Study On The Reaction Kinetics And Product Characteristics Of Siderite Magnetization Roasting In An Inert Atmosphere In A Microfluidized Be

With the sharp rise in international steel prices and the dwindling resources of high-grade iron ore,it is important to develop and utilize fully the low-grade iron ore to can alleviate effectively the shortage of resources in China’s steel industry.Due to the low content of iron,siderite（Fe CO3）,a significant iron ore resource in China,must be treated to generate iron oxide by magnetization roasting reaction before directly used in ironmaking.Currently,reported research on the characteristics and kinetics of siderite magnetization roasting are all based on the analysis of roasted solid products,and only the overall reaction kinetics can be obtained.There is no research on the release characteristics and the generation kinetics of each component of the gas products.In addition,the magnetization roasting reaction of siderite in an inert atmosphere is mainly divided into two steps according to the literatures,namely Fe CO3=FeO+CO₂ and 3FeO+CO₂=Fe3O4+CO.However,this mechanism has not been further verified.Therefore,it is necessary to study the process of siderite magnetization roasting in an inert atmosphere.In this thesis,at first,the time-series variation of relative content of the gas products（CO₂ and CO）produced by siderite magnetization roasting in an inert atmosphere was measured by micro fluidized bed reaction analyzer（MFBRA）and the isothermal kinetics of CO₂ and CO generation were obtained.It was found the content ratio of CO₂ to CO was close to 9:1,which was not consistent with the theoretical value reported in the literature.Then,the process of siderite magnetization roasting reaction was investigated using micro fluidized bed reaction analyzer with an on-line particle sampling system（MFBRA-P）.A new reaction mechanism of siderite magnetization roasting was deduced based on analyzing the release characteristics of CO₂ and CO,the roasting solid products collected in situ during the heating process and thermodynamic theoretical calculations.The non-isothermal kinetics of CO₂ and CO generation were also obtained.At last,the transient changes of the phase composition,pore structure and magnetism of solid products during the process of siderite magnetization roasting in an inert atmosphere were studied by MFBRA-P.This thesis provides an important theoretical data for the industrial application of siderite fluidized bed magnetization roasting,and also provides a new method for studying the mechanism and kinetics of gas-solid reaction.The results are as follows:（1）There was a slight time delay in CO generation than the releases of CO₂,and the delay time decreased gradually with increasing temperature,which is related to an increase in the reaction rate due to an increase in temperature.The time for complete decomposition of siderite was short as a few seconds in MFBRA at temperatures above 700 ℃,which was significantly less than the time measured by thermogravimetric analyzers（TGA）and the suspension roasting experiment apparatus with an isothermal reaction zone reported in the literature,revealing the difference between MFBRA and other apparatuses in measuring the reaction kinetics.The activation energies of CO₂ and CO generation were 103 k J/mol 和 116 k J/mol,respectively.The pre-exponential factors（lg A）were 12.76 s-1 and 14.35 s-1,respectively,and the most probable mechanism models were the shrinking core model（R2: G（x）= 1-（1-X）1/2）and the three-dimensional diffusion model（D3: G（x）= [1-（1-X）1/3]2）.The content ratio of CO₂ and CO was close to 9:1,which was much larger than the theoretical ratio obtained by the two-step reaction mechanism of siderite magnetization roasting reported in the literature（VCO₂:VCO=2:1）,indicating that there were other reactions except the two steps reaction reported in the literature.（2）The ratio of the content of CO₂ and CO generated calculated by the thermodynamic theoretical was closer to the result obtained by MFBRA,indicating that the result obtained by MFBRA was closer to thermodynamic equilibrium.Combined with the XRD results of the roasted solid products collected in situ by MFBRA-P and the release characteristics of CO₂ and CO analyzed online,it was inferred that the reaction mechanism of siderite magnetization roasting in an inert atmosphere was as follows: When the temperature is lower than 600 ℃,the reactions of 6FeO+CO₂=C+2Fe3O4 and 3FeO+CO=C+Fe3O4 may occur during magnetization roasting of siderite except the two reactions of Fe CO3=FeO+CO₂ and 3FeO+CO₂=CO+Fe3O4,while at the temperature of 600-700 ℃,only these two reactions of Fe CO3=FeO+CO₂ and 6FeO+CO₂=C+2Fe3O4 occur.Furthermore,the activation energies of CO₂ and CO generation obtained by the non-isothermal method in MFBRA-P were 177 k J/mol and 169 k J/mol,respectively,which were higher than those obtained by the isothermal method in MFBRA.The reason was that the larger particle size of the raw materials used in MFBRA-P than that in MFBRA increased the internal diffusion resistance of gases produced by siderite decomposition.The most probable mechanism models of CO₂ and CO generation were the nucleation and growth model（F1: G（x）=-ln（1-X））and the shrinking core model（R2: G（x）=1-（1-X）1/2）,respectively,different from the results obtained by the isothermal method.（3）It was shown that the intermediate product of FeO was firstly generated by the decomposition reaction of siderite,and the final product of Fe3O4 was generated through magnetization reaction of FeO.As the temperature increased and the reaction proceeded,the pore volume and specific surface area of the solid products increased first and then decreased,and reached the maximum of 3.03 cm3·g-1 and 13.19 m2·g-1 at 600 °C,respectively.The saturation magnetization of the roasted product increased significantly with the increase of temperature,but almost unchanged when the temperature was higher than 600 °C.