Fundamental Research On High Alumina Iron Ore By Gas-Based Direct Reduction

High alumina iron ore is a typical refractory ore, traditional beneficiation processes are found invalid to separate alumina and iron. To utilize such resources, a gibbsite-type high alumina iron ore, taken from Guigang city, Guangxi Zhuang Autonomous Region, is used in the thesis. First, the mineralogy characters of this ore are understood in detail. It is found that, iron element mainly occurs in goethite, and aluminium element mainly occurs in gibbsite, goethite contains aluminium and silicon elements, the content of Al2O3 within the goethite varies from 17% to 39%, and gibbsite also contains iron and silicon elements, the content of Fe2O3 within the gibbsite varies from 15% to 25%, the crystalline size of the aluminiferous minerals embedded in goethite and the ferrous minerals embedded in gibbsite are less than 5?m. Therefore, it is difficult to separate Fe, Al, Si from the ore by physical methods. Thus, reduction characteristics of the ore from thermodynamics, kinetics and microscopic view are studied systematically to provide the basic research for the smelting reduction ironmaking process on the ore.It is indicated that major iron minerals in the ore, with 30.82%(mass fraction, the same hereafter) TFe,23.32% Al2O3 and 12.27% SiO2, are hematite and goethite, and gangue minerals are gibbsite, quartz and kaolinite by the research on the physicochemical properties of materials.Thermodynamics investigation shows that under roasting condition, Al2O3 reacted with SiO2 impossible to generate Al2O3·2SiO2, others such as kyanite, andalusite, sillimanite as well as mullite may be generated. When CaO exists, CaO reacted with Fe2O3, Al2O3, and SiO2 to generate 2CaO·SiO2 easiest, followed by 2CaO·Fe2O3, last of 3CaO·Al2O3. At the range of temperature from 1123K to 1300K, the generation order of the ternary compounds is gehlenite (2CaO·Al2O3·SiO2),3CaO·Al2O3·3SiO2, CaO·Al2O3·SiO2,2CaO·Al2O3·SiO2, CaO·Al2O3·2SiO2. Above 1300K, the generation order of the ternary compounds is gehlenite (2CaO·Al2O3·SiO2), CaO·Al2O3·SiO2,3CaO·Al2O3·3SiO2 CaO·Al2O3·SiO2,CaO·Al2O3·SiO2. FeO generated during the reduction stage is most likely to react with Al2O3 to generate FeO·Al2O3, then it is likely to react with SiO2 to generate 2FeO·SiO2, last it is likely to react with SiO2 to generate FeO·SiO2. Moreover, the reaction of Al2O3 to replace SiO2 in 2FeO·SiO2 and FeO·SiO2 to generate FeO·Al2O3 is much easier than that of SiO2 to replace Al2O3 in FeO·Al2O3. And the reaction trend of the reduction of FeO·SiO2 is stronger than that of FeO·Al2O3 and 2FeO·SiO2.Gas-solid reduction experiments of high alumina iron ore pellets under isothermal condition are carried out. The results show that the maximum reduction degree of 1# pellets can reach 77.13% by reduction for 77 minutes with CO70%(volume fraction, the same hereafter) plus H230% at 1173K, and the maximum reduction degree of 2# pellets can reach 82.2% by reduction for 80 minutes with CO70% plus H230% at 1223K. The optimal prereduction temperature of the two kinds of pellets is 1173K by taking all into consideration. Hydrated lime is added to the pellets can improve the reduction effect to increase the reduction degree. Then the unreacted core model is adopted to process the reduction data of the experiments of the two kinds of pellets with dynamic regression and determine the controlling steps. The reduction processes are controlled by both chemical reaction and inner diffusion in the previous stage, and they are mainly controlled by inner diffusion in the later stage of the reduction. The corresponding kinetic parameters are obtained. The reduction rate constants of 1# pellets in the previous stage are as follows, k+=0.239 exp(-34.89/8.3147), when the reduction atmosphere is CO80% plus H220%, k+=0.443 exp (-31.03/8.314T), when the reduction atmosphere is CO70% plus H230%. The reduction rate constants of 2# pellets in the previous stage are as follows, k+=0.102 exp (-27.23/8.314T), when the reduction atmosphere is CO80% plus H220%, k+=0.148 exp (-24.75/8.314T), when the reduction atmosphere is CO70% plus H230%.X-Ray Diffraction (XRD) and Back Scattered Electron (BSE) imaging technique are used for identifying the phases and Energy Diffraction Spectrum (EDS) coupled with Scanning Electron Microscope (SEM) is used for microanalysis of the different phases of the high alumina iron ore in the reduction process and at the end of the reduction. It can help us to understand why high alumina iron ore is difficult to deoxidize and provide ways to improve the reduction degree of the high alumina iron ore pellets. In the process of reduction, there have been some phases of Fe2SiO4 and FeAl2O4 which will be difficult to be reduced further resulting in the low reduction degree. To clarify the phase change at different reduction moments, the high alumina iron ore pellets are deoxidized in pure carbon monoxide. The results show that the reduction process of the ferric oxide in this iron ore does not well accord with the reduction law step by step because of the earlier appearance of iron and the phases of Fe3O4 and wustite undetected in the reduction process. The effect of hydrated lime addition in pellets on the reduction in the carbon monoxide atmosphere shows that the sample with 2% Ca(OH)2, anorthite formed by the reaction of calcium oxide and the mineral phases was found. Gehlenite is also observed from the XRD pattern for the sample with the increase of the hydrated lime addition. These phases produced in the process can improve the reduction reaction activity of FeO to increase the reduction degree.Non-isothermal reduction of roasted Guangxi high alumina iron ore pellets with CO and H2 was conducted with NETZSCH STA 409C/CD. Chemical analysis, X-ray diffraction (XRD) examination and scanning electron microscope (SEM) analysis were adopted to analyze the roasted and reduced pellets. The results show that there are three main phases of Fe2O3, Al2O3 and Al3Fe5O12 in the roasted pellets. During reduction process, the iron bonded in hercynite and fayalite is difficult to be reduced by CO so that the final reduction degree is only 35.62% at the setting temperature of 1573K. But with H2 atmosphere, the reduction degree can reach 100% at about 1520K. It suggests that the ferric ion can completely be reduced to metallic iron by H2. Finally, non-isothermal kinetics analysis was carried out and kinetics data were obtained. The activation energy values of CO and H2 reduction are 295.82kJ·mol-1, and 185.42kJ·mol-1, respectively. Their reaction rate constants can be expressed by kCO=1.71*108e(-295820.5/T)and kH2= 1.36* 105e(-185422.4/T), respectively. When the temperature is lower than 1357K, the reaction rate constants of the two gases reduction are in good agreement. But when the temperature is higher than 1357K, the level of the reaction rate constant of H2 reduction higher than that of CO reduction increases sharply.