| Recently,increasing of Al content in raw materials of sinter decreases reduction degradation index(RDI)of iron ore sinter and increases viscoity of blast furnace slag.In order to improve the RDI of iron ore sinter and decrease the viscosity of blast furnace,Mg-bearing flux is added in sintering process.However,the increase of Mg content also leads to decrease of content of liquid phase and strength of iron ore sinter.In sintering process,formation and crystallization of calcium ferrite directly affects the quality of iron ore sinter.Therefore,it is very important to clarify mechanism of MgO on formation and crystallization of calcium ferrite for developing calcium ferrite theory and improving quality of iron ore sinter.In this work,studies were summarized as follows.Changing of initial liquid phase in MgO-CaO-Fe2O3 system formed in heating process was studied by DSC.In addition,depression mechanism of MgO on formation of initial liquid phase was also investigated by the changes of composition and microstructure of phases formed before liquid phase formation.It was found that MgO would react with Fe2O3 forming Mg-bearing magnetite.In addition,MgO addition also decreased thermal stability of CaFe2O4.It depressed the formation of CaFe2O4 leading to decrease of initial liquid phase content in sintering.The results clarified the main reason of decrease of initial liquid phase content in sintering which was led by MgO addition.Through assimilation experiment between MgO-CaO-Fe2O3 melt and hematite,mechanism of MgO on dissolution of hematite into MgO-CaO-Fe2O3 melt was investigated.The results showed that,viscosity of melt increased with increasing MgO content which was not favor for Fe2O3 to dissolve into melt.However,solubility of Fe2O3 in melt also increased with increasing MgO content.Effect of these two factors led the actual solution of Fe2O3 in melt firstly increased and then decreased.When MgO content exceeded 3 mass%,actual solution of Fe2O3 in melt decreased which was not favor for formation of calcium ferrite.In order to study effect of different gangue element on crystallization of CaO-Fe2O3 binary melt,composition of crystal products of MgO-CaO-Fe2O3,SiO2-CaO-Fe2O3 and Al2O3-CaO-Fe2O3 melt were investigated respectively by XRD and SEM.The results showed that MgO only exited in magnetite phase instead of forming binary calcium ferrite.MgO addition promoted the crystallization of magnetite in CaO-Fe2O3 melt which resulted in decrease of Fe content and depressed crystallization of CaFe2O4.On the contrary,both SiO2 and Al2O3 would promote and participate crystallization of calcium ferrite.However,when content of SiO2 or Al2O3 exceeded 3 mass%,Ca2SiO4 and Ca2(Al,Fe)2O5 began to crystallize leading to increase of viscosity of melt which was not favor to crystallization of calcium ferrite.On the basis of crystallization law of binary CaO-Fe2O3 melt,mechanism of MgO on crystallization of CaO-Fe2O3-SiO2-Al2O3 melt was investigated by using modified high-temperature quenching method.It was found that MgO addition promoted the crystallization of magnetite which resulted in decrease of Fe content in melt and depressed crystallization of silico-ferrite of calcium and aluminum(SFCA).Addition of Fe2O3 in melt increased the Fe content which promoted crystallization of SFCA.In addition,the crystal structure of SFCA transformed to SFCA-I with increasing Fe2O3 content.The results indicated the composition of crystal products in melt was related to the mass ratio of CaO to Fe2O3.The unified mechanism of MgO on crystallization of calcium ferrite from binary to multicomponent system was proposed.It was helpful to understand the crystallization theory of calcium ferrate and to provide a theoretical basis for improving the quality of iron ore sinter. |
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