Research On Oxidation Roasting And Metallurgical Behaviors Of Flux Pellets

With the increasingly prominent air pollution problem in China,the environmental protection department has formulated more stringent pollutant emission standards for the iron and steel enterprises.The sintering production has been one of the main pollution sources in the steel production process.The growth rate of sinter production has been decreasing year by year due to the restriction of emission standards.As the main raw material of blast furnace smelting,the decrease of sinter production will promote the burden structure of blast furnace to change.Compared with the sintering production,the pellet has significant advantages in energy consumption and pollutant emissions.Therefore,increasing the pellet ratio and responding to the limited production issue of sinter ore due to pollutant emissions and high energy consumption has become one of the important development directions of blast furnace smelting in China at present.To ensure the metallurgical properties of iron-containing raw materials,balance the slag basicity and improve the fluidity of slag,pellet will also be used as the carrier of basic and magnesium fluxes.The traditional acid oxidized pellets will be transformed into basic pellets,magnesium olivine pellets and magnesium flux pellets.However,the research and application of flux pellets still maintains at a lower level in China.The effect of fluxes on the oxidation roasting,high temperature reduction and softening-melting behaviors of pellets lacks systematic research.Based on the research objects of magnesium pellet and magnesium flux pellet,the effect mechanism of flux in the oxidation process of iron ore powder,the behaviors of reduction swelling index(RSI)in different types of pellets and the softening-melting properties for the slag of flux pellets were systemically investigated,which provides theoretical basis and practical guidance for optimal production and rational application of flux pellets.Based on the present situation of the iron ore concentrates with high SiO2 content in China,the effect of particle sizes on the oxidation behavior of iron ore concentrate was studied by thermogravimetric analysis after particle size classification with high-silicon iron ore concentrates as experimental material.The results show that with the refinement of the particle size,the SiO2 gangue content in the iron ore concentrates were dramatically decreased and the iron grade was increased.To promote the sufficient oxidation of iron ore concentrates,the lower heating rate should be used for the finer iron ore concentrates,while the higher heating rate was suitable for coarse iron ore concentrates.The influence of MgO and ternary basicity on the oxidation process for the pellet of high-silicon magnetite iron ore concentrates were studied by simulating the composition of magnetite with pure reagent and using "unreacted core model" calculation and phase analysis.The result shows that the oxidation process for the high-silicon magnetite followed the "unreacted core model".In addition,MgO has more evident inhibitory effect on the oxidation of the pellet of high-silicon magnetite iron ore concentrates compared with the pellet of low-silicon magnetite iron ore concentrates.The enhancement of MgO content not only increased the content of MF {(Fex·Mg1-x)O·Fe2O3} but also promoted the formation of FeSiO3,which could stabilize the lattice of magnetite iron ore and inhibited magnetite iron ore to be oxidized.When SiO2 content was constant,the oxidation of magnetite pellet was relatively adequate under lower ternary basicity and CaO was the main reaction flux in phase reaction.However,with the increase of ternary basicity,the ability of Mg2+ to replace Fe2+ was higher than that of Ca2+and MgO became the main reaction flux in phase reaction.Meanwhile,CaO played a catalyst role in the above substitution reaction process,which made the oxidation of magnetite pellet more difficult.Therefore,the increase of ternary basicity has a more significant inhibitory effect on the oxidation of high-silicon magnetite pellet,which is higher than that of the effect of single MgO.Reduction swelling index(RSI)is one of the most important performance indexes of pellets.Currently,RSI is characterized by the volume difference of cold pellet before and after reduction,which fail to actually show the volume change process of the pellets under high-temperature conditions.Therefore,the reduction expansion behaviors for the acid pellet,magnesium pellet and magnesium flux pellet under thermal state condition were investigated using visual high-temperature experiment combined with image analysis method.The results demonstrate that the RSI of pellets tested by Chinese standard method was lower than that tested by image analysis method.The RSI of magnesium pellet and the difference of RSI tested results between the two test methods was smallest in three types of pellets.The pellet with the minimum difference of RSI between the two test methods were still magnesium pellet.Therefore,Chinese standard test method was suitable for pellets with lower RSI.In addition,the RSI of acid pellet reached a maximum value and then remained stable until the end of reduction process during high-temperature reduction process.On the other hand,the change in the RSI of magnesium and magnesium flux pellet was first increased and then decreased.The RSI decline of magnesium pellet was larger and the magnesium flux pellet maintained a relatively stable after being decreased to a certain value.The RSI change of the three types of pellet under different reduction atmosphere at high temperature were studied.It can be found that regardless of the type of pellet,RSI of all pellets showed a stable upward trend under low reduction atmosphere.However,in addition to acid pellet,the RSI of magnesium and magnesium flux pellet were greatly decreased after reaching the maximum value under high reduction atmosphere.Consequently,the inhibition of MgO flux on the RSI of pellets is more prominent and its RSI measured by Chinese standard method is closer to the actual high-temperature reduction state.The addition of magnesium-bearing flux to pellet will promotes the increase of FeO content in slag phase.Consequently,the effect of MgO/SiO2,FeO,on the softening-melting behaviors of MgO-SiO2-FeO slag system in magnesium acid pellet were investigated in this paper.With the increase of MgO/SiO2,the softening and melting temperatures of slag phase were significantly increased.Conversely,the increase of FeO content could promote the decrease of the softening and melting temperatures of slag phase.The influence of Al2O3 contents on the softening-melting behaviors of Al2O3-MgO-SiO2-FeO slag system was further studied.The results shows that Al2O3 contents had little influence on the softening-melting performances of slag phase,but it could slightly increase the softening temperature.Based on the application background of magnesium flux pellet,the effect of quaternary basicity and reaction atmosphere on the softening-melting behaviors of CaO-MgO-SiO2-FeO-Al2O3 slag system were investigated.The results shows that there was little effect of CaO on the softening-melting characteristic temperatures of slag phase under lower CaO content.When the CaO content increased to above 30%,the softening,melting and permeation temperatures of slag phase increased and the softening-melting properties were improved.The increase of quaternary basicity could increase the softening and melting temperatures of slag phase.The softening and melting temperature had little changed when CO/CO2 ratio increased from 3/7 to 7/3.However,the permeation temperature correspondingly increased,which is attribute to the reason that the content of solid Fe obviously increased due to the sufficient reduction of FeO.Therefore,the increase of quaternary basicity is easier to improve the softening-melting properties of slag phase under high reduction atmosphere.