Magnesium aluminate spinel protective agent and preparation method thereof

Abstract

The invention relates to the technical field of catalyst protective agents, in particular to a magnesium aluminate spinel protective agent and a preparation method thereof, and the protective agent is prepared from magnesium-aluminum composite powder A, magnesium-aluminum composite powder B and an additive. The magnesium-aluminum composite powder A is formed by mixing magnesium peroxide and magnesium aluminate spinel A prepared by a specific method, and the magnesium-aluminum composite powder B relates to a plurality of magnesium-containing compounds and magnesium aluminate spinel B which is specifically mixed. The preparation process comprises the fine pretreatment of each raw material, such as the treatment of magnesium peroxide, aluminum oxide, aluminum powder and the like, and the precise control of parameters such as temperature, time, rotating speed and the like in the links of combustion synthesis, high-temperature sintering, mixed grinding and the like. Experiments show that the protective agent is large in specific surface area, high in porosity, good in mechanical strength and high in adsorption capacity for impurities and toxicants in complex feed gas, the carbon monoxide sulfur-tolerant shift catalyst can be effectively protected, the performance of the carbon monoxide sulfur-tolerant shift catalyst in industrial production is remarkably improved, and the service life of the carbon monoxide sulfur-tolerant shift catalyst in industrial production is remarkably prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst protectants, and particularly to a magnesium aluminate spinel protectant and a preparation method thereof. Background Art

[0002] In modern chemical production, carbon monoxide sulfur-tolerant shift catalysts play a crucial role in the production processes of important chemical products such as synthetic ammonia, methanol, and hydrogen production. The core reaction principle is to promote the catalytic reaction of carbon monoxide and steam to be converted into hydrogen and carbon dioxide. However, the composition of the raw material gas used in actual industrial production is extremely complex, inevitably containing solid impurities such as carbon black, tar, and dust, as well as toxic compounds such as As2O3 and P2O5. When these impurities and poisons enter the reaction system with the raw material gas and come into contact with the carbon monoxide sulfur-tolerant shift catalyst, a series of negative effects will occur. Solid impurities are easily deposited on the catalyst surface, blocking the microporous structure of the catalyst and hindering the effective contact between the reactants and the active centers of the catalyst, thereby causing the active sites of the catalyst to be covered and the activity to be significantly reduced. Poisonous substances such as As2O3 and P2O5 will chemically react with the active components of the catalyst, changing the chemical composition and crystal structure of the catalyst, causing the catalyst to be permanently deactivated and greatly shortening the service life of the catalyst.

[0003] Some existing catalyst protectants have many deficiencies in dealing with these complex working conditions. In terms of strength, they cannot withstand the pressure fluctuations and material scouring during the industrial reaction process, are easily broken, resulting in an increase in the bed resistance of the protectant and affecting the production continuity; the porosity design is unreasonable, unable to provide enough space to adsorb impurities, reducing the protection effect; the adsorption capacity for various impurities and poisons is limited, making it difficult to meet the requirements of efficient purification of the raw material gas; in terms of the protection effect on the catalyst, it cannot comprehensively prevent the erosion of impurities and poisons on the catalyst, and cannot fully play the role of the protectant, resulting in the existing protectants being difficult to meet the growing industrial production requirements.

[0004] In the prior art, to solve the above problems, magnesium aluminate spinel is generally used as a protective agent for carbon monoxide shift catalysts for the following reasons: 1. Magnesium aluminate spinel has a cubic crystal structure with a stable crystal structure. This stable structure enables magnesium aluminate spinel to maintain its structural integrity under harsh conditions such as high temperature and high pressure in the carbon monoxide sulfur-tolerant shift reaction, and is not prone to decomposition or phase change, so that it can continuously play the role of a protective agent; 2. Magnesium aluminate spinel usually has a large specific surface area, which can provide abundant surface active sites. This is conducive to its interaction with impurities and poisons in the raw materials, such as adsorption, so as to effectively remove and adsorb impurities such as carbon black, tar and dust in the raw material gas, prevent these substances from clogging or polluting the carbon monoxide sulfur-tolerant shift catalyst, and ensure the activity and stability of the catalyst; 3. Magnesium aluminate spinel has high mechanical strength, which enables the protective agent mainly composed of it to withstand the pressure changes and material erosion during the reaction, and is not easy to break and pulverize. In actual industrial applications, it can maintain good particle integrity, maintain the stability of the catalyst bed, ensure uniform gas passage through the bed, reduce the risk of bypass flow, and enable the reaction to proceed stably and efficiently; 4. The high porosity of magnesium aluminate spinel endows it with good adsorption performance and gas diffusion performance. On the one hand, a large number of pores can provide sufficient adsorption space for impurities and poisons, increasing the adsorption capacity for impurities in the raw material gas; on the other hand, it is conducive to the diffusion of reaction gases inside the protective agent particles, enabling the carbon monoxide sulfur-tolerant shift reaction to proceed smoothly, and also contributing to the diffusion and enrichment of adsorbed impurities and poisons in the pores, enhancing the protection effect; 5. Magnesium aluminate spinel has good chemical stability and is not prone to chemical reactions with the components in the raw material gas or reaction products in the environment of the carbon monoxide sulfur-tolerant shift reaction, and can maintain its chemical properties unchanged. This enables it to stably play a protective role for a long time without affecting the protection effect on the carbon monoxide sulfur-tolerant shift catalyst due to its own chemical changes; 6. Magnesium aluminate spinel itself has a certain organic sulfur hydrolysis performance. In the carbon monoxide sulfur-tolerant shift reaction, it can hydrolyze some organic sulfur in the raw material gas into inorganic sulfur, thereby reducing the poisoning effect of organic sulfur on the catalyst, further prolonging the service life of the carbon monoxide sulfur-tolerant shift catalyst, and improving the efficiency and stability of the entire reaction process.

[0005] Although magnesium aluminate spinel has many advantages, there are still many deficiencies in practical applications, such as: 1. Although magnesium aluminate spinel itself has certain sulfur resistance performance, in an environment with a high sulfur content, its sulfur poisoning resistance still has an upper limit. When the sulfur content in the feed gas is too high, it may cause changes in the structure and performance of magnesium aluminate spinel, resulting in a decrease in its protective effect on the catalyst, and even it may be sulfur poisoned itself and unable to continue to play a protective role; 2. Under the high-temperature conditions of the carbon monoxide sulfur-tolerant shift reaction, although the stability of magnesium aluminate spinel is relatively good, during a long-term high-temperature operation process, lattice distortion, grain growth and other phenomena may still occur, which may lead to a decrease in its specific surface area and changes in the pore structure, thereby affecting its adsorption performance and the protective effect on the catalyst; 3. In actual industrial applications, various additives or other auxiliary materials may be used in the carbon monoxide sulfur-tolerant shift reaction system, and there may be compatibility problems between the magnesium aluminate spinel protective agent and these substances. For example, it may chemically react with certain additives, or agglomeration and other phenomena may occur during the mixing process, thus affecting the performance of the protective agent and the entire reaction system.

[0006] Therefore, according to the relevant technologies described above, it is urgent to develop a magnesium aluminate spinel protective agent and its preparation method. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a magnesium aluminate spinel protective agent and its preparation method, so as to provide a catalyst protective agent with high performance and capable of effectively coping with complex feed gas conditions, thereby improving chemical production efficiency and reducing production costs.

[0008] Based on the above purpose, the present invention provides a magnesium aluminate spinel protective agent and its preparation method.

[0009] A magnesium aluminate spinel protective agent is prepared from the following raw materials in parts by mass: 45-65 parts of magnesium-aluminum composite powder A, 15-25 parts of magnesium-aluminum composite powder B, and 5-10 parts of additive;

[0010] The magnesium-aluminum composite powder A is prepared from magnesium peroxide and magnesium aluminate spinel A;

[0011] The magnesium aluminate spinel A is synthesized by combustion from magnesium peroxide, alumina and aluminum powder;

[0012] The magnesium-aluminum composite powder B is prepared from magnesium oxalate, magnesium citrate, magnesium stearate and magnesium aluminate spinel B;

[0013] The magnesium aluminate spinel B is obtained by mixing magnesium aluminate spinel B1, magnesium aluminate spinel B2 and magnesium-aluminum spinel B3;

[0014] The magnesium aluminate spinel B1 is prepared from magnesium oxalate and alumina;

[0015] The magnesium aluminate spinel B2 is prepared from magnesium stearate and alumina;

[0016] The magnesium aluminate spinel B3 is prepared from magnesium citrate and alumina.

[0017] Preferably, the preparation process of the magnesium aluminate spinel A is as follows:

[0018] Step A1, raw material pretreatment: The high-purity magnesium peroxide is pulverized so that its particle size reaches 180 - 200 mesh to obtain magnesium peroxide powder; after removing the surface impurities of the α-Al2O3 powder, it is soaked in a dilute hydrochloric acid solution for 1 - 2 h, then rinsed with deionized water until neutral, and then dried in an oven at 120 °C for 4 - 6 h to obtain alumina powder; the high-purity aluminum powder is soaked in a sodium hydroxide solution with a mass fraction of 10% for 3 - 5 min, then rinsed with deionized water until neutral, rinsed with absolute ethanol, and dried in a vacuum drying oven at 60 °C for 2 - 3 h to obtain aluminum powder;

[0019] Step A2, the magnesium peroxide powder, alumina powder and aluminum powder are mixed evenly, then added to a ball mill, and at the same time, zirconia balls are added as the grinding medium and absolute ethanol is added as the grinding aid. The rotation speed of the ball mill is set at 300 - 400 rpm, and the mixing and grinding time is 3 - 5 h to obtain a mixture a;

[0020] Step A3, the mixture a is evenly filled into a reaction crucible, and the filling height is 2 / 3 of the crucible height. After filling, it is placed in a combustion synthesis device for sealing, and the reaction is initiated by an ignition system. The reaction duration is 5 - 10 min. After the reaction ends, it is naturally cooled to room temperature to obtain a cooled product a;

[0021] Step A4, the cooled product a is taken out and roughly crushed to a particle size of 1 - 2 cm, then finely crushed, zirconia balls and absolute ethanol are added, the rotation speed of the ball mill is 400 - 500 rpm, and the crushing time is 4 - 6 h to make the particle size of the product reach 200 - 220 mesh to obtain a crushed material a;

[0022] Step A5, the crushed material a is soaked in a dilute hydrochloric acid solution for 2 - 3 h, then repeatedly rinsed with deionized water until neutral. Finally, the pickled product is dried in an oven at 120 °C for 6 - 8 h, and then screened through a vibrating screen to select the powder with a particle size between 100 - 300 mesh, that is, the magnesium aluminate spinel A is obtained.

[0023] Preferably, in step A1, the purity of the high-purity magnesium peroxide is ≥98%, the purity of the α-Al2O3 powder is ≥99%, the particle size of the α-Al2O3 powder is 1 - 5 μm, the mass fraction of the dilute hydrochloric acid solution is 5%, the purity of the high-purity aluminum powder is ≥99%, and the particle size of the high-purity aluminum powder is 100 - 200 mesh.

[0024] Preferably, the mass ratio of the magnesium peroxide powder, aluminum oxide powder and aluminum powder in step A2 is 30-40:30-40:30-35. When the zirconia balls are used as the grinding medium in step A2, the ball-to-material ratio is 80-100:10-15. The addition amount of absolute ethanol in step A2 is 3%-5% of the total mass of the materials.

[0025] Preferably, the ignition current of the ignition system in step A3 is 5-10 A, and the ignition time lasts for 3-5 s.

[0026] Preferably, when finely crushing in step A4, the ball-to-material ratio of the zirconia balls is 8-12:1-1.5, and the addition amount of absolute ethanol in step A4 is 3%-5% of the total mass of the materials.

[0027] Preferably, the preparation process of the magnesium aluminate spinel B1 is as follows:

[0028] Step B1: Mix magnesium oxalate and aluminum oxide powder and add them into a ball mill. Add zirconia balls and grind at a rotation speed of 300-500 rpm for 2-4 h to obtain a mixed powder b.

[0029] Step B2: Transfer the mixed powder b to a high-temperature furnace and heat it at a heating rate of 5-10 °C / min to 800-900 °C. Keep it at this temperature for 2-3 h to obtain a pre-sintered product b.

[0030] Step B3: Put the pre-sintered product b into the high-temperature furnace again and heat it at a biological rate of 5-10 °C / min to 1500-1600 °C. Keep it warm for 4-6 h. Take out the sintered product, crush it, and select the powder with a particle size between 1-5 μm, that is, the magnesium aluminate spinel B1 is obtained.

[0031] In step B1, the molar ratio of magnesium oxalate to aluminum oxide is 0.9-1.1:1-1.2. When the zirconia balls are used as the grinding medium in step B1, the ball-to-material ratio is 80-100:10-15.

[0032] Preferably, the preparation process of the magnesium aluminate spinel B2 is as follows:

[0033] Step C1: Add magnesium stearate and aluminum oxide powder into absolute ethanol and ultrasonically disperse for 15-30 min to obtain a stable suspension c.

[0034] Step C2: Transfer the suspension c to an evaporating dish and evaporate the absolute ethanol under a water bath heating at 60-80 °C. Then dry the evaporated material in an oven at 100-120 °C for 2-3 h to obtain a mixed powder material c.

[0035] Step C3: Put the mixed powder into a high-temperature furnace, heat it to 1400 - 1500 °C at a heating rate of 5 - 10 °C / min, keep it warm for 4 - 5 h. After the reaction ends, cool it to room temperature, crush the product, and collect the powder with a particle size between 1 - 3 μm, thus obtaining magnesium aluminate spinel B2;

[0036] In step C1, the molar ratio of magnesium stearate to alumina is 0.95 - 1.05:1 - 1.2, and the ratio of the addition amount of absolute ethanol to solid materials in step C1 is 80 - 100 mL:10 - 12 g.

[0037] Preferably, the preparation process of the magnesium aluminate spinel B3 is as follows:

[0038] Step D1: Add alumina powder into the magnesium citrate solution, dropwise add ammonia water, adjust the pH of the solution to 9 - 10, and stir for 1 - 2 h to obtain a mixed solution d;

[0039] Step D2: Filter the mixed solution d, collect the precipitate, and wash the precipitate with deionized water repeatedly for 3 - 5 times to obtain a precipitate d;

[0040] Step D3: Put the precipitate d into an oven, dry it at 120 - 150 °C for 4 - 6 h. Transfer the dried product to a high-temperature furnace, heat it to 1500 - 1600 °C at a heating rate of 5 - 10 °C / min, keep it warm for 5 - 7 h. After calcination, the product is crushed, and the powder with a particle size between 0.5 - 1.2 μm is collected, thus obtaining magnesium aluminate spinel B3;

[0041] In step D1, the molar ratio of the alumina powder to the magnesium ions in the magnesium citrate solution is 0.98 - 1.02:1 - 1.3;

[0042] In step D1, the concentration of the ammonia water is 25% - 28%.

[0043] A preparation method of a magnesium aluminate spinel protective agent, comprising the following steps:

[0044] Step S1: Mix magnesium peroxide and magnesium aluminate spinel A, add them to a high-speed mixer, and mix at a rotation speed of 800 - 1200 rpm for 30 - 60 min to obtain magnesium-aluminum composite powder A;

[0045] Step S2: Put magnesium aluminate spinel B1, magnesium aluminate spinel B2, and magnesium aluminate spinel B3 into a mixer, and mix for 2 - 3 h to obtain magnesium aluminate spinel B;

[0046] Step S3: Mix magnesium oxalate, magnesium citrate, magnesium stearate, and magnesium aluminate spinel B and put them into a stirred ball mill, and grind at a rotation speed of 300 - 500 rpm for 3 - 5 h to obtain magnesium-aluminum composite powder B;

[0047] Step S4: Mix magnesium-aluminum composite powder A, magnesium-aluminum composite powder B and the additive, then transfer them into a ball milling tank. Add zirconia balls as the grinding medium and grind at a rotation speed of 300 - 500 rpm for 60 - 70 h to obtain the magnesium-aluminum spinel protective agent;

[0048] In step S1, the mass ratio of magnesium peroxide to magnesium-aluminum spinel A is 3 - 4:6 - 7;

[0049] In step S2, the mass ratio of magnesium-aluminum spinel B1, magnesium-aluminum spinel B2 and magnesium-aluminum spinel B3 is 3 - 4:2 - 2.5:4 - 5;

[0050] In step S3, the mass ratio of magnesium oxalate, magnesium citrate, magnesium stearate and magnesium-aluminum spinel B is 2 - 3:2 - 3:1 - 1.5:5 - 6;

[0051] In step S4, the mass ratio of the grinding medium to the powder is 3 - 10:1 - 1.3.

[0052] The additive in step S4 is obtained by mixing titanium dioxide and yttrium oxide with a mass ratio of 2 - 3:3.5 - 4.2.

[0053] Advantages of the present invention:

[0054] The present invention provides a magnesium-aluminum spinel protective agent and its preparation method. Through specific raw material treatment and preparation processes, such as fine pretreatment of raw materials like magnesium peroxide, alumina, aluminum powder, etc., and precise control of conditions such as ball milling and sintering at different stages, the specific surface area of the prepared protective agent can reach 145 m 2 / g, and the porosity is as high as 50%. This provides a large number of adsorption sites and storage spaces for impurities and poisons, greatly enhancing the adsorption capacity of the protective agent for various impurities in complex raw material gases, such as the adsorption amounts of carbon black, tar, dust, and As2O3, P2O5, etc. are significantly increased;

[0055] The mechanical strength of the magnesium-aluminum spinel protective agent provided by the present invention reaches 9.8 MPa, which can effectively resist the pressure fluctuations and material scouring during the industrial reaction process, avoid the increase of the protective agent bed layer resistance caused by crushing, maintain the stability of the catalyst bed layer, and ensure the continuity and high efficiency of the reaction;

[0056] The present invention formulates detailed and highly targeted pretreatment steps for each raw material. For example, high-purity magnesium peroxide is crushed to a specific particle size, α - Al2O3 powder is soaked in dilute hydrochloric acid, rinsed, and dried, and high-purity aluminum powder is treated with sodium hydroxide solution, etc., to ensure the high purity and suitable activity of the raw materials, laying a good foundation for subsequent reactions;

[0057] The magnesia-alumina spinel protective agent provided by the present invention has excellent adsorption capacity for solid impurities such as carbon black, tar, dust, etc. and toxic compounds such as As2O3 and P2O5 in complex raw material gas. Experimental data shows that the adsorption capacity for carbon black can reach 15.5 mg / g, and the adsorption capacity for As2O3 reaches 200 μg / g, which can effectively purify the raw material gas, prevent these impurities and poisons from damaging the sulfur-tolerant carbon monoxide shift catalyst, and extend the service life of the catalyst;

[0058] Due to the stable cubic crystal structure of magnesia-alumina spinel and the optimization of material properties during the preparation process, the protective agent can maintain its own structural integrity and chemical stability under harsh conditions such as high temperature (e.g., 350 °C) and high pressure (e.g., 3 MPa) in the sulfur-tolerant carbon monoxide shift reaction, and continuously and stably play a protective role. Specific embodiments

[0059] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with specific embodiments.

[0060] Example 1: A preparation method of a magnesia-alumina spinel protective agent includes the following steps:

[0061] S1. Raw material pretreatment: The high-purity magnesium oxide with a purity of ≥98% is pulverized so that its particle size reaches 180 - 200 meshes to obtain magnesium peroxide powder; The α-Al2O3 powder with a particle size of 1 - 5 μm and a purity of ≥99% is soaked in a 5% mass fraction of dilute hydrochloric acid solution for 1 h after removing surface impurities, then rinsed with deionized water until neutral, and then dried in an oven at 120 °C for 4 h to obtain alumina powder; The high-purity aluminum powder with a particle size of 100 - 200 meshes and a purity of ≥99% is soaked in a 10% mass fraction of sodium hydroxide solution for 3 min, then rinsed with deionized water until neutral, rinsed with absolute ethanol, and dried in a vacuum drying oven at 60 °C for 2 h to obtain aluminum powder;

[0062] S2. 30 g of magnesium peroxide powder, 30 g of alumina powder and 30 g of aluminum powder are mixed evenly, then added to a ball mill, and at the same time zirconia balls are added as the grinding medium. When zirconia balls are used as the grinding medium, the ball-to-material ratio is 80:10. Absolute ethanol is added as the grinding aid, and the addition amount of absolute ethanol is 3% of the total mass of the materials. The rotation speed of the ball mill is set at 300 rpm, and the mixing and grinding time is 3 h to obtain mixture a;

[0063] S3. The mixture a is evenly filled into the reaction crucible, and the filling height is 2 / 3 of the crucible height. After filling, it is put into the combustion synthesis device for sealing, and the reaction is initiated through the ignition system. The ignition current is 5 A, the ignition time lasts for 3 s, the reaction duration is 5 min, and after the reaction ends, it is naturally cooled to room temperature to obtain the cooled product a;

[0064] S4. Take out the cooling product a and perform coarse crushing until the particle size reaches 1 - 2 cm, then perform fine crushing. Add zirconia balls and absolute ethanol. When performing fine crushing, the ball-to-material ratio of zirconia balls is 8:1, the addition amount of absolute ethanol is 3% of the total mass of the material, the rotational speed of the ball mill is 400 rpm, and the crushing time is 4 h to make the particle size of the product reach 200 - 220 mesh, obtaining crushed material a;

[0065] S5. Immerse crushed material a in dilute hydrochloric acid solution for 2 h, then repeatedly rinse with deionized water until neutral. Finally, dry the pickled product in an oven at 120 °C for 6 h, and then perform screening through a vibrating sieve to select the powder with a particle size between 100 - 300 mesh, obtaining magnesium aluminate spinel A;

[0066] S6. Mix magnesium oxalate and alumina powder and add them to a ball mill. Add zirconia balls and grind at a rotational speed of 300 rpm for 2 h to obtain mixed powder b, where the molar ratio of magnesium oxalate to alumina is 0.9:1, and when zirconia balls are used as the grinding medium, the ball-to-material ratio is 80:10;

[0067] S7. Transfer mixed powder b to a high-temperature furnace and heat it to 800 °C at a heating rate of 5 °C / min, and keep it at this temperature for 2 h to obtain pre-sintered product b;

[0068] S8. Put pre-sintered product b into the high-temperature furnace again and heat it to 1500 °C at a heating rate of 5 °C / min, keep it warm for 4 h. Take out the sintered product, break it, and select the powder with a particle size between 1 - 5 μm, obtaining magnesium aluminate spinel B1;

[0069] S9. Add magnesium stearate and alumina powder to absolute ethanol and ultrasonically disperse for 15 min. The molar ratio of magnesium stearate to alumina is 0.95:1, and the ratio of the addition amount of absolute ethanol to solid material is 80 mL:10 g, obtaining stable suspension c;

[0070] S10. Transfer suspension c to an evaporating dish and evaporate the absolute ethanol under a water bath heating at 60 °C. Then dry the evaporated material in an oven at 100 °C for 2 h to obtain mixed powder c;

[0071] S11. Put mixed powder c into a high-temperature furnace and heat it to 1400 °C at a heating rate of 5 °C / min, keep it warm for 4 h. After the reaction ends, cool it to room temperature, crush the product, and collect the powder with a particle size between 1 - 3 μm, obtaining magnesium aluminate spinel B2;

[0072] S12. Add alumina powder to the magnesium citrate solution. The molar ratio of the alumina powder to the magnesium ions in the magnesium citrate solution is 0.98:1. Dropwise add ammonia water with a concentration of 25% to adjust the pH of the solution to 9 - 10, and stir for 1 h to obtain a mixed solution d;

[0073] S13. Filter the mixed solution d, collect the precipitate, and wash the precipitate with deionized water repeatedly 3 times to obtain a precipitate d;

[0074] S14. Put the precipitate d into an oven and dry it at 120 °C for 4 h. Transfer the dried product to a high-temperature furnace and heat it to 1500 °C at a heating rate of 5 °C / min, hold for 5 h. After the calcined product is crushed, collect the powder with a particle size between 0.5 μm, and thus obtain magnesium aluminate spinel B3;

[0075] S15. Mix 30 g of magnesium peroxide and 60 g of magnesium aluminate spinel A, add them to a high-speed mixer, and mix at a speed of 800 rpm for 30 min to obtain magnesium-aluminum composite powder A;

[0076] S16. Put 30 g of magnesium aluminate spinel B1, 20 g of magnesium aluminate spinel B2, and 40 g of magnesium aluminate spinel B3 into a mixer and mix for 2 h to obtain magnesium aluminate spinel B;

[0077] S17. Mix 20 g of magnesium oxalate, 20 g of magnesium citrate, 10 g of magnesium stearate, and 50 g of magnesium aluminate spinel B and put them into a stirred ball mill, and grind at a speed of 300 rpm for 3 h to obtain magnesium-aluminum composite powder B;

[0078] S18. After mixing magnesium-aluminum composite powder A, magnesium-aluminum composite powder B, and an additive, transfer them into a ball milling tank, add zirconia balls as the grinding medium. The mass ratio of the grinding medium to the powder is 3:1. The additive is obtained by mixing titanium dioxide and yttrium oxide in a mass ratio of 2:3.5, and grind at a speed of 300 rpm for 60 h to obtain a magnesium aluminate spinel protective agent.

[0079] Example 2: A preparation method of a magnesium aluminate spinel protective agent, comprising the following steps:

[0080] S1. Raw material pretreatment: Crush high-purity magnesium peroxide with a purity of ≥98% to a particle size of 180 - 200 mesh to obtain magnesium peroxide powder; Remove the surface impurities of α-Al₂O₃ powder with a particle size of 1 - 5 μm and a purity of ≥99%, soak it in a 5% dilute hydrochloric acid solution for 2 h, then rinse it with deionized water until neutral, and then dry it in an oven at 120 °C for 5 h to obtain alumina powder; Soak high-purity aluminum powder with a particle size of 100 - 200 mesh and a purity of ≥99% in a 10% sodium hydroxide solution for 5 min, then rinse it with deionized water until neutral, rinse it with absolute ethanol, and dry it in a vacuum drying oven at 60 °C for 3 h to obtain aluminum powder;

[0081] S2. Mix 36 g of magnesium peroxide powder, 34 g of alumina powder, and 32 g of aluminum powder evenly, then add them to a ball mill, and at the same time add zirconia balls as the grinding medium. When zirconia balls are used as the grinding medium, the ball-to-material ratio is 90:12. Add absolute ethanol as the grinding aid, and the addition amount of absolute ethanol is 4% of the total mass of the materials. Set the ball mill rotation speed to 340 rpm and the mixing and grinding time to 4 h to obtain mixture a;

[0082] S3. Fill mixture a evenly into the reaction crucible, and the filling height is 2 / 3 of the crucible height. After filling, put it into the combustion synthesis device and seal the device. Initiate the reaction through the ignition system, with an ignition current of 7 A and an ignition time of 5 s. The reaction duration is 7 min. After the reaction ends, naturally cool it to room temperature to obtain cooled product a;

[0083] S4. Take out cooled product a and perform coarse crushing to a particle size of 1 - 2 cm, then perform fine crushing. Add zirconia balls and absolute ethanol. When performing fine crushing, the ball-to-material ratio of zirconia balls is 10:1.2, and the addition amount of absolute ethanol is 4% of the total mass of the materials. The ball mill rotation speed is 440 rpm, and the crushing time is 5 h to make the particle size of the product reach 200 - 220 mesh to obtain crushed material a;

[0084] S5. Soak crushed material a in a dilute hydrochloric acid solution for 3 h, then repeatedly rinse it with deionized water until neutral. Finally, dry the pickled product in an oven at 120 °C for 7 h, and then perform screening through a vibrating screen to select the powder with a particle size between 100 - 300 mesh, that is, obtain magnesium aluminate spinel A;

[0085] S6. Mix magnesium oxalate and alumina powder and add them to a ball mill. Add zirconia balls and grind at a rotation speed of 400 rpm for 3 h to obtain mixed powder b, where the molar ratio of magnesium oxalate to alumina is 1.1:1.2, and when zirconia balls are used as the grinding medium, the ball-to-material ratio is 90:12;

[0086] S7. Transfer the mixed powder b to a high-temperature furnace and heat it to 840 °C at a heating rate of 7 °C / min. Keep it at this temperature for 3 h to obtain the pre-sintered product b.

[0087] S8. Put the pre-sintered product b into the high-temperature furnace again and heat it to 1540 °C at a heating rate of 7 °C / min. Keep it for 5 h. Take out the sintered product, crush it, and select the powder with a particle size between 1 - 5 μm, namely, obtain magnesium aluminate spinel B1.

[0088] S9. Add magnesium stearate and alumina powder into absolute ethanol and ultrasonically disperse for 21 min. The molar ratio of magnesium stearate to alumina is 0.98:1.2, and the ratio of the addition amount of absolute ethanol to solid materials is 90 mL:11 g to obtain a stable suspension c.

[0089] S10. Transfer the suspension c to an evaporating dish and evaporate the absolute ethanol under a water bath at 70 °C. Then dry the evaporated material in an oven at 110 °C for 3 h to obtain the mixed powder c.

[0090] S11. Put the mixed powder into a high-temperature furnace and heat it to 1440 °C at a heating rate of 7 °C / min. Keep it for 5 h. After the reaction ends, cool it to room temperature, crush the product, and collect the powder with a particle size between 1 - 3 μm, namely, obtain magnesium aluminate spinel B2.

[0091] S12. Add alumina powder into the magnesium citrate solution. The molar ratio of alumina powder to magnesium ions in the magnesium citrate solution is 1.0:1.2. Dropwise add ammonia water with a concentration of 27% and adjust the pH of the solution to 9 - 10. Stir for 2 h to obtain the mixed solution d.

[0092] S13. Filter the mixed solution d, collect the precipitate, and wash the precipitate with deionized water repeatedly for 5 times to obtain the precipitate d.

[0093] S14. Put the precipitate d into an oven and dry it at 130 °C for 4 - 5 - 6 - 4 - 5 - 6 h. Transfer the dried product to a high-temperature furnace and heat it to 1540 °C at a heating rate of 7 °C / min. Keep it for 5 - 5.5 - 6 - 6.5 - 6 - 7 h. Crush the calcined product and collect the powder with a particle size of 0.9 μm, namely, obtain magnesium aluminate spinel B3.

[0094] S15. Mix 34 g of magnesium peroxide and 64 g of magnesium aluminate spinel A, add them to a high-speed mixer, and mix at a speed of 900 rpm for 50 min to obtain the magnesium-aluminum composite powder A.

[0095] S16. Put 34 g of magnesium aluminate spinel B1, 22 g of magnesium aluminate spinel B2, and 44 g of magnesium aluminate spinel B3 into a mixer and mix for 3 h to obtain magnesium aluminate spinel B.

[0096] S17. Mix 24 g of magnesium oxalate, 24 g of magnesium citrate, 12 g of magnesium stearate and 54 g of magnesium aluminate spinel B, and put them into a stirred ball mill. Grind at a speed of 400 rpm for 4 h to obtain magnesium-aluminum composite powder B;

[0097] S18. Transfer the magnesium-aluminum composite powder A, magnesium-aluminum composite powder B and the additive into a ball mill tank, add zirconia balls as the grinding medium, and the mass ratio of the grinding medium to the powder is 7:1.2. The additive is obtained by mixing titanium dioxide and yttrium oxide in a mass ratio of 2.4:3.7. Grind at a speed of 400 rpm for 64 h to obtain a magnesium aluminate spinel protective agent.

[0098] Example 3: A preparation method of a magnesium aluminate spinel protective agent, comprising the following steps:

[0099] S1. Raw material pretreatment: Crush high-purity magnesium peroxide with a purity of ≥98% to make its particle size reach 180-200 mesh to obtain magnesium peroxide powder; Remove the surface impurities of α-Al2O3 powder with a particle size of 1-5 μm and a purity of ≥99%, soak it in a 5% mass fraction of dilute hydrochloric acid solution for 2 h, then rinse it with deionized water until neutral, and then dry it in an oven at 120 °C for 6 h to obtain alumina powder; Soak high-purity aluminum powder with a particle size of 100-200 mesh and a purity of ≥99% in a 10% mass fraction of sodium hydroxide solution for 5 min, then rinse it with deionized water until neutral, rinse it with absolute ethanol, and dry it in a vacuum drying oven at 60 °C for 3 h to obtain aluminum powder;

[0100] S2. Mix 40 g of magnesium peroxide powder, 40 g of alumina powder and 35 g of aluminum powder evenly, then add them into a ball mill, and at the same time add zirconia balls as the grinding medium. When zirconia balls are used as the grinding medium, the ball-to-material ratio is 100:15. Add absolute ethanol as a grinding aid, and the addition amount of absolute ethanol is 5% of the total mass of the materials. Set the speed of the ball mill to 400 rpm and the mixing and grinding time to 5 h to obtain mixture a;

[0101] S3. Fill mixture a evenly into the reaction crucible, and the filling height is 2 / 3 of the crucible height. After filling, put it into the combustion synthesis device to seal the device, trigger the reaction through the ignition system, the ignition current is 10 A, the ignition time lasts for 5 s, the reaction duration is 10 min, and after the reaction ends, naturally cool it to room temperature to obtain cooled product a;

[0102] S4. Take out the cooling product a and perform rough crushing until the particle size reaches 1 - 2 cm, then perform fine crushing. Add zirconia balls and absolute ethanol. When performing fine crushing, the ball-to-material ratio of zirconia balls is 12:1.5, the addition amount of absolute ethanol is 5% of the total mass of the material, the rotation speed of the ball mill is 500 rpm, and the crushing time is 6 h to make the particle size of the product reach 200 - 220 mesh, obtaining crushed material a;

[0103] S5. Immerse the crushed material a in a dilute hydrochloric acid solution for 3 h, then repeatedly rinse it with deionized water until it is neutral. Finally, dry the pickled product in an oven at 120 °C for 8 h, and then perform screening through a vibrating sieve to select the powder with a particle size between 100 - 300 mesh, namely obtaining magnesium aluminate spinel A;

[0104] S6. Mix magnesium oxalate and alumina powder and add them to a ball mill. Add zirconia balls and grind at a rotation speed of 500 rpm for 4 h to obtain mixed powder b, where the molar ratio of magnesium oxalate to alumina is 1.1:1.2, and when zirconia balls are used as the grinding medium, the ball-to-material ratio is 100:15;

[0105] S7. Transfer the mixed powder b to a high-temperature furnace and heat it at a heating rate of 10 °C / min to 900 °C, and keep it at this temperature for 3 h to obtain pre-sintered product b;

[0106] S8. Put the pre-sintered product b into the high-temperature furnace again and heat it at a heating rate of 10 °C / min to 1600 °C, keep it warm for 6 h, take out the sintered product, break it, and select the powder with a particle size between 1 - 5 μm, namely obtaining magnesium aluminate spinel B1;

[0107] S9. Add magnesium stearate and alumina powder to absolute ethanol and ultrasonically disperse for 30 min. The molar ratio of magnesium stearate to alumina is 1.05:1.2, and the ratio of the addition amount of absolute ethanol to the solid material is 100 mL:12 g, obtaining a stable suspension c;

[0108] S10. Transfer the suspension c to an evaporating dish and evaporate the absolute ethanol under a water bath heating at 80 °C. Then dry the evaporated material in an oven at 120 °C for 3 h to obtain mixed powder c;

[0109] S11. Put the mixed powder into a high-temperature furnace and heat it at a heating rate of 10 °C / min to 1500 °C, keep it warm for 5 h. After the reaction ends, cool it to room temperature, crush the product, and collect the powder with a particle size between 1 - 3 μm, namely obtaining magnesium aluminate spinel B2;

[0110] S12. Add alumina powder to the magnesium citrate solution. The molar ratio of the alumina powder to the magnesium ions in the magnesium citrate solution is 1.02:1.3. Dropwise add ammonia water with a concentration of 28% to adjust the pH of the solution to 9 - 10, and stir for 2 h to obtain a mixed solution d;

[0111] S13. Filter the mixed solution d, collect the precipitate, and wash the precipitate with deionized water repeatedly 5 times to obtain a precipitate d;

[0112] S14. Put the precipitate d into an oven and dry it at 150 °C for 6 h. Transfer the dried product to a high-temperature furnace and heat it to 1600 °C at a heating rate of 10 °C / min, hold for 7 h. After the calcined product is crushed, collect the powder with a particle size between 1.2 μm, and thus obtain magnesium aluminate spinel B3;

[0113] S15. Mix 40 g of magnesium peroxide and 70 g of magnesium aluminate spinel A, add them to a high-speed mixer, and mix at a speed of 1200 rpm for 60 min to obtain magnesium aluminate composite powder A;

[0114] S16. Put 40 g of magnesium aluminate spinel B1, 25 g of magnesium aluminate spinel B2, and 50 g of magnesium aluminate spinel B3 into a mixer and mix for 3 h to obtain magnesium aluminate spinel B;

[0115] S17. Mix 30 g of magnesium oxalate, 30 g of magnesium citrate, 15 g of magnesium stearate, and 60 g of magnesium aluminate spinel B and put them into a stirred ball mill, and grind at a speed of 500 rpm for 5 h to obtain magnesium aluminate composite powder B;

[0116] S18. After mixing magnesium aluminate composite powder A, magnesium aluminate composite powder B, and an additive, transfer them into a ball milling tank, add zirconia balls as the grinding medium. The mass ratio of the grinding medium to the powder is 10:1.3. The additive is obtained by mixing titanium dioxide and yttrium oxide with a mass ratio of 3:4.2, and grind at a speed of 500 rpm for 70 h to obtain a magnesium aluminate spinel protective agent.

[0117] Comparative Example 1:

[0118] In this comparative example, compared with Example 1, only "magnesium aluminate composite powder A" is replaced by "magnesium aluminate composite powder B", and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0119] Comparative Example 2:

[0120] In this comparative example, compared with Example 1, only "magnesium aluminate composite powder B" is replaced by "magnesium aluminate composite powder A", and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0121] Comparative Example 3:

[0122] In this comparative example, compared with Example 1, only "magnesium aluminate spinel B2" is replaced with "magnesium aluminate spinel B1", and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0123] Comparative Example 4:

[0124] In this comparative example, compared with Example 1, only "magnesium aluminate spinel B3" is replaced with "magnesium aluminate spinel B1", and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0125] Comparative Example 5:

[0126] In this comparative example, compared with Example 1, only "magnesium-aluminum composite powder A" is replaced with "magnesium aluminate spinel A", and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0127] Comparative Example 6:

[0128] In this comparative example, compared with Example 1, only "magnesium-aluminum composite powder B" is replaced with "magnesium aluminate spinel B1, magnesium aluminate spinel B2 and magnesium aluminate spinel B3" with equal total amount, and the remaining steps and parameters are the same. This comparative example will not be repeated here, and finally a magnesium aluminate spinel protective agent is obtained.

[0129] Performance test: The following performance tests were carried out on the magnesium aluminate spinel protective agents prepared in Examples 1 - 6 and Comparative Examples 1 - 6:

[0130] Specific surface area: According to the BET (Brunauer - Emmett - Teller) theory, it was measured by the nitrogen adsorption method, and the standard was ASTM D4567 - 12. The larger the specific surface area, the more active sites the protective agent can provide, which is beneficial to adsorbing impurities and poisons. The protective agent sample was degassed at 200 °C for 4 h under vacuum conditions to remove the adsorbed impurities and moisture on the surface. Using a specific surface area analyzer, nitrogen was introduced into the sample tube at the liquid nitrogen temperature (77 K), and the adsorption amount of nitrogen on the sample surface at different relative pressures was measured. The specific surface area of the sample was calculated according to the BET equation;

[0131] Porosity: It was measured by the mercury intrusion method, and the standard reference was ASTM D4284 - 19. A higher porosity can accommodate more impurities and improve the adsorption capacity of the protective agent. The protective agent sample was made into particles with a particle size of about 2 - 3 mm, and a mercury intrusion instrument was used to press mercury into the pores of the sample. According to the injection amount of mercury and the pressure change, the porosity of the sample was calculated. The test pressure range was from 0.005 MPa to 400 MPa;

[0132] Mechanical strength: Compressive strength test is adopted. The protective agent is made into cylinders of a certain specification, and pressure is applied on a material testing machine until the sample breaks. Record the pressure value at the time of breakage, with the unit of MPa. The higher the mechanical strength, the more resistant the protective agent is to pressure fluctuations and material erosion in industrial reactions. Press the protective agent powder into cylinders with a diameter of 5 mm and a height of 10 mm. Place the prepared sample on the indenter of the material testing machine and apply pressure at a loading rate of 0.5 mm / min until the sample breaks. Record the pressure value at this time. Repeat the test 5 times and take the average value as the compressive strength of the sample;

[0133] Adsorption performance: Simulate the raw material gas containing impurities and poisons such as carbon black, tar, dust, As2O3, and P2O5, and determine the adsorption amount of various impurities by the protective agent through dynamic adsorption experiments. The higher the adsorption amount, the stronger the purification ability of the protective agent for the raw material gas. Mix carbon black, tar, dust, and impurities and poisons such as As2O3 and P2O5 in a certain proportion, use nitrogen as the carrier gas, and prepare the simulated raw material gas. Load the protective agent sample into the adsorption column, and let the simulated raw material gas pass through the adsorption column at a certain flow rate. Regularly collect the outlet gas, and use analytical instruments such as gas chromatography and atomic absorption spectrometry to measure the concentration of impurities and poisons in the outlet gas. Calculate the adsorption amount of various impurities by the protective agent according to the concentration difference between the inlet and outlet and the gas flow rate;

[0134] Protection effect on the catalyst: Load the protective agent and the carbon monoxide sulfur-tolerant shift catalyst into a fixed-bed reactor in a certain proportion, introduce the simulated raw material gas for reaction, and measure the change in the activity of the catalyst before and after the reaction. The protection effect of the protective agent on the catalyst is expressed by the catalyst activity retention rate. The higher the catalyst activity retention rate, the better the protection effect. The catalyst activity is measured by the carbon monoxide conversion rate, and the calculation formula is:

[0135]

[0136] In the fixed-bed reactor, first load a certain amount of the protective agent, and then load the carbon monoxide sulfur-tolerant shift catalyst. The mass ratio of the protective agent to the catalyst is 1:5. The reaction temperature is 350 °C, the pressure is 3 MPa, and the space velocity of the simulated raw material gas is 1000 h-1; Before the reaction, measure the initial carbon monoxide conversion rate of the catalyst. After the reaction proceeds for 100 h, measure the carbon monoxide conversion rate of the catalyst again, and calculate the catalyst activity retention rate. The results are shown in Table 1 and Table 2 as follows:

[0137] Table 1

[0138]

[0139]

[0140] Table 2

[0141]

[0142] As can be seen from Table 1, the magnesium-aluminum spinel protective agent prepared in Examples 1-6 is superior to Comparative Examples 1-6 in all performance indicators. The protective agent in the embodiment has a higher specific surface area, porosity and mechanical strength, a larger adsorption capacity for various impurities and poisons, and a better protection effect on the catalyst, indicating that the method of the present invention can effectively improve the performance of the magnesium-aluminum spinel protective agent: the higher specific surface area of ​​Examples 1-6 means more active sites. The large specific surface area can provide a wider space for the adsorption of impurities and poisons. The larger specific surface area increases the number of atoms or molecules that can be contacted on the surface of the protective agent, increases the probability of collision and adsorption with As2O3 molecules, and thus helps to increase the adsorption of As2O3. The specific surface area of ​​Comparative Examples 1-6 is relatively small, which leads to insufficient surface active sites and limited adsorption capacity for various impurities. Taking carbon black adsorption as an example, due to the small number of active sites, the positions where carbon black molecules can be attached are limited, making the carbon black adsorption amount much lower than that of Example 6; the high porosity of the embodiment provides more storage space for impurities. When processing tar, the porosity of Example 5 reaches 49%, and the tar molecules can diffuse into the pores more easily and be adsorbed. The connectivity between the pores is also good, which is conducive to the transmission of gas inside the protective agent, so that the tar adsorption amount reaches 9.2 mg / g. The relatively low porosity of the comparative example reduces the space for accommodating impurities, and the tar molecules may be hindered during the diffusion process, resulting in its tar adsorption amount being significantly lower than the data in the embodiment; the high mechanical strength of the embodiment enables the protective agent to maintain structural integrity when subjected to pressure fluctuations and material scouring. When the pressure of the reaction system changes suddenly, the protective agent particles are not easy to break, which maintains the stability of the catalyst bed and ensures that the gas passes evenly, which is conducive to improving the catalyst activity retention rate. The comparative example may be broken due to being unable to withstand the pressure, and the broken protective agent particles will increase the bed resistance and affect the gas flow, resulting in the catalyst activity retention rate being far lower than the average level of the embodiment; the carbon black adsorption amount of the embodiment is relatively high, wherein the synergistic effect of specific surface area and porosity makes the carbon black molecules easier to be captured and attached to the surface and pores of the protective agent, while the comparative example 1 has a lower specific surface area and porosity, and tar adsorption: the tar adsorption amount of the embodiment is relatively high, and the high specific surface area, suitable porosity and chemically active sites work together; the suitable pore size distribution and surface properties in the embodiment are conducive to the retention and adsorption of dust particles, and the comparative example 4 has a lower dust adsorption amount due to structural defects; the As2O3 adsorption amount of the embodiment is relatively high, and the specific preparation process enables the protective agent surface to produce more active sites with affinity for As2O3, and the adsorption amount is high. Due to the different preparation process, comparative example 5 has fewer active sites and low As2O3 adsorption; the embodiment has a higher P2O5 adsorption, and the optimized preparation method enables the protective agent to have a more efficient ability to adsorb P2O5, while comparative example 6 has a low P2O5 adsorption due to differences in raw materials and processes; the catalyst activity retention rate of the embodiment is higher.This is due to the excellent performance of the protective agent in terms of specific surface area, porosity, mechanical strength, and adsorption performance. The high specific surface area and porosity enable it to effectively adsorb impurities and poisons, and the high mechanical strength ensures stability during the reaction process, with a relatively high catalyst activity retention rate. In Comparative Example 1, due to relatively poor performance indicators in various aspects, the catalyst cannot be fully protected, resulting in a low activity retention rate. Although Comparative Example 6 has certain adsorption performance, it has deficiencies in mechanical strength and other aspects, making the catalyst activity retention rate lower than the best effect of the examples.

[0143] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

[0144] The present invention aims to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A magnesium aluminum spinel protective agent, characterized in that: It is prepared from the following raw materials in parts by weight: 45-65 parts of magnesium-aluminum composite powder A, 15-25 parts of magnesium-aluminum composite powder B, and 5-10 parts of additives; The magnesium-aluminum composite powder A is prepared from magnesium peroxide and magnesium-aluminum spinel A; The magnesium aluminum spinel A is synthesized by combustion of magnesium peroxide, aluminum oxide and aluminum powder; The magnesium-aluminum composite powder B is prepared from magnesium oxalate, magnesium citrate, magnesium stearate and magnesium-aluminum spinel B; The magnesium aluminum spinel B is obtained by mixing magnesium aluminum spinel B1, magnesium aluminum spinel B2 and Alcoa spinel B3; The magnesium aluminum spinel B1 is prepared from magnesium oxalate and aluminum oxide; The magnesium aluminum spinel B2 is prepared from magnesium stearate and aluminum oxide; The magnesium aluminum spinel B3 is prepared from magnesium citrate and aluminum oxide.

2. The magnesium aluminum spinel protective agent according to claim 1, characterized in that: The preparation process of the magnesium aluminum spinel A is as follows: Step A1, raw material pretreatment: crush high-purity magnesium peroxide to a particle size of 180-200 mesh to obtain magnesium peroxide powder; remove impurities from the surface of α-Al2O3 powder and soak it in a dilute hydrochloric acid solution for 1-2 hours, then rinse it with deionized water until it is neutral, and then dry it in a 120°C oven for 4-6 hours to obtain aluminum oxide powder; soak high-purity aluminum powder in a 10% sodium hydroxide solution by mass for 3-5 minutes, then rinse it with deionized water until it is neutral, rinse it with anhydrous ethanol, and then dry it in a vacuum drying oven at 60°C for 2-3 hours to obtain aluminum powder; Step A2, mixing magnesium peroxide powder, aluminum oxide powder and aluminum powder evenly, and then adding them into a ball mill, adding zirconium oxide balls as grinding media, adding anhydrous ethanol as a grinding aid, setting the ball mill speed to 300-400 rpm, and mixing the grinding time to 3-5 hours, to obtain a mixture a; Step A3, evenly filling the mixture a into the reaction crucible, the filling height being 2 / 3 of the height of the crucible, placing the mixture into the combustion synthesis device after the filling is completed, and initiating the reaction through the ignition system. The reaction duration is 5-10 minutes. After the reaction is completed, naturally cool to room temperature to obtain a cooling product a; Step A4, taking out the cooled product a and coarsely crushing it to a particle size of 1-2 cm, and then finely crushing it, adding zirconium oxide balls and anhydrous ethanol, the ball mill speed is 400-500 rpm, the crushing time is 4-6 hours, so that the product particle size reaches 200-220 mesh, and the crushed material a is obtained; Step A5, soak the crushed material a in a dilute hydrochloric acid solution for 2-3 hours, then repeatedly rinse with deionized water until neutral, and finally dry the acid-washed product in an oven at 120°C for 6-8 hours, and then sieve it through a vibrating screen to select a powder with a particle size between 100-300 meshes to obtain magnesium aluminum spinel A.

3. The magnesium aluminum spinel protective agent according to claim 2, characterized in that: The purity of the high-purity magnesium oxide in step A1 is ≥98%, the purity of the α-Al2O3 powder is ≥99%, the particle size of the α-Al2O3 powder is 1-5μm, the mass fraction of the dilute hydrochloric acid solution is 5%, the purity of the high-purity aluminum powder is ≥99%, and the particle size of the high-purity aluminum powder is 100-200 mesh.

4. The magnesium aluminum spinel protective agent according to claim 2, characterized in that: The mass ratio of magnesium peroxide powder, aluminum oxide powder and aluminum powder in step A2 is 30-40:30-40:30-35, the ball-to-material ratio of zirconium oxide balls as grinding media in step A2 is 80-100:10-15, and the amount of anhydrous ethanol added in step A2 is 3%-5% of the total mass of the materials.

5. The magnesium aluminum spinel protective agent according to claim 2, characterized in that: The ignition current of the ignition system in step A3 is 5-10A, and the ignition time lasts 3-5s.

6. The magnesium aluminum spinel protective agent according to claim 2, characterized in that: The ball-to-material ratio of the zirconium oxide balls during the fine crushing in step A4 is 8-12:1-1.5, and the amount of anhydrous ethanol added in step A4 is 3%-5% of the total mass of the material.

7. The magnesium aluminum spinel protective agent according to claim 1, characterized in that: The preparation process of the magnesium aluminum spinel B1 is as follows: Step B1, adding magnesium oxalate and alumina powder to a ball mill, adding zirconium oxide balls, and grinding at a speed of 300-500 rpm for 2-4 hours to obtain a mixed powder b; Step B2, transfer the mixed powder b to a high temperature furnace, heat it to 800-900°C at a heating rate of 5-10°C / min, and keep it at this temperature for 2-3 hours to obtain a pre-burned product b; Step B3, the pre-burned material b is put into the high temperature furnace again, heated to 1500-1600°C at a biological rate of 5-10°C / min, and kept warm for 4-6 hours, the sintered product is taken out, crushed and powder with a particle size of 1-5 μm is selected to obtain magnesia-alumina spinel B1; The molar ratio of magnesium oxalate to aluminum oxide in step B1 is 0.9-1.1:1-1.2, and the ball-to-material ratio of zirconium oxide balls as grinding media in step B1 is 80-100:10-15.

8. The magnesium aluminum spinel protective agent according to claim 1, characterized in that: The preparation process of the magnesium aluminum spinel B2 is as follows: Step C1, adding magnesium stearate and aluminum oxide powder into anhydrous ethanol, and ultrasonically dispersing for 15-30 minutes to obtain a stable suspension c; Step C2, transfer the suspension c to an evaporating dish, evaporate the anhydrous ethanol in a water bath at 60-80°C, and then dry the evaporated material in an oven at 100-120°C for 2-3 hours to obtain a mixed powder c; Step C3, placing the mixed powder into a high-temperature furnace, heating to 1400-1500°C at a heating rate of 5-10°C / min, and keeping the temperature for 4-5 hours. After the reaction is completed, cooling to room temperature, crushing the product, and collecting powder with a particle size of 1-3 μm to obtain magnesium aluminum spinel B2; The molar ratio of magnesium stearate to aluminum oxide in step C1 is 0.95-1.05:1-1.2, and the ratio of anhydrous ethanol to solid material added in step C1 is 80-100 mL:10-12 g.

9. The magnesium aluminum spinel protective agent according to claim 1, characterized in that: The preparation process of the magnesium aluminum spinel B3 is as follows: Step D1, adding aluminum oxide powder to magnesium citrate solution, adding ammonia water dropwise, adjusting the pH of the solution to 9-10, stirring for 1-2 hours, and obtaining a mixed solution d; Step D2, filtering the mixed solution d, collecting the precipitate, and repeatedly washing the precipitate with deionized water for 3-5 times to obtain a precipitate d; Step D3, placing the precipitate d in an oven and drying it at 120-150°C for 4-6 hours, transferring the dried product to a high-temperature furnace, heating it to 1500-1600°C at a heating rate of 5-10°C / min, and keeping it warm for 5-7 hours. The calcined product is crushed and powder with a particle size of 0.5-1.2 μm is collected to obtain magnesium aluminum spinel B3; The molar ratio of the aluminum oxide powder to the magnesium ions in the magnesium citrate solution in step D1 is 0.98-1.02:1-1.3; The concentration of the ammonia water in step D1 is 25%-28%.

10. The method for preparing the magnesium aluminum spinel protective agent according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step S1, mixing magnesium peroxide and magnesium aluminum spinel A, adding them into a high-speed mixer, and mixing them at a speed of 800-1200 rpm for 30-60 min to obtain magnesium aluminum composite powder A; Step S2, putting magnesium aluminum spinel B1, magnesium aluminum spinel B2 and magnesium aluminum spinel B3 into a mixer, and mixing for 2-3 hours to obtain magnesium aluminum spinel B; Step S3, mixing magnesium oxalate, magnesium citrate, magnesium stearate and magnesium aluminum spinel B and putting them into a stirring ball mill, and grinding them at a speed of 300-500 rpm for 3-5 hours to obtain magnesium aluminum composite powder B; Step S4, mixing the magnesium-aluminum composite powder A, the magnesium-aluminum composite powder B and the additive, transferring the mixture into a ball mill, adding zirconium oxide balls as grinding media, grinding at a speed of 300-500 rpm for 60-70 hours, and obtaining a magnesium-aluminum spinel protective agent; The mass ratio of magnesium peroxide to magnesium aluminum spinel A in step S1 is 3-4:6-7; In step S2, the mass ratio of the magnesium aluminum spinel B1, magnesium aluminum spinel B2 and magnesium aluminum spinel B3 is 3-4:2-2.5:4-5; The mass ratio of magnesium oxalate, magnesium citrate, magnesium stearate and magnesium aluminum spinel B in step S3 is 2-3:2-3:1-1.5:5-6; The mass ratio of the grinding medium to the powder in step S4 is 3-10:1-1.3; The additive in step S4 is titanium dioxide and yttrium oxide mixed in a mass ratio of 2-3:3.5-4.2.

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