CN101736368A - Noble metal ceramic composite coating inert anode for aluminum electrolysis and preparation method thereof - Google Patents

Abstract

The noble metal ceramic composite coating inert anode for aluminum electrolysis and its preparation method relate to the field of nonferrous metal molten salt electrolysis and the preparation technology of metal ceramic composite materials. The inert anode is composed of an alloy matrix, an oxide film on the surface of the alloy and an outer noble metal-alumina composite layer, and the noble metal-alumina composite layer has a structure of noble metal-coated alumina particles. This kind of noble metal ceramic composite coating inert anode has the ideal inert anode characteristics of noble metals. During the aluminum electrolysis process, only the reaction of oxygen evolution occurs on the surface of the electrode, which can eliminate _CO2 pollution and protect the environment; it also has the characteristics of metal ceramic composite materials, such as Wear-resistant, erosion-resistant, thermal shock-resistant, can prevent interdiffusion between precious metals and alloy substrates, etc.; has the characteristics of long life, energy saving, and simplified industrial operations. The novel noble metal-ceramic composite coating inert anode can save a large amount of noble metal compared with the noble metal coating inert electrode, and thus has industrial applicability.

Description

Noble metal ceramic composite coating inert anode for aluminum electrolysis and preparation method thereof

technical field

The invention relates to the field of non-ferrous metal molten salt electrolysis and the preparation technology of cermet composite materials, in particular to the low-temperature aluminum electrolysis inert anode and the preparation technology thereof.

Background technique

The anode of the current Hall-Héroult aluminum electrolysis uses carbon material. The reaction of carbon anode is: Al ₂ O ₃ +3/2C→2Al+3/2CO ₂ , resulting in a large amount of carbon anode consumption. The carbon anode consumption per ton of aluminum exceeds 400kg, making the aluminum electrolysis industry a high-carbon industry. Economically, it is necessary to invest in the construction of a huge carbon anode factory, which increases the production cost. In terms of technology, due to the continuous consumption of carbon anodes, the pole distance is unstable, and complex mechanical devices are required to adjust the pole distance, which complicates the aluminum electrolysis process; Stable operation of the electrolyzer. In terms of environmental protection, a large amount of CO ₂ and a small amount of CO, as well as carcinogenic CF _n substances are produced during the electrolysis reaction, which seriously pollutes the atmospheric environment. It is necessary to build a huge flue gas purification treatment system, which increases the production cost of aluminum electrolysis.

Due to the above reasons, since the birth of the aluminum electrolysis industry, people have been developing inert anodes to replace the current general-purpose carbon anodes (that is, active anodes), which is considered to be a revolution in the aluminum electrolysis industry. The inert anodes used in the aluminum electrolysis industry refer to those anodes that are not consumed or consumed in a small amount in the current common cryolite-alumina molten salt electrolysis. The inert anode reaction of aluminum electrolysis is: Al ₂ O ₃ → 2Al+3/2O ₂ . The great significance of developing an inert anode lies in: the electrode is not consumed during the electrolysis process, no additional carbon processing plant is required, and the production cost is reduced; the electrode is not consumed, the pole distance is stable, easy to control, the number of anode replacements is small, and the labor intensity is reduced; it can be used The higher anode current density increases the capacity of the electrolytic cell; the anode product is oxygen, which avoids environmental pollution, and oxygen can also be used as a by-product. It is estimated that the recovered oxygen may be 3% of the value of the original aluminum product. Aluminum ingot production costs can be reduced by about 30%.

Since the 1980s, the research on inert anodes has mainly focused on metal oxide anodes, alloy anodes and cermet anodes (Edited by Liu Yexiang et al., Modern Aluminum Electrolysis, Metallurgical Industry Press, 2008). However, so far, there is still no enterprise in the world that uses inert anodes for industrial aluminum electrolytic production. This is because metal oxide anodes, alloy anodes and cermet anodes all rely on the premise that the oxides or oxides formed on the surface of the electrodes have a certain resistance to cryolite-alumina molten salt corrosion. However, except for precious metals such as gold and platinum, all oxides have a certain solubility in cryolite-alumina molten salt, resulting in electrode corrosion that cannot meet the requirements of industrial aluminum electrolyte volume. As Donald R. Sadoway said, the research on the inert anode of aluminum electrolysis is still a challenge for human beings to extreme materials. He believes that the greatest hope of success lies in minimizing the experience already gained. (Donald R. Sadoway, Inert Anodes for the Hall-Héroult Cell: The Ultimate Materials Challenge, JOM, May 2001, 34-35).

Noble metals such as gold and platinum are ideal inert anode materials. Due to the high cost of noble metals, they cannot be directly used as inert anodes in industry. However, preparing noble metals as coatings is a promising route to exploit the properties of noble metals to create inert anodes. In the invention patent "noble metal coated inert anode for aluminum production" (patent publication number: CN 1612776A), SCX sputtering process is used to coat noble metal such as platinum on the surface of alloy wire as an inert electrode for aluminum electrolysis. The electrode can maintain its structural integrity after being kept in molten fluoride at 900 °C for 17 h. But this noble-metal-coated inert electrode has not found industrial application, suggesting that the technology still has some drawbacks. For example, when the noble metal is coated on the alloy substrate, the alloy substrate and the coated noble metal will undergo interdiffusion under the high-temperature working environment of aluminum electrolysis, forming an alloy layer containing noble metal, and the more active metal elements in the alloy layer can be selected. Oxidation, dissolved in molten salt, reduced at the cathode and entered into the aluminum product; in addition, the coated precious metal layer is only 1-10 μm thick, which is easy to be worn and scratched, resulting in high-speed corrosion of the alloy matrix. The current general view is that precious metal materials such as gold and platinum can be used as inert anodes in the laboratory, but they are not suitable for industrial aluminum production (Qiu Zhuxian, Aluminum Smelting in Prebaked Tanks, Metallurgical Industry Press, 2005). Therefore, preparing noble metals as coatings and utilizing the properties of noble metals to manufacture inert anodes that can be used in industry still requires new research ideas and technical approaches.

Contents of the invention

The purpose of the invention is to propose a novel noble metal ceramic composite coating inert anode for aluminum electrolysis and a preparation method thereof. The inert anode has ideal inert anode characteristics of noble metals, excellent mechanical properties, long service life and low overall cost, and has broad application prospects in the aluminum electrolysis industry.

The present invention is achieved through the following technical solutions:

A noble metal ceramic composite coating inert anode for aluminum electrolysis is composed of an alloy substrate, an oxide film on the surface of the alloy and a noble metal-alumina composite layer on the outer layer.

The alloy matrix of the inert anode is an iron-based alloy or a nickel-based alloy with a Cr content of 10-25% and a rare earth element content of 0-0.5%, which is formed by casting or machining;

The noble metal content in the noble metal-alumina composite layer is 5% to 30%, and the noble metal is Au, or Au-Pt alloy, or Au-Pd alloy, or Au-Rh alloy, and the content of Pt or Pd in the alloy is The content is 0%-40%, the content of Rh is 0%-10%, and the thickness of the noble metal-alumina composite layer is 10-100 μm.

The noble metal-alumina composite layer has a structure of noble metal-coated alumina particles, and the particle size of the alumina particles used is 10nm-100μm. The mass ratio is 9:1-5:5; the precious metal content in the noble metal-alumina composite layer is 5%-30% (mass percentage); the thickness of the noble metal-alumina composite layer is 10-100 μm.

The preparation method of the noble metal ceramic composite coating inert anode for aluminum electrolysis, the preparation of the noble metal-alumina composite layer on the outer layer of the inert anode comprises the following steps:

1) After uniformly mixing alumina particles and noble metal powder, ball milling in a high-energy ball mill for 1 to 10 hours to obtain a powder body of noble metal-coated alumina particles;

2) preparing an oxide film on the surface of the alloy substrate by pre-oxidizing in air;

3) Electrophoresis a layer of paint film on the surface of the above-mentioned oxide film; apply the powder of precious metal coated alumina particles on the paint film, apply pressure to make the paint film coat the precious metal coated alumina particle powder evenly Adhere to the oxide film surface to form a lacquer-noble metal-oxide composite coating;

4) Using a hot-pressing treatment device, the alloy substrate coated with a lacquer-noble metal-oxide composite coating on the surface is embedded in the mold of the hot-pressing treatment device equipped with alumina microspheres, through the hot-pressing treatment device The pressure head applies a pressure of 20-100 MPa, the temperature of the furnace of the hot-pressing treatment device is controlled at 900-1200°C, and the treatment is performed for 1h-10h to burn off the paint film, and the precious metal-coated alumina particle powder is sintered into a dense precious metal - Aluminum oxide composite layer, and maintain the structure of precious metal coated alumina particles;

5) Soak the alloy matrix forming the dense noble metal-alumina composite layer in 1-10% NaOH aqueous solution for 10-60 minutes, and wash with water to remove the aluminum oxide and other impurities attached to the surface.

The oxide film on the surface of the alloy substrate of the inert anode is roughened by physical and chemical methods such as surface microporation, shot peening, sandblasting and other physical and chemical methods of the iron-based alloy or nickel-based alloy containing rare earths processed and formed; after cleaning, Pre-oxidize in air at 800-1000°C for 1-10 hours to obtain a conductive oxide film.

The oxide film on the surface of the base alloy of the inert anode is roughened by using physical and chemical methods such as surface microporation, shot peening, sandblasting and other physical and chemical methods on the iron-based alloy or nickel-based alloy that does not contain rare earths; after cleaning , apply a rare earth oxide film or zirconia film with a thickness of 10-200nm on the surface, and then pre-oxidize in air at 800-1000°C for 1-10 hours to obtain a conductive oxide film. Adding rare earth to the alloy or applying a rare earth oxide film or zirconia film with a thickness of 10-200nm on the surface can change the growth mechanism of the oxide film formed by pre-oxidation on the alloy surface. New oxides are formed at the interface between the alloy and the oxide film, which can Improve the bonding force between the oxide film and the base alloy.

The structure of the noble metal-alumina composite layer obtained by this method makes the electrode have good electrical conductivity. The noble metal is coated with nano-scale alumina powder, and the composite of noble metal-coated nano-scale alumina powder is coated with micron-scale alumina powder. Aluminum particles, this precious metal-coated alumina particle structure coating, once the precious metal on the surface is destroyed, the exposed alumina particles will dissolve into the molten salt, and then expose the underlying precious metal, thus making the precious metal ceramic composite coating an inert anode The surface of the electrode is kept as a noble metal, so that the electrode has the ideal inert anode characteristics of the noble metal, and only the reaction of oxygen evolution occurs on the electrode surface during the aluminum electrolysis process.

The beneficial effects of the present invention are:

1. The noble metal-alumina composite layer of the present invention has a structure in which the noble metal coats the alumina particles so that the electrode has good electrical conductivity.

2. The structure of the precious metal-coated alumina particles of the present invention makes the coating have mechanical properties completely different from precious metals, and exhibits the characteristics of metal-ceramic composite materials, such as wear resistance, erosion resistance, and thermal shock resistance.

3. The structure of the noble metal-coated alumina particles in the present invention can greatly reduce the content of noble metal in the coating, and can significantly reduce the manufacturing cost of the new type of inert electrode.

4. The oxide film on the surface of the alloy of the present invention can prevent the interdiffusion between the noble metal and the base alloy, and keep the coating structure stable.

Description of drawings

Fig. 1 is the structural representation of the noble metal ceramic composite coating inert anode for aluminum electrolysis;

Fig. 2 is a structural schematic diagram of a hot-pressing treatment device;

in

1. Alloy matrix 5. Mold 2. Oxide film 6. Pressure head 3. Lacquer-noble metal-oxide composite coating 7. Stove 4. Alumina microspheres

Detailed ways

Fig. 1 is a schematic diagram of the structure of an inert anode with a noble metal ceramic composite coating for aluminum electrolysis. The inert anode is composed of an alloy substrate, an oxide film on the surface of the alloy and an outer noble metal-alumina composite layer. The noble metal-alumina composite layer has The structure of noble metal-coated alumina particles, in which the alumina particles are divided into micro-scale and nano-scale.

Fig. 2 is a structural schematic diagram of a hot-pressing treatment device, as shown in the figure: a hot-pressing treatment device is used to embed an alloy substrate 1 coated with a paint-noble metal-oxide composite coating 3 on the surface into an aluminum oxide microsphere 4 In the mold 5, a pressure of 20-100 MPa is applied through the indenter 6, and the temperature of the furnace 7 is controlled at 900-1200 ° C. After 1-10 hours of treatment, the paint film is burned off, and the precious metal-coated alumina particle powder is sintered into a dense precious metal -Alumina composite layer and maintain the structure of noble metal coated alumina particles.

Example 1:

Au powder, alumina powder with an average particle size of 5 μm, and alumina powder with an average particle size of 80 nm, with a mass ratio of 20:60:20, were high-energy ball milled for 6 hours to obtain a composite powder of Au-coated alumina. Process the Fe-20%Cr-0.5%Ce alloy into a sample with a diameter of 10mm and a length of 500mm; the surface is sandblasted; then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped Cr ₂ O ₃ film; Then cathodic electrophoresis is used to deposit a layer of polyurethane paint film; the powder of Au-coated alumina particles is applied on the paint film, and pressure is applied to make the paint film evenly adhere the Au-coated alumina particle powder to the surface of the sample. Using the hot-pressing treatment device shown in Figure 2, the alloy substrate 1 coated with the paint-Au-oxide composite coating 3 on the surface is embedded in the mold 5 equipped with alumina microspheres 4, and applied by the pressure head 6. The pressure is 80MPa, the temperature of the furnace 7 is controlled at 900°C, and the paint film is burned off after treatment for 3 hours. The Au-coated alumina particle powder is sintered to form a dense Au-alumina composite layer, and the Au-coated alumina particle is maintained. structure. Soak in 5% NaOH aqueous solution for 10-60min, wash with water to remove aluminum oxide and other impurities attached to the surface of the sample. The obtained Au-coated alumina composite layer had a thickness of 20 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , 35% K ₃ AlF ₆ , 35% Na ₃ AlF at 700°C; 26% AlF ₃ , 29.6% K ₃ AlF ₆ , 44.4% Na at 800°C ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total mass of the electrolyte was added to each electrolyte. The anode current density was 0.8mA/cm ² , the temperature was controlled at 700°C, 800°C and 900°C, respectively, and the electrolysis was performed for 6 hours. After the electrolysis, the Au content in the molten salt and the produced aluminum were tested respectively, and the test accuracy was 10 ^-6 g/g, and no Au was detected.

Example 2:

Au powder, Pt powder, alumina powder with an average particle size of 5 μm, and alumina powder with an average particle size of 80 nm, with a mass ratio of 16:4:60:20, were high-energy ball milled for 6 hours to obtain Au-20%Pt Composite powder coated with alumina. Process the Fe-20%Cr-0.5%Ce alloy into a sample with a diameter of 10mm and a length of 500mm; the surface is sandblasted; then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped Cr ₂ O ₃ film; Then use cathodic electrophoresis to deposit a layer of polyurethane paint film; apply the powder of Au-20% Pt coated alumina particles on the paint film, apply pressure to make the paint film uniformly coat the Au-20% Pt coated alumina particle powder adhere to the sample surface. Using the hot-pressing treatment device shown in Figure 2, the alloy matrix 1 coated with the paint-Au-20%Pt-oxide composite coating 3 on the surface is embedded in the mold 5 that alumina microspheres 4 are housed, and passed The pressure head 6 applies a pressure of 80MPa, the temperature of the furnace 7 is controlled at 1100°C, and the paint film is burned off after the treatment for 3 hours, and the Au-20%Pt-coated alumina particle powder is sintered into a dense Au-20%Pt-alumina Composite layer, and maintain the structure of Au-20%Pt coated alumina particles. Soak in 5% NaOH aqueous solution for 10-60min, wash with water to remove aluminum oxide and other impurities attached to the surface of the sample. The thickness of the obtained Au-20%Pt _- coated _Al2O3 composite layer is 20 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , 35% K ₃ AlF ₆ , 35% Na ₃ AlF ₆ at 700°C, 26% AlF ₃ , 29.6% K ₃ AlF ₆ , 44.4% at 800°C Na ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total mass was added to each electrolyte. The anode current density is 0.8mA/cm ² , the temperature is controlled at 700°C, 800°C and 900°C respectively, and electrolysis is performed for 6h. After the electrolysis, the contents of Au and Pt in the molten salt and the produced aluminum were tested respectively. The test accuracy was 10 ^-6 g/g, and neither Au nor Pt was detected.

Example 3:

Au powder, Pd powder, alumina powder with an average particle size of 5 μm, and alumina powder with an average particle size of 80 nm, with a mass ratio of 16:4:60:20, were high-energy ball milled for 6 hours to obtain Au-20% Pd Composite powder coated with alumina. The Ni-20%Cr-0.5%Ce alloy is processed into a sample with a diameter of 10mm and a length of 500mm; the surface is sandblasted; then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped Cr ₂ O ₃ film; Then use cathodic electrophoresis to deposit a layer of polyurethane paint film; apply the powder of gold-coated alumina particles on the paint film, and apply pressure to make the paint film evenly adhere the Au-20%Pd-coated alumina particle powder on the paint film sample surface. Using the hot-pressing treatment device shown in Figure 2, the alloy matrix 1 coated with the paint-Au-20%Pd-oxide composite coating 3 on the surface is embedded in the mold 5 that alumina microspheres 4 are housed, and passed The pressure head 6 applies a pressure of 80MPa, the temperature of the furnace 7 is controlled at 900°C, and the paint film is burned off after 3 hours of treatment, and the Au-20%Pd-coated alumina particle powder is sintered into a dense Au-20%Pd-alumina Composite layer, and maintain the structure of Au-20%Pd coated alumina particles. Soak in 5% NaOH aqueous solution for 10-60 minutes, wash with water, and remove aluminum oxide and other impurities attached to the surface of the sample. The thickness of the obtained Au-20%Pd _- coated _Al2O3 composite layer is 20 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The specific mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , ω(K ₃ AlF ₆ )=35%, 35% Na ₃ AlF ₆ at 700°C; 26% AlF ₃ , 29.6% K ₃ AlF at 800°C _6. 44.4% Na ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total electrolyte was added to each electrolyte. The anode current density is 0.8mA/cm ² , the temperature is controlled at 700°C, 800°C and 900°C respectively, and electrolysis is performed for 6h. After the electrolysis, the contents of Au and Pd in the molten salt and the produced aluminum were tested respectively. The test accuracy was 10 ^-6 g/g, and neither Au nor Pd was detected.

Example 4:

Au powder, Rh powder, alumina powder with an average particle size of 5 μm, and alumina powder with an average particle size of 80 nm, with a mass ratio of 19:1:60:20, were high-energy ball milled for 6 hours to obtain Au-5% Rh Composite powder coated with alumina. Process the Ni-20%Cr-0.5%Ce alloy into a sample with a diameter of 10mm and a length of 500mm; _the surface is sandblasted; then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped _Cr2O3 film ; Then use cathodic electrophoresis to deposit a layer of polyurethane paint film; apply the powder of Au-5% Rh coated alumina particles on the paint film, apply pressure to make the paint film coat the Au-5% Rh coated alumina particle powder Evenly adhere to the sample surface. Using the hot-pressing treatment device shown in Figure 2, the alloy matrix 1 coated with the paint-Au-5%Rh-oxide composite coating 3 on the surface is embedded in the mold 5 that alumina microspheres 4 are housed, and passed The indenter 6 applies a pressure of 80 MPa, the temperature of the furnace 7 is controlled at 1100 ° C, and the paint film is burned off after treatment for 3 hours. The gold-coated alumina particle powder is sintered into a dense Au-5%Rh-alumina composite layer, and The structure of Au-20%Rh coated alumina particles is maintained. Soak in 5% NaOH aqueous solution for 10-60 minutes, wash with water to remove alumina and other impurities attached to the surface of the sample. The thickness of the obtained Au-5%Rh-coated _{Al2O3 composite} layer is 30 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , 35% K ₃ AlF ₆ , 35% Na ₃ AlF ₆ at 700°C; 26% AlF ₃ , 29.6% K ₃ AlF ₆ , 44.4% at 800°C Na ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total mass of the electrolyte was added to each electrolyte. The anode current density is 0.8mA/cm ² , the temperature is controlled at 700°C, 800°C and 900°C respectively, and electrolysis is performed for 6h. After the electrolysis, the Au and Rh contents in the molten salt and the produced aluminum were tested respectively, and the test accuracy was 10 ^-6 g/g, and neither Au nor Rh was detected.

Example 5:

Au powder, Pt powder, alumina powder with an average particle size of 5 μm, and alumina powder with an average particle size of 80 nm, with a mass ratio of 8:2:70:20, were high-energy ball milled for 6 hours to obtain Au-20%Pt Composite powder coated with alumina. The 1Cr18Ni9Ti alloy is processed into a sample with a diameter of 10mm and a length of 500mm; the surface is sandblasted; in a 0.2M ethanol solution of cerium nitrate, the alloy sample is used as the cathode, the graphite rod is used as the anode, the pole spacing is 10mm, the applied voltage is 20V, and the electrolysis is 30 seconds, and then treated at 300°C for 30min to obtain a CeO ₂ film, and then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped Cr ₂ O ₃ film; a polyurethane paint film was deposited by cathodic electrophoresis; The powder coated with alumina particles is applied on the paint film, and pressure is applied to make the paint film adhere the Au-20%Pt coated alumina particle powder evenly to the surface of the sample. Using the hot-pressing treatment device shown in Figure 2, the alloy matrix 1 coated with the paint-Au-20%Pt-oxide composite coating 3 on the surface is embedded in the mold 5 that alumina microspheres 4 are housed, and passed The indenter 6 applies a pressure of 80MPa, the temperature of the furnace 7 is controlled at 1000°C, and the paint film is burned off after the treatment for 3 hours. The gold-coated alumina particle powder is sintered into a dense Au-20%Pt-alumina composite layer, and The structure of Au-20%Pt coated alumina particles is maintained. Soak in 5% NaOH aqueous solution for 10-60 minutes, wash with water to remove alumina and other impurities attached to the surface of the sample. The thickness of the obtained Au-20%Pt _- coated _Al2O3 composite layer is 30 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , 35% K ₃ AlF ₆ , 35% Na ₃ AlF ₆ at 700°C; 26% AlF ₃ , 29.6% K ₃ AlF ₆ , 44.4% at 800°C Na ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total mass of the electrolyte was added to each electrolyte. The anode current density is 0.8mA/cm ² , the temperature is controlled at 700°C, 800°C and 900°C respectively, and electrolysis is performed for 6h. After the electrolysis, the contents of Au and Pt in the molten salt and the produced aluminum were tested respectively. The test accuracy was 10 ^-6 g/g, and neither Au nor Pt was detected.

Embodiment 6:

Au powder, alumina powder with an average particle size of 5μm, and alumina powder with an average particle size of 80nm, with mass ratios of 10:65:25 and 50:35:15, respectively, were high-energy ball milled for 6 hours to obtain different precious metal ceramics. Proportion of gold-coated alumina composite powder. Process the Fe-20%Cr-0.5%Ce alloy into a sample with a diameter of 10mm and a length of 500mm; the surface is sandblasted; then pre-oxidized in air at 900°C for 5h to obtain a conductive Ce-doped Cr ₂ O ₃ film; Then cathodic electrophoresis is used to deposit a layer of polyurethane paint film; the powder of Au-coated alumina particles with different mass ratios of precious metals and alumina is applied on the paint film in three layers, and the Au-coated alumina particle powder used in the bottom layer The content of Au in the middle layer is high, the mass fraction is 50%; the Au content in the Au-coated alumina particle powder used in the middle layer is low, the mass fraction is 10%; and the outermost layer adopts pure Au particle powder. Apply pressure to make the paint film evenly adhere the Au-coated alumina particle powder to the surface of the sample. Using the hot-pressing treatment device shown in Figure 2, the alloy matrix 1 coated with the paint-Au-oxide gradient composite coating 3 on the surface is embedded in the mold 5 equipped with alumina microspheres 4, and passed through the pressure head 6 Apply a pressure of 80MPa, control the temperature of the furnace 7 at 900°C, treat for 3 hours, burn off the paint film, and sinter the Au-coated alumina particle powder to form a dense Au-alumina gradient composite layer, and keep the Au-coated alumina The structure of the particles. Soak in 5% NaOH aqueous solution for 10-60 minutes, wash with water to remove alumina and other impurities attached to the surface of the sample. The obtained Au-coated alumina gradient composite layer has a thickness of 40 μm.

The electrode is used as the anode, the graphite crucible is used as the cathode, and the electrolyte components are AlF ₃ , K ₃ AlF ₆ and Na ₃ AlF ₆ and the electrolytic raw material Al ₂ O ₃ . The mass percentages of electrolyte components at different electrolysis temperatures are as follows: 30% AlF ₃ , 35% K ₃ AlF ₆ , 35% Na ₃ AlF ₆ at 700°C; 26% AlF ₃ , 29.6% K ₃ AlF ₆ , 44.4% at 800°C Na ₃ AlF ₆ ; 24% AlF ₃ , 7.6% K ₃ AlF ₆ , 68.4% Na ₃ AlF ₆ at 900°C. Alumina powder accounting for 8% of the total mass of the electrolyte was added to each electrolyte. The anode current density was 0.8mA/cm ² , the temperature was controlled at 700°C, 800°C and 900°C, respectively, and the electrolysis was performed for 6 hours. After the electrolysis, the Au content in the molten salt and the produced aluminum were tested respectively, and the test accuracy was 10 ^-6 g/g, and no Au was detected.

Claims (7)

1. The noble metal ceramic composite coating inert anode for aluminum electrolysis is characterized in that: the inert anode is composed of an alloy substrate, an oxide film on the surface of the alloy substrate and an outer precious metal-alumina composite layer; The alloy matrix of the inert anode is an iron-based alloy or a nickel-based alloy with a Cr content of 10-25% and a rare earth element content of 0-0.5%, which is formed by casting or machining; The noble metal-alumina composite layer has a structure of noble metal-coated alumina particles; The thickness of the noble metal-alumina composite layer is 10-100 μm. 2. The inert anode according to claim 1, characterized in that: the noble metal in the noble metal-alumina composite layer of the inert anode outer layer is Au, Au-Pt alloy, Au-Pd alloy or Au-Rh alloy, in the alloy The mass percent contents of Pt and Pd are respectively 0-40%, and the mass percent contents of Rh are 0-10%. 3. The inert anode according to claim 1, characterized in that: the particle size of the alumina particles is 10 nm to 100 μm, and the mass of micron-scale alumina and nano-scale alumina in the noble metal-alumina composite layer is The ratio is 9:1-5:5, and the content of the noble metal in the noble metal-alumina composite layer is 5%-30%. 4. The preparation method of the noble metal ceramic composite coating inert anode for aluminum electrolysis is characterized in that: the preparation of the noble metal-alumina composite layer of the outer layer of the inert anode comprises the following steps: 1) After uniformly mixing alumina particles and noble metal powder, ball milling in a high-energy ball mill for 1 to 10 hours to obtain a powder body of noble metal-coated alumina particles; 2) preparing an oxide film on the surface of the alloy substrate by pre-oxidizing in air; 3) Electrophoresis a layer of paint film on the surface of the above-mentioned oxide film; apply the powder of precious metal coated alumina particles on the paint film, apply pressure to make the paint film coat the precious metal coated alumina particle powder evenly Adhere to the oxide film surface to form a lacquer-noble metal-oxide composite coating; 4) Using a hot-pressing treatment device, the alloy substrate coated with a lacquer-noble metal-oxide composite coating on the surface is embedded in the mold of the hot-pressing treatment device equipped with alumina microspheres, through the hot-pressing treatment device The pressure head applies a pressure of 20-100 MPa, the temperature of the furnace of the hot-pressing treatment device is controlled at 900-1200°C, and the treatment is performed for 1h-10h to burn off the paint film, and the precious metal-coated alumina particle powder is sintered into a dense precious metal - Aluminum oxide composite layer, and maintain the structure of precious metal coated alumina particles; 5) Soak the alloy substrate forming a dense noble metal-alumina composite layer in 1-10% NaOH aqueous solution for 10-60 minutes, and wash with water to remove alumina and other impurities attached to the surface. 5. The method for preparing an inert anode according to claim 4, characterized in that: in the step 2), the oxide film on the surface of the alloy substrate is prepared by pre-oxidizing in air at 800-1000° C. for 1-10 hours. 6. The preparation method of the inert anode according to claim 5, characterized in that: in the step 2), preparing the oxide film on the surface of the alloy substrate is to apply a rare earth oxide film with a thickness of 10 to 200 nm on the surface of the alloy substrate Or obtained by pre-oxidizing zirconia film in air at 800-1000°C for 1-10 hours. 7. according to the preparation method of the inert anode described in any one in claim 4 to 6, it is characterized in that: In the step 1), the precious metal powder and alumina powder in various mass ratios are ball-milled in a high-energy ball mill for 1 to 10 hours to obtain powders of precious metal-coated alumina particles in various mass proportions; In the step 2), the powders of the various mass ratios of precious metal-coated alumina particles are applied on the paint film in multiple layers to form a noble metal-oxide gradient composite coating; a gradient-structured precious metal-ceramic composite coating is obtained Coated inert anode.

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