Anti-oxidation coating for anode carbon block and preparation method of anti-oxidation coating

Abstract

The invention provides an anti-oxidation coating for an anode carbon block and a preparation method of the anti-oxidation coating, and belongs to the technical field of anode anti-oxidation coatings. Comprising an inner-layer coating and an outer-layer coating, the inner coating is prepared from the following components in parts by weight: 45 to 55 parts of aluminum oxide, 3 to 4 parts of silica sol, 4 to 5 parts of aluminum phosphate, 0.5 to 1.0 part of boron nitride nanosheet, 0.5 to 1.0 part of CeO2, 3 to 5 parts of TiB2 graphene and 1.2 to 1.5 parts of polyacrylic acid dispersing agent; the outer-layer coating comprises the following components in parts by weight: 10-15 parts of aluminum oxide, 6-9 parts of silicon carbide whiskers, 8-10 parts of aluminum calcium magnesium silicate glass powder, 1-2 parts of boron nitride nanosheets, 1-2 parts of Y2O3, 0.5-1.0 part of CeO2, 1.5-2.5 parts of aluminum phosphate and 2-3 parts of mullite fibers. The high-temperature stability and environmental protection property of the coating can be remarkably improved, meanwhile, the gradient structure design is introduced, conductivity and permeability resistance are both considered, and the pole changing period is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of anode anti-oxidation coatings, and in particular to an anti-oxidation coating for an anode carbon block and a preparation method thereof. Background Art

[0002] Electrolytic aluminum is the final aluminum product obtained by electrochemical reaction at the two electrodes in the electrolytic cell at 950℃~970℃, using aluminum oxide as solute, molten cryolite as solvent, carbon body as anode, and aluminum liquid as cathode. After passing strong direct current, the electrochemical reaction takes place at 950℃~970℃. Anode carbon block is one of the main raw materials for aluminum electrolysis production. It plays an important role in electrolysis production and is also an important component of the production cost of aluminum electrolysis. In the process of electrolytic aluminum production, the consumption of carbon anode includes the following aspects: (1) Electrochemical consumption: In high-temperature electrolysis (about 950℃), the carbon of the anode reacts with the oxygen ions in the electrolyte to generate CO2 gas, resulting in direct oxidation loss of carbon atoms. This is the main form of anode consumption. Theoretically, 1 ton of aluminum requires about 330~400kg of anode carbon. (2) High-temperature oxidation: The anode surface exposed to high-temperature air (especially the part of the anode in the electrolytic cell that is not immersed in the electrolyte) will undergo thermal oxidation reaction with oxygen (C+O2→CO2), resulting in non-reactive loss. (3) Mechanical shedding (slagging or breakage): The anode may crack, peel, or even partially break due to electrolyte penetration, thermal stress, or uneven current distribution. Residues fall into the electrolyte, increasing additional losses. (4) Cryolite, as a flux in aluminum electrolysis, is highly corrosive, especially at high temperatures. It can corrode all metals and non-metals except carbon and aluminum.

[0003] At present, the common means to improve the oxidation resistance of the anode is to apply an antioxidant coating on the anode surface. The antioxidant coating can form a dense oxide film at high temperature, isolating oxygen from direct contact with the anode surface, inhibiting high-temperature oxidation reactions, and reducing thermal oxidation losses; reducing carbon oxidation and slagging can significantly extend the anode replacement cycle and reduce production costs; reducing the amount of residual anodes produced can reduce the pressure on solid waste treatment and reduce CO2 emissions.

[0004] Existing anode carbon block antioxidant coatings present several problems. First, they rely on fluoride or chloride salts. While they can enhance the anode's antioxidant properties by manipulating the crystal structure, fluoride salts are prone to volatilization or decomposition at high temperatures (approximately 950°C), releasing HF (hydrogen fluoride) and fluorine-containing dust, while chloride salts may produce Cl2 (chlorine gas) or HCl (hydrogen chloride). Fluoride salts may dissolve into the aluminum electrolyte (Na3AlF6-AlF3 system), causing electrolysis temperature fluctuations and reduced current efficiency. Chloride salts easily generate AlCl3 during high-temperature electrolysis, increasing electrolyte viscosity and adding impurities to the molten aluminum (resulting in Cl- contaminated aluminum products). Second, the use of water-soluble phenolic / epoxy resins as binders is prone to carbonization and decomposition at temperatures of 950°C, increasing the coating's porosity and weakening its protective effectiveness. Third, the use of albite and nepheline feldspar as fluxes, which have high Na / K contents, may penetrate into the electrolyte (Na3AlF6 system) after melting, leading to sodium contamination of the molten aluminum. Increased sodium content in aluminum deteriorates the metal's ductility. For example, Chinese invention patent publication number CN115895302B discloses an anti-oxidation coating for prebaked anodes used in electrolytic aluminum and its preparation method; Chinese invention patent publication number CN116606561B discloses an anti-oxidation and anti-corrosion coating for carbon blocks in electrolytic aluminum anodes; and Chinese invention patent publication number CN114806231B discloses an anti-oxidation coating for prebaked anodes used to reduce energy consumption in aluminum electrolytic cells and its preparation and application methods. These issues are all present. Furthermore, these existing anti-oxidation coatings are single-layer coatings with limited functionality, failing to balance electrical conductivity and permeability.

[0005] Based on this, there are still many problems that need to be overcome in the development of antioxidant coatings for anode carbon blocks. Summary of the Invention

[0006] The purpose of the present invention is to provide an antioxidant coating for anode carbon blocks and a preparation method thereof in response to the above-mentioned problems. By combining a fluorine-free crystal control system with a multi-mechanism collaborative bonding system, the high-temperature stability and environmental friendliness of the coating are significantly improved. At the same time, a gradient structure design is introduced to take into account both conductivity and anti-permeability.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] An anti-oxidation coating for an anode carbon block comprises an inner coating and an outer coating. The inner coating comprises, by weight, 45 to 55 parts of aluminum oxide, 3 to 4 parts of silica sol, 4 to 5 parts of aluminum phosphate, 0.5 to 1.0 parts of boron nitride nanosheets, 0.5 to 1.0 parts of CeO2, 3 to 5 parts of TiB2@graphene, 1.2 to 1.5 parts of a polyacrylic acid dispersant, and 10 to 15 parts of anhydrous ethanol. The outer coating comprises, by weight, 10 to 15 parts of aluminum oxide, 6 to 9 parts of silicon carbide whiskers, 8 to 10 parts of calcium magnesium aluminosilicate glass powder, 1 to 2 parts of boron nitride nanosheets, 1 to 2 parts of Y2O3, 0.5 to 1.0 parts of CeO2, 1.5 to 2.5 parts of aluminum phosphate, 2 to 3 parts of mullite fiber, and 8 to 12 parts of water.

[0009] In the present invention, preferably, the TiB2@graphene is prepared by the following method: weighing raw materials according to a molar ratio of tetrabutyl titanate to boric acid of 1:2, dissolving boric acid in anhydrous ethanol, then adding tetrabutyl titanate and stirring until completely dissolved; then adding citric acid and continuing to stir to form a transparent mixed solution, wherein the molar ratio of citric acid to boric acid is 1:10; maintaining a water bath temperature of 60±2°C, adding deionized water dropwise to the transparent mixed solution so that the volume ratio of deionized water to anhydrous ethanol is 2:1, adjusting the pH to 3-4 (adjusting with dilute hydrochloric acid), increasing the stirring rate to 1000-1200rpm, and continuously stirring for more than 6 hours to hydrolyze and generate TiO2-B2O3 gel; and adding 2m A TiO2-B2O3 gel is added to an ethanol dispersion of 0.12 g / mL graphene oxide, with the mass of GO accounting for 0.12-0.15 wt% of the total mass of the gel, and the mixture is stirred continuously at 60°C in a sealed container for 10-12 hours to form a homogeneous composite sol without precipitation; the composite sol is washed and transferred into a vacuum drying oven, and dried under reduced pressure at 80°C for 20-24 hours to obtain a porous dry gel; the porous dry gel is placed in a tubular furnace, and in an atmosphere of Ar / H2 with a volume ratio of 95:5, the temperature is first raised to 800°C and kept warm for 2 hours to reduce the graphene oxide to graphene; then the temperature is raised to 1450-1500°C and kept warm for 6 hours, and TiO2 and B2O3 react in the presence of a carbonaceous reducing agent to form TiB2.

[0010] In the present invention, preferably, the calcium magnesium aluminosilicate glass powder contains 55-60wt% SiO2, 20-25wt% Al2O3, 10-15wt% CaO, and 5-8wt% MgO in terms of oxide conversion. During preparation, raw materials quartz sand, alumina, calcium carbonate, and basic magnesium carbonate are prepared according to weight ratios. The raw materials are mixed and ground, melted at 1500-1600°C, and then quenched into glass slag with water, and ball milled to D50 <10μm.

[0011] In the present invention, preferably, the α-Al2O3 crystal form accounts for more than 80% of the alumina.

[0012] In the present invention, preferably, the aspect ratio of the silicon carbide whiskers is greater than 50.

[0013] In the present invention, preferably, the aspect ratio of the mullite fiber is greater than 30.

[0014] The present invention also provides a method for preparing an anti-oxidation coating for an anode carbon block, comprising the following steps:

[0015] S1. Preparation of inner coating;

[0016] S1.1 Add polyacrylic acid dispersant to anhydrous ethanol and stir at high speed to form a transparent solution; add ball-milled alumina powder and stir at low speed to pre-wet for 3 to 8 minutes;

[0017] S1.2 Add aluminum phosphate and CeO2 successively, increasing the stirring speed to 600-900 rpm and continuing for 10-25 minutes;

[0018] S1.3 Add boron nitride nanosheets, TiB2@graphene, and silica sol, and stir at a low speed of 200-300 rpm until the slurry is uniform;

[0019] S2. Preparation of outer coating;

[0020] S2.1 Pre-sinter the calcium magnesium aluminosilicate glass powder to 720-780℃ and keep it at this temperature for 45-75 minutes to increase the melting activity;

[0021] S2.2 Add aluminum oxide, aluminum phosphate, and silicon carbide whiskers to deionized water in sequence and disperse them by high-speed shearing.

[0022] S2.3 Add Y2O3, CeO2, and boron nitride nanosheets, and mix by ball milling. The diameter of the zirconium beads is 1 mm, the rotation speed is 400-500 rpm, and the ball milling time is 100-150 min.

[0023] S2.4 Add pre-burnt calcium magnesium aluminosilicate glass powder and mullite fiber, and stir at a low speed of 200-300 rpm for 20-40 minutes.

[0024] In the present invention, preferably, the alumina powder is pre-milled to a D50 particle size of ≤5 μm.

[0025] In the present invention, the working principle of each component is as follows:

[0026] In the inner coating, alumina forms the coating skeleton, maintaining structural rigidity at high temperatures, supporting the porous anode substrate, and reducing electrolyte penetration; silica sol acts as a low-temperature binder to improve the density of the coating and enhance the initial bonding strength; aluminum phosphate is a high-temperature inorganic binder, which decomposes at high temperatures to generate AlPO4, enhancing the chemical bonding between the coating and the carbon block at high temperatures, and the temperature resistance can reach 1200°C; boron nitride nanosheets spread vertically to block the oxygen diffusion path, and the lateral heat conduction can improve the thermal stress distribution, playing a role in resisting thermal shock and oxygen diffusion; CeO2 is a dynamic antioxidant, Ce 3+ / Ce 4+ The redox cycle absorbs gaseous O2 and reduces the oxidation reaction of carbon on the anode surface (C+O2→CO2); TiB2@graphene can improve the conductivity of the coating, while the graphene coating can prevent high-temperature oxidation of TiB2 (the oxidation starting temperature is raised to above 700°C); polyacrylic acid dispersant is adsorbed on the surface of nanoparticles to prevent the agglomeration of TiB2@graphene and boron nitride.

[0027] In the outer coating, the alumina and inner aluminum-based phases have the same structure, reducing the mismatch in interfacial thermal expansion and preventing interlayer peeling. SiC whiskers act as a three-dimensional reinforcing skeleton, interpenetrating to form a fiber network, greatly improving fracture toughness. Calcium magnesium aluminosilicate glass powder can melt at around 900°C to form a low-viscosity glass phase, filling the pores and generating an Al2SiO5-CaSiO4 fluoride salt corrosion barrier, which melts the sealing layer and blocks electrolyte penetration. Boron nitride nanosheets form internal and external oxygen barrier channels throughout the coating. The thermal expansion coefficient of mullite (3Al2O3·2SiO2) fibers is thermally matched to the anode carbon block, absorbing coating-anode interface stress and improving thermal shock buffering capabilities.

[0028] Y2O3-CeO2 co-doping can stabilize the alumina crystal, reduce the anodic oxidation rate, and delay the electrolyte penetration rate; 3+ Partially replace Al 3+ Afterwards, its larger ionic radius forms local distortion in the Al2O3 lattice, resulting in the obstruction of grain boundary migration. 3+ The doping of CeO2 introduces additional oxygen vacancy defects, which form a continuous diffusion network with the oxygen vacancies generated by CeO2, promoting the rapid diffusion of oxygen ions along the grain boundaries rather than directly penetrating the coating to attack the carbon matrix, thereby reducing the anodic oxidation rate; CeO2 also has reversible oxygen storage capacity, which can dynamically absorb or release oxygen through its own redox reaction, reducing the active oxygen (O - 、O2 -The concentration of Y2O3-CeO2 can be reduced to a low concentration, thus inhibiting the high-temperature oxidation reaction of the carbon anode. Y2O3-CeO2 co-doping also forms YAlO3 (yttrium aluminate) and CeAlO3 (cerium aluminate) nanophases at the alumina grain boundaries. These compounds increase the wetting angle of fluoride molten salts (such as Na3AlF6), significantly slowing the electrolyte penetration rate.

[0029] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0030] 1. The present invention adopts a double-layer gradient structure design with inner and outer layers of coating. The inner layer of coating is added with highly conductive TiB2@graphene, combined with the oxidation passivation effect of aluminum oxide and CeO2 to construct a low resistance (<5×10 -3 The outer coating comprises calcium magnesium aluminosilicate glass powder (CaO-MgO-Al2O3-SiO2) which melts at 900-950°C to form a continuous amorphous phase, filling pores with a self-healing effect and preventing oxygen diffusion. Y2O3-CeO2 co-doping stabilizes the alumina crystal form, and combined with the toughening effect of silicon carbide whiskers and mullite fibers, a dense barrier is formed to resist molten salt penetration and thermal shock, significantly blocking the infiltration of electrolytes (Na3AlF6-AlF3) and improving thermal shock resistance. Through precise component design and multi-level synergy, the present invention achieves breakthroughs in the multi-dimensional performance of anti-oxidation, electrical conductivity, corrosion resistance, thermal shock resistance, and bonding strength.

[0031] 2. The inner and outer coatings of the present invention are scientifically designed, with matching thermal expansion gradients, to achieve a continuous gradient design with gradually decreasing thermal expansion coefficients of the outer layer, inner layer, and anode substrate, thereby reducing thermal stress concentration. The inner layer focuses on electrical contact and basic anti-oxidation: TiB2@graphene optimizes current distribution (current density fluctuation <5%), and CeO2 and boron nitride inhibit bulk oxidation. The outer layer focuses on mechanical protection and resistance to molten salt corrosion, with SiC whiskers / mullite blocking crack propagation and calcium magnesium aluminosilicate glass blocking fluorine penetration. The inner layer of BN sheets slides to dissipate thermal stress, while the outer layer of SiC whiskers / mullite fibers bridges cracks, giving the overall structure significant impact resistance. Rare earth oxides produce a synergistic effect, with rare earth Y 3+ Stabilizes the grain boundary structure of alumina, Ce 3+ / Ce 4+ Dynamically adjust oxygen activity, surface CeO2 preferentially consumes F - The internal Y2O3 consolidates the anti-corrosion base, and the double barrier delays the failure of the coating.

[0032] In general, the antioxidant coating of the present invention is beneficial to reducing anodic oxidation consumption, extending the pole-changing cycle, reducing the number of pole-changing times, lowering resistance, improving current efficiency, and greatly reducing the generation of carbon slag in the electrolytic cell. DETAILED DESCRIPTION

[0033] In order to more clearly express the present invention, the present invention is further described below through specific embodiments. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0034] In some embodiments of the present invention, the α-Al 2 O 3 crystal form accounts for more than 80% of the alumina.

[0035] In some embodiments of the present invention, calcium magnesium aluminosilicate glass powder is prepared from quartz sand, alumina, calcium carbonate, and basic magnesium carbonate. Specifically, the raw materials are prepared according to a weight ratio, mixed and ground, melted at 1500-1600°C, then quenched into glass slag, and ball milled to D50 <10μm.

[0036] Example 1

[0037] An anti-oxidation coating for anode carbon blocks comprises an inner coating and an outer coating. The inner coating comprises: 45 parts aluminum oxide, 3 kg silica sol, 4 kg aluminum phosphate, 0.5 kg boron nitride nanosheets, 1.0 kg CeO2, 3 kg TiB2@graphene, 1.2 kg polyacrylic acid dispersant, and 10 kg anhydrous ethanol. The outer coating comprises: 10 kg aluminum oxide, 6 kg silicon carbide whiskers, 8 kg calcium magnesium aluminosilicate glass powder, 2 kg boron nitride nanosheets, 1 kg Y2O3, 0.5 kg CeO2, 1.5 kg aluminum phosphate, 2 kg mullite fiber, and 8 kg water. The silicon carbide whiskers have an aspect ratio greater than 50, the mullite fiber has an aspect ratio greater than 30, and the calcium magnesium aluminosilicate glass powder contains, in oxide equivalents, 55 wt% SiO2, 25 wt% Al2O3, 12 wt% CaO, and 8 wt% MgO.

[0038] TiB2@graphene was prepared by the following method: 5.00 kg of tetrabutyl titanate and 1.812 kg of boric acid were weighed at a molar ratio of 1:2; the boric acid was dissolved in 20 L of anhydrous ethanol, and then tetrabutyl titanate was added and stirred until completely dissolved; 565 g of citric acid was added and continued to stir to form a transparent mixed solution; the water bath temperature was maintained at 60 ± 2 ° C, 20 L of deionized water was added to the transparent mixed solution, the pH was adjusted to 4 with 1 mol / L dilute hydrochloric acid, the stirring rate was increased to 1200 rpm, and stirring was continued for more than 6 hours to hydrolyze to form TiO2-B2O3 gel; 2 kg of ethanol dispersion of graphene oxide with a concentration of 1 mg / mL was added to the mixture. TiO2-B2O3 gel, in which GO accounts for 0.12wt% of the total mass of the gel, is continuously stirred at 60°C in a sealed container for 10 hours to form a homogeneous composite sol without precipitation; the composite sol is washed and transferred to a vacuum drying oven, and dried under reduced pressure at 80°C for 20 hours to obtain a porous dry gel; the porous dry gel is placed in a tubular furnace, and in an atmosphere with an Ar / H2 volume ratio of 95:5, the temperature is first raised to 800°C and kept for 2 hours to reduce graphene oxide to graphene; then the temperature is raised to 1450°C and kept for 6 hours, and TiO2 and B2O3 react in the presence of a carbonaceous reducing agent to form TiB2.

[0039] A method for preparing an anti-oxidation coating for an anode carbon block comprises the following steps:

[0040] S1. Preparation of inner coating;

[0041] S1.1 Pre-ball-mill alumina powder to a D50 particle size of ≤5μm. Add polyacrylic acid dispersant to anhydrous ethanol and stir at high speed to form a transparent solution. Add the ball-milled alumina powder and stir at low speed for 3 minutes to pre-wet.

[0042] S1.2 Add aluminum phosphate and CeO2 successively, increase the stirring speed to 600 rpm, and continue for 25 minutes;

[0043] S1.3 Add boron nitride nanosheets, TiB2@graphene, and silica sol, and stir at 200 rpm until the slurry is uniform;

[0044] S2. Preparation of outer coating;

[0045] S2.1 Alumina powder is pre-ball-milled to a D50 particle size of ≤5μm. Calcium magnesium aluminosilicate glass powder is pre-sintered to 720℃ for 75min to increase melting activity.

[0046] S2.2 Add aluminum oxide, aluminum phosphate, and silicon carbide whiskers to deionized water in sequence and disperse them by high-speed shearing.

[0047] S2.3 Add Y2O3, CeO2, and boron nitride nanosheets and mix by ball milling. The diameter of the zirconium beads is 1 mm, the rotation speed is 400 rpm, and the ball milling time is 150 min.

[0048] S2.4 Add pre-calcined calcium magnesium aluminosilicate glass powder and mullite fiber, and stir at a low speed of 200 rpm for 40 minutes.

[0049] Example 2

[0050] An anti-oxidation coating for anode carbon blocks includes an inner coating and an outer coating. The inner coating comprises: 50 kg alumina, 3.5 kg silica sol, 4.5 kg aluminum phosphate, 0.8 kg boron nitride nanosheets, 0.8 kg CeO2, 4 kg TiB2@graphene, 1.3 kg polyacrylic acid dispersant, and 12 kg anhydrous ethanol. The outer coating comprises: 12 kg alumina, 8 kg silicon carbide whiskers, 9 kg calcium magnesium aluminosilicate glass powder, 1.5 kg boron nitride nanosheets, 1.5 kg Y2O3, 0.8 kg CeO2, 2 kg aluminum phosphate, 2.5 kg mullite fiber, and 10 kg water. The silicon carbide whiskers have an aspect ratio greater than 50, and the mullite fiber has an aspect ratio greater than 30. The calcium magnesium aluminosilicate glass powder contains, in oxide equivalents, 57 wt% SiO2, 22 wt% Al2O3, 13 wt% CaO, and 8 wt% MgO.

[0051] TiB2@graphene was prepared by the following method: 5.00 kg of tetrabutyl titanate and 1.812 kg of boric acid were weighed at a molar ratio of 1:2; the boric acid was dissolved in 20 L of anhydrous ethanol, and then tetrabutyl titanate was added and stirred until completely dissolved; 565 g of citric acid was added and stirred continuously to form a transparent mixed solution; the water bath temperature was maintained at 60 ± 2 ° C, 20 L of deionized water was added dropwise to the transparent mixed solution, the pH was adjusted to 3.5 with 1 mol / L dilute hydrochloric acid, the stirring rate was increased to 1000 rpm, and stirring was continued for 6 hours to hydrolyze and generate TiO2-B2O3 gel; 2 kg of ethanol dispersion of graphene oxide with a concentration of 1 mg / mL was added to the mixture. TiO2-B2O3 gel, with GO mass accounting for 0.15wt% of the total mass of the gel, was stirred continuously at 60°C in a sealed container for 11 hours to form a homogeneous composite sol without precipitation; the composite sol was washed and transferred to a vacuum drying oven, and dried under reduced pressure at 80°C for 22 hours to obtain a porous dry gel; the porous dry gel was placed in a tubular furnace, and in an atmosphere of Ar / H2 volume ratio of 95:5, first heated to 800°C and kept warm for 2 hours to reduce graphene oxide to graphene; then heated to 1480°C and kept warm for 6 hours, and TiO2 and B2O3 reacted with a carbonaceous reducing agent to form TiB2.

[0052] A method for preparing an anti-oxidation coating for an anode carbon block comprises the following steps:

[0053] S1. Preparation of inner coating;

[0054] S1.1 Pre-ball-mill alumina powder to a D50 particle size of ≤5μm. Add polyacrylic acid dispersant to anhydrous ethanol and stir at high speed to form a transparent solution. Add the ball-milled alumina powder and stir at low speed to pre-wet for 5 minutes.

[0055] S1.2 Add aluminum phosphate and CeO2 successively, increase the stirring speed to 800 rpm, and continue for 15 minutes;

[0056] S1.3 Add boron nitride nanosheets, TiB2@graphene, and silica sol, and stir at 200 rpm until the slurry is uniform;

[0057] S2. Preparation of outer coating;

[0058] S2.1 Alumina powder is pre-ball-milled to a D50 particle size of ≤5μm. Calcium magnesium aluminosilicate glass powder is pre-sintered to 750℃ for 60min to improve melting activity.

[0059] S2.2 Add aluminum oxide, aluminum phosphate, and silicon carbide whiskers to deionized water in sequence and disperse them by high-speed shearing.

[0060] S2.3 Add Y2O3, CeO2, and boron nitride nanosheets and mix by ball milling. The diameter of the zirconium beads is 1 mm, the rotation speed is 400 rpm, and the ball milling time is 120 min.

[0061] S2.4 Add pre-calcined calcium magnesium aluminosilicate glass powder and mullite fiber, and stir at a low speed of 200 rpm for 30 minutes.

[0062] Example 3

[0063] An anti-oxidation coating for anode carbon blocks comprises an inner coating and an outer coating. The inner coating comprises: 55 kg alumina, 4 kg silica sol, 5 kg aluminum phosphate, 1.0 kg boron nitride nanosheets, 0.5 kg CeO2, 5 kg TiB2@graphene, 1.5 kg polyacrylic acid dispersant, and 15 liters of anhydrous ethanol. The outer coating comprises: 15 kg alumina, 9 kg silicon carbide whiskers, 10 kg calcium magnesium aluminosilicate glass powder, 1 kg boron nitride nanosheets, 2 kg Y2O3, 1.0 kg CeO2, 2.5 kg aluminum phosphate, 3 kg mullite fiber, and 12 kg water. The silicon carbide whiskers have an aspect ratio greater than 50, the mullite fiber has an aspect ratio greater than 30, and the calcium magnesium aluminosilicate glass powder contains, in oxide equivalents, 60 wt% SiO2, 20 wt% Al2O3, 15 wt% CaO, and 5 wt% MgO.

[0064] TiB2@graphene was prepared by the following method: 5.00 kg of tetrabutyl titanate and 1.812 kg of boric acid were weighed at a molar ratio of 1:2; the boric acid was dissolved in 20 L of anhydrous ethanol, and then tetrabutyl titanate was added and stirred until completely dissolved; 565 g of citric acid was added and continued to stir to form a transparent mixed solution; the water bath temperature was maintained at 60 ± 2 ° C, 20 L of deionized water was added dropwise to the transparent mixed solution, the pH was adjusted to 3 with 1 mol / L dilute hydrochloric acid, the stirring rate was increased to 10000 rpm, and stirring was continued for more than 6 hours to hydrolyze and generate TiO2-B2O3 gel; 2 kg of ethanol dispersion of graphene oxide with a concentration of 2 mg / mL was added to the mixture. TiO2-B2O3 gel, in which GO mass accounts for 0.15wt% of the total mass of the gel, is continuously stirred at 60°C in a sealed container for 12 hours to form a homogeneous composite sol without precipitation; the composite sol is washed and transferred to a vacuum drying oven, and dried under reduced pressure at 80°C for 24 hours to obtain a porous dry gel; the porous dry gel is placed in a tubular furnace, and in an atmosphere of Ar / H2 volume ratio of 95:5, first heated to 800°C and kept warm for 2 hours to reduce graphene oxide to graphene; then heated to 1500°C and kept warm for 6 hours, and TiO2 and B2O3 react in the presence of a carbonaceous reducing agent to form TiB2.

[0065] A method for preparing an anti-oxidation coating for an anode carbon block comprises the following steps:

[0066] S1. Preparation of inner coating;

[0067] S1.1 Pre-ball-mill alumina powder to a D50 particle size of ≤5μm. Add polyacrylic acid dispersant to anhydrous ethanol and stir at high speed to form a transparent solution. Add the ball-milled alumina powder and stir at low speed to pre-wet for 8 minutes.

[0068] S1.2 Add aluminum phosphate and CeO2 successively, increase the stirring speed to 900 rpm, and continue for 10 minutes;

[0069] S1.3 Add boron nitride nanosheets, TiB2@graphene, and silica sol, and stir at 300 rpm until the slurry is uniform;

[0070] S2. Preparation of outer coating;

[0071] S2.1 Alumina powder is pre-ball-milled to a D50 particle size of ≤5μm. Calcium magnesium aluminosilicate glass powder is pre-sintered to 780℃ and kept at this temperature for 45min to increase melting activity.

[0072] S2.2 Add aluminum oxide, aluminum phosphate, and silicon carbide whiskers to deionized water in sequence and disperse them by high-speed shearing.

[0073] S2.3 Add Y2O3, CeO2, and boron nitride nanosheets and ball mill to mix. The diameter of the zirconium beads is 1 mm, the rotation speed is 500 rpm, and the ball milling time is 100 min.

[0074] S2.4 Add pre-calcined calcium magnesium aluminosilicate glass powder and mullite fiber, and stir at a low speed of 300 rpm for 20 minutes.

[0075] Comparative Example 1

[0076] The difference between this comparative example and Example 2 is that the inner layer coating and the outer layer coating are mixed for use.

[0077] Comparative Example 2

[0078] The difference between this comparative example and Example 2 is that no Y2O3 and CeO2 are added to the outer coating.

[0079] Comparative Example 3

[0080] The difference between this comparative example and Example 2 is that TiB2@graphene is not added to the outer coating;

[0081] Comparative Example 4

[0082] The difference between this comparative example and Example 2 is that an equal amount of TiB2 in the outer coating is used instead of TiB2@graphene.

[0083] Comparative Example 5

[0084] The difference between this comparative example and Example 2 is that the composition of the calcium magnesium aluminosilicate glass powder is different, specifically 57wt% CaO, 22wt% Al2O3, 13wt% SiO2, and 8wt% MgO.

[0085] Comparative Example 6

[0086] The difference between this comparative example and Example 2 is that no boron nitride nanosheets are added to the inner coating and the outer coating.

[0087] Antioxidant test:

[0088] The coatings of Examples 1-3 and Comparative Examples 1-6 were subjected to an antioxidant test. The specific test method is as follows:

[0089] Prepare multiple identical anode carbon blocks with a diameter of 50 mm and a height of 30 mm, prepare the inner and outer coatings according to the examples and comparative examples, use an airless spray gun to spray the anode carbon blocks at a working pressure of 15 MPa, so that the anode carbon blocks are evenly covered with an inner coating with a thickness of 300 μm, and dry at room temperature for 40 minutes; then spray the outer coating (the same coating was sprayed twice in comparative example 1), and cure at room temperature for 5 hours. All anode carbon blocks are placed together in a high-temperature furnace, and a layer of cryolite powder is covered on each anode carbon block to simulate electrolytic aluminum liquid. Then, air is introduced into the high-temperature furnace for a dynamic oxidation test at a test temperature of 900°C for 10 hours. The state of the coating after high-temperature oxidation is observed, and the high-temperature oxidation weight loss rate is calculated to test the high-temperature anti-oxidation and anti-corrosion effect of the coating on the anode carbon blocks. The high-temperature oxidation weight loss rate is calculated by the following formula.

[0090] High temperature oxidation weight loss rate = (mass before calcination - mass after calcination) / mass before calcination × 100%

[0091] Each group was repeated three times and the average value was taken. The calculation results are shown in Table 1 below.

[0092] Table 1 High temperature oxidation test results

[0093]

[0094] The coating of Example 2 is complete; after the inner and outer coatings are mixed in Comparative Example 1 (mixed coating), it is impossible to control oxygen and resist penetration by stratification. The high Al2O3 in the inner layer has poor bonding ability with the SiC whiskers in the outer layer, resulting in micropores at the interface; cryolite penetrates through the pores, accelerating the oxidation corrosion of the substrate. Comparative Example 2 lacks the oxygen ion transport inhibition effect of Y2O3 and the oxidation self-repair ability of CeO2, resulting in a decrease in the density of the molten glass phase, a mismatch in the thermal expansion coefficient, and no rare earth oxide to buffer thermal stress; Comparative Example 3 lacks the conductivity of the TiB2@graphene inner layer (graphene is missing), and the uneven current distribution leads to local overoxidation. Comparative Example 4 Ordinary TiB2 has weaker oxidation resistance than the graphene-coated type, but is better than Comparative Example 3 without TiB2. The oxidation of TiB2 to TiO2 causes volume expansion and induces stress cracking. The composition of the glass powder in Comparative Example 5 is unbalanced, resulting in a decrease in melt viscosity. The coating cannot effectively seal the pores after melting. The high CaO promotes the reaction of cryolite (Na3AlF6) with the coating to generate volatile phases such as CaF2, resulting in obvious cracking.

[0095] Comparative Example 6: BN nanosheets have both lubricity and thermal conductivity, but their absence leads to a decrease in the thermal shock resistance of the coating. Cyclic thermal stress concentration induces network cracks.

[0096] Application test

[0097] The anode carbon block obtained in Example 2 was tested for its application using the anti-oxidation coating on an electrolytic aluminum anode carbon block. The application method is as follows:

[0098] The dimensions of the anode carbon block are 1770mm (length) * 770mm (width) * 665mm.

[0099] In the experimental group, a uniformly dispersed inner coating was sprayed onto the top and sides of the anode carbon blocks, evenly applied one or two times to a thickness of approximately 300 μm. The coating was then dried at room temperature for 40 minutes. The outer coating was then sprayed to a thickness of approximately 300 μm and cured at room temperature for 5 hours. Five similarly treated anode carbon blocks were used in a 400 kVA electrolytic cell for aluminum production.

[0100] Control group: anode carbon blocks without anti-oxidation coating treatment, 5 identical anode carbon blocks were used in the electrolytic aluminum production process of a 400KVA electrolytic cell.

[0101] The anode carbon blocks of the experimental group and the control group were electrolyzed in different electrolytic cells using the same conditions.

[0102] The application results show that the operating cycle of the anode carbon block in the control group is 35 days. After the carbon anode is removed and cleaned, it is found that the anode is severely oxidized and corroded, the surface is loose and porous, and even flaking and angular defects occur.

[0103] The carbon anode treated with the antioxidant coating of Example 2 of the present invention can still work normally after 37 days of use. After forcibly stopping the electrolysis, the electrodes are pulled out and cleaned, and it is found that the remaining carbon anodes are sharp and the size of the remaining residual electrodes is also larger than that of the control group. Therefore, the effect of extending the use of each anode by more than 2 days can be achieved.

[0104] The average cell voltage of the experimental group was 4.05±0.03, and that of the control group was 4.20±0.05, with the cell voltage reduced by 0.15V; the DC power consumption (kWh / t-Al) of the experimental group was 13350±50, and that of the control group was 13800±100, with the power consumption reduced by 450kWh / t-Al; after deducting the production cost of the coating, the production cost per ton of aluminum decreased by approximately RMB 185-230.

[0105] The above description is a detailed description of the preferred embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. Any equivalent changes or modifications completed under the technical spirit suggested by the present invention should fall within the patent scope covered by the present invention.

Claims

1. An anti-oxidation coating for anode carbon blocks, characterized by: The invention comprises an inner coating and an outer coating; the inner coating comprises, by weight, 45 to 55 parts of aluminum oxide, 3 to 4 parts of silica sol, 4 to 5 parts of aluminum phosphate, 0.5 to 1.0 parts of boron nitride nanosheets, 0.5 to 1.0 parts of CeO2, 3 to 5 parts of TiB2@graphene, 1.2 to 1.5 parts of polyacrylic acid dispersant, and 10 to 15 parts of anhydrous ethanol; the outer coating comprises, by weight, 10 to 15 parts of aluminum oxide, 6 to 9 parts of silicon carbide whiskers, 8 to 10 parts of calcium magnesium aluminosilicate glass powder, 1 to 2 parts of boron nitride nanosheets, 1 to 2 parts of Y2O3, 0.5 to 1.0 parts of CeO2, 1.5 to 2.5 parts of aluminum phosphate, 2 to 3 parts of mullite fiber, and 8 to 12 parts of water.

2. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The TiB2@graphene is prepared by the following method: weighing raw materials according to a molar ratio of tetrabutyl titanate to boric acid of 1:2, dissolving boric acid in anhydrous ethanol, then adding tetrabutyl titanate and stirring until it is completely dissolved; then adding citric acid and continuing to stir to form a transparent mixed solution, wherein the molar ratio of citric acid to boric acid is 1:10; maintaining a water bath temperature of 60±2°C, adding deionized water dropwise to the transparent mixed solution to make a volume ratio of deionized water to anhydrous ethanol of 2:1, adjusting the pH to 3-4 (adjusting with dilute hydrochloric acid), increasing the stirring rate to 1000-1200 rpm, and continuously stirring for more than 6 hours to hydrolyze and generate TiO2-B2O3 gel; and adding 1 mg / mL TiO2-B2O3 gel is added to an ethanol dispersion of graphene oxide, with the mass of GO accounting for 0.12-0.15wt% of the total mass of the gel. The mixture is stirred continuously at 60°C in a sealed container for 10-12 hours to form a homogeneous composite sol without precipitation. The composite sol is washed and transferred into a vacuum drying oven, dried under reduced pressure at 80°C for 20-24 hours to obtain a porous dry gel. The porous dry gel is placed in a tubular furnace and heated to 800°C and kept warm for 2 hours in an atmosphere with an Ar / H2 volume ratio of 95:5 to reduce the graphene oxide to graphene. The mixture is then heated to 1450-1500°C and kept warm for 6 hours to allow TiO2 and B2O3 to react in the presence of a carbonaceous reducing agent to generate TiB2.

3. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The calcium magnesium aluminosilicate glass powder contains 55-60wt% SiO2, 20-25wt% Al2O3, 10-15wt% CaO, and 5-8wt% MgO in terms of oxide conversion. During preparation, raw materials including quartz sand, aluminum oxide, calcium carbonate, and basic magnesium carbonate are prepared according to weight ratios. The raw materials are mixed and ground, melted at 1500-1600°C, then water-quenched into glass slag, and ball-milled to a D50 of less than 10μm.

4. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The α-Al2O3 crystal form accounts for more than 80% of the alumina.

5. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The aspect ratio of the silicon carbide whiskers is greater than 50.

6. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The aspect ratio of the mullite fiber is greater than 30.

7. The method for preparing an anti-oxidation coating for an anode carbon block according to claim 1, characterized in that: The following steps are involved: S1. Preparation of inner coating; S1.1 Add polyacrylic acid dispersant to anhydrous ethanol and stir at high speed to form a transparent solution; add ball-milled alumina powder and stir at low speed to pre-wet for 3 to 8 minutes; S1.2 Add aluminum phosphate and CeO2 successively, increasing the stirring speed to 600-900 rpm and continuing for 10-25 minutes; S1.3 Add boron nitride nanosheets, TiB2@graphene, and silica sol, and stir at a low speed of 200-300 rpm until the slurry is uniform; S2. Preparation of outer coating; S2.1 Pre-sinter the calcium magnesium aluminosilicate glass powder to 720-780℃ and keep it at this temperature for 45-75 minutes to increase the melting activity; S2.2 Add aluminum oxide, aluminum phosphate, and silicon carbide whiskers to deionized water in sequence and disperse them by high-speed shearing. S2.3 Add Y2O3, CeO2, and boron nitride nanosheets, and mix by ball milling. The diameter of the zirconium beads is 1 mm, the rotation speed is 400-500 rpm, and the ball milling time is 100-150 min. S2.4 Add pre-burnt calcium magnesium aluminosilicate glass powder and mullite fiber, and stir at a low speed of 200-300 rpm for 20-40 minutes.

8. The anti-oxidation coating for anode carbon blocks according to claim 1, characterized in that: The alumina powder is pre-ball-milled to a D50 particle size of ≤5 μm.

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