Effect Of Gangue Composition And Alkalinity On Consolidation Strength And Reduction Swelling Of Fe₃O₄ Pellets

Stepping with domestic carbon oxide emission peak and cabon neutrality,the steel industry with high carbon emissions develop necessarily low-carbon smelting technology.Pellet ores are a high-quality ironmaking concentrate with high iron content.Increasing pellet proportion in blast furnace can reduce energy consumption and decrease CO₂ and pollutant emissions,which is beneficial to energy saving and emission reduction of steel industry.But there are also some shortcomings,such as the volume expansion and pulverization of pellets in blast furnace,reducing the permeability of blast furnace burden which restricting the increase of pellet ratio.The complex gangue components（such as Si O₂,Al₂O₃,Mg O and Ca O）in pellet are the key factors affecting pellet strength and reduction swelling.Therefore,in this paper,pure magnetite reagents were used as materials,and pure reagents（Si O₂,Al₂O₃,Mg O and Ca O）were used to simulate gangue composition to explore the influence of gangue composition and alkalinity on the consolidation strength and reduction expansion of pellets.The conclusions are as follows:（1）As referred to pure Fe₃O₄ pellets,elevating calcination temperature appropriately is beneficial to improve the compressive strength and reduce swelling rate.When the roasting temperature increases to 1230°C,the compressive strength is the largest and the reduction swelling rate is the smallest.This is because increasing the roasting temperature reduces the porosity of the pellets and hinders the contact between the reducing gas and the iron oxide inside the pellets,which is beneficial to reduce the reduction degree and volume swelling of pellets.（2）As referred to containing a single gangue component pellets,although more gangue will reduce the strength and reduction swelling rate of pellets,but the influencing mechanisms are different.For example,Si O₂ and Al₂O₃ mainly exist in the form of free,which destroys the recrystallized structure of Fe₂O₃ grains.More high melting point Si O₂and Al₂O₃ can not only support the internal structure of pellets,but also react with the reduction intermediate product Fe O to form 2Fe O·Si O₂ and Fe O·Al₂O₃,Hinder the conversion of Fe O to metallic iron,reduce the content of metallic iron particles in the reduced pellets,and reduce the pellet expansion rate.Mg O inhibits magnetite oxidation in the oxidation roasting process,which not only reduces the recrystallization degree of Fe₂O₃,but also reduces the volume expansion caused by the crystal transformation of iron oxides,and reduces the strength and swelling rate of pellets.Ca O can react with Fe₂O₃ to form Ca O·Fe₂O₃ during the roasting process,which will form a large amount of liquid phase at higher temperature,resulting in the increase of pellets pores in cold state and the deterioration of compressive properties of pellets.More pores are beneficial to improve the reduction degree of the pellets,which makes the metal particles inside the pellets form a dense metal iron layer,forcing the reducing gas to react with the iron oxide in the pores,so that the metal iron particles fill part of the voids and reduce the swelling rate of the pellets.（3）As referred to containing different SiO₂ content pellets.The analysis of Fact Sage software shows that when alkalinity of pellets increases,the liquid production decreases.Under the condition of constant alkalinity,the increase of Si O₂ content will increase the liquid production and increase the number of pores in the pellets.When the Si O₂ content was 3%and the calcination temperature was 1200°C,the strength of the pellets first decreased and then increased with the increase of alkalinity,and the swelling rate first increased and then decreased.When the alkalinity was 0.6,the strength of the pellets was the lowest,and the swelling rate was the highest.The reduction of the alkaline pellets was good,and the metal iron whiskers were also observed in the microstructure.With the increase of roasting temperature,the strength of the pellets increases first and then decreases,and the expansion decreases first and then increases,and the optimum roasting temperature is 1210°C.When the roasting temperature increases to 1230°C,the strength of the pellets decreases greatly and the swelling rate increases greatly.This is because increasing the roasting temperature is beneficial to the formation of liquid phase,which increases the porosity of the pellets,resulting in the decrease of the strength of the pellets and the increase of the reduction degree.When the Si O₂ content was 5%,the compressive properties and reduction swelling properties of the pellets were worse than those of the pellets with 3%Si O₂.With the increase of calcination temperature,the strength of the pellets decreased and the expansion rate increased.This was because the higher Si O₂ content would produce more liquid phases in the pellets,which increased the porosity of the pellets and reduced the inter-grain bonding force,resulting in the decrease of the strength of the pellets and the increase of the expansion rate.