phosphate.

2. The antioxidant surface treatment agent according to claim 1, characterized in that: The components and weight ratio of the auxiliary additives are: 30wt%-100wt% rare earth oxide, 0wt%-50wt% bentonite, 0wt%-30wt% nano-TiO2, 0-50wt% silicon carbide fiber and 0-20wt% ethanol.

3. The antioxidant surface treatment agent according to claim 2, characterized in that: The rare earth oxide is one or both of La2O3 and Y2O3.

4. The antioxidant surface treatment agent according to claim 1, characterized in that: The aluminum sol or silica sol has a solid content of 20%-40%, a pH value of 3.5-5.0, a sol particle size of 10-50nm, and a specific surface area of ​​>200m 2 / g.

5. The antioxidant surface treatment agent according to claim 1, characterized in that: The silicon carbide fiber has a fiber diameter of 5-10 μm and a length of 100-200 μm.

6. The method for preparing an antioxidant surface treatment agent according to claim 2 or 3, characterized in that: (1) Premixing and dispersion: Mix the base material with solvent water or ethanol and stir until uniform; (2) Powder addition: Add the powder components of the antioxidant, reinforcing phase and auxiliary additives in sequence and disperse until there is no agglomeration; (3) Introduction of binder: Add aluminum dihydrogen phosphate solution as binder dropwise and stir evenly to form a stable slurry.

7. The method for preparing an antioxidant surface treatment agent according to claim 6, characterized in that: The stable slurry is passed through a 150-200 mesh sieve and subjected to vacuum defoaming to improve the slurry uniformity.

8. The method for preparing an antioxidant surface treatment agent according to claim 6, characterized in that: If nano-TiO2 is used as an auxiliary additive, it needs to be dispersed after adding water and ultrasonic treatment. If silicon carbide fiber is used as an auxiliary additive, add it after adding the binder and stir to disperse it evenly to form a stable slurry.

9. Use of the antioxidant surface treatment agent according to any one of claims 1 to 5 in antioxidant surface treatment of prebaked anodes.

10. The method for using the antioxidant surface treatment agent prepared according to claim 6, characterized in that: (1) Application: Apply the stable slurry to the surface of the pre-baked anode workpiece by spraying, rolling or scraping, and control the wet film thickness to 100-200 μm; (2) Curing: Step heating or constant temperature drying to form a dense protective layer.