Mechanism Of Al₂O₃ Effect In Processes Of Formation And Reduction Of Complex Calcium Ferrites?SFCA?

Silico-ferrite of calcium and aluminum(SFCA)is a significant crystalline phase having better strength and reduciility as a bonding phase in high basicity sinter.Al2O3 plays an important role in the formation of SFCA in the Fe2O3-CaO-SiO2-Al2O3 system.As a solid solution,the composition of SFCA is affected by the Al2O3 and SiO2 contents in iron ore fines.In this work,the formation processes of SFC and CFA as the precurser of SFCA formation is studied to understand the competing effect on the formation of SFCA.Sencondly,the effect mechanism of Al2O3 content on reduction of SFCA solid solutions was investigated by changing the temperature,the time and the gas concentration of reduction.Simultaneously,Rietveld method was combined to refine the structure of SFCA in the process of reduction.The results were as follows.By investigation of CFA and SFC competing effects on formation of SFCA,it shows that CaFe2O4(CF)as precursors of CFA and SFC reacted with Al2O3 to form firstly the FCA but with SiO2 to form Ca2SiO4 before the SFC under the same conditions.The CFA participated in SFCA formation over 1100?,while SiO2 participated in SFCA formation over 1150?.It reveals that the CFA formed so easier than SFC that it was preferential as a precursor in process of SFCA formation.By TG-DSC method for studying the effect of SFCA on the formation of liquid phase,it was found that the eutectic temperature of SFCA and CF is 1194?lower than others,so the SFCA forming in the solid-phase reaction could reduce the formation temperature of liquid phase to promote the liquid phase quantity of sinter.Ca2Fe2O5 is hard to be reduced as an intermediate product appeared in the reduction process of SFCA.By thermogravimetric experiment combining XRD and SEM-EDS determinations,it showed that with increasing of Al2O3 dissolved into Ca2Fe2-xAlxO5,it's reduction was promoted.It also was found that the Al3+had migrated into the unreacted Ca2Fe2-xAlxO5 until the formation of Ca3Al2O6 during the reduction process.By experimental result in the reduction process of SFCA with different alumina content,it was found that the solid solution of Al2O3 into the SFCA brought more lattice distortion promoting the reduction of SFCA on kinetics although it increased the starting reduction temeperatue on thermodynamics.In addition,as the Al3+content in SFCA increases,it inhibites the formation of Ca2Fe2O5 due to Al3+ combining easily with Ca2+ and Si4+to form Ca2Al2SiO7 during the reduction process,this reveals the main reason why Al2O3 could improve the performance of SFCA reduction.