Intermittent Microscopic Observation of Structure Change and Mineral Reactions of High Phosphorus Oolitic Hematite in Carbothermic Reduction

Change of the typical structure of high phosphorous oolitic hematite during carbothermic reduction process have been studied using intermittent observation. It indicated that there were four change stages, including crack formation and growth, mineral diffusion, mineral crystalization, mineral crystal migration and reaction. Eventually, inherent oolitic structure was completely destroyed. Mineral reactions were also investigated and discussed along the structural change, giving much understanding of thermal behavior of the oolitic hematite during reduction. The results showed a variety of mineral reactions occurred, such as decomposition of CaCO3, reduction of iron oxide, dehydration and decomposition of chlorite, decomposition of fluorapatite, and transformation of gangues. SiO2 and Al2O3 decomposed from chlorite can not only combine with FeO to form Fe2SiO4 and FeAl2O4, which hindered the reduction of iron oxide, but also promoted the decomposition of fluorapatite to form Ca3(PO4)2 in the presence of SiO2. Then, Ca3(PO4)2 was reduced to form CaSiO3 and P2, which was the main process of dephosphorization. However, a small amount of fluorapatite diffused into iron oxides, which made it difficult to separate by mechanical crushing and screening.