Knowledge Management System Of Institute of process engineering,CAS
|ADEWOLE ADETORO AJALA|
随着高品位含钛矿石资源的快速消耗，近年来低品位含钛矿石如钒钛磁铁矿中的钛资源开发利用受到了广泛关注。但在传统高炉-转炉流程中，由于工艺条件限制，只能利用铁钒资源，钛资源无法有效富集提取。直接还原-电炉熔分两步法流程作为新一代非高炉冶炼工艺，被认为是实现钒钛磁铁矿铁钒钛资源综合利用的有效方法。在现有直接还原设备中，相比竖炉、回转窑、转底炉，流态化直接还原具有直接利用粉矿和传热传质效率高的优点，被认为是钒钛磁铁矿的非高炉冶炼最有前景的方法。钒钛磁铁矿由于自身复杂矿相组成和含钛铁氧化物特性，其直接还原过程相比较普通铁精矿也更为复杂，所需还原势高、还原效率低。因此，如何突破含钛铁氧化物的直接还原热力学限制，并提高其直接还原效率，是钒钛磁铁矿两步法流程的一个关键所在。本论文以南非某钒钛磁铁矿为实验对象，系统研究钒钛磁铁矿原矿气基直接还原特性，考察了氧化预处理对还原过程物相结构以及流化特性的影响规律。取得的主要研究结果如下：（1）钒钛磁铁矿气基直接还原过程物相转变规律研究。在750-950 oC范围内，钒钛磁铁矿的直接还原过程按顺序可分为三个阶段，包括一般铁氧化物还原（第一阶段），钛铁晶石还原（第二阶段）和钛铁矿还原（第三阶段）。直接还原过程中FeO会与FeO?TiO2发生化合反应生成2FeO?TiO2，大幅降低第一阶段平衡金属化率15.4 %。固溶杂质氧化物（如MgO、MnO）会增加FeO还原至金属铁的还原势要求。据此，探明了包含杂质氧化物在内的钒钛磁铁矿复杂含钛铁氧化物物相转变路径，建立了实际还原势-平衡金属化率计算模型。（2）钒钛磁铁矿高温（825-950 oC）预氧化强化直接还原机制研究。通过预氧化处理，可以实现钒钛磁铁矿中难还原的含钛铁氧化物钛铁晶石和钛铁矿的解离，得到易还原的赤铁矿物相，进而提高直接还原过程反应效率约10 %，并可提高第一、二阶段平衡金属化率约14.5 % 和4.5 %。但在900oC以上氧化时，由于铁氧化物产物晶粒尺寸增加，反应效率增加效果有所降低。同时由于高温下Fe2O3会与TiO2发生化合反应生成Fe2TiO5，因此高温预氧化平衡金属化率存在单峰最优值。据此，建立了钒钛磁铁矿预氧化强化含钛铁氧化物物相解离-还原的转变路径，以及对应还原势-平衡金属化率计算模型。（3）钒钛磁铁矿低温（700-825 oC）预氧化强化直接还原机制研究。低于800 oC时，铁板钛矿化合反应会得到抑制，因此随着预氧化时间的延长，后续还原过程平衡金属化率呈单调增加趋势，可超越高温预氧化所得金属化率最佳值。当温度高于800 oC时，由于存在铁板钛矿化合反应与氧化产物自由赤铁矿生成速率间的竞争关系，预氧化平衡金属化率存在双峰变化趋势。据此，建立了完整的钒钛磁铁矿高低温预氧化强化含钛铁氧化物物相解离-还原的转变路径，为钒钛磁铁矿直接还原工艺的优化设计和选择提供了理论支撑。（4）预氧化抑制粘结失流研究。在前述实验研究过程中发现，预氧化除可强化钒钛磁铁矿直接还原外，还可提升其流化稳定性。研究发现预氧化处理改变了钒钛磁铁矿颗粒表层铁氧化物赋存状态，从而改变后续还原过程表面金属铁的形貌，进而影响颗粒的粘结性。绘制了预氧化稳定流化的区域图，分为易失流区域（弱氧化条件）、稳定流化区域（中氧化条件）和不稳定流化区域（强氧化条件）。即使是采用易失流区域预氧化参数，其流化性能也明显强于原矿。经稳定流化区域预氧化处理后，在900 oC 100 % CO的直接还原条件下，稳定流化气速与原矿相比大幅降低56 %，为钒钛磁铁矿流态化直接还原过程防失流开拓了新方法。;The utilization of low-grade minerals such as titanomagnetite (TTM) has gained more attention in recent years due to the rapid depletion of high-grade natural resources. However, due to the limitation of blast furnace operating condition in the modern blast furnace-converter process, only iron and vanadium resource can be utilized, titanium in TTM cannot be effectively enriched and extracted. The direct reduction-electric arc furnace (EAF) melting separation process, (the two-step short process), has been proposed as a more effective and promising approach for comprehensive utilization of the iron, vanadium, and titanium resources in TTM. The two-step process can be classified into the rotary kiln process, the rotary hearth furnace process, the shaft furnace process, and the fluidized bed direct reduction process according to the reduction reactor employed. By taking advantage of the high heat and mass transfer efficiency, and direct use of ore fines, the fluidized bed (FB) process exhibits a high potential to achieve high-efficient reduction of TTM. Compared with the direct reduction of common iron ores, the reduction of titania-ferrous oxides in TTM ores requires a much higher reduction potential than ordinary iron oxides, resulting in a lower reduction efficiency and high product cost. Therefore, how to break through the thermodynamic limitations of titanium-containing iron oxides and to improve the reaction efficiency in the direct reduction process are the basic key problems that must be solved for efficient utilization of titanium resources in the TTM two-step short process.In the present thesis, TTM from South Africa was used as the experimental material to study the FB direct reduction characteristic of the TTM. The influence of the pre-oxidation on the reduction efficiency as well as the fluidization characteristic was systematically investigated. The main findings and conclusions of this thesis are as follows:(1) Characteristic research on the phase transformation of TTM by gas-phase direct reduction. In the temperature range of 750 - 950 oC, the direct reduction of TTM can be divided into three sequential steps: the reduction of ordinary iron oxides (the first step), ulvospinel reduction (the second step) and ilmenite reduction (the third step). During the reduction process, FeO combines with FeO?TiO2 to form 2FeO?TiO2, which greatly reduces the equilibrium metallization degree of the first reduction step by about 15.4 %. Also, solid solution impurity oxides (such as MgO, MnO) increase the required reducing gas potential for FeO to Fe reduction. Based on this, the phase transformation path of the impurity-containing titania-ferrous oxides in TTM and the corresponding quantitative model of the reduction potential-balance metallization degree have been built.(2) The mechanism of high temperature (825-950 oC) pre-oxidation on the improvement of TTM direct reduction. By oxidation pretreatment, the titania-ferrous oxides difficult to be reduced in TTM are dissociated to easily reducible free Fe2O3, resulting in about 10 % increase in the reduction efficiency. Additionally, the equilibrium metallization degree of the first and second reduction step can be relatively increased by 14.5 and 4.5 % respectively. However, when the peroxidation temperature is greater than 900 oC, the improvement effect on the reduction rate became weak, due to the high-temperature sintering and the larger crystallite size of the oxidation product. Also, Fe2O3 combines with TiO2 to form Fe2TiO5 which decreases the amount of free hematite available for reduction. Thus, the equilibrium metallization degree at high oxidation temperature has a single optimum value. Based on this, the transformation path of the pre-oxidation enhanced the titania-ferrous oxides dissociation-direct reduction and the corresponding quantitative model of the reduction potential-balance metallization degree have been developed.egion, the fluidization quality is significantly better than that of the raw ore. At the stable fluidization region, the steady state fluidization gas velocity was reduced by 56 % as compared with the raw concentrate under 100 % CO gas at 900 oC. The peroxidation method provides a novel approach to suppressing defluidization via pre-oxidation treatments.(3) The mechanism of low temperature (700-825 oC) pre-oxidation on the improvement of TTM direct reduction. At oxidation temperature < 800 oC, where the formation of pseudobrookite phase can be prevented, the equilibrium metallization degree increased linearly with the increase in the oxidation time and can exceed the optimum metallization degree obtained at high pre-oxidation temperature. Whereas, at the oxidation temperature ≥ 800 oC, there exist two peak values for the maximum effect of pre-oxidation on reduction improvement due to the competitive relationship between the amount of generated pseudobrookite and the free hematite. Finally, the enhanced titania-ferrous oxides dissociation-direct reduction transformation path pre-oxidized both at the low and high temperatures, and the corresponding quantitative model of the reduction potential-balance metallization degree have been built, which provides a theoretical guide for the optimal design of the direct reduction process.(4) Research on preventing defluidization by peroxidation method. In the previous study, it was found that in addition to improving the reduction efficiency of TTM, pre-oxidation can also improve the fluidization quality. It was found that peroxidation modifies the morphology of the as-reduced metallic iron on the particle surfaces, thereby changing the sticking behavior of the as-reduced particles. Based on the relationship between oxidation product morphology and the resultant iron morphology after reduction, the fluidization behavior of pre-oxidized SA TTM can be divided into three operating regions: defluidization (low oxidation condition), stable fluidization (intermediate oxidation condition), and unstable fluidization (high oxidation condition). Even at the defluidization region, the fluidization quality is significantly better than that of the raw ore. At the stable fluidization region, the steady state fluidization gas velocity was reduced by 56 % as compared with the raw concentrate under 100 % CO gas at 900 oC. The peroxidation method provides a novel approach to suppressing defluidization via pre-oxidation treatments.
|ADEWOLE ADETORO AJALA. 钒钛磁铁矿流态化气固直接还原基础研究[D]. 中国科学院大学,2019.|
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