Magnesia carbon bricks are widely used in metallurgical processes, but their service life is still very problematic due to their harsh working conditions, especially in the slag line of the ladle, where the damage to magnesite carbon bricks is particularly serious.
(1) Slag erosion of magnesia carbon refractory bricks:
In the ladle, due to the complex physical and chemical environment of the slag line, the lining of this part is most susceptible to damage. 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 alkalinity: The lower the alkalinity of slag, the more favorable it is to erode MgO-C bricks. If the alkalinity 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 alkalinity, 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 alkalinity, the slower the erosion of MgO-C bricks. 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 more easily penetrate from the grain boundary of magnesia sand, and make periclase separate from the matrix of magnesia carbon bricks.
4. Effect of FeO: First, FeO in slag can easily react with graphite in magnesia carbon brick at high temperature to produce bright white iron beads, forming a decarburization layer as shown in Figure 1. Secondly, periclase in magnesia carbon brick will also react with FeO in slag to form low melting point products.
(2) Oxidation of carbon in magnesia carbon brick:
When magnesia carbon brick contacts with slag, carbon will react with FeO and other oxides in slag to form a decarburization layer under certain conditions, resulting in loose structure of the working surface of magnesia carbon brick, which is the main reason for the damage of magnesia carbon brick. 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 decarburization layer produces larger cracks and pores under the action of thermal expansion and scouring of slag, making it easier for slag to penetrate and form a low melting point phase with MgO. At the same time, the surface structure of the magnesia carbon brick 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 the magnesia carbon brick to be severely damaged. When the temperature exceeds a certain value, the brick body will be damaged and rapidly corroded. This is because MgO and graphite begin to react with self-consumption at high temperature.
(3) The influence of pores:
Due to the presence of micropores inside and on the surface of magnesia carbon bricks, erosion of magnesite carbon bricks is more likely to occur. During the use of mag-c bricks, pores play an accelerating role in the formation of the decarburization layer, which in turn makes the slag erode the refractory material of magnesium carbon bricks more serious. When the outside air enters the pores in the magnesite carbon 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 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 to 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 magnesite 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 magnesia carbon fire bricks. Due to the repetitive nature of the gas generation process, the damage to the magnesia carbon firebricks continues to intensify.
