Thermochemistry and microstructures of MgO–C refractories containing various antioxidants

Reactions on firing MgO–C refractories with added Al, Si and B4C have been predicted by thermodynamic calculations and observations made of microstructures given equivalent treatments. At 1200 and 1500oC, addition of Al leads to generation of Al4C3, AlN, Al2O3 and magnesium aluminate spinel, MA. Gaseous species, such as Al(g), participate in their formation processes as predicted and suggested by their morphologies. For example, AlN occurs as whiskers and MA as fine precipitates in the matrix and as surface layers on MgO aggregates. Additionally, Al2O3 and Al4C3 shells occur surrounding porous cores at the location of the original Al particles. At 1200oC, addition of Si leads to formation of SiC, Si3N4, SiO2 and forsterite, M2S. These phases are still present at 1500oC except for Si3N4 which is not thermodynamically stable. Gaseous species such as SiO(g) were also involved in formation of these product phases as predicted and suggested by their morphologies. M2S occurred as fine precipitates in the matrix and as a layer on MgO aggregate surfaces. SiO2 formed either directly as a layer on Si particles or indirectly as a shell around a SiC core. Thermodynamic calculations predict that at 1200 and 1500oC B4C reacts with N2 from the atmosphere to form BN and/or with CO from the atmosphere to form B2O3 which further reacts with MgO to form low melting 3MgO.B2O3, M3B. M3B becomes liquid >1350oC and takes up impurities from MgO and/or graphite raw materials, forming more liquid, which will be detrimental to the refractories corrosion resistance. M3B was detected in the microstructures but there was no direct evidence of BN formation and its reaction with CO. Gaseous species such as B2O3(g) were also involved in M3B formation.