Effect Of MgO On Mineralization Mechanism Of Sinter For Inhibiting The Low-temperature Reduction Degradation

It has been recently reported that the low-temperature reduction degradation of sinter can be improved by adding MgO during the sintering process through stabilizing the lattice of magnetite and depressing the formation of secondary hematite.However,there are still many problems on the mechanisms.For examples,the reasons to promote the transformation from hematite to magnetite to depress the formation of secondary hematite by addition of MgO are not clear,it has restricted the optimization of sintering process and the development of sintering theory.In this work,a simulation experiment for secondary hematite formation was carried out to follow quantitatively the transformation between hematite and magnetite,and XRD,SEM-EDS,TG-DSC and ferrous analysis,etc were combined to investigate the effect of MgO on formation mechanism of Mg-bearing magnetite and secondary hematite.In addition,the relationship between MgO added in sintering process and the low-temperature reduction degradation of sinter in blast furnace was established by experiments to study the effect of MgO on formation of calcium ferrite during multicomponent system sintering process and on reduction of the sintered product at low temperature,finally the effect of MgO on mineralization mechanism for improving the low-temperature reduction degradation of sinter was revealed.In this experiment,it was found that the magnesium ferrite(MgO·Fe2O3)forms firstly as an intermediary during reaction between Fe2O3 and MgO,subsequently Mg-bearing magnetite appears,resulting the decrease of transformation temperature from hematite to magnetite to 900? in air.On the basis of this result,the formation mechanism of Mg-bearing magnetite was suggested i.e.the Mg2+ in magnesium ferrite is replaced by the Fe3+ from hematite to form the Mg-bearing magnetite.By the simulation experiment of secondary hematite formation it shows that with increase of MgO addition from 0%to 5wt%,the index of secondary hematite formation decreases from 0.890 to 0.318 and cracks in the sintered product decrease from 37.89%to 8.25%obviously.It reveals that the cracks are the insufficient initially on characteristics of low-temperature reduction degradation caused by secondary hematite.It is also found that the cracks are mainly formed in the transformation process from hematite to magnetite.After MgO addition,the decrease of transformation temperature enlarges the temperature range of transformation from hematite to magnetite resulting in decrease of cracks due to reduction of the stress concentration.In sintering experiment of MgO-bearing multicomponents it shows that the MgO addition promoted the formation of SFC and SFCA,but had effect no more on the formation of CFA in solid phase reaction,while it promoted the formation of SFC,inhibited the formation of CFA and had effect no more on formation of SFCA in liquid phase reaction.Nevertheless,MgO addition had been still helpful to formation of magnetite.It was also found that Al2O3 addition promotes the formation of calcium ferrite in liquid phase reaction,simultaneously weakens the effect of MgO.Finally,the reduction experiment of sintered products at low temperature verified that the crack produced in reduction process is related to the reduction ratio,it increases with the reduction ratio.In addition,it also gives some enlightenment,that is,the cracks decrease with the increase of calcium ferrite and magnetite mainly due to the decrease of hematite.It also reveals that the effect of MgO addition can not be ignored for improving of the low-temperature reduction degradation.Simultaneously,the resisting ability of the stress produced during the transformation from hematite to magnetite could also be improved by increasing the calcium ferrite.