【作者】 黄艳芳；

【导师】 姜涛； 李光辉；

【作者基本信息】 中南大学 ， 钢铁冶金， 2012， 博士

【摘要】 铁矿球团具有铁品位高、粒度均匀等优点,被越来越多地用于钢铁生产。作为制备球团不可或缺的原料之一,粘结剂对球团质量和生产过程具有重要影响,新型粘结剂的开发与应用成为高效利用劣质铁矿资源、提高钢铁生产效益的重要课题。中南大学开发的新型高效有机-无机复合粘结剂已成功应用于直接还原球团的生产,但由于缺乏系统的研究,该粘结剂目前尚未用于氧化球团的生产。此外,关于该粘结剂对球团还原焙烧动力学以及体积膨胀行为的影响及其作用机制尚不完全清楚。1本论文以膨润土粘结剂作为对比,深入研究了复合粘结剂对铁矿球团氧化焙烧与还原行为的影响,为复合粘结剂在氧化球团生产中的应用提供理论和技术依据,并为完善和发展复合粘结剂球团直接还原工艺提供理论指导。本论文的主要结论和创新点如下：(1)深入研究了复合粘结剂物化特性及热态行为。研究发现,复合粘结剂中有机组分和无机组分的含量分别为75.72%和24.28%,有机组分是具有芳香结构、带有羟基、羧基官能团的高分子化合物,在加热过程中,有机组分发生系列化学反应,释放出CO、H2、CO2、CxHy气体。(2)系统研究了复合粘结剂磁铁精矿球团的氧化动力学行为。研究发现,复合粘结剂降低了磁铁精矿球团氧化反应的表观速率常数,增大了气体扩散阻力。在800℃～1000℃时,球团氧化过程受界面化学反应和内扩散混合控制,表观活化能为44.70kJ/mol;但是温度超过1000℃时,球团内部还原气氛增强,氧化速率下降。(3)查明了复合粘结剂氧化球团矿制备的工艺特征,获得了优化的球团工艺技术参数。与膨润土粘结剂相比,复合粘结剂用量小、适宜成球水分高；复合粘结剂生球干燥速率快,适宜的预热和焙烧温度低。以磁铁精矿为原料时,复合粘结剂球团的预热温度由920℃降低至900℃,焙烧温度由1250℃降至1220℃。以赤铁精矿为原料时,焙烧温度由1280℃下降至1250℃。(4)深入研究了两种粘结剂铁矿球团的还原动力学行为。研究发现,还原所用球团的种类和粘结剂类型对动力学行为有重要影响。与焙烧球团相比,预热球团的还原速率明显较高,而且动力学控制特性也显著不同；虽然复合粘结剂不改变球团还原过程的控制特性,但可降低还原后期浮氏体转化为金属铁的表观活化能,提高还原速率。采用预热球团还原,当还原度低于50%时,复合粘结剂和膨润土球团的表观活化能分别为44.02kJ/mol和38.24kJ/mol,表明球团还原过程受混合控制；当还原度大于50%时,两种粘结剂球团还原反应的表观活化能分别为16.43kJ/mol和28.40kJ/mol,还原过程受内扩散控制。采用焙烧球团还原,当还原度低于50%时,复合粘结剂和膨润土球团的还原均受界面化学反应控制,表观活化能为47.03kJ/mol和50.48kJ/mol;还原度大于50%时,还原过程受界面化学反应和内扩散混合控制,表观活化能为29.67kJ/mol和39.11kJ/mol。(5)查明了复合粘结剂球团的还原膨胀行为。研究结果表明,以焙烧球团为原料时,两种粘结剂球团的体积膨胀率均随着还原温度的升高逐渐增大,且最大体积膨胀率低于40%。采用预热球团还原时,两种粘结剂球团的体积膨胀率随温度的升高和气氛中CO浓度的增加显著下降,当还原温度低于1000℃时,球团发生异常膨胀。(6)揭示了还原过程中球团体积异常膨胀机理。研究表明,球团在还原过程中的膨胀行为主要由金属铁晶粒的长大模式所决定。预热球团具有赤铁矿颗粒结晶不完善、表面存在晶格缺陷的特点,当在较低的温度下还原时,在晶格缺陷处优先形成少量铁晶核,随还原过程的推进,金属铁在少量铁晶核上以线性的纤维状方式长大,导致球团发生异常膨胀；但在较高的温度下和较强的还原气氛中还原时,由于金属铁的形成速率加快,铁晶粒发生兼并、互连,金属铁主要以平面的片状方式长大,球团体积膨胀率降低。焙烧球团中的赤铁矿结晶完善,表面活性低,还原过程中金属铁以层状形式向球团内部逐渐推进,在还原过程中的体积膨胀主要是由于还原初期Fe203向Fe304的晶形转变应力导致的。在较低的还原温度下,晶形转变应力随还原过程的进行逐渐消失,球团不发生异常膨胀；但在较高的还原温度条件下,晶形转变应力来不及消失,导致球团体积膨胀增大。更多还原

【Abstract】 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.更多还原