The reduction shaft furnace is a countercurrent moving bed cylindrical reactor in which hematite pellets are reduced by a gas mixture of H 2 and CO. Most of the reduction shaft furnaces use hydrogen rich gas as reducing gas. The materials and heat balances show that the heat balance determines the gas consumption. Compared with the gas requirement by reduction reaction, a much greater quantity of gas has to be blown into the furnace to satisfy the heat balance, leading to a high energy consumption and high excess of reduction potential of top gas. To solve this problem, a new approach is proposed where some oxygen is blown into the upper zone of the shaft furnace. As the feasibility study of such an approach, a static model was developed first to calculate input gas flowrate and oxygen rate. Then, based on conservations of mass and heat, a kinetic model for a typical MIDREX shaft furnace was developed to investigate the proper position for oxygen blowing. The model predictions were validated by comparison with production data. Finally, a kinetic model was developed to investigate the process of shaft furnace with oxygen blowing. The results show that the gas consumption can be reduced from 1897 to 1405 N m 3 tDRI 21 with oxygen blowing of 20?3 N m 3 tDRI 21 . After oxygen blowing, the temperature increases sharply, the concentration of CO and H 2 decreases, and reduction potential of top gas decreases from 1?48 to 0?69, showing that the gas utilisation is greatly improved. These models can be used for process optimisation development.
List of symbolsA cross-section area, m 2 C P specific heat capacity, J mol 21 K 21
To understand the reduction process of pellets made from high alumina iron ore from Guangxi Zhuang Autonomous Region, kinetic experiments, X-ray diffraction examination and scanning electron microscope analysis were adopted. The results show that temperature, a critical factor, increasing from 850 to 1050uC can increase the reduction degree from 31?03 to 55?01%. With increasing hydrated lime addition, the reduction degree increases. Calcium oxide can participate in the solid reaction and affect the mineral structure of ore, but cannot completely prevent the formation of fayalite and hercynite. As a result, the reduction improvement with calcium hydroxide addition is limited.
Non-stoichiometry influences both the thermodynamic and kinetic analyses of the iron oxide redox processes. The thermochemical data of iron oxide redox reactions in various textbooks are not consistent, and the kinetic characteristics are not well understood because of the nonstoichiometry. To clarify such confusions, some famous thermodynamic data are compared, and highly precise experimental work conducted for verification. It is shown that the thermodynamic data for the pure iron oxide reduction reactions from JANAF agree well with the experimental results; the eutectoid temperature of iron oxides was proven to be 576uC; Dieckmann's defect model of magnetite was proven in good agreement with the experimental results only at high oxygen activities but not low oxygen activities; and the dependences of iron deficiency on Dwt-% (weight loss ratio) and Fe 2z % (ferrous ratio) were calculated and experimentally verified in pure iron oxides reduction processes.
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