Fundamental Research On Gas-based Direct Reduction Of Iron Ore Pellets With Carbon Monoxide And Hydrogen Mixtures

Abstract:Direct reduction iron (DRI) with properties of pure and stable is an ideal material for quality steel production. In recent years with the rapid development of steelmaking by EAF process, as an indispensable charge to replace scrap, the production of DRI has been increasing year by year. However, due to the restriction of natural gas resource DR industry in China mainly are coal-based processes with characteristic of small yield but high energy consumption, which leads to the no more than0.6million tons annual output of DRI. In EAF process of developed countries the usage of DRI has reached50%, but the proportion in China is only about5%. So, developing the direct reduction technology with coal gasification will promote the sustainable development of short route technology in iron and steel industry.In this paper, iron ore pellets from a domestic palletizing plant were taken as main research object. Typical gas compositions of coal gasification, Midrex and Hyl-III processes were simulated in reduction experiments. The degradation, swelling, compressive strength and sticking behaviors of pellets during reduction were deeply investigated based on the research of thermodynamics and kinetics. And relative sticking prevention technology was developed. The following conclusions were obtained.Thermodynamics research showed that the reaction H2+CO2=CO+H2O attained equilibrium at810℃, the reduction capability of CO is superior to H2below810℃, however the H2is superior to CO above810℃. The comprehensive utilization of CO-H2mixture in the reduction process was determined by the2nd reduction stage (Fe3O4â†'FeO) below650℃. However, above650℃the utilization was determined by the3rd stage (FeOâ†'Fe). As It DRI is produced, the minimum need of reducing gas in coal gasification-shaft furnace process is100m3more than Midrex process, but the energy consumption can be decreased by0.34GJ and energy utilization increased by3.3%. Rise of hydrogen proportion in reducing gas from0to H2/CO=1.0leads to a rapid increase of reduction rate. But reduction rate tends to be stable with hydrogen proportion further rise from H2/CO=1.0to H2. Raising temperature can effectively improve the reduction rate. However, with temperature raising to950℃, reduction rate significantly slowed. Under conditions of800-950℃and H2/CO<1, the prophase of reduction process is controlled by interfacial chemical reaction and then controlled by the combination of interfacial chemical reaction and diffusion of reducing gas in the final stage. However, under conditions of1000℃or H2/CO>1, the whole reduction process is controlled by interfacial chemical reaction. Apparent activated energy of reaction is24.98kJ/mol in CO atmosphere and finally reaches59.63kJ/mol as the rise of hydrogen proportion in reducing gas.Reduction degradation (RDI<3.15mm) of pellets reaches the maximum in the coal gasification atmosphere at550℃The thick extended cracks formed around pores and thin ones formed by the reduction of dense plate-like hematite both lead to the degradation. And RDI decreases significantly with the increase of SiO2and CaO content in pellets. However, effect of MgO and Al2O3on degradation is much weaker.Volume swelling reches the maximum in the reduction degree range of20%-40%. Swelling of pellets decreases with the rise of H2proportion but increases with the reduction temperature. Moreover, pellet contracts rapidly with the rise of H2proportion after the maximum volume swelling. Mechanism research showed that during the reduction with CO, swelling is firstly caused by the reduction of Fe2O3to Fe3O4and FexO and then due to the internal gas pressure as well as the iron whiskers precipitation pellets maintains the swelling state. But pellets reach the maximum swelling due to the formation of FexO and then contract with the aggregation of metallic iron. FexO formation increases with the temperature, therefore volume swelling of pellets increases accordingly.Compressive strength of pellets increases with H2proportion but decreases with temperature. The strength change corresponds well with volume swelling of pellets and reaches the minimum at10-20min. Compressive strength of oxide pellets drops sharply from2973.3N/pellet to742.0N/pellet as reduce for lmin. Increase of porosity causes damage to bond phase between particles and cracks in particles leads to grain defects, both of which result in the strength loss in the initial reduction. And those determinants of strength gradually shift to the inner part of pellets with the reduction proceed. Fine pores and cracks form widely in the whole pellet as hematite is reduced to magnetite, which cause the largest strength loss of75.85%. With further reduction of magnetite to wustite the pores and cracks expand to be larger and strength drops slightly. Finally, cracks heal gradually with wustite reduce to iron and strength improves.Sticking of pellets at lower reduction degree mainly depends on the bonding of wustite phase on the interface. The crystallization of iron phase and formation of low melting eutectic phase becomes the dominant factor afterward as reduction degree increases, which correspondingly leads to higher sticking index. Sticking practically occurres on micro-convexities and edge angles on the surface at lower reduction degree (R<70%), and then develops to connection in large area as edge angles transform to flat surface with the rise of reduction degree (R<70%).With increasing temperature the iron precipitation, which acts as a bonding phase, found to be various. Fibrous iron precipitates on the sticking interface hooks the pellets together at lower temperatures. While at high temperature sintering of freshly precipitated iron with high activity and surface energy leads to the rise of sticking index. Pellets reduced by CO exhibit highest sticking index for the dense iron layer precipitated on the interface. Addition of H2to reducing gas considerably decreases the sticking index due to the formation of porous and fibrous iron precipitation on the interface, which can be used as an effective inhibition of sticking in iron ore reduction process.Coating pellets with CaO, not only physically seprates the pellets with each other but also transforms the precipitation of metallic iron, can decrease the sticking index but slow down the reduction process unfavorably. Charcoal acts as reductant, physical sepration and burden looser during reduction process. However, its suppression of sticking is not as well as the coating method. Recombination of coatings and charcoal, can improve the reduction rate and suppression of sticking at the same time, is an ideal method for sticking prevention.