Effect Of Basicity On Thermal Properties And Mineralogical Structure Of Pellets

Steel production in China has increased year by year,but domestic iron ore reserves is in shortage,so domestic steelmaking enterprises have to purchase foreign high grade iron ore,and the profit of the iron and steel industry is seriously reduced.Therefore, the optimization of the metallurgical performance of raw materials and rational utilization of domestic resources have drawn much attention.Pellets with high quality can be produced by screening abundant domestic low grade ores, which can improve the grade of iron burden and reduce pollution emissions and coke rate etc.The amount of pellets into the furnace as charges accounts to 30% because its thermal state metallurgical property is worse than that of sinter.This study aims to find optimum basicity pellets by adding dolomite to adjust the pellet basicity in order to improve the metallurgical properties of pellets.The experiment was carried out using the ore metallurgical property comprehensive measuring instrument to study the reduction disintegration property and changes of strength and reduction swelling index of acid pellets at400?,450?,500?,550? and 600?,and reduction swelling index at800?,850?,900?,950? and 1000?.Results shows that the low temperature reduction powder of acid pellets at 500? was the worst and RSI was the worst at950?.In order to improve the thermal metallurgical properties of pellets,the dolomite wase added into pellets, and the basicity of pellets was adjusted to 0.4,0.6,0.8,1.0,1.2and 1.4,and metallurgical properties of pellets at 500? and 950? were studied.The results showed that the reduction swelling index of the pellets with dolomite was inhibited to a certain extent, and the reduction disintegration of pellets increased with the increase of basicity. By taking into account various factors, it is found that R=0.4is the best pellet basicity.The thermal metallurgical properties of pellets depend on its types of mineral phase and structures, and a Zeiss ore microscope was used to analysis changes rules of phases and structures.Studies have shown that Fe3O4 is transformed into Fe2O3 when acid pellets are reducted at 500?.As the crystal structure of hematite and magnetite is different, the crack and the hole near the magnetite will increase, whichcan aggravating powdering of the pellet; At the reduction temperature of 950?, the expansion index of pellets reaches the maximum and the volume of pellets increases rapidly. The main reason is the transformation of FeO into Fe,and the particle shaped metal iron destroys the stress balance inside the pellet, which causes dilation and crack of the pellet. For those pellets with the addition of dolomite, its internal ore phase structure will occur a series of changes when basicity(MgO) increases.At the reduction temperature of 500?, the iron crystal phase inside the pellets with different basicity has not yet formed a network structure, and the strength of slag phase(two calcium silicate) is low,which leads to, the degradation of powdering when pellets are reduced at low temperature;At the reduction temperature of 950?,the iron calcium ferrite and metal iron inside the pellets are connected in grids, which can effectively alleviate the reducing stress, so the change of the size decreases when pellets with different basicity are reduced.