Influence of reducing gas composition on disintegration behavior of iron ore agglomerates

H₂ injection through the shaft into blast furnace (BF) is a potential option for a further reduction of CO₂ emission from BF. H₂ promotes reduction reaction of burden materials, but its influence on their reduction disintegration behavior remains unknown in detail. This study has investigated the influence on the iron ore sinter, self-fluxed and acid pellets and specified essential factors governing the reduction disintegration behavior. Mass ratios of particles below 3 mm in size were measured as an index for reduction disintegration (RDI) after the reduction of burden samples applying the gas mixtures of CO-H₂-CO₂-N₂ at 823 K. The reduced samples were observed by an optical-microscope and an electron probe micro-analyzer for evaluation of reaction mode. Further, using the measurement results, stress, strain energy and crack area generated during reduction were calculated and formation mechanism of cracks was examined. RDI value of self-fluxed pellet increased under higher H₂ condition (40%N₂-20%H₂-10%CO-30%CO₂) and reached to 26 mass%. Such an increase was larger than expected from the standard RDI measure without H₂. Sample observation revealed a fact that reaction mode governed RDI value, that is, disintegration worsened under the reduction with non-topochemical mode. The fact was explained by the calculation as an influence of crack formation and propagation. In a case of reduction with topochemical reaction, cracks generated in a concentric fashion. Meanwhile, reduction with non-topochemical reaction tended to generate cracks in radial direction, which causes pellet chipping and further degradation. Calculated crack areas showed good correlation with RDI values. The Crack area of non-toochemical reaction was more than double than that of topochemical reaction. This result indicates that disintegration does not much progress when crack area is less than the certain limit value, but it proceeds drastically when it exceeded the limit value.