Alumina and Magnesia-Based Castables Containing Graphite: Comparison

摘要

Castables are being selected more and more as a valuable replacement of bricks in steel-making ladles. The alumina-spinel castables and recently alumina-MgO castables, are commonly in use in the barrel and/or the bottom of ladles. At the same time, considerable changes have occurred in the nature of bricks also in common usage today. The alumina bricks (60-75% Al_2O_3), as well as the doloma bricks, are being replaced by alumina-magnesia-carbon bricks. The only position in ladles, where neither castables nor any other bricks have been really proven to be an effective replacement for the magnesia-carbon brick, is the slag line. At any rate, the carbon-containing refractories have been gaining a valid place in steel ladle applications. In zones, where the service life is much longer than at the slag line, the penetration of refractories by slag rather than the corrosion/erosion process becomes very critical. The exposition to slag attack is more static, and does not really result in erosion. It is rather the initial physical impregnation and the damage by thermo-mechanical stresses and structural spalling which are predominant. To improve the performance, penetration has to be minimized and the structural characteristics of refractory material designed to minimize the damages caused by thermo-mechanical stresses. In this type of refractories, the content of carbon, if any, is usually low (below 5 %) and its purpose is to minimize the penetration and the structural spalling. In slag line application, where the service conditions are more oxidizing than it is in the barrel and bottom, corrosion/erosion prevails. The refractory material is exposed to severe dynamic attack by the slag at steel-making temperatures, which may exceed 1650°C, and all phenomena including the penetration with the physical dissolution and recrystallization, as well as the chemical reaction, the mechanical erosion and the thermo-mechanical stress, act simultaneously. Here, the role of carbon addition into refractories is more complex than it is in the previous case. In magnesia-carbon bricks, the flake graphite addition is commonly around the level between 9% - 12%, and antioxidants are used for its protection. To achieve the same properties and performances that are common for magnesia-carbon brick with magnesia-carbon castables of the same The overall economy of doing so will be the decisive factor. It is outside the scope of this paper to tackle such an analysis. Instead, in the next 3 paragraphs, some of the recent progress made in our group will be reviewed. composition is a very difficult, almost impossible, task with the present state of knowledge, in the technology of castables. Thus the reasons for replacement of magnesia-carbon bricks in ladles by castables, particularly by basic carbon-containing castables, should be more complex than simply trying to match the properties of bricks. The overal economy of doing so will be the decisive factor. It is outside the scope of theis paper to tackle such an analysis. Instead, in the next 3 paragraphs, some of the recent progress made in our group will be reviewed.