Process Intensification For Direct Reducion Of Fine Iron Ore Concentrates Via Conical Fluidized Beds

When adopted as the main reactor of direct reduction(DR)processes,fluidized beds share the merit of all the non-blast furnace iron-making processes,which is coke-free.More significantly,it can utilize fine iron ore directly;has better gas-solid contact and mass/heat transfer efficiency.Therefore fluidized beds are regarded as the most efficient DR reactors.However,the sticking and defluidization issue during the direct reduction process is a serious obstacle to the continuous and stable operation of the fluidization process,especially for the fine iron ore concentrates less than 100 ?m.Although it is abundantly produced through the beneficiation of the low-grade iron ore in China and has the properties of faster reaction kinetics,this kind of iron ore is difficult to be applied in the DR processes.In view of the above problem,this thesis solved the defluidization problem during direct reducing the fine iron ore concentrates(<100 ?m)in a conical fluidized bed,systematically studied the mechanism of preventing defluidization,investigated the effect of the design parameters of the conical fluidized bed on the fluidization behavior of the direct reduction process,and achieved process intensification via a two-step DR process.The main innovative findings of this paper are as follows:(1)The fluidization behaviors and reduction efficiencies during the DR of fine iron ore concentrates(<100 ?m)was compared in a conical fluidized bed and a cylindrical fluidized bed.In the cylindrical fluidized bed,stable fluidization can only be achieved within 700 ? due to the limitation of the terminal gas velocity.Defluidization would happen with the metallization degree(MD)lower than 40.0%at temperatures higher than 700?.While in the conical fluidized bed,the reduction temperature was enhanced to 800? without the occurrence of defluidization.In the condition of high temperature and gas velocity(Ug),the reduction rate of fine iron ore concentrates is considerably improved,such that direct reduced iron(DRI)with MD higher than 90.0%can be obtained within 20min.The results showed that the conical fluidized bed can prevent the defluidization issue during the utilization of fine iron ore concentrates without any inert addition,which provides a clean and efficient method for intensifying the DR process.(2)The behaviors and mechanisms of whisker growth on the surface of the DRI were investigated in a conical fluidized bed with H2 gas.Contrary to the existing research,the results showed that H2 atmosphere is not a sufficient condition to eliminate the growth of iron whiskers;the observed surface morphology is closely related to the H2 content in the reducing gas,fluidization gas velocity,and operating temperature.It was found that lower H2 content,lower Ug,and higher operating temperature promotes the growth of iron whiskers.Further exploration verified that the growth of iron whiskers could be eliminated only when the reduction rate is more dominate compared with the diffusion rate of iron atoms on the surface of the particles.The results indicated that the use of high gas velocity in the conical fluidized bed can effectively avoid the appearance of whisker morphology on the surface of the fine iron ore concentrate at high temperatures,thus preventing defluidization via controlling the surface morphology of the DRI products.(3)The agglomeration behavior of DRI particles in a conical fluidized bed was systematically studied.The results showed that the fine iron ore formed steady agglomerates by the sticking effects.Three different aggregation behavior were identified,i.e.,the rapid formation,stable growth and secondary agglomeration,among which the secondary agglomeration behavior was seldom reported before.The mechanism of the secondary agglomeration behavior was analyzed.It was found that this special agglomeration behavior is related to the hydrodynamic properties of conical fluidized bed.When the particles enter the partially fluidized bed regime,the separation force will decrease considerably at the edge of the conical fluidized bed,which results in the secondary agglomeration phenomenon.Further investigation revealed that the secondary agglomeration phenomenon is the main reason for the unstable fluidization and gradual leads to defluidization.Clarifying the mechanism of the formation and growth of agglomerates can give a better understanding of the defluidization issue and provide necessary basis for the optimization of operating the conical fluidized beds.Stable fluidization could be achieved for a long time by controlling the fluidization state in a fully fluidized bed regime.(4)The influence of the design parameters(bed height and cone angle)of the conical fluidized bed on the fluidization behavior of the DR of the fine iron ore concentrates was experimentally investigated.It is found that with the increase of the cone angle and the bed height,the fluidization time and the metallization rate of the fine iron ore would decrease,and the Ug at the bottom of the conical fluidized bed needed to prevent defluidization would increase accordingly.A modified agglomeration fluidization model was established,which was used to analyze the interparticle forces during the DR of the fine iron ore in the fluidized beds and to explain the different agglomeration behaviors at different stages in the conical fluidized bed.The model was also applied to predict the critical fluidizing gas velocity(Uc)for different designed conical fluidized beds,and results showed that the theoretical predicted U,are in good agreement with the experimental values,which means the model can provide guidance for designing conical fluidized bed for the DR process.(5)A two-stage DR process of "self-granulation and deep reduction" was explored to further intensify the reduction of fine iron ore concentrates.The effects of pre-reduction conditions on the fluidization behavior of deep-reduction stage were investigated.It was found that reducing the proportion of single particles in the system is the main reason for preventing the high-temperature defluidization.The high-temperature reduction results showed that the agglomerates after the pre-reduction could achieve steady flurdization at temperatures as high as 900?.The MD of the fine iron ore reached 90.0%within 10 min,and the utilization degree of H2 is more than 20.0%.Compared with the one-stage process,the total reduction time can be reduced by more than 50.0%,and the gas utilization degree can be increased by more than 100%.The two-stage DR process can further improve the reduction efficiency of fine iron ore concentrate,and further intensifying the DR process.