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Magnesia carbon bricks are made of high melting point basic oxide magnesium oxide (melting point 2800℃) and high melting point carbon material that is difficult to be wetted by slag as raw materials, and various non-oxide additives are added. It is a non-burning carbon composite refractory material combined with a carbon binder. Magnesia carbon bricks are mainly used for the lining of converters, EAF, The slag line of the ladle and other parts.
As a composite refractory material, magnesia carbon bricks effectively utilize the strong resistance to slag erosion of magnesia sand and the high thermal conductivity and low expansion of carbon, compensating for the biggest disadvantage of magnesia sand, which is poor spalling resistance.
Its main features are: good high temperature resistance, strong slag resistance, good thermal shock resistance and low high temperature creep.
Preparation process
Traditional magnesia carbon bricks made with synthetic tar binders according to the cold mixing process harden and obtain the necessary strength during the tar damage process, thus forming isotropic glassy carbon. This carbon does not show thermoplasticity, which can timely eliminate a large amount of stress during lining baking or operation. Magnesium carbon bricks produced with asphalt binders have higher high-temperature plasticity due to the formation of anisotropic graphitized coke structure during asphalt carbonization.
Production process
raw material
The main raw materials of MgO-C bricks include fused magnesia or sintered magnesia, flake graphite, organic binder and antioxidant.
Magnesia
Magnesia is the main raw material for the production of MgO-C bricks, which can be divided into fused magnesia and sintered magnesia. Compared with sintered magnesia, fused magnesia has the advantages of coarse periclase crystal grains and high particle volume density, and is the main raw material used in the production of magnesia-carbon bricks. In the production of ordinary magnesia refractory materials, the main requirements for magnesia raw materials are high temperature strength and corrosion resistance. Therefore, the purity of magnesia and the C/S ratio and B2O3 content in the chemical composition are emphasized. With the development of the metallurgical industry, smelting conditions are becoming increasingly harsh. In addition to the chemical composition, the magnesia used in MgO-C bricks used in metallurgical equipment (converters, electric furnaces, ladles, etc.) also requires high density and large crystals in terms of organizational structure.
Carbon source
Whether in traditional MgO-C bricks or in the low-carbon MgO-C bricks used in large quantities, flake graphite is mainly used as its carbon source. Graphite, as the main raw material for the production of MgO-C bricks, mainly benefits from its excellent physical properties:
① Non-wetting of slag.
② High thermal conductivity.
③ Low thermal expansion.
In addition, graphite and refractory materials do not melt together at high temperatures and have high refractoriness. The purity of graphite has a great influence on the performance of MgO-C bricks. Generally, graphite with a carbon content greater than 95%, preferably greater than 98%, is used.
In addition to graphite, carbon black is also widely used in the production of magnesium carbon bricks. Carbon black is a highly dispersed black powdered carbonaceous material obtained by thermal decomposition or incomplete combustion of hydrocarbons. The carbon black particles are fine (less than 1μm), with a large specific surface area, a carbon mass fraction of 90-99%, high purity, high powder resistivity, high thermal stability, low thermal conductivity, and is difficult to graphitize carbon. The addition of carbon black can effectively improve the anti-stripping property of MgO-C bricks, increase the residual carbon content, and improve the density of bricks.
Binder
Common binders used in the production of MgO-C bricks include coal tar, coal tar and petroleum tar, as well as special carbonaceous resins, polyols, asphalt-modified phenolic resins, synthetic resins, etc. The binders used are of the following types:
1) Asphalt substances. Tar asphalt is a thermoplastic material with strong affinity with graphite and magnesium oxide, high residual carbon rate after carbonization, and low cost. It has been used in large quantities in the past; however, tar asphalt contains carcinogenic aromatic hydrocarbons, especially high content of benzo[a]-[b]-[e]; due to the strengthening of environmental awareness, the use of tar asphalt is now decreasing.
2) Resin substances. Synthetic resin is made by the reaction of phenol and formaldehyde. It can be well mixed with refractory particles at room temperature. It has a high residual carbon rate after carbonization and is the main binder for the current production of MgO-C bricks; however, the glassy network structure formed after carbonization is not ideal for the thermal shock resistance and oxidation resistance of refractory materials.
3) Substances obtained by modification based on asphalt and resin. If the binder can form a mosaic structure and in-situ carbon fiber material after carbonization, then this binder will improve the high temperature performance of the refractory material.
Antioxidants
In order to improve the oxidation resistance of MgO-C bricks, a small amount of additives are often added. Common additives include Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B4C, BN and the recently reported Al-B-C and Al-SiC-C series additives. The working principle of additives can be roughly divided into two aspects: on the one hand, from the thermodynamic point of view, that is, at the working temperature, additives or additives react with carbon to generate other substances, which have a greater affinity with oxygen than carbon and oxygen, and are oxidized before carbon, thereby protecting carbon; on the other hand, from the kinetic point of view, the compounds generated by the reaction of additives with O2, CO or carbon change the microstructure of carbon composite refractory materials, such as increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products.
Application
In the early days, the refractory materials used in the slag line of the ladle were high-quality alkaline bricks such as directly bonded magnesia-chrome bricks and electric melting and then bonded magnesia-chrome bricks. After the successful use of MgO-C bricks on the converter, MgO-C bricks were also used in the slag line of the refining ladle, and good results were achieved. my country and Japan generally use resin-bonded MgO-C bricks with a carbon content of 12% to 20%, while Europe mostly uses asphalt-bonded MgO-C bricks with a carbon content of about 10%.
The Kokura Steel Plant of Sumitomo Metal Corporation of Japan uses MgO-C bricks with a MgO content of 83% and a C content of 14-17% instead of directly bonded magnesia-chrome bricks in the VAD slag line, and the life of the slag line is increased from 20 times to 30-32 times. The LF refining ladle of Sendai Steel Plant in Japan uses MgO-C bricks instead of magnesia-chrome bricks, and the life of the slag line is increased from 20-25 times to 40 times, achieving good results. Osaka Ceramics Refractory Co., Ltd. studied the effects of carbon content and antioxidant types on the oxidation resistance, slag resistance and high-temperature flexural strength of MgO-C bricks. The study shows that MgO-C bricks made of a mixture of fused magnesia and sintered magnesia, with 15% phosphorus flake graphite and a small amount of magnesium-aluminum alloy as antioxidants, have a good use effect. When used in the 100-ton LF ladle slag line, the damage rate is reduced by 20-30% compared with MgO-C bricks with a C content of 18% without antioxidants, and the average erosion rate is 1.2-1.3mm/furnace.
Since the use of MgO-C bricks instead of magnesia-chrome bricks in my country's refined ladle slag line bricks, the comprehensive use effect is obvious. Baosteel Group Corporation's 300t ladle slag line began using MT-14A magnesia carbon bricks in July 1989, and the life of the slag line has remained above 100 times; the 150T electric furnace ladle slag line uses low-carbon magnesia carbon bricks to smelt cord steel, with a steel-out temperature of 1600℃~1670℃, achieving significant results.
Low carbon magnesia carbon brick
With the advancement of smelting technology and the new requirements for refractory materials, traditional magnesia carbon bricks have been found to have the following problems in the long-term application practice:
① Due to the increase in heat loss due to high thermal conductivity, the tapping temperature is increased, which leads to increased energy consumption and a series of problems such as increased erosion of refractory materials;
② As a lining material for special refining furnaces, such as when smelting high-quality clean steel and ultra-low carbon steel in VOD refining ladles, it will cause carbon increase problems;
③ Consume a large amount of precious graphite resources. In view of the above situation, in recent years, the development of low-carbon magnesia carbon bricks with low carbon content and excellent performance for refining ladles has received attention from the domestic and foreign industries.
The main problem caused by the reduction of carbon content in magnesia carbon bricks is the decrease in thermal shock stability and slag permeability.
As we all know, when the carbon content in magnesia carbon bricks is reduced, the thermal conductivity of the bricks decreases and the elastic modulus increases, thereby deteriorating the thermal shock stability of the bricks. After the carbon content is reduced, the wettability of slag and molten steel with the material is enhanced, and the material's resistance to slag and molten steel permeability is deteriorated.
The understanding of solving these problems mainly includes the following three aspects:
① Improving the thermal shock stability of magnesium carbon bricks by improving the carbon structure of the binding carbon: the binder of traditional magnesium carbon bricks is mostly phenolic resin. The carbon structure of this binder after carbonization is isotropic glassy, so the magnesium carbon brick is brittle and has a high elastic modulus, which is not conducive to the thermal stability of the product, and the high-temperature strength of the product is also low. After introducing a graphitizable carbon precursor into the phenolic resin, this composite binder can be carbonized into a secondary carbon with a flowing or mosaic structure, or in situ to form nano-carbon fibers under the use environment of magnesium carbon bricks. The thermal shock stability and high-temperature strength of low-carbon magnesium carbon bricks are improved by improving the carbon structure and the strengthening effect of the formation of nano-carbon fibers;
② Optimizing the matrix structure of magnesium carbon bricks: The thermal shock stability and slag permeability resistance of magnesium carbon bricks mainly depend on the composition and structure of the matrix. When the carbon content is greatly reduced, how to increase the contact frequency between aggregate particles and carbon particles, that is, to reduce the size of carbon particles and ensure their high dispersion, is one of the important measures to improve the thermal shock stability and slag permeability resistance of low-carbon magnesium carbon bricks. By adjusting the particle size composition of the matrix ingredients to control the size, shape and distribution of the pores, the thermal conductivity of the material will also be significantly affected;
③ Use of high-efficiency antioxidants: As the carbon content in MgO-C bricks decreases, the oxidation protection of carbon is particularly important, so the use of suitable high-efficiency antioxidants is also very necessary.
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