Fundamental Study On Direct Reduction Of Oxidized Pellets By Gas-based Shaft Furnace

After taking into account the conditions of the domestic iron resources, the development of iron and steel industry, and the abundance of domestic non-coking coal resources, the coal-gasification shaft furnace direct reduction becomes the main direction of developing DRI production in China. The dominant gas-based shaft furnace direct reduction process in the world includes Midrex and HYL, which both put forward strict requirements on the properties of oxidized pellets. In addition, although Midrex and HYL process both owes successful production experience, their reduction conditions (mainly the H2/CO ratio of the reducing gas) are different. At the same time, the composition of the effective reduction gas in the syngas generated by several coal gasefication processes, are quite different, and industrial production costs and energy consumption are also distinctness. Thus, the following tasks are very critical and urgent for China to develop the process of coal gasification-shaft furnace direct reduction, including producing of the oxidized pellets with the qualified compositon and metallurgical properties, rationally choosing the gasification process, systematically studying the reduction mechanism and reduction conditions in the shaft furnace.This thesis focuses on direct reduction of oxidized pellets by gas-based shaft furnace to carry out experiment. At first, full tests of relevant metallurgical properties have been made for the oxidized pellet that is produced on basis of the domestic resources, which is aimed to verify the feasibility of the oxidized pellet using as the burden of gas-based direct reduction shaft furnace. Next, carried out gas-based reduction reaction of the oxidized pellet by changing the temperature and atmosphere, researched the influences on the reduction reaction rate and swelling of the pellets in different experimental conditions, analyzed the kinetics mechanism of gas-solid reaction in a system, and clarified the effects of the control step of reduction reaction, the utilization of the gas thermodynamics and comprehensive thermal effects under different temperature and reducing atmosphere. Thus, it provide basis for rationally choosing the gasification process and determining the appropriate reduction conditions of oxidized pellets by gas-based shaft furnaceThrough this study, the following conclusions could be drawn, including:(1) The oxidized pellets which are prepared based on the domestic resources, and have a good performance meeting the performance indicators of the HYL, can be used as the pellets of gas direct reduction in shaft furnace;(2)The pellet metallization ratio could be achieved 95% within two hours under the experimental conditions, which is satisfied with the actual production requirement for shaft furnance. The reduction swelling ratio of pellet is less than 20% when the mixture reducing gas contains 30%～75% H2 and temperature is below 950℃. It is in line with necessity of swelling performence by gas-based shaft furnace. Thus, in order to achieve smooth running of shaft furnace and meet the requirements of stable production, H2 concentration in effective reduction gas components of the syngas generated by the relevant coal gasefication process, is suitable to maintain within 35%～75%;(3) Under the experimental temperature, both the rise of temperature and the increase of H2 concentration in the reduction atmosphere are benefit to the reduction reaction. With the temperature rising, the impact of the increase of H2 concentration in the atmosphere on the reaction rate is strengthened. But when the H2 concentration reached 50%, the trend of the reaction rate increased with the increasing of H2 concentration is gradually weakened. When H2 is used as reducing agent, the interfacial chemical reaction is restricted link; When CO is used as a reducing agent, the reaction process is mixed control by the gas intrinsic diffusion through the product layer and interfacial chemical reaction;(4) Under the experimental temperature, the thermodynamics utilization ratio of the reducing gas during the reaction process is mainly determined by the composition of the balance gas phase of the third (FeO-Fe) reduction process. When the integrated heat effect of the reduction reaction of is zero, the H2 concentration is about 45% in the mixed reduction atmosphere. When the H2 concentration less than 45%, the reduction process is exothermic reaction, while higher than 45%, the reduction process is endothermic reaction.