With the maturity of magnesia carbon brick production technology, the application scope of magnesia carbon bricks is getting wider and wider. Magnesite carbon bricks are used as the lining of electric arc furnaces, and the service life of the lining is greatly improved. Although magnesia carbon fire bricks are widely used in metallurgical processes, their service life is still very problematic due to their harsh working conditions, especially in the slag line of the ladle, where the damage of magnesium carbon bricks is particularly serious.
In the ladle, the chemical composition of the slag is complex and changeable, and the temperature changes violently and frequently, especially in the slag line of the ladle. Therefore, mgo-c bricks with excellent performance are often used in the slag line. The corrosion mechanism of magnesia carbon refractory bricks in slag in ladle has been deeply studied at home and abroad, and the detailed summary is as follows.
PART.01 Slag erosion of magnesia carbon bricks
In the ladle, due to the complex physical and chemical environment of the slag line, the lining of this part is most easily damaged. The chemical erosion of slag on MgO-C bricks is mainly through the dissolution of MgO and the oxidation of carbon in the matrix of MgO-C bricks. Under the combined action of the following factors, MgO-C bricks are damaged:
1. The influence of basicity: The lower the basicity of slag, the more favorable it is to erode MgO-C bricks. If the basicity of slag increases, the activity of SiO2 in slag decreases, which can reduce the oxidation of carbon. At the same time, with the increase of basicity, the activity of FeO in slag decreases, which relatively slows down the erosion of slag on MgO-C bricks;
2. The influence of MgO: Osbom et al. found that the content of MgO in the slag layer was as high as 30% when analyzing the composition of LF slag line. They believed that the higher the content of MgO in slag, the slower the erosion of MgO-C bricks. The higher the basicity, the slower the erosion of MgO-C bricks by slag.
3. Effect of Al2O3: Al2O3 in slag will reduce the melting point and viscosity of slag, increase the wettability of slag and refractory materials, make slag easier to penetrate from the magnesia grain boundary, and make periclase separate from the magnesia carbon brick matrix.
4. Effect of FeO: First, FeO in slag can easily react with graphite in magnesia carbon brick at high temperature, and produce bright white iron beads to form a decarburized layer. Secondly, periclase in magnesia carbon brick will also react with FeO in slag to form low melting point products.
During the repeated heating and cooling of the ladle, due to the inconsistent thermal expansion rate between the formed magnesia-iron composite low melting point product and magnesia iron ore, the magnesium oxide on the surface of the refractory material is broken, which leads to the dissolution of the brick. Foreign scholars also believe that the increase of iron content in steel slag is not good for the life of magnesium carbon bricks. First, iron FeO accelerates the oxidation of carbon on the surface of magnesite carbon bricks. Secondly, FeO will react with MgO to make the working surface structure of magnesia carbon refractory bricks loose. Under the combined action of these two points, the erosion of magnesia carbon fire bricks is accelerated.
PART.02 Oxidation of carbon in mgo carbon bricks
When magnesia carbon bricks come into contact with slag, carbon will react with oxides such as FeO in the slag to decarburize, forming a decarburized layer under certain conditions, which makes the working surface structure of magnesium carbon bricks loose, which is the main reason for the damage of mgo carbon bricks. Carbon reacts with oxides such as CO2, O2 and SiO2 and is continuously oxidized by iron oxides in the slag; secondly, the loose structure formed by the decarburized layer produces larger cracks and pores under the action of thermal expansion and scouring of slag, making it easy for slag to penetrate and form a low melting point phase with MgO. At the same time, the surface structure of mgo carbon bricks changes under the action of violent mechanical stirring of the molten pool and violent scouring of steel slag, and eventually gradually damages from the outside to the inside, causing serious damage to magnesite carbon bricks. After the temperature exceeds a certain value, the brick body structure will be damaged and rapidly corroded, which is because MgO and graphite begin to self-consume at high temperature.
PART.03 Influence of pores
Due to the presence of micropores inside and on the surface of magnesia carbon bricks, erosion of mgo c refractories bricks is more likely to occur. During the use of mgo c fire bricks, pores play an accelerating role in the formation of the decarburization layer, which makes the slag corrode the refractory material of magnesia carbon bricks more seriously. When the external air enters the pores in the mgo c bricks for cooling, the oxygen in the air reacts with the surrounding carbon to generate CO gas and is discharged through the micropores. The continuous occurrence of the two processes gradually increases the porosity and pore size. The most important factor in the generation of pores is the selection of binders in magnesia carbon fire bricks. Phenolic resin is generally used as the binder. If a small amount of phenolic resin is added to the magnesia carbon brick, the porosity will not be too high in the cold state, about 3%, but the phenolic resin will decompose and produce water, hydrogen, methane, carbon monoxide (carbon dioxide) and other gases after heating, and form pores under the flow of these gases, increasing the porosity. Therefore, the magnesium carbon bricks are corroded by the slag passing through the pores, making the oxidation of carbon and the dissolution of MgO more intense, thereby damaging the magnesite carbon bricks. Due to the repetitive nature of the gas generation process, the damage of magnesia carbon firebricks continues to intensify.
The damage process of magnesia carbon bricks can be summarized as: oxidation, decarburization, loosening, erosion, scouring, shedding, and damage. First, the graphite on the working surface of the magnesia carbon brick is oxidized to form a decarburized layer. The magnesia in the decarburized layer is gradually eroded and shed under the conditions of thermal stress (the thermal expansion rates of graphite and magnesia at 1000°C are 1.4% and 0.2%, respectively), chemical erosion, and mechanical scouring. After shedding, the graphite is exposed and continues to be oxidized to form a decarburized layer, and then the magnesia dissolution process occurs. Under repeated action, the magnesia carbon brick is damaged.
