Optimization of antioxidant additives in carbon-containing castables

Oxidation of carbon is a major drawback for oxide-carbon composite refractories, especially for a castable which is inherently more porous than a brick. Magnesia-based material system hold a potential to be used in slag line of steelmaking ladles due to their high corrosion resistance to basic slag and are chosen as the material of study in this thesis. Overcoming the difficulties of introducing flake graphite into such castables and of protecting it from oxidation have constituted the principal goals of this work.; The results have demonstrated that important factors to minimize oxidation of carbon-containing castables are the nature of carbon source, the antioxidants and the densification of the materials.; In this thesis it has been shown that:; 1. The mode of incorporating flake graphite into castables plays a decisive role in developing graphite containing castables. Modified flake graphite pellets by extruding (EG) and coating (CG) efficiently reduce water addition and pore size, increasing bulk density, CMOR, thermal shock parameter and oxidation resistance of the castables.; 2. Packaging natural flake graphite by incorporating antioxidants and oxide fillers into extruded graphite pellets sufficiently improve densification, pore size distribution, new compound formations and oxidation resistance of the pellets.; The best EG pellets for the MgO-MA-C system contains 80% graphite, 15% Al₂O₃ and 5% B₄C or 80% graphite, 15%Al and 5% B₄C. The best EG pellets for the MgO-M₂S-C system contains 80% graphite and 20% Si or 80% graphite and 20% SIC.; 3. Specific antioxidants sufficiently improve oxidation resistance both locally in extruded graphite pellets and overall in the matrix of the castables. Based upon our findings it is more appropriate to use aluminium buried in EG pellets and a minimum amount into the castable matrix part.; 4. The antioxidants not only protect graphite from oxidation but also improve the mechanical properties of the EG pellets and the castables, with either spinel or forsterite bonding.; 5. It is possible to obtain MgO-based graphite-containing castables with high magnesia content up to 90% MgO, cement free, 6% of carbon, 1 to 2% metallic antioxidants.; 6. To achieve such performances in MgO-M₂ S-C system, the deflocculants used are a combination of sodium hexametaphosphate (S6P), Galoryl (G) and sodium dihydrogencitrate (SD).; 7. Among the two systems tested, the MgO-M ₂S-C system seems to be the most promising to optimize, taking into consideration the hydration resistance of magnesia fines in the two systems, the workability and the permanent linear change of the castable mixes. (Abstract shortened by UMI.)...