Fundamentals For Coal-based Reduction Of High-phosphorus Oolitic Iron Ore

High-phosphorus oolitic iron ore is considered to be one of the most refractory iron ores in the world due to its complex mineral composition,ultrafine-grain iron minerals,high phosphorous content and unique internal structure.Coal-based reduction has recently proven a feasible way to recover metallic iron from high-phosphorus oolitic iron ores.However,most of the existent studies only examined the reduction and magnetic separation conditions,and little research focused on the key scientific issues in the reduction,such as thermodynamics and kinetics mechanisms,phase and microstructure evolution,metallic phase formation and growth,phosphorus interphase transfer,and so on.Consequently,in order to solve the above problems,the coal-based reduction of a high-phosphorus oolitic iron ore collected form Guandian iron mine,Hubei Province,China were investigated using scanning electron microscope(SEM),X-ray diffraction analysis(XRD),electron probe microanalysis(EPMA),etc.A systematical study was carried out by theoretical analysis,experimental researches and computing simulation,and some achievements,having obviously scientific and technological significance,were obtained.The investigations on thermo mechanical analysis,phase transformation and micro structure evolution indicated that the process of coal-based reduction of high-phosphorus oolitic iron ore is extremely complex,it not only including phase transformations of the minerals,but also microstructure evolution of the ore.Iron minerals was reduced to metallic iron along the chemical reaction sequence of Fe2O3?Fe3O4?FeO(Fe2SiO4,FeAl2O4)?Fe and the spatial sequence from the outer layer within the particle to its inner core.Impurities such as SiO2,Al2O3 and CaO formed slag phase along the sequence of Fe-Al-Si-O?Fe-Ca-Al-Si-O?Ca-Al-Si-O.The micro oolitic structure of the ore was gradually destroyed starting from the outer layer to the inner layer,and the evolution process could be divided into the marginal,internal,and entire destruction three stages.Based on these findings,a simplified model was proposed to describe the coal-based reduction of high-phosphorus oolitic iron ore.The observations of kinetics revealed that the reduction mechanisms changed as the reduction progressed.At the early stage,Fe2O3,Fe3O4 and FeO were reduced to metallic iron in a stepwise manner,and the reduction is controlled by interfacial chemical reaction.At the later stage,FeAl2O4 and Fe2SiO4 were the main phase that was reduced to metallic iron,and solid-state diffusion is the reaction determining step.The isothermal reduction process could be divided into three stages(namely,the initial stage,middle stage and final stage),and the mechanism functions for these three stages weref(?)=4(1-?)[-ln(1-?)]3/4,f(?)=(1-?)2(C/O molar ratio of 1.5 and 2.0)and f(?)=2(1-?)3/2(C/O molar ratio of 2.5 and 3.0),and 3/2(1-?)4/3[(1-?)-1/3-1]-1,.The most probable mechanism function for non-isothermal reduction was chemical reaction model f(?)3/2(1-?)2/3[1-(1-?)1/3]-1.Based on the determined mechanism functions,the corresponding apparent activation energy and pre-exponential factor were obtained,and the reduction kinetic models of oolitic iron ore by coal were proposed.The researches on formation and growth of metallic iron particle showed that the generated iron atoms firstly separate out in the surface of the ore particle and formed tenuous metallic protuberance in irregular shape,which would become the nucleus for the growth of metallic iron.Subsequently,the reduced iron diffused to the surface of the metallic protuberance and confluently grew into spherical metallic iron particle.At early stage of the reduction,the reaction of iron minerals was the restrictive step for the growth of metallic iron particle.At the later stage,the growth of metallic iron particle was controlled by mixed influence of surface diffusion and solid diffusion of iron atoms in slag.According to the characterization of metallic iron particles in reduced material,optical image analysis is proposed to measure the size of metallic iron particles,and the data obtained were analyzed using frequency and cumulative distributions.On the basis of the general equation for kinetics of grain growth,the kinetics model was proposed to describe the growth of metallic iron particle during the coal-based reduction of high-phosphorus oolitic iron ore.The studies on phosphorus interphase transfer revealed that the onset temperature of reduction of phosphate minerals was significantly decreased by SiO2,and phosphate minerals in the ore were reduced to phosphorus.On the macro level,the phosphorus distributed in the metal,slag,and gas phases after the reduction process,and most easily enriched in metallic iron phase.Phosphorus was present in the form of Fe3P and Fe-P solid solution in metallic iron phase and in the form of unreduced apatite and CaO-SiO2-P2O5 solid solution in slag phase.The phosphorus micro transfer path could be expressed as the reduced phosphorus diffusing from slag to slag-metal interface,reacting with metallic iron and generating Fe3P,diffusing from Fe3P to inner of metallic iron phase and forming Fe-P solid solution.Based on the principles of mass transfer,the kinetic equation for phosphorus interphase transfer was established,and the diffusion coefficient,diffusion activation energy and diffusion constant were calculated.As mentioned above,a new method namely coal-based reduction and phosphorus enrichment technology was proposed to treat high-phosphorus oolitic iron ore,and the reduction conditions were optimized.The experiments show that the optimal conditions were reduction temperature of 1523 K,reduction time of 50 min,C/O molar ratio of 2.0 and CaO dosage of 6%.After magnetic separated,the metallic iron powder containing 89.86%Fe with the iron recovery of 96.04%and metallization degree of 97.46%was obtained.The phosphorus content and recovery could reach 1.74%and 64.27%,respectively.The analysis of metallic iron powder indicated that the contents of acid impurities and harmful element S were relatively low.When the iron powder was refined by a dephosphorization steelmaking process,both steel products and a slag containing more than 10%P2O5 would be obtained.This slag could be used as a fertilizer or as a phosphorus resource,which created the possibility for the phosphorus recovery.The results presented in this dissertation can provide insights on the process of coal-based reduction of high-phosphorus oolitic iron ore and advances on theoretical system of coal-based reduction,thus will be of great significance in solving key technical problems in coal-based reduction of high-phosphorus oolitic iron ore and other refractory iron ores.