Effect of cooling rate on the controlled solidification of "Fe2O3"- CaO-SiO2 liquids in air in synthetic iron ore sinter

Abstract

The cooling rate of the liquid oxide can be controlled in industrial sintering processes through the draft pressure and has the potential to influence microstructure formation. The solidification of a liquid within the hematite primary phase field in the ternary "Fe2O3"-CaO-SiO2 system in air was undertaken at different cooling rates to determine the impact of cooling rate on the formation of product microstructures. Samples with a bulk composition of 72.7 wt% Fe2O3 and a CaO/SiO2 ratio of 3.46, were cooled from 1623 K (1350°C) at 2 K/s, 0.5 K/s, 0.1 K/s and 0.01 K/s and quenched at 5 K temperature intervals from 1533 K (1260°C) to 1453 K (1180°C). During cooling, four stages of phase assemblage formation were consistently observed at all cooling rates; in order of formation these are, Liquid +hematite (I), Liquid+hematite+dicalcium silicate(C2S)(II), Liquid+C2S+calcium diferrite (CF2)(III) and C2S+CF2+calcium ferrite (CF)(IV). An intergrowth of silico-ferrite of calcium and aluminium-I (SFCA-I) and Ca7.2Fe2+0.8Fe3+30O53 was observed to form in some conditions in regions free of hematite, present in liquids solidifying at 0.5 K/s and 0.1 K/s. The sizes and shapes of microstructures were observed to systematically change with cooling rate, with a slower cooling rate typically resulting in coarser coupled microstructures and larger individual crystals. A larger proportion of coupled microstructures are observed at slower cooling rates, this appears to be related to the degree of undercooling prior to the nucleation of new phases. The equilibrium silico-ferrite of calcium (SFC) phase was not observed at any of the cooling rates investigated.