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 microstruc tures given equivalent treatments. At 1200 and 1500 degreesC. addition of A l leads to generation of Al4C3. AIN. Al2O3 and magnesium aluminate spinel, MA. Gaseous species. such as Al(g). participate in their formation processe s as predicted and suggested by their morphologies. For example. AlN occurs as whiskers and MA as fine precipitates in the matrix and as surface layer s on MgO aggregates. Additionally, Al2O3 and Al4C3 shells occur surrounding porous cores at the location of the original Al particles. At 1200 degrees C. addition of Si leads to formation of SiC. Si3N4. SiO2 and Forsterite, M2 S. These phases are still present at 1500 degreesC except for Si3N4 which i s not thermodynamically stable. Gaseous species such as SiO(g) were also in volved in formation of these product phases as predicted and suggested by t heir 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 cal culations predict that at 1200 and 1500 degreesC B4C reacts with N-2 from t he atmosphere to form BN and/or with CO from the atmosphere to form B2O3 wh ich further reacts with MgO to form low melting 3MgO.B2O3, M3B. M3B becomes liquid > 1350 degreesC and takes up impurities from MgO and/or graphite ra w materials, forming more liquid. which will be detrimental to the refracto ries corrosion resistance. M3B was detected in the microstructures but ther e was no direct evidence of BN formation and its reaction with CO. Gaseous species such as B2O3(g) were also involved in M3B formation. (C) 2001 Elsev ier Science Ltd. All rights reserved.