A comparison of the effect of nanostructured MgCr2O4 and FeCr2O4 additions on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories

Abstract

The effect on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories of additions of nanostructured MgCr2O4 and FeCr2O4 is reported. The nanostructured additives, synthesized by the citrate-nitrate route and calcined at several different temperatures, were characterized by XRD, BET and TEM. Additions of 0.5 and 1 wt % of these nanostructured oxides were made to magnesia-chrome refractories and calcined at 1650 OC in a shuttle kiln. Their microstructures were analyzed by SEM/EDX and their physical and mechanical properties (permanent linear change (PLC), bulk density, apparent porosity, cold crushing strength (CCS) and hot modulus of rupture (HMOR) were determined according to the respective DIN standards. The addition of the nanostructured oxides to the magnesia-chrome refractories facilitated the formation of secondary spinels, influencing the physical and mechanical properties. FeCr2O4 additions increased the size of the secondary spinel due to liquid phase formation in the presence of magnetite impurities in the FeCr2O4 nano-powder. The addition of nano-sized MgCr2O4 and FeCr2O4 to the base formulation of the refractory increased the CCS from 67.4 MPa to 82.8 MPa and 81.0 MPa respectively, while nano-sized MgCr2O4 increased the HMOR value from 5.48 MPa to 5.91 MPa and nano-sized FeCr2O4 increased the HMOR from 5.48 MPa to 5.72 MPa. This smaller increase than that obtained with FeCr2O4 additions is attributed to liquid phase formation in the presence of magnetite, as observed by XRD.