Mechanisms Of Calcium Ferrite Formation In Sintering Process And Low Temperature Reduction Degradation For Al-bearing Hematite

Silico-ferrite of calcium and aluminum is the main bonding phase in high basicity sinter,as one of its main chemical composition,aluminum not only affects the formation of calcium ferrite,but also aggravates the low temperature reduction degradation of sinter by dissolving into the iron oxides.It is significant to study the action mechanism of aluminum in sintering process.In this work,the formation process of CFA and its effect on liquid phase formation were investigated,and the mechanism of low temperature reduction degradation of hematite solid solution was revealed through simulating the solid-solid and solid-liquid sintering process of different type of Al-bearing hematite,and reduction process of hematite solid solution with aluminum.The XRD,OM,SEM-EDS,DSC and TG were adopted to characterize the mineral,heat and mass change in the reactions.The experimental results show that the ways in which aluminum in different occurrence states participate in the formation of CFA were different.When the Al2O3 was added to Fe203-CaO sample directly,two kinds of binary calcium ferrite CF and CF3.9 generated in the early stage and reacted with Al2O3 respectively to form CFA.However,a calcium aluminate layer formed on the surface of Al2O3 particles during both the reaction processes,which inhibiting the further diffusion of Al3+ and leading to a small formation rate of CFA.When the aluminum was added in form of Al-containing hematite solid solution,CFA generated directly on the interface between CF and hematite solid solution without the formation of CF3.9 and calcium aluminate layer.So,the CFA formation rate was significantly larger than that of adding Al2O3.In particular,the SEM-EDS results show that almost no aluminum was contained in CF.Therefore,it can be deduced that CF and CFA generated directly when Al-bearing hematite solid solution reacted with CaO in the sintering process,and the formation of CF3.9 and calcium aluminate were inhibited.TG-DSC test was carried out to study the effect of CFA on the liquid phase formation.The results show that CF3.9 generated in sample without aluminum had a eutectic temperature of 1206? with CF.When aluminum was added,the liquid formation temperature can be reduced to 1201? by co-melting of CFA and CF in the product.As the diffusion coefficient in the liquid phase was three orders of magnitude greater than that in the solid phase,it was revealed that the formation of bonding phase can be promoted through decreasing the formation temperature of liquid phase by adding Al-bearing hematite in sintering process.The verification experiment at 1230? shows that the formation of CFA was significantly promoted by the aluminum dissolved in hematite after the formation of the liquid phase.Moreover,it was advantageous to form a favored mineral microstructure when the dissolving amount of Al2O3 in hematite was 1.15-2.33%due to the adequate quantity of liquid phase.Therefore,it can be said that a small amount of aluminum in hematite solid solution is helpful to promote the sintering process of iron ore fines.The results of low temperature TG experiment show that the increase of aluminum content in hematite led to the acceleration of reduction process and the increase of stress generated in unit time.Moreover,the stress in product released in form of cracking after a rapid rise in the early stage of reduction,leading to the degradation of hematite.However,several common complex calcium ferrites didn't reduce below 600?.Therefore,the low temperature reduction degradation of sinter could be improved by promoting the transformation of Al-bearing hematite to complex calcium ferrite in sintering process through adding aluminum in solid-solution state.