Fundamental Investigation Of The Characteristics Of Bed Porous Structure And The Resistance Of Flame Front In Iron Ore Sintering

packed bed of granulated mixture undergoing a high temperature partial melting processIn most integrated steel mills,the iron ore sintering is an important pre-process technology to provide sinter burden to blast furnaces for ironmaking.The iron ore sinter is a porous lumpy material formed by.And sintering can be viewed as a two-stage process of cold packed bed preparation followed by hot bed transformation.The scarce iron ore resources,the fluctuant materials supply and the stricter standards for energy consumption and pollutant emission in China result in great challenges for sinter plants therefore is necessary to optimize the operation and control of sintering including productivity,product quality etc.Fundamental understanding of the granulation and packing in sintering for cold bed preparation,the progress of flame front and its properties during the hot bed transformation stage is especially important for exploring more efficient and environmentally-friendly sintering technologies.Under this prospect,the present work is proposed and special attention was paid to the bed porous structure and its effects,the details are introduced as follows.Firstly,based on the mathematical description of granule structure of nuclei with an adhering fines layer by population balance,granulation and packing experiments were conducted to investigate the influence of conditions including moisture levels,hydrated lime dosage levels as well as magnetite concentrate substitution levels on granule structure and green bed properties.The results showed that the granule Sauter mean diameter is linked proportionally to the increasing moisture content,and the introduction of micro-particles such as hydrated line and magnetite concentrate resulted in a higher granule growth rate and larger adhering mass ratio of granules when providing sufficient moisture.Both the bed voidage-moisture curve and the bed permeability-moisture curve have three regions.Adding hydrated lime could improve the green bed permeability effectively for all blends tested while introducing magnetite concentrate into the sinter blend is not good for the bed permeability.Secondly,the deformation of the granule adhering layer greatly affects the bed voidage during packing,which is related to the strength of the adhering layer and the force exerted on the granules.To get a thorough understanding of the mechanical properties of the adhering layer of iron ore granules,simulated tablets were manufactured using proportions of components in each size range based on Litster's granulation model.Uniaxial compression tests and direct shear tests were performed to characterize the compression and shear strength of the simulated tablets respectively.The hydrated lime could act as solid binder and increase the strength of granule adhering layer while the concentrate plays an opposite role and deteriorates the adhering layer strength.Compared to the stress exerted on the granules during the packing process in a lab-scale permeability pot,the apparent cohesion of the adhering layer(1-3 kPa)is quite weak,suggesting that the granules could be easily compacted or sheared.Thirdly,a more mechanistic model of green bed voidage was developed taking the granule size distribution,the cohesive forces and the deformation of granule into consideration.On the basis of previous research,the voidage model was validated using data covering single ore blends,mixed blend with various hydrated lime and concentrate addition levels.Most of the data is located in the ± 10%error zone.Combined with the widely accepted population balance granulation model developed by Litster,the voidage model can give accurate predicted voidage values for modeling the sintering process and optimizing actual production from the properties of raw materials and moisture content.Fourthly,nondestructive X-ray microtomography was applied to characterize the pore structure of sinter and following simulations for estimating the sinter effective thermal conductivity were performed.The reconstructed sinter cakes have various complex pore distributions,leading to remarkable anisotropic thermal conductivities and complicated temperature field.The pores smaller than 300 ?m dominate the number frequency in sinter cake(around 45-55%),while a small amount of pores larger than 1 mm account for around 95%of the total pore volume,which determine the thermal behavior greatly.Comparing the predicted sinter effective thermal conductivities with values of similar iron agglomerates in the literature,some empirical correlations and analytical models,the results proved that XCT-simulation is a valid approach for capturing the peculiar details of the real sinter porous structure,thus to predict thermal behavior with higher accuracy than the simplified geometric models.Fifthly?CFD simulations were also performed on the real geometry of sinter reconstructed by high resolution X-ray tomography to observe the inhomogeneous flow through the sintered zone in sintering bed.The fluid velocity is accelerated obviously in the regions where the solid acts as obstacle and the channels are narrowed.A high porosity sinter bed has more gas passages and the flow distribution is well developed.The pilot-scale pot tests showed that the sintering bed permeability is greatly linked to the green bed permeability.The sintering airflow remains at a relatively constant level during sintering,indicating that the whole sintering bed has a self-adjusted permeability under the fixed suction condition(16 kPa)and the?100 mm high-temperature zone has a controlling effect on sintering bed permeability.Finally,to provide better understanding of the growth of gas channel relating to melt formation in flame front and its resistance to flow,The progress of flame front and its properties have been tracked by both the pressure and thermocouple measurements in pilot-scale pot tests under the conditions of a wide range of fuel rate.The actual porous structures of green bed and sintered bed have been characterized in terms of pore distribution and pore parameters by high resolution X-ray computed tomography.The reconstruction and image analysis showed that the sintered bed has some large gas channels and many more closed pores due to solid-melt-gas coalescence.More melt is generated when the heat is accumulated along the bed or inputted by higher coke content,showing a propensity to suppress the gas channel growth and amplify the mismatch of gas transportation along the bed.Higher coke rate leads to higher resistance in flame front,resulting in slower flame front speed.The specific pressure drop value in high-temperature zone increases from?3 kPa in upper bed to-7 kPa in bottom bed,which varies with the bed temperature and structure properties.