Behaviores Of Carbon In Preparation Of Oxidized Pellet From Hematite Concentrate

Hematite concentrates burdened with certain proportion solid fuel to produce oxidized pellet by straight-grate process, is able to greatly reduce combustion consumption, increase pellet strength, and improve metallurgical properties. However, industrial application has not been developed in grate-kiln process. Recent twenty years, Chinese scholars mainly studied into reductive pellet for non-blast-furnace iron making prepared from carbon-burden pellet, but few for blast furnace iron making. With wide application of the hematite in pellet industry of our country, study on technology of carbon-burdened hematite pellet prepared in grate-kiln process has important theoretical and practical significance.Through simulating grate-kiln process, the difference of pelletizing process, green ball quality, drying property and compressive strength between carbon-burdened and non-carbon-burdened hematite pellet is studied in this paper. Especially, for formation mechanism of iron oxides with low valence in roasting process for carbon-burdened hematite pellet and effect of roasting atmosphere, air flow, roasting temperature, roasting time and anthracite ratio on FeO content, to clarify the effect of carbon in roasting process of hematite pellet, and based on microstructure analysis, reveals induration mechanism of carbon-burdened hematite pellet in roasting processThe results show that compressive strength of green ball and roasted pellet can be increased by adding certain proportion anthracite, however, the drop strength and thermal shock temperature do not change significantly; The suitable amount of anthracite is 1%～1.5% (fixed carbon: 0.8%～1.2%). The suitable balling parameters of carbon-burdened pellet are: 1.25% bentonite, 1.0%～2.0% anthracite, 7.5%～8.0% moisture of raw material, 10～12min balling time and 8.5%～9.0% moisture of green ball. Drop strength of green ball is more than 3times/0.5m, compressive strength is greater than 13N/P, and thermal shock temperature is higher than 500℃.The influence of temperature and wind speed on dehydration rate is different for carbon-burdened and non-carbon-burdened pellet. The wind speed is more obvious effect than temperature on dehydration rate of carbon-burdened pellet, but in contrast to non-carbon-burdened pellet.The suitable roasting process of carbon-burdened pellet is: 15%～20% oxygen partial pressure, 4L/min～8L/min airflow, 1280℃～1350℃roasting temperature and 15min～20min roasting time. Compared with non-carbon-burdened pellet, roasting temperature is able to be decreased 20℃and roasting time is able to be shortened 5min～10min. Meanwhile, the interval range of roasting temperature is able to be expanded 50℃.The role of carbon in anthracite during roasting includes: Firstly, combustion and releasing heat. The heat needed in roasting process can be provided by carbon combustion, which makes up the problem of heat shortage in pellet. Secondly, reduction. Hematite is able to be reduced because of carbon. Carbon-burdened hematite pellet follows a consecutive reduction and oxidation procedure in oxidizing atmosphere, but follows reduction process in inert atmosphere.The result indicates that some original hematite is able to be reduced to magnetite or wustite firstly, and be reoxidized to secondary hematite in preheating and roasting process of carbon-burdened hematite pellet. Because the activity of Fe₂O₃ of secondary hematite (oxidized from magnetite concentrate) is higher than from original hematite, Fe₂O₃ recrystallization bonds between particles can be enhanced by the secondary hematite grains at lower roasting temperature, which changes induration process of original hematite through growth and recrystallization of Fe₂O₃ grain at high temperature, decreases roasting temperature, improves roasting properties, reduces combustion consumption and provides theoretical basis to improve pelletizing process for hematite.