【作者】 李建；

【导师】 朱德庆； 邱冠周；

【作者基本信息】 中南大学 ， 钢铁冶金， 2007， 博士

【摘要】 近年来,随着电炉炼钢短流程的兴起,作为废钢替代品和生产优特钢不可或缺的原料,直接还原铁倍受青睐,直接还原技术及直接还原铁产量发展迅速。我国煤资源及铁精矿丰富,适宜发展球团煤基直接还原法。与传统的煤基“二步法”球团直接还原相比,“一步法”直接还原流程短、投资省、成本低、产品质量好,因而在我国获得广泛地应用。继北京密云6.2万t直接还原铁(DRI)、鲁中5万t DRI投产并取得良好效果后,单窑年产15万t DRI的“一步法”直接还原工艺在新疆也即将投产。但对铁精矿复合粘结剂球团在大型链篦机—回转窑内强度变化、还原行为及微观和宏观结构演变机制缺乏系统深入的研究,相关工艺流程和参数有待进一步优化。本论文以新疆磁铁精矿和进口赤铁矿为研究对象,开展了铁精矿复合粘结剂球团直接还原法工艺及机理研究,对“一步法”直接还原投产及其推广应用具有重要的指导作用。系统研究了铁精矿种类和配比、预处理方式、粘结剂种类和用量、内配煤种类和用量对生球制备、球团预热固结及还原行为的影响,进一步优化了工艺参数。结果表明,润磨预处理可明显降低干球粉末率,消除回转窑结圈的隐患;复合粘结剂CB可大幅度提高干球抗压强度,增大预热球团孔隙率,促进球团还原。对磁铁矿复合粘结剂球团和磁铁矿氧化球团进行了还原动力学研究。等温还原动力学研究表明,还原温度800℃～1050℃范围内,复合粘结剂球团还原反应受界面化学反应和内扩散混合控制。氧化球团在800℃～900℃还原阻力主要为化学反应阻力,1000℃～1050℃,还原反应受界面化学反应和内扩散混合控制。当还原温度为800～1000℃,复合粘结剂球团反应速率常数和气体有效扩散系数远远大于氧化球团;当还原温度为1050℃时,复合粘结剂球团化学反应速率常数略低于氧化球团,气体扩散系数大于氧化球团。当还原温度大于900℃时,复合粘结剂球团可快速还原,而氧化球团需1000℃上才能快速还原。非等温还原实验结果表明,复合粘结剂球团还原速率比氧化球团快一倍。动力学研究结果为铁精矿复合粘结剂球团“一步法”直接还原采用热球入窑和全窑高温(＞900℃、)的热工制度,提供了有力的理论支撑。研究了两种磁铁矿球团在还原过程强度的变化规律。结果表明,复合粘结剂球团还原过程中,球团抗压强度低谷区(抗压强度小于500 N/个)持续时间仅30 min,氧化球团还原过程中抗压强度“低谷区”持续时间长达70 min。即使处于强度“低谷区”,复合粘结剂球团结构全部保持完整,氧化球团50%裂为碎片。越过抗压强度“低谷区”后,复合粘结剂球团抗压强度迅速提高,氧化球团抗压强度升高缓慢。复合粘结剂球团还原产品抗压强度为2570 N/个,球团结构完整,外表光滑;氧化球团还原产品抗压强度仅为775 N/个,球团表面布满裂纹,部分球团裂为碎片。两种球团还原过程中抗压强度变化规律及球团结构变化的差别是由球团还原过程中微观和宏观结构演变机制所决定。采用光学显微镜、X射线衍射仪、扫描电镜、图像分析软件对球团还原过程微观结构演变机制进行了研究;采用热膨胀仪、热传导系数测定仪对复合粘结剂团块和氧化团块的热物性参数进行了测定;运用X射线衍射仪结合MDI Jade软件对两种球团内矿物晶体的微观应变进行了测定。揭示了复合粘结剂球团和氧化球团还原过程中应力累积和释放机制,首次阐述了两种球团还原过程强度和还原行为差异的本质原因。结果表明,球团内部裂纹生成和扩展机制的差异决定了强度的差异。还原过程中,复合粘结剂球团裂纹主要由相变应力产生,裂纹形态以同心裂纹为主。同心裂纹出现后,很快被金属铁外壳包裹、约束,随着还原过程球团体积膨胀,裂纹得到“自愈合”,还原产品结构完整、表面光滑、强度高。相变应力和热应力耦合作用导致氧化球团产生大量径向裂纹,球团整体结构被破坏后,金属铁壳才出现;此外,还原后期,球团体积沿球面和径向收缩,径向裂纹甚至扩展(大),故无法消除。还原产品布满裂纹,强度差。磁铁精矿复合粘结剂球团“一步法”直接还原扩大试验主要技术指标为:直接还原铁全铁含量91.48%,金属铁含量87.05%,金属化率95.16%;铁回收率为93.58%,矿耗为1.442 t/t·DRI,煤耗为0.854 t/t·DRI,磁性粉率(-3 mm)为0.3%,烟尘0.161 t/t·DRI。磁铁矿氧化球团冷球直接还原对应的技术指标为:直接还原铁全铁含量89.24%,金属铁含量83.22%,金属化率93.25%;铁回收率90.66%,矿耗为1.508t/t·DRI,煤耗为0.889 t/t·DRI,烟尘0.223 t/t·DRI。复合粘结剂球团与氧化球团直接还原相比,全铁和金属铁品位高、金属化率高;还原过程产生含铁粉尘少,烟尘量小,铁回收率高;DRI抗压强度高。通过扩大试验,确定了新疆直接还原铁厂适宜的工艺流程,并为其建厂设计提供了工艺参数和依据。首次对赤铁矿复合粘结剂球团还原进行工艺优化试验和扩大试验研究。结果表明,印度赤铁矿可以任意配比与磁铁精矿混合,采用“一步法”直接还原法生产金属化球团。100%印度赤铁矿复合粘结剂球团直接还原后,可得到全铁87.83%,金属化率93.30%的金属化球团。研究结果扩大了原料来源,提升了“一步法”直接还原工艺的适应性。更多还原

【Abstract】 In recent years, with the rapid development of steelmaking by EAF process, direct reduced iron (DRI) has gained world-wide recognition as an indispensable charge to replace scrap for quality steel production. DRI technology has been making great progress meanwhile global DRI output has been surging year by year. In China, there are abundant resources of high grade magnetite concentrate and noncoking coal, which provides advantages to develop direct reduction process of pellets by coal-based rotary kiln. In comparison with the traditional direct reduction of fired pellets (two-step process), direct reduction of pellets made from concentrate and composite binder (one-step process) possesses such advantages as: shorter flowsheet, lower capital investment, greater economic profits and better quality of DRI. One step process has been finding more application in China. Miyun DRI plant with a capacity of 62 ktpa and Luzhong DRI plant with a capacity of 50 ktpa have been under operation successfully, and a brand-new plant with 150 ktpa DRI using one-step direct process will put into operation in Xinjiang in July 2007. However, no systematic and mechanism study have been carried out on strength change, reduction behaviors, variations of macrostructure and microstructure of pellets made from concentrate and composite binder during reduction in huge grate-kiln. In this paper, Xinjiang magnetite and imported hematite concentrate are chosen as pellet feed, a study on mechanism and process of direct reduction of pellets made from concentrate and composite binder was conducted, which enhances the spread and application one-step process for DRI production.The effects of concentrate types and pellet blends, pretreating methods, types and dosage of binder, and internal coal on green ball making, pellet preheating concretion and direct reduction behavior have been studied systemically. It was shown that damp milling can reduce fines ratio of dry pellets to eliminate potential kiln accretion. Using composite binder CB can dramatically increase compressive strength of dry pellets, enhance the porosity of preheated pellets and accelerate the reduction.The isothermal and non-isothermal kinetics of the reduction of pellets with CB as binder and fired pellets with bentonite as binder were investigated respectively. Results from isothermal reduction kinetics show that the reduction rate of CB pellets is controlled by a mixed mechanism of chemical reaction and diffusion when the reduction temperature ranges from 800 to 1050℃. However, the reduction rate of fired pellets is primarily controlled by chemical reaction in the range of 800 ~ 900℃, but by both chemical reaction and diffusion from 1000 to 1050℃. In the range of 800 to 1000℃, the chemical reaction rate and effective gas diffusivity of CB pellets are much bigger than that of fired pellets. When the temperature is above 900℃, CB pellets can be reduced rapidly, while high-speed reduction of fired pellets only happens at temperatures higher than 1000℃.Results from non-isothermal reduction research showed that the reduction rate of CB pellets is twice as fast as that of fired pellets. Based on the outcomes of reduction kinetics, one-step direct reduction of pellets made from concentrate and composite binder is significantly improved by using preheated hot pellets as feed and reducing at high temperatures higher than 900℃inside whole kiln.During the reduction, an investigation of strength changes of two kinds of pellets was carried out. It is shown that CB pellets present the valley of compressive strength (less than 500 Newton per pellet) for 30 mins during reduction. However, the valley of compressive strength lasts for 70 mins during reduction of fired pellets. Even more, CB pellets can maintain as whole pellets after reduction finished, but 50% of fired pellets broke into many fragments. The compressive strength of CB pellets increases dramatically when reduction proceeds beyond the valley, while that of fired pellets increases slowly. DRI products of CB pellets possess firm structure and smooth surface, with an average compressive strength 2570 Newton per pellet. However, cracks were bestrewn on the surface of DRI products of fired pellets and some pellets even turned into many pieces, with compressive strength of 775 Newton per pellet.During the reduction, there are some differences in compressive strength and structure changes between two types of pellets, which is determined by microstructure and macrostructure variations. Microstructure development of pellets during the reduction was studied by using microscope, XRD, SEM and image analysis software. Thermal properties of two types of compact were measured by using thermal dilatometer and thermal conductivity measurer. The mineral crystal microstrain of two types of pellets was investigated by employing XRD and MDI Jade software. The mechanism of stress accumulation and releasing was unveiled for two kinds of pellets and for the first time, the essential reasons to explain the difference on compressive strength and reduction behavior between two kinds of pellets were discovered. It is demonstrated that the strength difference is caused by different crack generation and extension inside pellets. During the reduction, the concentric cracks occurred inside CB pellets as a result of only phase change stress. After the appearance of concentric cracks, CB pellets are enwrapped, constricted by shell of metallic iron. In the latter stage of reduction, cracks can be self-cicatrized with pellet volume shrinking, which makes DRI pellets present whole structure, smooth surface and high strength. However, inside fired pellets, some radial cracks appeared at early stage of reduction due to coupling stress of phase change and thermal stress. The layer of metallic iron formed after pellets structure has been destroyed by radial cracks. Furthermore, in the latter reduction, the direction of pellet shrinking was vertical to the direction of cracks, resulting in wider radial cracks instead of cracks close. So the DRI products of fired pellets possess many cracks with low compressive strength.The main technical and economic results of scale-up test for one-step direct reduction process of CB pellets in a rotary kiln were achieved as follows: total iron content of DRI 91.48%, metallic iron content 87.05%, metallization degree 95.16%, Fe recovery 93.58%, ore concentrate consumption 1.442 t/t·DRI, dry coal consumption 0.854 t/t·DRI, magnetic fines ratio (-3 mm) 0.3%, dust 161 Kg/t·DRI. In contrast, the results of for direct reduction process of fired pellets were as follows: total iron content of DRI 89.24% , metallic iron content 83.22%, metallization degree 93.25%, Fe recovery 90.66%, ore concentrate consumption 1.508 t/t·DRI, dry coal consumption 0.889 t/t·DRI, dust 223 Kg/t·DRI. Direct reduction of CB pellets has such advantages as: higher total iron and metallic iron grade, higher metallization degree, less dust, higher recovery of iron and higher compressive strength of DRI compared with direct reduction of fired pellets. Through the scale-up tests, the direct reduction of Xinjiang DRI plant is using one-step process certified as feasible, and process parameters were utilized for designing the plant.One-step direct reduction process of pellets made from hematite concentrate and composite binder was also optimized in scale-up tests for the first time. It is shown that Indian hematite can be used to produce metallic pellets by one-step direct reduction process when mixing with magnetite concentrate at any ratio. High quality metallic pellets with iron grade of 87.83% and metallization degree of 93.30% were manufactured using 100% Indian hematite as feed. Which further extend the raw material of direct reduction feed and enhance the applicability of one-step direct reduction process.更多还原