Understanding coalescence in iron ore sintering using two bench-scale techniques

With the formation of melt, the structure of a sintering bed transforms because of material coalescence. The drivers of coalescence were studied using two bench-scale techniques. Analogue sinter mixes of varying the basicity and gangue levels were taken to high temperatures using thermomechanical analysis (TMA) and in an ash fusion test (AFT). In TMA, the penetration of a piston into the sinter mix as melt was generated provided information on the deformation, shrinkage, densification and flow of the sample as a function of temperature. Projected sample shapes in the AFT were used to determine sample density and densification level. A computer model FactSage and reported equations were used to provide estimates of melt volume and viscosity. Trends indicated by the TMA and AFT results were similar and large changes in results were only obtained with significant melt generation. Differences in results between the samples could not always be explained because varying the composition of the sinter mix altered the porosity of the sample. Increasing sample porosity meant that the generated melts were not as connected and more work is required to achieve the same level of densification. On a sinter strand, coalescence occurs under a normal load and this effect is simulated in the TMA. However, the excessive flow of melt from the crucible and chemical reactions means that TMA results are unreliable at temperatures greater than 1300°C. For this reason, the AFT is the preferred technique to understand the factors that cause material coalescence on a sinter strand.