Research On Effect Of Gangue Composition On Reduction Swelling Index Of Hematite Pellets

With the implement of environmental protection policies and supply side structural reforms,low cost blast furnace ironmaking is the goal pursued by many steel companies.The rational allocation of the furnace burden structure is one of the most effective ways to reduce ironmaking costs.High basicity sinter and acid pellet is the burden structure commonly used in blast furnaces in China.With the increase of proportion of pellets into the furnace,acid pellets can no longer meet the demand of blast furnace production.Therefore,the development of fluxed pellets is particularly important.The reduction swelling index of fluxed pellets is the key factor affecting the feed ratio of pellets,and the gangue composition is the key to affect the reduction swelling index of the pellets.It is of great significance to study the effect of gangue composition on the reduction swelling index of pellets and further explore its swelling mechanism.The theoretical analysis of the roasting process of hematite pellets with different gangue composition was carried out.The experiment used analytically pure reagents to suppress hematite pellets.The effect of gangue composition?CaO,MgO,SiO₂,Al₂O₃?and basicity?CaO/SiO₂?on reduction swelling index of pellets was investigated,and the mechanism of reduction swelling of pellets was explored.The main conclusions are as follows:The results of FactSage thermodynamic calculation show that when the pellets contain CaO,appropriate roasting temperature should be selected to ensure the recrystallization temperature of pellets.And liquid phase production and formation temperature should be taken into account.When the pellets containing MgO and Al₂O₃is roasted,a high melting point substance such as magnesium aluminate spinel is formed,so that the roasting temperature needs to be appropriately increased.For pellets of different basicity,when the basicity of pellet is 0.2-0.6,the main mineral composition in the pellet is liquid phase,iron oxide and a small amount of spinel.When the basicity of pellet is increased from 0.6 to 1.0,no liquid phase is produced in the pellet,and its main mineral composition is kirschsteinite,iron oxide and melilite.Different gangue composition and basicity have different effects on the reduction swelling index of pellet.For the pellet with a single gangue,when the reduction time is60 min,the reduction swelling index of pure hematite pellets is about 18.0%.When the content of CaO in the pellet increases from 0.25%to 2.00%,the reduction swelling index of pellet increased from 40.82%to 60.24%.When the content of MgO in the pellet increased from 0.25%to 2.00%,the reduction swelling index of pellet first increased and then decreased,and the maximum reduction swelling index of the pellet is only 16.85%.With the content of SiO₂ and Al₂O₃ increases,and the reduction swelling index of pellets remains between 4.0%and 11.0%.It indicates that CaO can promote the reduction swelling of pellets.On the contrary,MgO,SiO₂ and Al₂O₃ can inhibit the reduction swelling of pellets.For pellets with different basicity,as the basicity of the pellet increases,the reduction swelling index of the pellet increases first and then decreases.When the basicity of the pellet is 0.2 to 0.8,the reduction swelling index of the pellet is increases lineary.When the pellet basicity is 0.8 to 1.2,the swelling index of the pellet slowly decreases.The surface microstructures of pellets after reduction were observed by scanning electron microscopy.It is found that the precipitation forms of metal iron from pellets with different gangue compositions after reduction were different.For the pellets with a single gangue,the surface of the CaO pellets is all in the form of iron whiskers.The surface of the MgO and SiO₂ pellets is in the form of granular metallic iron and a small amount of iron whiskers,and the surface of the Al₂O₃ pellets is entirely in the form of granular metallic iron.For the pellets of different basicity,as the basicity of the pellet increases,the metallic iron on the surface of the pellet gradually changes from granular metallic iron to iron whisker.The reduction swelling index of pellets is related to the porosity of the pellets,the structure of the mineral phase and the precipitation morphology of metallic iron.1)The reason why CaO promotes the reduction swelling of pellets is as follows:First,the liquid phase of calcium ferrite is formed during the calcination of CaO pellets,and the porosity of the pellets is increased at the same time,and the contact area of gas-solid reaction during the reduction reaction is increased,which is beneficial to increase the degree of reduction of the pellets.Second,during the reduction process of CaO pellets,the metal iron on the surface of the pellets is all in the form of iron whiskers.With the increase of CaO content,the number of iron whiskers increases and the whisker length becomes longer.There is anisotropy in the growth process of iron whiskers.Different forms of iron whiskers occupy different spaces in pellets.With the increase of the porosity of pellets,the reduction swelling index of pellets increases.2)The reason why MgO,SiO₂ and Al₂O₃ inhibit the reduction swelling of pellets is as follows:First,poor reducing substances such as spinel,SiO₂ and Al₂O₃ formed during pellet roasting.And the porosity of pellets is smaller than that of pure hematite pellet.The contact area of gas-solid decreases during the reduction reaction,resulting in the degree of reduction of the pellet decrease.Second,during the reduction process of the pellets,the metallic iron exists in the form of a granular metallic iron layer,and substantially no iron whiskers are produced.The compact structure and small porosity of the pellets reduce the reduction swelling index of the pellets.