A high-temperature resistant magnesia-carbon composite refractory brick and its preparation method

Abstract

The present invention provides a high-temperature resistant magnesia-carbon composite refractory brick and a preparation method thereof, belonging to the technical field of magnesia refractory wall bricks. The raw materials of the refractory brick include double-layer modified magnesia, modified flake graphite, zirconium silicate powder, modified liquid phenolic resin, and benzoxazine cross-linking agent. The double-layer modified magnesia is obtained by first coating magnesia with powder phenolic resin, alumina, and zirconia together, and then coating it with pitch, micron magnesia, zirconia, and calcium oxide together. The modified flake graphite is obtained by coating flake graphite with strontium hydroxide, pitch, and boron carbide together. The modified liquid phenolic resin is prepared by blending liquid phenolic resin with silane coupling agent and polyacrylate cross-linked polymer. The present invention uses special components and methods to prepare double-layer modified magnesia, modified flake graphite, and modified liquid phenolic resin, and then uses them in combination with zirconium silicate powder and benzoxazine cross-linking agent, and designs reasonable forming and sintering processes, so that the refractory brick has good properties such as high-temperature stability, oxidation resistance, and thermal shock resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnesia refractory wall bricks, and particularly relates to a high-temperature resistant magnesia-carbon composite refractory brick and a preparation method thereof. Background Art

[0002] With the rapid development of high-temperature industries such as steel and non-ferrous metal smelting, traditional refractory materials are gradually difficult to meet the increasingly stringent usage conditions. During the steel smelting process, steelmaking equipment such as converters and electric furnaces need to withstand high temperatures above 1600 °C, and at the same time, they have to endure the strong erosion of molten steel and slag, as well as mechanical scouring and thermal shock. For example, early used high-aluminum bricks and clay bricks, although they have certain refractory properties, have poor erosion resistance and thermal shock resistance at high temperatures, short service life, and frequent replacement not only affects production efficiency but also increases a large amount of labor and material costs.

[0003] To solve these problems, researchers began to explore the research and development of new refractory materials. Magnesia materials have become one of the key research focuses due to their high melting point (the melting point of magnesia is about 2800 °C), good chemical stability, and resistance to alkaline slag erosion. However, pure magnesia refractory materials have poor thermal shock resistance and are prone to cracking and spalling when the temperature changes rapidly. At the same time, carbon materials have high thermal conductivity, low expansion coefficient, and good thermal shock resistance, but their oxidation resistance is insufficient and they are easily oxidized in a high-temperature aerobic environment. Based on the respective advantages and disadvantages of magnesia materials and carbon materials, the idea of preparing magnesia-carbon composite refractory bricks by combining the two emerged. Through reasonable formula design and preparation process, the magnesia material and the carbon material form a complementary structure in the brick body, giving full play to the high refractoriness and erosion resistance of the magnesia material and the thermal shock resistance of the carbon material.

[0004] With the continuous progress of steel smelting technology, such as the widespread application of new processes such as secondary refining and continuous casting, higher requirements are put forward for the performance of magnesia-carbon composite refractory bricks. On the one hand, it is required that magnesia-carbon bricks have lower porosity and higher density to improve their erosion resistance and impermeability; on the other hand, it is necessary to further enhance their oxidation resistance and extend their service life. For this reason, various antioxidants are added. However, improper use of various additive components and improper preparation processes will still affect the properties of refractory bricks such as high-temperature stability, oxidation resistance, and thermal shock resistance. Therefore, in order to meet the high-performance and high-quality usage requirements, it is necessary to continuously develop magnesia-carbon brick products with better balanced performance. Summary of the Invention

[0005] In order to further balance and improve the high-temperature stability, oxidation resistance, thermal shock resistance and other indicators of magnesia-carbon bricks, the present invention provides a high-temperature resistant magnesia-carbon composite refractory brick and its preparation method. By using special components and methods, double-layer modified magnesium oxide, modified flake graphite and modified liquid phenolic resin are prepared, and then used in combination with zircon powder and benzoxazine cross-linking agent in a specific proportion. A reasonable forming and sintering process is designed to prepare a refractory brick with good high-temperature stability, oxidation resistance and thermal shock resistance, etc., effectively improving the refractory service life of magnesia-carbon bricks. The specific technical solutions are as follows:

[0006] A high-temperature resistant magnesia-carbon composite refractory brick, the refractory brick comprises the following raw materials in parts by mass: 65 to 75 parts of double-layer modified magnesium oxide, 15 to 18 parts of modified flake graphite, 0.5 to 2 parts of zircon powder, 5 to 7 parts of modified liquid phenolic resin, 0.5 to 1 part of benzoxazine cross-linking agent; the double-layer modified magnesium oxide is obtained by first coating magnesium oxide with powder phenolic resin, alumina and zirconia, and then coating with asphalt, micron magnesium oxide, zirconia and calcium oxide; the modified flake graphite is obtained by coating flake graphite with strontium hydroxide, asphalt and boron carbide; the modified liquid phenolic resin is prepared by blending liquid phenolic resin with silane coupling agent and polyacrylate cross-linked polymer.

[0007] Further, the preparation method of the double-layer modified magnesium oxide comprises the following steps: according to the mass ratio, magnesium oxide: thermosetting powder phenolic resin: alumina: zirconia = 100: (4 - 6): (4 - 6): (2 - 4), adding the thermosetting powder phenolic resin into magnesium oxide and mixing evenly, then adding alumina and zirconia and mixing evenly, sintering in an oxygen-free environment at 500°C - 600°C for 1h - 2h, and then dispersing to obtain a layer of modified magnesium oxide; according to the mass ratio, a layer of modified magnesium oxide: asphalt: micron magnesium oxide: zirconia: calcium oxide = 100: (5 - 8): (2 - 3): (4 - 6): (2 - 3), adding asphalt into the layer of modified magnesium oxide and mixing evenly, then adding micron magnesium oxide, zirconia and calcium oxide and mixing evenly, sintering in an oxygen-free environment at 400°C - 500°C for 1h - 2h, and then dispersing to obtain double-layer modified magnesium oxide.

[0008] Further, the magnesium oxide is capacitor magnesia; the particle size distribution of the magnesium oxide in parts by mass is: 25 to 35 parts with a particle size between 3mm and 5mm, 35 to 45 parts with a particle size between 1mm and 3mm, 30 to 40 parts with a particle size between 0.5mm and 1mm, and 10 to 15 parts with a particle size less than 0.5mm.

[0009] Further, the alumina and zirconia are premixed evenly before adding; the micron magnesium oxide, zirconia and calcium oxide are premixed evenly before adding.

[0010] Further, the median particle size of the thermosetting phenolic resin powder is below 5 μm; the pitch is medium-temperature modified pitch, the median particle size of the pitch is below 5 μm, the pitch contains 2 wt% - 5 wt% zinc phenolsulfonate, and the median particle size of the zinc phenolsulfonate is below 5 μm; the micron magnesium oxide is capacitor magnesite with a median particle size below 5 μm; the median particle sizes of the alumina, zirconia, and calcium oxide are all below 5 μm.

[0011] Further, the preparation method of the modified flake graphite includes the following steps: by mass ratio, flake graphite:pitch:boron carbide:strontium hydroxide = 100:(4 - 6):(3 - 5):(1 - 2). First, add strontium hydroxide to the flake graphite and mix evenly, then add the pitch and mix evenly, and finally add boron carbide and mix evenly. Sinter in an oxygen-free environment at 800°C - 900°C for 1 h - 2 h, and then break up to obtain the modified flake graphite.

[0012] Further, the median particle size of the flake graphite is 10 μm - 30 μm; the pitch is medium-temperature modified pitch, the median particle size of the pitch is below 5 μm; the median particle sizes of the strontium hydroxide and boron carbide are both below 3 μm.

[0013] Further, the modified liquid phenolic resin contains 4 wt% - 6 wt% of the silane coupling agent KH-560 and 1.5 wt% - 3 wt% of the polyacrylate cross-linked polymer in the liquid phenolic resin; the liquid phenolic resin is a thermosetting phenolic resin; the median particle size of the zircon powder is 10 μm - 50 μm.

[0014] The preparation method of the above-mentioned high-temperature resistant magnesia-carbon composite refractory brick includes the following steps: by mass fraction, mix the double-layer modified magnesium oxide, modified flake graphite, and zircon powder evenly to obtain a mixed powder; mix the modified liquid phenolic resin and the benzoxazine cross-linking agent evenly, and then add them to the mixed powder and mix evenly to obtain a mixed material; fill the mixed material into a mold and press it into shape to obtain a brick blank; sinter the brick blank in an oxygen-free environment at 250°C - 300°C for 8 h - 10 h, then raise the temperature to 600°C - 800°C and sinter in an oxygen-free environment for 4 h - 5 h, then raise the temperature to 1300°C - 1400°C and sinter in an oxygen-free environment for 2 h - 2.5 h, and then cool to obtain the refractory brick.

[0015] Further, the pressure of the pressing die head is 240 MPa - 260 MPa, and the heating rate is 4°C / min - 6°C / min.

[0016] A high-temperature resistant magnesia-carbon composite refractory brick and its preparation method provided by the present invention have the following beneficial effects:

[0017] I. Preparation of double-layer modified magnesium oxide: Alumina and zirconia are premixed and then mixed with thermosetting powder phenolic resin and magnesium oxide followed by anaerobic sintering. Alumina and zirconia can improve the high-temperature stability and erosion resistance of magnesium oxide. The thermosetting powder phenolic resin forms a carbonaceous skeleton during the sintering process, enhancing the bonding strength of the material. Based on a layer of modified magnesium oxide, asphalt, micron-sized magnesium oxide, zirconia, and calcium oxide are added and sintered anaerobically again. Asphalt further improves the densification and bonding of the material. Micron-sized magnesium oxide can fill pores and increase the density. Zirconia continues to enhance the high-temperature performance, and calcium oxide helps improve the slag resistance of the material.

[0018] Among them, first adding the thermosetting powder phenolic resin to magnesium oxide for mixing is beneficial for the phenolic resin to uniformly wrap the magnesium oxide particles. During the subsequent sintering process, the carbonaceous skeleton formed by the phenolic resin can better combine with magnesium oxide, enhancing the overall structural strength of the material. Then, alumina and zirconia are added for mixing, enabling them to be evenly distributed among the magnesium oxide particles wrapped by the phenolic resin, giving full play to the role of alumina and zirconia in improving the high-temperature stability and erosion resistance of magnesium oxide, while avoiding the agglomeration of alumina and zirconia and ensuring the synergistic effect of each component in the system.

[0019] Among them, first adding asphalt to a layer of modified magnesium oxide for mixing can make the asphalt evenly penetrate into the pores and surface of the layer of modified magnesium oxide. During the subsequent sintering, the carbonaceous structure formed by the carbonization of asphalt can effectively fill the pores, improving the densification and bonding of the material. Then, micron-sized magnesium oxide, zirconia, and calcium oxide are added. These substances can further fill the micro-pores on the basis of the layer of modified magnesium oxide wrapped by asphalt, increasing the density. At the same time, zirconia continues to enhance the high-temperature performance, and calcium oxide improves the slag resistance. And such an order is conducive to the full contact and reaction of each component, forming a uniform microstructure and exerting the best modification effect.

[0020] The asphalt contains zinc phenolsulfonate. Zinc phenolsulfonate can react with certain components in the asphalt or decompose at high temperatures to produce some active substances, promoting the combination of components, connecting them more tightly together, reducing pores, enhancing the structural stability of the refractory brick, and enabling it to withstand greater pressure. During the anaerobic sintering process, it can catalyze the carbonization reaction of organic substances such as asphalt, promoting the formation of a more stable carbon structure, improving the high-temperature resistance of the refractory brick, and enhancing the high-temperature flexural strength. It makes the internal structure of the refractory brick uniform and tightly combined. When the temperature changes rapidly, it can better resist the action of thermal stress, reduce the generation and expansion of cracks, and improve the thermal shock stability. The stable and tight structure can reduce the contact between oxygen and internal components, slowing down the oxidation rate.

[0021] II. Preparation of Modified Flake Graphite: Strontium hydroxide is added to flake graphite. Strontium hydroxide will react with some groups on the surface of flake graphite, improving its surface properties, enhancing the bonding force with other components, and also generating a composite protective layer of strontium oxide and other phases. Asphalt is added. During the sintering process, the asphalt carbonizes to form a carbonaceous network, enhancing the structural stability and oxidation resistance of flake graphite. The addition of boron carbide can increase the hardness and wear resistance of the modified flake graphite, and at the same time, it has a synergistic effect with other components at high temperatures to further improve the performance of the material.

[0022] Among them, first, strontium hydroxide is added to flake graphite and mixed to make strontium hydroxide fully act on the surface of flake graphite, improving the surface properties of flake graphite, increasing its surface active sites, and enhancing the bonding force with the subsequent added components. Then, asphalt is added and mixed evenly. The asphalt can spread and adhere better on the surface of the flake graphite treated with strontium hydroxide, forming a uniform carbonaceous network during the sintering process, enhancing the structural stability and oxidation resistance of flake graphite. Finally, boron carbide is added to make it evenly distributed in the system formed by flake graphite and asphalt, playing the role of increasing hardness and wear resistance, and at the same time, avoiding boron carbide from being overly wrapped by other components during the mixing process, which affects the full play of its performance.

[0023] III. In the modified liquid phenolic resin, the silane coupling agent can form chemical bonding between inorganic materials such as magnesium oxide and flake graphite and phenolic resin, improving the interfacial bonding force, thereby enhancing the overall performance of the refractory brick. The polyacrylate cross-linked polymer can increase the cross-linking density of phenolic resin, enhance the mechanical properties and heat resistance of the resin, and also has the effect of promoting material fusion, improving the uniformity and density, making the refractory brick more stable at high temperatures.

[0024] IV. Zircon powder has a high melting point, high hardness, and good chemical stability. It can fill the pores in the magnesia-carbon composite refractory brick, improving the density of the brick body, enhancing the erosion resistance and permeability resistance, and at the same time, it can stably exist at high temperatures, contributing to maintaining the structural stability of the brick body.

[0025] V. The benzoxazine cross-linking agent can react with phenolic resin to form a three-dimensional network structure, increasing the cross-linking degree and heat resistance of the resin, and further enhancing the strength and high-temperature stability of the refractory brick, enabling it to better withstand high temperatures and mechanical stresses.

[0026] VI. The polyacrylate cross-linked polymer makes the resin matrix more dense by increasing the cross-linking density of phenolic resin. The benzoxazine cross-linking agent forms a stable three-dimensional network structure with phenolic resin. The two act synergistically to further improve the heat resistance, mechanical properties, and stability of phenolic resin, enabling the refractory brick to maintain good structural integrity and performance at high temperatures, and improving its oxidation resistance, thermal shock resistance, and other indicators.

[0027] VII. After the refractory bricks are pressed into shape, gradient temperature sintering is carried out to allow the material to fully complete physical and chemical changes at different temperature stages, gradually remove moisture and organic matter, and promote crystal growth and densification.

[0028] In summary, the double-layer modified magnesium oxide has undergone multi-layer modification treatment. The thermosetting powder phenolic resin, alumina, zirconia, and additives such as micron magnesium oxide and calcium oxide form a tight and complex composite structure with magnesium oxide through sintering, and this structure can effectively disperse stress. When heated, the additives in the double-layer modified magnesium oxide can stabilize the crystal structure, inhibit the abnormal growth of magnesium oxide crystals, and enhance the grain boundary bonding force. The multi-layer structure of the double-layer modified magnesium oxide can buffer thermal stress. When the temperature changes rapidly, the thermal expansion differences of each layer of material can compensate each other, reducing the internal stress concentration. Some components in the double-layer modified magnesium oxide can form a dense protective film on the surface to prevent oxygen from diffusing inward. The additives in the process of modifying magnesium oxide fill the pores between magnesium oxide particles and promote the densification of the material during the sintering process. Additives such as asphalt, boron carbide, and strontium hydroxide in the modified flake graphite, during the modification process, the asphalt carbonizes at high temperature to form a bonding phase, enhancing the bonding force between the flake graphite and other raw materials; boron carbide improves the hardness and oxidation resistance of the material; strontium hydroxide can participate in the reaction to form a composite protective layer of strontium oxide and other phases. The polyacrylate cross-linked polymer can cross-link with the liquid phenolic resin to form a denser and more stable three-dimensional network structure, enhancing the bonding effect of the resin on other raw materials. The polyacrylate cross-linked polymer also has the effect of promoting material fusion and improving the uniformity and densification. The benzoxazine cross-linking agent can promote the cross-linking reaction of phenolic resin, increase the cross-linking density, and enhance the curing degree and stability of the material. The production of the refractory bricks of the present invention is simple and feasible, and has excellent performance in various aspects, having good practical value. Specific Embodiments

[0029] The present invention will be further described below in conjunction with specific implementation cases, but the present invention is not limited to these embodiments.

[0030] Example 1

[0031] A high-temperature resistant magnesia-carbon composite refractory brick, the refractory brick comprises the following raw materials in parts by mass: 70 parts of double-layer modified magnesium oxide, 16 parts of modified flake graphite, 1.2 parts of zircon powder, 6 parts of modified liquid phenolic resin, and 0.8 part of benzoxazine cross-linking agent. The modified liquid phenolic resin contains 5 wt% of silane coupling agent KH-560 and 2.2 wt% of polyacrylate cross-linked polymer in the liquid phenolic resin; the median particle size of the zircon powder is 24 μm.

[0032] The preparation method of double-layer modified magnesium oxide comprises the following steps: According to the mass ratio, magnesium oxide: thermosetting powder phenolic resin: alumina: zirconia = 100:5:5:3, premix alumina and zirconia evenly, add the thermosetting powder phenolic resin into magnesium oxide and mix evenly, then add alumina and zirconia and mix evenly, heat up to 550 °C at a heating rate of 5 °C / min for anaerobic sintering for 1.5 h, and break up to obtain one-layer modified magnesium oxide; According to the mass ratio, one-layer modified magnesium oxide: asphalt: micron magnesium oxide: zirconia: calcium oxide = 100:6:2.5:5:2.5, premix micron magnesium oxide, zirconia and calcium oxide evenly, add the asphalt containing 3 wt% zinc phenolsulfonate into the one-layer modified magnesium oxide and mix evenly, then add micron magnesium oxide, zirconia and calcium oxide and mix evenly, heat up to 450 °C at a heating rate of 5 °C / min for anaerobic sintering for 1.5 h, and break up to obtain double-layer modified magnesium oxide.

[0033] Among them, the magnesium oxide is capacitor magnesia; The mass fraction of the particle size distribution of magnesium oxide is: 30 parts with a particle size between 3 mm and 5 mm, 40 parts with a particle size between 1 mm and 3 mm, 35 parts with a particle size between 0.5 mm and 1 mm, and 12 parts with a particle size less than 0.5 mm.

[0034] The preparation method of modified flake graphite comprises the following steps: According to the mass ratio, flake graphite: asphalt: boron carbide: strontium hydroxide = 100:5:4:1.5, first add strontium hydroxide into flake graphite with a median particle size of 10 μm and mix evenly, then add asphalt and mix evenly, and finally add boron carbide and mix evenly, heat up to 850 °C at a heating rate of 5 °C / min for anaerobic sintering for 1.5 h, and break up to obtain modified flake graphite.

[0035] The preparation method of the above-mentioned high-temperature resistant magnesia-carbon composite refractory brick comprises the following steps: According to the mass parts, mix double-layer modified magnesium oxide, modified flake graphite and zircon powder evenly to obtain a mixed powder; Mix the modified liquid phenolic resin and benzoxazine cross-linking agent evenly, then add them into the mixed powder and mix evenly to obtain a mixed material; Fill the mixed material into a mold and press it into shape at a die head pressure of 250 MPa to obtain a brick blank; Sinter the brick blank at 280 °C for anaerobic sintering for 9 h, then heat up to 700 °C at a heating rate of 5 °C / min for anaerobic sintering for 4.5 h, and then heat up to 1350 °C at a heating rate of 5 °C / min for anaerobic sintering for 2 h, and cool to obtain a refractory brick.

[0036] Example 2

[0037] A high-temperature resistant magnesia-carbon composite refractory brick, the refractory brick comprising the following raw materials in parts by mass: 65 parts of double-layer modified magnesia, 15 parts of modified flake graphite, 0.5 part of zircon powder, 5 parts of modified liquid phenolic resin, and 0.5 part of benzoxazine cross-linking agent. The modified liquid phenolic resin is a liquid phenolic resin containing 4 wt% of silane coupling agent KH-560 and 1.5 wt% of polyacrylate cross-linked polymer; the median particle size of the zircon powder is 50 μm.

[0038] The preparation method of the double-layer modified magnesia comprises the following steps: according to the mass ratio, magnesia: thermosetting powder phenolic resin: alumina: zirconia = 100:4:4:2, premix alumina and zirconia evenly, add the thermosetting powder phenolic resin into magnesia and mix evenly, then add alumina and zirconia and mix evenly, heat up to 500 °C at a heating rate of 4 °C / min for anaerobic sintering for 1 h, break up, to obtain a layer of modified magnesia; according to the mass ratio, a layer of modified magnesia: pitch: micron magnesia: zirconia: calcium oxide = 100:5:2:4:2, premix micron magnesia, zirconia and calcium oxide evenly, add the pitch containing 2 wt% of zinc phenolsulfonate into a layer of modified magnesia and mix evenly, then add micron magnesia, zirconia and calcium oxide and mix evenly, heat up to 400 °C at a heating rate of 4 °C / min for anaerobic sintering for 1 h, break up, to obtain double-layer modified magnesia.

[0039] Among them, the magnesia is capacitor magnesia; the mass fraction of the particle size distribution of magnesia is: 25 parts with a particle size between 3 mm and 5 mm, 35 parts with a particle size between 1 mm and 3 mm, 30 parts with a particle size between 0.5 mm and 1 mm, and 10 parts with a particle size less than 0.5 mm.

[0040] The preparation method of the modified flake graphite comprises the following steps: according to the mass ratio, flake graphite: pitch: boron carbide: strontium hydroxide = 100:4:3:1, first add strontium hydroxide into the flake graphite with a median particle size of 22 μm and mix evenly, then add pitch and mix evenly, and finally add boron carbide and mix evenly, heat up to 800 °C at a heating rate of 4 °C / min for anaerobic sintering for 1 h, break up, to obtain modified flake graphite.

[0041] The preparation method of the above-mentioned high-temperature resistant magnesia-carbon composite refractory brick comprises the following steps: according to the parts by mass, mix the double-layer modified magnesia, modified flake graphite and zircon powder evenly to obtain a mixed powder; mix the modified liquid phenolic resin and the benzoxazine cross-linking agent evenly, then add them into the mixed powder and mix evenly to obtain a mixed material; fill the mixed material into a mold and press it into shape at a die head pressure of 240 MPa to obtain a brick blank; sinter the brick blank anaerobically at 250 °C for 8 h, then heat up to 600 °C at a heating rate of 4 °C / min for anaerobic sintering for 4 h, and then heat up to 1300 °C at a heating rate of 4 °C / min for anaerobic sintering for 2 h, and cool to obtain the refractory brick.

[0042] Example 3

[0043] A high-temperature resistant magnesia-carbon composite refractory brick, the refractory brick comprising raw materials in the following parts by mass: 75 parts of double-layer modified magnesia, 18 parts of modified flake graphite, 2 parts of zircon powder, 7 parts of modified liquid phenolic resin, and 1 part of benzoxazine crosslinking agent. The modified liquid phenolic resin contains 6 wt% of silane coupling agent KH-560 and 3 wt% of polyacrylate crosslinked polymer in the liquid phenolic resin; the median particle size of the zircon powder is 10 μm.

[0044] The preparation method of the double-layer modified magnesia comprises the following steps: according to the mass ratio, magnesia: thermosetting powder phenolic resin: alumina: zirconia = 100:6:6:4, premix alumina and zirconia evenly, add the thermosetting powder phenolic resin into magnesia and mix evenly, then add alumina and zirconia and mix evenly, heat up to 600 °C at a heating rate of 6 °C / min for anaerobic sintering for 2 h, break up, and obtain a layer of modified magnesia; according to the mass ratio, a layer of modified magnesia: pitch: micron magnesia: zirconia: calcium oxide = 100:8:3:6:3, premix micron magnesia, zirconia and calcium oxide evenly, add the pitch containing 5 wt% of zinc phenolsulfonate into the layer of modified magnesia and mix evenly, then add micron magnesia, zirconia and calcium oxide and mix evenly, heat up to 500 °C at a heating rate of 6 °C / min for anaerobic sintering for 2 h, break up, and obtain the double-layer modified magnesia.

[0045] Among them, the magnesia is capacitor magnesite; the mass fraction of the particle size distribution of the magnesia is: 35 parts with a particle size between 3 mm and 5 mm, 45 parts with a particle size between 1 mm and 3 mm, 40 parts with a particle size between 0.5 mm and 1 mm, and 15 parts with a particle size less than 0.5 mm.

[0046] The preparation method of the modified flake graphite comprises the following steps: according to the mass ratio, flake graphite: pitch: boron carbide: strontium hydroxide = 100:6:5:2, first add strontium hydroxide into the flake graphite with a median particle size of 30 μm and mix evenly, then add pitch and mix evenly, and finally add boron carbide and mix evenly, heat up to 900 °C at a heating rate of 6 °C / min for anaerobic sintering for 2 h, break up, and obtain the modified flake graphite.

[0047] The preparation method of the above-mentioned high-temperature resistant magnesia-carbon composite refractory brick comprises the following steps: by mass parts, double-layer modified magnesia, modified flake graphite and zircon powder are uniformly mixed to obtain a mixed powder; a modified liquid phenolic resin and a benzoxazine cross-linking agent are uniformly mixed, and then added to the mixed powder and uniformly mixed to obtain a mixture; the mixture is filled into a mold and pressed into shape under a die head pressure of 260 MPa to obtain a brick blank; the brick blank is sintered in an oxygen-free environment at 300 °C for 10 h, then heated to 800 °C at a heating rate of 6 °C / min and sintered in an oxygen-free environment for 5 h, and then heated to 1400 °C at a heating rate of 6 °C / min and sintered in an oxygen-free environment for 2.5 h, and then cooled to obtain the refractory brick.

[0048] The raw materials used in the above Examples 1 to 3: the liquid phenolic resin is a thermosetting phenolic resin, sourced from Jinan Dahui Chemical Technology Co., Ltd., model 8022. The median particle size of the thermosetting powdered phenolic resin is below 5 μm, and the thermosetting powdered phenolic resin is sourced from Henan Jinrun New Materials Co., Ltd., model JR2123. The pitch is medium-temperature modified pitch, and the median particle size of the pitch after pulverization is below 5 μm. The medium-temperature modified pitch is sourced from Jining Chaolian New Materials Technology Co., Ltd., number 34066350. The median particle size of zinc phenolsulfonate after pulverization is below 5 μm, and the zinc phenolsulfonate is sourced from Zhongshan Yuanda New Materials Co., Ltd. The micron magnesia is capacitor magnesia with a median particle size of 3.2 μm. The median particle size of alumina is 2.8 μm. The median particle size of zirconia is 4.5 μm. The median particle size of calcium oxide is 3.7 μm. The median particle size of strontium hydroxide is 1.4 μm. The median particle size of boron carbide is 2.6 μm. The silane coupling agent KH-560 is sourced from Dongguan Shanyi Plastic Co., Ltd., model KH560. The polyacrylate cross-linked polymer is polyacrylate cross-linked polymer-6, sourced from Guangzhou Luyuan Chemical Co., Ltd. The benzoxazine cross-linking agent is allyl phenol type benzoxazine resin, sourced from Shandong Jiaying Chemical Technology Co., Ltd., model JH-2102. The zircon powder (zirconium silicate) is sourced from Shijiazhuang Qadian New Materials Technology Co., Ltd.

[0049] Comparative Example 1

[0050] In the refractory brick raw materials, the double-layer modified magnesia is replaced with magnesia; other methods and parameters are the same as those in Example 1.

[0051] Comparative Example 2

[0052] In the refractory brick raw materials, the second layer modification of the double-layer modified magnesia is not carried out; other methods and parameters are the same as those in Example 1.

[0053] Comparative Example 3

[0054] In the refractory brick raw materials, the first layer modification of the double-layer modified magnesia is not carried out; other methods and parameters are the same as those in Example 1.

[0055] Comparative Example 4

[0056] In the preparation method of double-layer modified magnesium oxide, the thermosetting powder phenolic resin of one layer of modified magnesium oxide is replaced by asphalt; other methods and parameters are the same as those in Example 1.

[0057] Comparative Example 5

[0058] In the preparation method of double-layer modified magnesium oxide, zinc phenolsulfonate is not added to the asphalt; other methods and parameters are the same as those in Example 1.

[0059] Comparative Example 6

[0060] In the raw materials of refractory bricks, modified flake graphite is replaced by flake graphite; other methods and parameters are the same as those in Example 1.

[0061] Comparative Example 7

[0062] In the preparation method of modified flake graphite, strontium hydroxide is not added; other methods and parameters are the same as those in Example 1.

[0063] Comparative Example 8

[0064] In the preparation method of modified flake graphite, boron carbide is not added; other methods and parameters are the same as those in Example 1.

[0065] Comparative Example 9

[0066] In the modified liquid phenolic resin, polyacrylate cross-linked polymer is not added; other methods and parameters are the same as those in Example 1.

[0067] Comparative Example 10

[0068] In the raw materials of refractory bricks, benzoxazine cross-linking agent is not added, and the mass fraction of benzoxazine cross-linking agent is replaced by liquid phenolic resin; other methods and parameters are the same as those in Example 1.

[0069] Comparative Example 11

[0070] Both polyacrylate cross-linked polymer and benzoxazine cross-linking agent are not added, and the mass fraction of benzoxazine cross-linking agent is replaced by liquid phenolic resin; other methods and parameters are the same as those in Example 1.

[0071] Comparative Example 12

[0072] In the preparation method of refractory bricks, gradient temperature rise sintering is not carried out, and it is directly heated from room temperature to 1350 °C at a heating rate of 5 °C / min for anaerobic sintering for 2 h; other methods and parameters are the same as those in Example 1.

[0073] I. Specimen Specification

[0074] A cube specimen with a processing size of 50 mm × 50 mm × 50 mm is used for the room temperature compressive strength test.

[0075] A cuboid specimen with processing dimensions of 150 mm × 25 mm × 25 mm is used for the high-temperature flexural strength test.

[0076] Standard brick specimens with processing dimensions of 230 mm × 114 mm × 65 mm are used for the thermal shock stability, oxidation resistance, bulk density, and apparent porosity tests.

[0077] II. Detection Items and Methods

[0078] 1. Cold Crushing Strength: Place the cube specimen at the center of the loading platform of the universal material testing machine, apply pressure until the specimen is damaged, record the failure load, and calculate the cold crushing strength. The results are shown in Table 1 below.

[0079] 2. High-Temperature Flexural Strength (1400 °C): Place the cuboid specimen on the flexural fixture in the high-temperature furnace of the high-temperature flexural testing machine, heat it up to 1400 °C at a rate of 10 °C / min, hold for 30 min, then apply a bending load, record the load when the specimen breaks, and calculate the high-temperature flexural strength. The results are shown in Table 1 below.

[0080] 3. Thermal Shock Stability: Put the brick specimen into the high-temperature furnace, heat it up to 1400 °C at a rate of 15 °C / min, hold for 30 min, then quickly take it out and put it into the water tank for rapid cooling for 3 min, take it out and dry it. Repeat this process until through cracks appear in the brick specimen or the spalling amount exceeds 5%, and record the number of thermal shock times. The results are shown in Table 1 below.

[0081] 4. Oxidation Resistance: First, weigh the initial mass m0 of the brick specimen, put it into the high-temperature furnace and hold it for 10 h in an air atmosphere at 1400 °C, take it out and cool it to room temperature, then weigh the mass m1; calculate the oxidation weight loss rate. The results are shown in Table 1 below.

[0082] 5. Bulk Density:

[0083] Weigh the mass m of the dry brick specimen d , put it into water and boil for 5 h, take it out, dry the surface moisture with a wet cloth, and then weigh the saturated surface-dry mass m s , measure the mass m of the displaced water by the drainage method w = m s - m d , calculate the volume V of the displaced water (water density is calculated as 1 g / cm³); calculate the bulk density ρ of the brick specimen as ρ = m d / V. The results are shown in Table 1 below.

[0084] 6. Apparent Porosity: First, use a true density tester to measure the true density ρt of the brick specimen, and then combine with the measured bulk density ρ to calculate the apparent porosity P = (1 - ρ / ρt) × 100%. The results are shown in Table 1 below.

[0085] Table 1 Test Results of Items

[0086]

[0087] As can be seen from the above results, the refractory bricks of Examples 1 to 3 have good high-temperature stability, oxidation resistance, and thermal shock resistance. The double-layer modified magnesium oxide has undergone multi-layer modification treatment. Additives such as thermosetting powder phenolic resin, alumina, zirconia, micron magnesium oxide, and calcium oxide form a tight and complex composite structure with magnesium oxide through sintering, and this structure can effectively disperse stress. When heated, the additives in the double-layer modified magnesium oxide can stabilize the crystal structure, inhibit the abnormal growth of magnesium oxide crystals, and enhance the grain boundary bonding force. The multi-layer structure of the double-layer modified magnesium oxide can buffer thermal stress. When the temperature changes rapidly, the thermal expansion differences of each layer of materials can compensate each other, reducing the internal stress concentration. Some components (such as zirconia) in the double-layer modified magnesium oxide can form a dense protective film on the surface to prevent oxygen from diffusing inward. The additives in the process of modifying magnesium oxide fill the pores between magnesium oxide particles and promote the densification of the material during the sintering process. Zinc phenolsulfonate can react with some components in the asphalt or decompose at high temperature to produce some active substances, promoting the combination of components, connecting more tightly together, reducing pores, enhancing the structural stability of the refractory brick, and being able to withstand greater pressure. During the anaerobic sintering process, it can catalyze the carbonization reaction of organic substances such as asphalt, promote the formation of a more stable carbon structure, improve the high-temperature resistance of the refractory brick, and improve the high-temperature flexural strength. It makes the internal structure of the refractory brick uniform and tightly combined. When the temperature changes rapidly, it can better resist the action of thermal stress, reduce the generation and expansion of cracks, and improve the thermal shock stability. The stable and tight structure can reduce the contact between oxygen and internal components and slow down the oxidation rate.

[0088] During the modification process of additives such as asphalt, boron carbide, and strontium hydroxide in the modified flake graphite, the asphalt carbonizes at high temperature to form a bonding phase, enhancing the bonding force between the flake graphite and other raw materials; boron carbide improves the hardness and oxidation resistance of the material; strontium hydroxide participates in the reaction to form a composite protective layer of strontium oxide and other phases.

[0089] The polyacrylate cross-linked polymer can cross-link with the liquid phenolic resin to form a denser and more stable three-dimensional network structure, enhancing the bonding effect of the resin on other raw materials. The polyacrylate cross-linked polymer also has the effect of promoting material fusion and improving the uniformity and denseness. The benzoxazine cross-linking agent can promote the cross-linking reaction of phenolic resin, increase the cross-linking density, and enhance the curing degree and stability of the material.

[0090] Comparative Example 1: Magnesium oxide was used to replace the double-layer modified magnesium oxide. The magnesium oxide was not modified, had a single crystal structure, and lacked the strengthening and toughening effects brought by the composite structure. The binding force between ordinary magnesium oxide crystals was relatively weak. When subjected to pressure, the crystals were prone to relative slip and dislocation and could not effectively resist external forces like the modified structure. The grain boundaries of ordinary magnesium oxide were significantly weakened at high temperatures, and the crystals were prone to slip and fracture, resulting in poor high-temperature stability. The thermal expansion coefficient of ordinary magnesium oxide was single. Without the barrier and buffering of the modifying substances, stress could not be effectively released during thermal shock, leading to poor thermal shock stability. In terms of oxidation resistance, the binding property of the refractory brick component of ordinary magnesium oxide was poor, and oxygen was easy to diffuse inward, affecting the overall oxidation resistance. During the preparation process of ordinary magnesium oxide, the particle packing was not tight enough, there were many internal pores, and the loose structure resulted in a low bulk density and a high apparent porosity.

[0091] Comparative Example 2: The modified magnesium oxide was not subjected to the second-layer modification, and the protective layer ability decreased. Additives such as micron magnesium oxide, zirconia, and calcium oxide in the second-layer modification further filled the pores after the first-layer modification, refined the grains, and formed stronger binding bridges between the particles. Without the second-layer modification, there were more and larger pores and weak interfaces inside the material. When stressed, stress concentration was likely to occur at these defect sites, leading to the initiation and propagation of cracks. Without the refined structure and stress buffering mechanism formed by the second-layer modification, the material could not efficiently disperse and absorb thermal stress during thermal shock. The dense structure formed by the second-layer modification could block the penetration of oxygen. Without this layer of protection, oxygen was more likely to diffuse into the brick body. Due to the lack of filling and densification by the second-layer modification additives, the internal pores of the material increased.

[0092] Comparative Example 3: The modified magnesium oxide was not subjected to the first-layer modification. The thermosetting powder phenolic resin in the first-layer modification carbonized to form a carbon skeleton during sintering, enhancing the binding force between magnesium oxide particles. Alumina and zirconia improved the stability of the material through solid solution strengthening and other effects. Without the first-layer modification, the binding between magnesium oxide particles mainly relied on simple physical packing and a small amount of chemical bonding. Under the action of force and high temperature, the particles were prone to relative displacement and separation. The structure formed by the first-layer modification helped buffer thermal stress and block oxygen. The lack of it exacerbated the stress concentration phenomenon during thermal shock of the material. The microstructure of the material was not uniform and dense enough, and oxygen was easy to penetrate.

[0093] Comparative Example 4 uses asphalt to replace the thermosetting powder phenolic resin in one layer of modified magnesium oxide. The chemical structures and properties of asphalt and the thermosetting powder phenolic resin are different. The carbonization process of asphalt at high temperatures is relatively complex, and the carbon structure formed is not as stable and uniform as the carbon skeleton formed after the carbonization of the thermosetting powder phenolic resin. The bonding and strengthening effect on magnesium oxide particles is relatively weak. When stressed or heated, the bonding between particles is easily damaged. The structure formed after the carbonization of the thermosetting powder phenolic resin has a certain barrier effect on oxygen. The decomposition and volatilization behavior of asphalt during the sintering process is different from that of the thermosetting powder phenolic resin, resulting in changes in the internal pore structure of the material.

[0094] Comparative Example 5 lacks zinc phenolsulfonate in the asphalt. The structural stability and density of the asphalt after sintering are relatively poor, and it is easily damaged and exfoliated during the mixing and pressing processes, resulting in deteriorated performance.

[0095] Comparative Example 6 uses unmodified flake graphite to replace modified flake graphite. The combination of unmodified flake graphite with other components is not tight enough and is easily detached when stressed. In a high-temperature aerobic environment, unmodified flake graphite is easily oxidized, reducing the overall antioxidant property of the material. The synergistic effect between flake graphite and other materials weakens during the thermal shock process. The bonding state of unmodified flake graphite with other raw materials affects the overall structure of the material, and the apparent porosity increases.

[0096] In the preparation method of modified flake graphite in Comparative Example 7, strontium hydroxide is not added. Strontium hydroxide undergoes chemical reactions with other components during the modification process to form some protective compounds and improve the interfacial bonding between flake graphite and other additives. Without adding strontium hydroxide, this protective and interfacial optimization effect is missing, affecting the overall performance and causing it to decline.

[0097] In the preparation method of modified flake graphite in Comparative Example 8, boron carbide is not added. Boron carbide has high hardness and good antioxidant properties. In modified flake graphite, it can enhance the hardness of the material, improve its ability to resist external forces, and inhibit the oxidation of flake graphite at high temperatures. The absence of boron carbide disrupts the performance balance of modified flake graphite, resulting in varying degrees of decline in the mechanical and antioxidant properties of the material.

[0098] In the modified liquid phenolic resin of Comparative Example 9, polyacrylate cross-linked polymer is not added. The network structure after the curing of phenolic resin is not perfect enough, and the bonding force is relatively weak. When stressed, the synergistic effect between the components inside the material weakens. The lack of polyacrylate cross-linked polymer makes the structure of the bonding phase not tight enough, reducing the fusion of each material and affecting the uniformity and density, thus affecting the overall performance of the material.

[0099] In the raw materials of refractory bricks in Comparative Example 10, no benzoxazine cross-linking agent was added. After replacing the benzoxazine cross-linking agent with liquid phenolic resin, the degree of cross-linking reaction decreased, and the network structure inside the material was not firm enough. Under stress and high-temperature environment, the structure was prone to deformation and damage. The lack of benzoxazine cross-linking agent damaged the originally designed cross-linking system, thus affecting the various properties of the material.

[0100] In Comparative Example 11, neither polyacrylate cross-linked polymer nor benzoxazine cross-linking agent was added. The cross-linking reaction of phenolic resin was greatly restricted, and an ideal three-dimensional network structure could not be formed. The adhesion between the components inside the material was greatly weakened, and the resistance of the material decreased sharply under stress, thermal shock and oxidation environment. The overall performance balance of the material was completely broken, and all properties were severely affected.

[0101] In the preparation method of refractory bricks in Comparative Example 12, gradient temperature rise sintering was not carried out. Gradient temperature rise sintering was to allow the material to fully complete physical and chemical changes at different temperature stages, gradually remove moisture and organic matter, and promote crystal growth and densification. Directly heating to high temperature for sintering, the moisture and organic matter inside the material did not have time to be fully discharged, which would form a large number of pores and defects inside the material; at the same time, the reaction and diffusion between the components were uneven, resulting in stress concentration in the internal structure of the material. Direct heating sintering led to chaotic internal structure, uneven pore distribution and imperfect crystal development of the material, thus severely affecting the performance of the material.

Claims

1. A high-temperature resistant magnesia-carbon composite refractory brick, characterized in that, The refractory brick comprises the following raw materials in parts by mass: 65 - 75 parts of double - layer modified magnesium oxide, 15 - 18 parts of modified flake graphite, 0.5 - 2 parts of zirconium powder, 5 - 7 parts of modified liquid phenolic resin, and 0.5 - 1 part of benzoxazine cross - linker; the double - layer modified magnesium oxide is obtained by first coating magnesium oxide with powder phenolic resin, alumina and zirconia together, and then coating it with pitch, micron - sized magnesium oxide, zirconia and calcium oxide together; the modified flake graphite is obtained by coating flake graphite with strontium hydroxide, pitch and boron carbide together. The modified liquid phenolic resin is a liquid phenolic resin blended with a silane coupling agent and a polyacrylate cross - linked polymer.

2. The high-temperature resistant magnesia-carbon composite refractory brick according to claim 1, characterized in that, The preparation method of the double - layer modified magnesium oxide comprises the following steps: According to the mass ratio, magnesium oxide: thermosetting powder phenolic resin: alumina: zirconia = 100: (4 - 6): (4 - 6): (2 - 4), add the thermosetting powder phenolic resin into magnesium oxide and mix evenly, then add alumina and zirconia and mix evenly, sinter in an oxygen - free environment at 500℃ - 600℃ for 1h - 2h, and then break up to obtain a layer of modified magnesium oxide; According to the mass ratio, a layer of modified magnesium oxide: pitch: micron - sized magnesium oxide: zirconia: calcium oxide = 100: (5 - 8): (2 - 3): (4 - 6): (2 - 3), add pitch into a layer of modified magnesium oxide and mix evenly, then add micron - sized magnesium oxide, zirconia and calcium oxide and mix evenly, sinter in an oxygen - free environment at 400℃ - 500℃ for 1h - 2h, and then break up to obtain the double - layer modified magnesium oxide.

3. The high-temperature resistant magnesia-carbon composite refractory brick according to claim 2, characterized in that, The magnesium oxide is fused magnesite; the mass fraction of the particle size distribution of the magnesium oxide is: 25 - 35 parts with a particle size between 3mm and 5mm, 35 - 45 parts with a particle size between 1mm and 3mm, 30 - 40 parts with a particle size between 0.5mm and 1mm, and 10 - 15 parts with a particle size less than 0.5mm.

4. A high-temperature resistant magnesium-carbon composite refractory brick according to claim 2, characterized in that, The alumina and zirconia are premixed evenly before being added; the micron - sized magnesium oxide, zirconia and calcium oxide are premixed evenly before being added.

5. The high-temperature resistant magnesia-carbon composite refractory brick according to claim 2, wherein The median particle size of the thermosetting powder phenolic resin is less than 5μm; the pitch is medium - temperature modified pitch, and the median particle size of the pitch is less than 5μm; the pitch contains 2wt% - 5wt% zinc phenolsulfonate, and the median particle size of the zinc phenolsulfonate is less than 5μm; the micron - sized magnesium oxide is fused magnesite with a median particle size of less than 5μm; the median particle sizes of the alumina, zirconia and calcium oxide are all less than 5μm.

6. A high-temperature resistant magnesium-carbon composite refractory brick according to claim 1, characterized in that, The preparation method of the modified flake graphite comprises the following steps: According to the mass ratio, flake graphite: pitch: boron carbide: strontium hydroxide = 100: (4 - 6): (3 - 5): (1 - 2), first add strontium hydroxide into flake graphite and mix evenly, then add pitch and mix evenly, and finally add boron carbide and mix evenly, sinter in an oxygen - free environment at 800℃ - 900℃ for 1h - 2h, and then break up to obtain the modified flake graphite.

7. The high-temperature resistant magnesia-carbon composite refractory brick according to claim 6, characterized in that, The median particle size of the flake graphite is 10μm - 30μm; the pitch is medium - temperature modified pitch, and the median particle size of the pitch is less than 5μm; the median particle sizes of the strontium hydroxide and boron carbide are both less than 3μm.

8. A high-temperature resistant magnesia-carbon composite refractory brick according to claim 1, characterized in that, The modified liquid phenolic resin is a liquid phenolic resin containing 4wt% - 6wt% of silane coupling agent KH-560 and 1.5wt% - 3wt% of polyacrylate cross-linked polymer; the liquid phenolic resin is a thermosetting phenolic resin; the median particle size of the zircon powder is 10μm - 50μm.

9. A method for preparing the high-temperature resistant magnesia-carbon composite refractory brick according to claim 1, characterized in that, It includes the following steps: by mass fraction, mix double-layer modified magnesium oxide, modified flake graphite and zircon powder evenly to obtain a mixed powder; mix the modified liquid phenolic resin and benzoxazine cross-linking agent evenly, and then add them to the mixed powder and mix evenly to obtain a mixed material; fill the mixed material into a mold and press-mold it to obtain a brick blank; sinter the brick blank in an oxygen-free environment at 250°C - 300°C for 8h - 10h, then raise the temperature to 600°C - 800°C and sinter in an oxygen-free environment for 4h - 5h, then raise the temperature to 1300°C - 1400°C and sinter in an oxygen-free environment for 2h - 2.5h, and cool to obtain a refractory brick.

10. The preparation method of a high-temperature resistant magnesia-carbon composite refractory brick according to claim 9, characterized in that, The pressure of the pressing die head is 240MPa - 260MPa, and the heating rate is 4°C / min - 6°C / min.

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