The role of magnesia carbon brick:
Magnesia-carbon bricks are made of high melting point alkaline oxide magnesia (melting point 2800C) and high melting point carbon materials that are difficult to be infiltrated by slag as raw materials, and various non-oxide additives are added. Non-burning carbon composite refractory material combined with carbon binder. Magnesia-carbon bricks are mainly used for the lining of converters, AC electric arc furnaces, DC electric arc furnaces, and the slag line of ladle.
As a composite refractory material, magnesia-carbon brick effectively utilizes the strong slag erosion resistance of magnesia and the high thermal conductivity and low expansion of carbon, which compensates for the biggest disadvantage of poor spalling resistance of magnesia.
Its main features are: good high temperature resistance, strong slag resistance, good thermal shock resistance, and low high temperature creep.
The preparation process of magnesia carbon brick:
Traditional magnesia-carbon bricks made with synthetic tar binders according to the cold mixing process harden and acquire the necessary strength during tar damage, thus forming isotropic glassy carbon. This carbon exhibits no thermoplasticity, which in time relieves substantial stress during liner baking or handling. The magnesia-carbon brick produced with pitch binder has high high temperature plasticity due to the formation of anisotropic graphitized coke structure during the carbonization of pitch.
The production process of magnesia carbon brick:
- Raw material
The main raw materials of MgO-C bricks include fused magnesia or sintered magnesia, flake graphite, organic binders and antioxidants.
Magnesia is the main raw material for the production of MgO-C bricks. There are fused magnesia and sintered magnesia. Compared with sintered magnesia, fused magnesia has the advantages of coarse periclase crystal grains and large particle bulk density. It is the main raw material used in the production of magnesia-carbon bricks.
The production of ordinary magnesia refractory materials mainly requires high temperature strength and erosion resistance for magnesia raw materials. Therefore, attention should be paid to the purity of magnesia and the C/S ratio and B2O3 content in the chemical composition. With the development of the metallurgical industry, the smelting conditions are becoming more and more severe, and the magnesia used in the MgO-C bricks used in metallurgical equipment (converter, electric furnace, ladle, etc.), except for the chemical composition。 In terms of organizational structure, high density and large crystallinity are also required.
- Carbon source
Whether in traditional MgO-C bricks or low-carbon MgO-C bricks used in large quantities. It mainly uses flake graphite 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 to slag.
②High thermal conductivity.
③ Low thermal expansion.
In addition, graphite and refractory do not eutectic at high temperature, and the refractoriness is high. 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 commonly used in the production of magnesia-carbon bricks. Carbon black is a highly dispersed black powdery carbonaceous material obtained by thermal decomposition or incomplete combustion of hydrocarbon-like compounds. Carbon black has small particles (less than 1um), large surface area, carbon mass fraction of 90~99%, high purity, high powder resistivity, high thermal stability, and low thermal conductivity. It is difficult to graphitize carbon. The addition of carbon black can effectively improve the spalling resistance of MgO-C bricks, increase the amount of residual carbon, and increase the density of bricks.
- Binding agent
The commonly used binders for the production of MgO-C bricks are coal tar, coal pitch and petroleum pitch, as well as special carbonaceous resins, polyols, pitch-modified phenolic resins, synthetic resins, etc. The binders used are of the following types:
1) Asphalt substances.
Tar pitch is a thermoplastic material, which has the characteristics of high affinity with graphite and magnesium oxide, high residual carbon rate after carbonization, and low cost. It has been widely used in the past. However, tar pitch contains carcinogenic aromatic compounds, especially benzoβ content; due to the strengthening of environmental awareness, the use of tar pitch is now decreasing.
2) Resin substances.
Synthetic resin is prepared by the reaction of phenol and formaldehyde. It can be well mixed with refractory particles at room temperature, and has a high residual carbon rate after carbonization. It is the main binder for the current production of MgO-C bricks. However, the glassy network structure formed after carbonization is not ideal for thermal shock resistance and oxidation resistance of refractory materials.
3) Substances obtained by modification on the basis of asphalt and resin.
If the binder is carbonized to form a mosaic structure and in-situ formation of carbon fiber mass, then this binder will improve the refractory material
In order to improve the oxidation resistance of MgO-C bricks, a small amount of additives are often added. Common additives are Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B4C , BN and recently reported additives such as AI-B-C and AI-SiC-C [5-7]. The principle of action of additives can be roughly divided into two aspects:
On the one hand, it is from a thermodynamic point of view, that is, at the working temperature, the additive or the additive and carbon react to form other substances. They have a greater affinity for oxygen than carbon, are oxidized in preference to carbon. So as to play a role in protecting carbon.
On the other hand, from a kinetic point of view, the compounds generated by the reaction of additives with o2, CO or carbon change the microstructure of carbon composite refractories. Such as increasing the density, blocking pores, hindering the diffusion of oxygen and reaction products.
Application of magnesia carbon brick:
The refractory material used in the early ladle slag line was directly combined magnesia-chrome bricks, Electrofusion combined with high-quality alkaline bricks such as magnesia-chrome bricks. After the MgO-C brick was successfully used in the converter, the MgO-C brick was also used in the refining ladle slag line, and good results were obtained. China 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, and the carbon content is generally around 10%.
Japan Sumitomo Metal Corporation Kokura Steel Works used MgO content of 83% and C content of 14-17% MgO-C in VAD line section 1M1 instead of directly bonded magnesia-chrome bricks, and the life of the slag line was increased from 20 times to 30- 32 times. Japan Sendai Steel Plant LF refining ladle, using Mgo-C bricks instead of magnesia-chrome bricks, the life of the slag line has been increased from 20-25 times to 40 times, and good results have been achieved. Osaka Kiln 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 mixture of fused magnesia and sintered magnesia, plus 15% phosphorus flake graphite and a small amount of magnesium aluminum alloy as antioxidants to makes MgO-C bricks, which have good use effects. When used in LF ladle slag line with a capacity of 100 tons, compared with MgO-C bricks with 18% C content without antioxidants, the damage rate is reduced by 20-30%, and the average erosion rate is 1.2-1.3mm/furnace.
How to deal with the problem of reducing the carbon content of magnesia carbon bricks:
With the progress of smelting technology and new requirements for refractory materials, traditional magnesia-carbon bricks have found the following problems in the long-term application and practice process:
①Due to the increase of heat loss due to the high thermal conductivity, the tapping temperature is increased, resulting in an increase in energy consumption, and at the same time, a series of problems such as increased erosion of refractory materials;
②As the lining material of a special refining furnace, such as smelting high-quality clean steel and ultra-low carbon steel in a VOD refining ladle, it will cause carbon increase problems;
③ Consume a lot 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 ladle has received attention from domestic and foreign industries.
The main problems caused by the reduction of carbon content in magnesia-carbon bricks are the decrease in thermal shock stability and resistance to slag penetration. As we all know, after the carbon content in magnesia-carbon bricks decreases, the thermal conductivity of the bricks decreases and the elastic modulus increases, so that the thermal shock resistance of the bricks deteriorates. After the carbon content is reduced, the wettability of slag and molten steel and the material is enhanced, and the permeability of the material to slag and molten steel is deteriorated.