Study Of The Mechanism On Formation Of Calcium Ferrite In The Fe₂O₃-CaO-SiO₂System

Silico-ferrite of calcium (SFC) is an important intermediate phase in the sintering process of fine iron ores, which provides the structural basis for the formation of complex silico-ferrites of calcium and aluminium (SFCA-Ⅰ and SFCA), and to study it will improve the sintering theory and optimize the sintering production in the promotion of the formation of SFCA. In order to understand the formation mechanism of SFC, SiO2involved in the formation of SFC, the construction process of the crystal lattice of SFC, and the role of SFC on the formation of the low melting mineral in the Fe2O3-CaO-SiO2system were made clearly by the fundamental experiments in the paper.The results show that SiO2reacts with the intermediate product of CaO·Fe2O3(CF) to form2CaOSiO2(C2S) at approximately1000℃; and C2S reacts with Fe2O3and CF to form SFC when the temperature reaches to approximately1100℃; among them, the amount of SFC reaches maximum when the mole ratio of Fe2O3to CF is approximately1:0.55. In addition, the formation process of SFC and the construction process of the crystal lattice of SFC were determined by the reaction of SiO2with Fe2O3and CF combining with Rietveld structure refinement. It shows that Fe2O3will reacts with CF firstly to form orthorhombic Ca2.5Fe15.5O25when Fe2O3is sufficient, then, Si4+and Ca2+ions take some place of Fe3+ion in preference located on the octahedral layers which belongs to (0018) plane of Ca2.5Fe15.5O25, and the crystal structure gradually transforms to triclinic SFC.Further, X-ray absorption fine structure (XAFS) was used to study SFC and found that the solution of Si4+is closely related to the Ca channel composed of Ca octahedron in the crystal structure of SFC, Si4+mainly occupies the center positions of the upper and lower tetrahedron adjacent to Ca channel. The length of Ca-Ca bond in Ca channel increases with the increasing of Si4+solution which determines the stability of the crystal structure of SFC.The effects of Al2O3and MgO on the formation of SFC show that the bond length of Fe-0in octahedron increases after Al2O3solution in Ca2.5Fe15.5O25, and the solution of Si4+was promoted so that the transformation of Ca2.5Fe15.5O25to SFC is easy to occur. But the amount of SFC reaches maximum when the addition of Al2O3is1.72wt%due to the existence of solid solubility limit. The addition of MgO was not conducive to the formation of SFC during the solid state reaction stage. On the contrary, it tends to be more reacted with iron oxide to form the solid solution of Mg2+in Fe3O4.Finally, the role of SFC on the formation of low melting mineral in the Fe2O3-CaO-SiO2system was studied by sintering reaction at high temperature and DSC analysis. The results show that the amount of liquid phases appeared by adding SiO23.0wt%to CF is maximum at1200℃in air for4h, and the composition consistent with50wt%CF-50wt%SFC. DSC experiment found that the melting temperature comparing that of CF decreases35℃when the addition of SiO2is3.0wt%; and the melting temperature of CF and50wt%SFC is relatively lower34℃than CF, it can be considered above that caused by the eutectic structure formed by CF and SFC.