【作者】 李慧敏；

【导师】 朱德庆；

【作者基本信息】 中南大学 ， 钢铁冶金， 2009， 硕士

【摘要】 随着我国钢铁企业的快速发展,国内各大钢铁企业对进口矿的依赖程度大幅度上升,2008年进口铁矿石的消费量达到4.4亿吨左右。尤其是近年来链篦机-回转窑球团工艺的迅速发展与大型化,以至铁品位高、粒度细和焙烧性能好的磁铁精矿日益短缺,开发和应用进口高品位赤铁矿资源已成为我国球团工业持续发展的新途径。巴西赤铁矿虽然可烧性能差,但是其铁品位高,有害杂质含量低,储量丰富,越来越受到国内各大球团厂的青睐。因此,致力于进口赤铁矿球团新技术的开发和应用,尤其是使用添加剂强化球团焙烧,可提高其利用率,对我国钢铁企业的发展将具有十分重要的现实意义。本文通过对巴西赤铁矿PFCJ中添加含硼复合添加剂制备氧化球团及其机理的研究。系统研究了添加剂种类和用量对生球制备、球团预热焙烧固结及冶金性能的影响,进一步优化了工艺参数。最佳工艺参数如下:膨润土用量1.8%,造球时间15min,生球水分11%,预热温度1000℃,预热时间8min,焙烧温度1280℃,焙烧时间15min。在此条件下,添加5.0%含硼复合添加剂时,生球落下强度为6.5次/0.5m,抗压强度为12.3N/个,爆裂温度为509℃,预热球团抗压强度为773N/个,焙烧球团抗压强度为2843N/个,球团冶金性能优良:还原性指数为81.67%,还原膨胀率为14.42%,低温还原粉化指数RDI_+3.15mm为99.98%;而无添加剂时,虽然生球性能和球团冶金性能均较好,但是球团焙烧性能差,焙烧球团抗压强度仅为1792N/个,即使提高焙烧温度和延长焙烧时间,所生产球团的机械强度均不能达到球团生产的要求。结果表明,该赤铁矿添加含硼复合添加剂后,其焙烧性能得到明显改善,在较低的焙烧温度和较短的焙烧时间就可制备出机械强度高、冶金性能优良的优质氧化球团。对硼砂、含硼复合添加剂强化赤铁矿氧化球团固结机理的研究表明,添加硼砂或含硼复合添加剂均可显著降低液相生成温度,促进液相生成和赤铁矿晶粒长大和互连,从而大大提高球团的致密程度和固结强度。对赤铁矿晶粒长大的动力学研究结果表明,在焙烧温度1150～1300℃,焙烧时间20～50min范围内,硼砂、含硼复合添加剂球团的赤铁矿晶粒生长速率常数为分别为7.59×10⁵（μm）ⁿ·min^-1和7.22×10⁵（μm）ⁿ·min^-1,远大于无添加剂赤铁矿球团的赤铁矿晶粒生长速率常数5.04×10⁵（μm）ⁿ·min^-1。研究了硼砂、含硼复合添加剂对赤铁矿球团还原行为的影响。等温还原动力学结果表明,在还原温度范围850℃～950℃,还原时间为180min,以CO/N₂=30/70为还原气体的条件下进行还原,球团受界面化学反应控制,含硼复合添加剂球团的表观活化能为32.192kJ/mol,小于硼砂添加剂球团的表观活化能43.233kJ/mol,表观活化能越低,还原反应越容易进行,即含硼复合添加剂球团的还原性优于硼砂添加剂球团的还原性。添加硼砂使球团还原膨胀恶化,硼砂导致赤铁矿晶格畸变,促使球团体积膨胀;硼砂使液相的熔点降低,还原过程中的机械强度降低,抗膨胀能力减弱;添加硼砂的氧化球团结构致密,阻碍热量由表及里的传递,使球团内部热应力增大,球团膨胀加剧。添加含硼复合添加剂时,球团中的Fe₂O₃与MgO发生固溶,在还原时转变为Fe₃O₄的过程中,不会引起晶格变化;MgO使液相熔点升高,还原过程中机械强度提高,抗膨胀能力增强;另外,含硼复合添加剂的加入使球团孔隙增多,可抵消一部分球团内部膨胀所产生的内应力,进一步抑制膨胀。更多还原

【Abstract】 In recent years,with the rapid development of steelmaking,the demand for imported iron ores has been greatly growing all over the country.In 2008,the consumption of imported iron ores was about 440 Million Tons.Particularly,with the development and large-scale of grate-kiln process,the growing shortage of the magnetite concentrate with high grade iron,fine size and good firing behavious become more serious.The utilization of imported hematite concentrate is an effective way for sustainable development of pellets industry in our country. Though Brazilian specularite is characterized by poor firing behaviours,it has gained world-wide recognition as an indispensable charge to replace magnetite for pelletizing plant due to its high grade iron and abundant. Therefore,the utilization of imported hematite concentrate as pellet feed by new technology,particularly,using additives to improve the firing behavious of pellets,will be of practical significance for Chinese iron and steel enterprises.In this paper,the process and mechanism of improving the pelletization of Brazilian PFCJ hematite by adding boron bearing compound additives have been studied.The effects of additives on balling, pellet preheating and firing,metallurgical properties have been studied systemically.It is shown that the optimum conditions are as follows: 1.8%bentonite in pellet feed,balling for 15min,11%moisture, preheating at 1000℃for 14min,and firing at 1280℃for 15min. Greenballs were manufactured with drop strength of 6.5 times from 0.5m hight,compression strength of 12.3 N/P and thermal shock temperature of 509℃,the compression strength of preheated and fired pellets are 773 and 2843 N/P,respectively.Good metallurgic properties of fired pellets are the following:RI 81.67%,RSI 14.42%,and ROI_+3.15mm 99.98%. Though the properties of green balls and metallyrgic properties of fired pellets are excellent without additives,the firing performance is poor,the compression strength of fired pellets is only 1792 N/P.Even if increasing the fire temperature and extending the fire time,the compression strength of the fired pellets without additives can not meet the product requirement. The results show that good quality fired pellets can be made from Brazilian hematite by adding boron bearing compound additives,which possess high mechanical strength and good metallyrgic properties at lower firing temperature for a short time.The consolidation mechanism of fired pellets with borax or boron bearing compound additives were studied.It is revealed that adding borax into the hematite pellet helpes to form more liquid phases at lower temperature,which enhance grain growth,leading to higher strength of fired pellets due to forming more coarse and dense hematite pellets. Results from the kinetics of grain growth are shown that borax or boron compound additive can fastens the grain growth rates when firing at 1150 to 1300℃for 20 to 50min.The effects of borax or boron bearing compound additives on the reduction of hematite pellets were studied.Resultes from isothermal reduction kinetics show that the reduction rate of pellets containing borax or boron bearing compound additives is controlled by chemical reactions when the reduction temperature ranges from 850 to 950℃,and reduction lasts for 180min in CO/N₂=30/70 atmospher.The apparent activation energy of boron bearing compound additives pellets is 32.192kJ/mol,less than that of borax pellets with 43.233kJ/mol.The lower the apparent activation energy,the easier the reduction reaction take place.So the reduction properties of pellets containing boron bearing compound additives are better than that containing borax.Adding borax into pellets worsens reduction swelling.It is probably that the reason is the sodium ion of borax leads to lattice distortion,resulting in anormal reductive swelling.The low melting point of liquid phases from borax bring about lower mechanical strength during reducting,leading to poorer resistance against swelling stress.The borax containing pellets are too coarse and dense,which impedes heat transfer and increases the thermal stress inside the pellets,leading to higher swelling.When adding boron bearing compound additives,magnesium from the additives not only can stabilize hematite to restrain the lattice distortion and increase the melting point to improve the mechanical strength during reducting,but also increase the porosity,which can offset part of thermal stress.Therefore,the RSI of boron bearing compound additives is lower than that containing borax.更多还原