Iron Particle Size Control In Coal-based Reduction Of High Phosphorous Oolitic Hematite

The rapid development of steel industry in China has significantly stimulated the serious shortage state of domestic iron ore market. The development of domestic steel industry closely depends on the import of high-quality iron ore from foreign countries. Therefore, to develop and utilize the low grade and complex domestic iron ore efficiently is of great importance in ensuring the smooth development of steel industry in our country.In the complex and refractory iron ore resources of our country, the reserves of high phosphorous oolitic hematite are considerably enormous. The ore being difficult to separate is due to the extremely fine dissemination size, complex associated relationship and properties. Plenty of testing results have shown that it is very difficult to separate oolitic hematite by traditional beneficiating methods. If the bestowed states of iron in the ore were not changed, it would be impossible to upgrade the iron concentrate. In recent years, relevant researchers have adopted coal-based reduction followed by magnetic separation to deal with the ore and have acquired certain breakthrough in both theory and practice. One of the factors affecting the efficient benefication of coal-based reduction is the size of iron particles formed during the process of coal-based reduction. Only under the condition of suitable iron particle size, can the best index be obtained. Therefore, there is great significant in realizing the control of iron particle size.Based on the statistical analysis of iron particles granularity characteristics in reduction materials, this paper mainly discussed the influence of the reduction temperature, reduction time and binary basicity on iron particle nucleation and growth. Apart from that, this paper also analyzed the growth process and mechanism of metallic iron particles and obtained the regression model of iron particle size, which provided theoretical foundation for iron particle size control in coal-based reduction process. Through the chemical analysis, X-ray diffraction (XRD) analysis, etc. we can know that, the total iron grade of ore is42.21%. The primary mineral form of iron is hematite, while the gangue minerals are mainly quartz and chamosite, and phosphorus is in the form of collophanite. The oolitic in the ore is comprised of the original chemical sedimentary hematite, chamosite and a small amount of collophane, among which hematite and chamosite are embedded closely with ultra-fine size. The content of fine fraction in run-of-mine is very high, while the content of coarse fraction is rather low.The paper investigated the influence of process conditions (such as reduction temperature, reduction time, binary basicity, etc.) on the metallization rate of reduction materials and the nucleation and growth of metallic iron particles during the process of coal-based reduction of high phosphorous oolitic hematite. Within an appropriate range of process condition, the metallization rate of reduced materials increases with the improvement of the reduction temperature and the extending of the reduction time. However, if the temperature were over high or the time were overlong, the metallization rate would fluctuate or even decline. Overhigh binary basicity is not conducive to the reduction of iron ore.Within a certain range, the higher the reduction temperature and the longer the reduction time, the larger the sizes of iron particles are. Improving the reduction temperature can accelerate the nucleation and growth of iron particles, while extending reduction time can provide the dynamic conditions for fine particles diffusing, migrating, gathering and growing. The mechanism of CaO promoting the nucleation and growth of iron particles is that CaO can exchange FeO from the complex compounds. It can not only effectively increase the activity of FeO and speed up the rate of the reaction. It can also promote the re-deoxidization of fayalite and hercynite. However, CaO has the effect of slagging. If the basicity were overhigh, the slag phase of the system would increase remarkably, which is not conducive to accumulation and grow up of iron particles. The results have revealed that the suitable condition of iron grain growth is the reduction temperature of1225℃, reduction time of80min and binary basicity of0.2.According to the experimental data obtained from the tests of coal-based reduction, combining the regression analysis and the powerful calculate function of MATLAB, the regression model between the iron particle size D80(μm) and reduction temperature T(℃) and reduction time t(min) is D80=481.3+221.8T+188.1t+63.5T2+72.3Tt-10.1t2. Through adjusting T and t, we can predict the variation trend of iron particle size, which can realize the iron particle control in coal-based reduction process.Combining the reaction thermodynamics, XRD, SEM and EDS spectrum analysis of reduction materials, we can know that the reduction process of iron oxides is in line with the order of Fe2O3→Fe3O4→FeO→Fe in coal-based reduction of the high phosphorus oolitic hematite. In the coal-based reduction process of high phosphorus oolitic hematite, during the same period of the high price iron oxides being reduced to metallic iron, FeO is liable to have solid phase reaction with the gangue minerals (such as SiO2, Al2O3, CaO, etc.) and generate complex compound such as fayalite (FeO·SiO2), hercynite (FeO·Al2O3), iron cordierite (2FeO·2Al2O3·5SiO2), calcium ferrite (2CaO-Fe2O3), etc.The growth process of iron particles during the process of coal-based reduction is the process of iron atoms continuously migrating and gathering. The growth process can be divided into three stages, which are the stages of nucleation, rapid growth and slow growth. Iron grain growth process is a conversion process from the stochastic diffusion control mechanism to the definitive curvature driven mechanism. When the grain size in the system is small, its growth is mainly controlled by the diffusion mechanism. When the gain size is larger, the effect of curvature driven mechanism become more apparent. In addition to the coarsening process of grains, the rapid growth of grains contains the accumulation of grains after the grain boundary disappearing. The gathering of adjacent grains, especially the reunion between iron grains of preferred orientation, leads to the fast growth of grains.The research results of the paper have laid the foundation for iron particle size control during the process of coal-based reduction of high phosphorus oolitic hematite, which has certain guiding significance to the efficient development and utilization of high phosphorus oolitic hematite.