An Experimental Study On The Reduction Kinetics Of Iron Ore Fines By CO-H₂ Gases In A Micro-Fluidized Bed

In recent years,national policies related to resources,energy,and environment have prompted China's steel industry to improve and upgrade existing technologies to achieve sustainable development.The FINEX ironmaking technology based on fluidized reduction has certain advantages sucha as handling iron ore fines,getting rid of coke,shortening the ironmaking process,energy conservation and environmental protection,and has become one of the most potential new ironmaking processes.The fluidized reduction ironmaking technology is based on fluidized beds that work as the main reaction equipments,gases are used as the fluidization medium and reducing agents,thus the mass transfer,momentum transfer,heat transfer and chemical reaction process between the reducing gas and the iron ore powders are significantly enhanced,and the kinetic conditions for the reduction reactions have been greatly improved,thus the chemical reactions are promoted.Study on the reduction behavior and physical mechanism of the iron ore fines reduction in a mixed reducing gas composed of CO and H₂ is of critical importance to the development and application of new ironmaking technologies.Therefore,it is necessary to study the reduction process of iron ore fines by both experimental and theoretical methods,and the topic of this thesis has great significance.In this thesis,the reduction kinetics of iron ore fines by CO/H₂ gases are systematically studied based on the micro-fluidized bed gas-solid reaction analysis system.Firstly,the rules of the key reaction process parameters such as reaction rate,conversion rate and reaction time are analyzed with the effects of various factors such as temperature,reducing gas type,gas concentration and particle size in iron oxide reduction process.Then,the reaction kinetic parameters of the iron ore fines are calculated by different methods,the controlling steps and kinetic models are determined.On this basis,the analysis shows that there is a change in the rate-controlling step during the reduction process.Finally,the transition point of the controlling steps of iron ore fines reduction process is determined by comparing with multiple kinetic models and experimental results,as well as their ranges.The main contents and conclusions are given as follows:(1)Increasing temperature and gas concentration are both conducive to the process of the reaction.Increasing the H₂ gas content can further shorten the reaction time,but excessively high CO gas content will promote the carbon evolution reaction and hinder reaction process.The reduction reaction rate under each condition generally conforms that the reduction rate increases first and then decreases.When CO gas is used,the reaction rate reaches the maximum when the conversion is about 0<X<0.2,and0.2<X<0.6 when H₂ gas is used;the reaction rate in the range of 0.2<X<0.6 increases at first and then decreases due to the the main reaction gradually transforms from Fe₂ O₃?Fe O to Fe O?Fe.Increasing the gas concentration can facilitate the reaction,especially,increasing the H₂ gas content can further reduce the reaction time.Appropriate atmosphere composition,higher reduction temperature,and lower particle size of iron ore fines can speed up the reaction and improve the reduction efficiency.Therefore,increasing the temperature,using smaller particle size iron ore,and using hydrogen-rich gas can improve the production efficiency,which can provide a certain reference for the actual production.(2)The calculation and analysis of the kinetic parameters show that the reduction conditions have an effect on both the activation energy and reaction mechanism.The activation energy for CO reduction is 36.2?85.5 KJ/mol,and the activation energy for H₂ reduction is 24.2?56 KJ/mol.The activation energy for CO-H₂ mixture gas reduction of Brazilian iron ore is 14?41.8 KJ/mol,for the reduction of Australia iron ore it is13.6?60.6 KJ/mol,the controlling mechanism is mainly composed of chemical reaction and gas internal diffusion.The particle size of iron ore has a significant influence on the reaction time and the internal diffusion.The reduction time becomes longer as the particle size increases,and the reaction tends to be controlled by the internal diffusion in the final period.Australian iron ore powder is also tends to the internal diffusion control in the middle and late stages due to the high density.(3)On this basis,different methods are used to analyze the transition and working range of the controlling steps in reduction process using mixture gas.The research indicates that during the reduction process of Brazilian iron ore by mixture gases,when the conversion is 0.1(27)X(27)0.5,the reaction is controlled by chemical reaction and gas diffusion.When the conversion rate is 0.6(27)X(27)0.9,the reaction is controlled by gas diffusion,and the transformation point belongs to 0.5(27)X(27)0.6.During the reduction process of Australian iron ore by mixture gases,the reaction control mechanism is the chemical reaction control in the early stage:0<X<0.4.In the later stage:0.6<X<1,it is controlled by the internal diffusion.The transition from the chemical reaction to diffusion control occurs at about 0.4<X<0.6.The mechanism can be described by the three-dimensional phase interface reaction model of G(X)=1-(1-X)^1/3 for 973k,and G(X)=1-(2/3)X-(1-X)^2/3 for 1023K and 1073K,which is a three-dimensional gaseous diffusion model.The iron ore fines reduction process with different particle sizes basically conforms to G(X)=1-(1-X)^1/3,i.e.the three-dimensional phase interface reaction model,when particle size is about 400-500?m,the reaction model is more consistent with G(X)=1-(2/3)X-(1-X)^2/3,i.e.the three-dimensional diffusion model when the conversion is 0.8<X<1.