Study On The Characteristics And Kinetics Of Gas-Based Direct Reduction Of Hematite Ore Fines In Rotary Bed

In order to temper the pressure on the cost of blast-furnace ironmaking enterprises causing by the decline of high-grade iron ore reserves and increasingly severe environmental policies,it is an important exit of transformation and upgrading for the iron and steel enterprises in China to produce direct reduced iron by direct reduction process with the features of low pollution,low energy consumption and short process based on the low-grade limonite ore with abundant reserves.In view of the current lack of the related experimental research of limonite ore by the gas-based direct reduction.In this paper,the limonite ore Australian PB powder was used as the raw material,and we investigated the effects of reduction temperature,time and composition of reducing gas on the reduction of ore powder and carbon deposition behavior in the self-developed rotary drum reactor.And then we analyzed the transformation process of phase composition,structure and morphology of the particles during the reduction process of ore fines.We also explored the formation and growth mechanism of dense iron layer and filamentous carbon.Finally,model fitting and parameter calculation of reduction kinetics of ore powder were carried out in different temperature ranges and reduction atmospheres.The main conclusions obtained by the study are as follows:1?The basic properties of limonite ore Australian PB powder were studied by different analytical methods.During the preparation of porous hematite ore powder,the change rules of the physical properties of limonite ore powder were also investigated and analyzed.The results showed that after calcination at 500?the specific surface area of limonite ore was increased from 5.69 m²/g to 24.82 m²/g of hematite ore.The macroporous structure in the limonite ore transformed to the more uniform mesoporous structure and some micropores in the hematite ore due to the calcination effect.All the FeOOH transformed to Fe₂O₃ after dehydration.2?In the process of gas-based direct reduction,reduction gas flow rate,particle size,reduction temperature,time and other factors both have important influence on the reduction behavior of ore fines.Under the same experimental conditions,the reduction degree of ore fines increased with the increase of reduction gas flow rate or the decrease of particle size.The optimal gas flow rate and particle size range were 200 mL/min and 44⁸9?m,respectively.3?In the 50%H₂-N₂ reduction atmosphere,when the temperature was improved from 700?to 800?,the iron oxide particles on the surface of the product particle transformed gradually to metal iron particles with the diameter of 2?m.In the initial reduction reaction,a large number of newly generated near-spherical iron oxide particles with the diameter of 0.15⁰.6?m on the surface of product particles were reduced gradually to metallic iron particles,which then gathered together to reduce the surface energy at 900?.Furthermore,the O atoms in the FeO particles diffused from the FeO/Fe interface to the Fe/reduction gas interface to react with reduction gas causing by the dense iron shell formed on the periphery of the FeO particles,leading to the solid diffusion of O atoms became the limiting step at the later stages.4?Under the condition of 750?and pure CO atmosphere,the reduction degree in the initial reaction increased rapidly to 51.89%at 60 min,and then decreased rapidly to 21.53%at 120 min.The Fe₃C content of reduced products increased first and then decreased with the time.When the reduction time reached 60 min,the graphite-mold free carbon occurred in the reduced products.5?Under the above-mentioned condition,the surface of the product particles was very rough when reduced for 60 min,and there were some tiny filamentous carbons on the surface of several product particles.The surface of product particles became very soft,and some particles completely broken down,and the filamentous carbons disappeared when reduced for 90 and 120min.In view of the phenomena of the formation and disappearance of the filamentous carbons,the content change of the Fe₃C and graphite carbon,and the fracture of the product particles,the following mechanism was proposed in this paper and it can be divided into eight steps:?1?The Fe layer was formed on the surface of product particles;?2?The decomposition and carburization of CO and subsequent nucleation and growth of Fe₃C occurred on the surface of iron layer;?3?Graphite was deposited on the surface of the Fe₃C layer;?4?The Fe atoms from the decomposition of Fe₃C gathered and formed metallic iron particles on the surface of the graphite layer;?5?Graphite and iron particles were carburized to form catalytic particles;?6?Catalytic particles catalyzed CO decomposition and led to the growth of the filamentous carbon;?7?The rupture of the product particles led to the disappearance of filamentous carbon;?8?The surface of metal iron particles in the product particles repeated the steps of?2?⁴.6?The reduction degree in the pure CO was greater than that in the 25%H₂-CO atmosphere before reducing for 14 min at 750?.However,it was opposite with increasing the time.This is because at the early stage of the reaction,the H₂ in the 25%H₂-CO atmosphere reduced FeO to Fe,which catalyzed CO to decompose to deposited carbon.And it also reacted with CO to form graphite-mold free carbon.The presence of the two carbon sources led to the higher carbon content in the initial stage of the reaction.However,as the reaction went on,the free carbon entered and deposited on the large number of nanoscale pores in the particles to participate in the reduction reaction,and thus improved the reaction rate and reduced the carbon content in the product.In the 50%H₂-CO reduction atmosphere,the product particles all showed the shape of ellipsoid at all reduction temperatures and times.A sintering phenomenon between the metal iron particles has occurred at the later stages of reduction at 850?and 900?.7?In the simulated COG atmosphere,the reduction process can be divided into reduction section and carbon deposition section in the temperature range of 750⁹00?.In the reduction section,the reduction degree increased with the increase of temperature and the time,and reached the maximum of94.69%at 900?for 10 min.In the carbon deposition section,the reduction degree presented the opposite trend,and the rate of carbon deposition increased from 0.0712 g/min at 750?to 0.7024 g/min at 900?.8?At 800?,with the extension of the time the iron oxide particles on the particle surface transformed to tabular Fe3C,whereas at 900?the diameter of the newly generated metallic iron on the particle surface increased from no more than 0.5?m to 1.5?m.The deposition carbon on the particle surface existed in the two forms of porous structure and cluster structure when reducing for 30 min at 900?.9?The results of model fitting and dynamics calculation showed that in temperature range of 700⁹00?,the dynamic models of reduction of the ore fines in the 50%H₂-N₂ atmosphere,50%CO-N₂ atmosphere,50%H₂-CO atmosphere and simulated COG atmosphere were first-order reaction model?g???=-ln?1-???,2D diffusion model?g???=?+?1-??ln?1-???,3D diffusion model(g???=[1-?1-??^1/3]²)and 1D diffusion/3D phase boundary reaction mixed controlled model(g???=?²+1-?1-??^1/3),respectively.The activation energy were51.01?104.4?59.88?50.79 k J/mol and the pre-exponential factor were 24.12?199.1?8.134?29.76 min^-1,respectively.