Early Gaseous Oxygen Enrichment to Enhance Magnetite Pellet Oxidation
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RON oxide pellets, produced from natural magnetite, are the main iron source for North American blast furnace ironmaking. Agglomeration and induration are two major steps in the industrial pelletizing process. Agglomeration involves the formation, by balling drum or disk, of green balls (diameter from 9 to 13 mm). Green balls are formed from ore concentrate, with added fluxes (if required), approximately 0.5 pct by mass binder (typically bentonite), and water. Before feeding to the ironmaking process, induration (improving pellet strength) is performed by oxidation and sintering at high temperature [maximum temperature approximately 1500 K to 1600 K (1227 °C to 1327 °C)]. Pellet strength is obtained by high-temperature oxidation of the magnetite to hematite, forming iron oxide bridges and bonding between particles. Induration is typically performed using grate-kiln or straight-grate units; the process steps include drying, preheating, sintering, and cooling. During the drying and preheating period, the pellets are exposed to a gas atmosphere containing oxygen and water vapor, causing oxidation of magnetite to hematite. In the grate-kiln process, preheating takes place, while the pellets are on the grate. The heating rate during preheating is typically 200 K/min, with a maximum temperature of approximately 1200 K to 1300 K (approximately 900 °C to 1000 °C) at the end of the MING TANG, Research Assistant, and PETRUS CHRISTIAAN PISTORIUS, Professor, are with Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA. Contact e-mail: [email protected] HYEON JEONG CHO, formerly Graduate Student with the Department of Materials Science and Engineering, Carnegie Mellon University, is now Assistant Manager with the Technical Research Center, Hyundai Steel, Dangjin, South Korea. Manuscript submitted January 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
preheating zone.[1] Typical molar percentages of oxygen in the preheating zone are 16 to 18 pct.[2] It has been found previously that as little as 50 pct oxidation (of magnetite to hematite) takes place during preheating.[1] Incomplete oxidation might be caused by the short oxidation time (associated with the high heating rate of 200 K/min) and limited diffusion of molecular oxygen into the pellets. Partially oxidized pellets have a duplex structure of a hematite shell around an unreacted magnetite core (see Figure 1). The presence of unreacted magnetite tends to affect the macrostructure, because of differences in the sintering behavior of magnetite pellets: Based on isothermal testing of magnetite pellets for times of 30 to 120 minutes, magnetite particles in unoxidized pellets sinter together at significantly lower temperatures than do oxidized (hematite) particles; the sintering temperature of magnetite is approximately 1120 K (approximately 850 °C), much lower than that of hematite [approximately 1370 K (approximately 1100 °C)].[3] The result is that if an unoxidized core is present in pellets at the end of the preheating stage, the unox
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