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Investigation On Oxidizing Roasting And Reduction Behaviors Of Iron Concentrate Pellets Prepared With Composite Binder

Posted on:2013-07-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y F HuangFull Text:PDF
GTID:1261330401979234Subject:Iron and steel metallurgy
Abstract/Summary:PDF Full Text Request
The proportion of iron ore pellet in steelmaking production keeps growing because of its high iron grade and uniform size. As one of the most indispensable ingredients for iron ore pelletizing, binder is very important for the production process of pellets and the quality of final products. The development and application of new binder is an important subject to make comprehensively use of inferior iron ores and increase the economic benefit of iron-making and steel-making plants. One new type composite binder, developed by Central South University, has been successfully applied in the production of direct reduction iron (DRI). However, the binder has not been used in the production of oxidized pellets due to the shortage of systematic research. On the other hand, the effects of composite binder on the reduction behavior of pellets and its mechanisms have not been clearly expounded.Aiming at providing theoretical guidance and technical support for its application in the production of oxidized pellets and reduced pellets, the effects of composite binder on the behaviors of oxidizing roasting and reduction roasting of pellets are investigated in contrast with bentontie in this research. The main conclusions and innovation points are as follows:(1) The physical-chemical properties show that composite binder is composed of organic component and inorganic component with mass fraction of75.72%and24.28%. Organic component is a kind of polymer compounds with aromatic structure and hydroxyl and carboxyl functional groups. There are a series of chemical reactions during the heating process of organic component. The pyrolysis gas products include CO, H2, CO2and CxHy.(2) The oxidizing kinetics demonstrates that composite binder can reduce the apparent rate constant of pellets and increase the gas diffusion resistance. Under the condition of the reaction temperature lower than1000℃, the oxidation process of pellets with composite binder is mixed controlled by chemical reaction and internal diffusion with apparent activation energy of44.70kJ/mol. The oxidation rate decreases due to the existence of reducing reaction of composite binder when the reaction temperature is higher than1000℃.(3) From the results of preparation technique for oxidized pellets, it can be concluded that the dosage of composite binder can be reduced. Meantime, the suitable moisture content in balling is increased by the addition of composite binder. By contrast with pellets prepared by bentonite, the drying of pellets with composite binder can be accelerated. Furthermore, the suitable preheating temperature and roasting temperature are decreased. For pellets prepared from magnetite concentrates, the suitable preheating temperature is decreased to900℃from920℃, and so does the roasting temperature to1220℃from1250℃. For pellets prepared from hematite concentrates, the roasting temperature is decreased to1250℃from1280℃.(4) The reduction dynamics of pellets is studied. The results show that the type of binders and pellets used for reducing has significant effect on reduction rate. For those pellets with different binder, the reduction rate of preheated pellets is higher and that of oxidized pellets, while the kinetic mechanism is different between those pellets with two different binders. The reduction rate of pellets with composite binder is higher than pellets with bentonite.Using preheated pellets for reduction, the reduction process of pellets with composite binder and bentonite are controlled by the combined effect of both chemical reaction andgas diffusion mechanisms when reduction degree is less than50%. And the apparent activation energy is44.02kJ/mol and38.24kJ/mol, respectively. When reduction degree is more than50%, the apparent activation energy respectively is16.43kJ/mol and28.4kJ/mol, which indicates reducing reactions are controlled bythe gas diffusion mechanism.Using oxidized pellets for reduction, the reducing process of pellets with composite binder or bentonite is both controlled by chemical reaction with apparent activation energy of47.03kJ/mol or50.48kJ/mol when reduction degree is less than50%. While the apparent activation energy of pellets with two different binders is decreased to29.67kJ/mol and39.11kJ/mol, which indicates that process is hybrid controlled when reduction degree is more than50%.(5) Study on reduction swelling behavior of pellets shows that the swelling index of oxidized pellets is increased with increasing the reaction temperature and CO content. And the maximum swelling indexes of oxidized pellets are all less than40%. For the reducing of preheated pellets, the swelling index is obviously decreased accompany with the increase in temperature and CO content, and abnormal swelling is obtained when temperature is lower than1000℃.(6) The reduction swelling mechanisms of pellets with composite binder are investigated. The results reveal that the swelling behavior is determined by propagation mode of metallic iron grains.For preheated pellets, the initial Fe2O3is not fully crystallized with a little crystal lattice disfigurement, which gives priority to the formation of ferrous nucleus once reduced at low temperature. With the advancement of reduction process, metallic iron will grow up in the form of fibors, leading to catastrophic swelling. With the increasing of reducing temperature and CO content, the formation rate of metallic iron is speed up and iron grains merge with one another, growing up in flat-sheet mode, further resulting into decreasing of the swelling index.For oxidized pellets, Fe2O3is well-crystallized and with low surfactivity. Layered metallic iron is formed from outside to inside and the swelling of pellets is mainly caused by the disruptive stresses set-up during the transformation of hematite to magnetite. When oxidized pellets are reduced at lower temperature, the disruptive stresses will disapperare during the reduction progress, resulting in the normal swelling of pellets. With the increasing of reducing temperature, abnormal swelling of pellets will be obtained before the the disruptive stresses disappeare.
Keywords/Search Tags:Composite binder, pellet, oxidizing roasting, directreduction, reduction swelling
PDF Full Text Request
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