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Kinetics of direct reduction of unagglomerated iron-ore with coal char Roman-Moguel, Guillermo Julio


The kinetics of direct reduction of a commercial unagglomerated iron ore, with low-rank coal chars, have been investigated in the temperature range of 800-950°C (1073-1223 K) using a laboratory-size rotary reactor. The variables studied were temperature, coal type, particle size of coal and ore, fixed carbon-to-iron ratio, rotational speed-and percent filling. In addition the effects of a catalyst on the Boudouard reaction and of inert gas flushing on the reduction rate were determined. Mixing studies at room temperature and at reduction temperature yielded the best mixing conditions prior to the kinetics determinations. Agglomeration between the reduced particles was also studied. The mixing experiments at room temperature yielded the following. The degree of mixing depends almost entirely on the coal-to-ore size ratio. Best mixing is achieved with values of this ratio of 1 and smaller, for ore particles larger than 254 µm; for smaller sizes than this, good mixing can be obtained at higher coal-to-ore size ratios. At reduction temperature, improvement in the reduction rate was not obtained either by further increasing the fixed carbon-to-iron ratio from 0.32 to 0.64 or by varying the rotational speed from 7 to 20 r.p.m.. In the kinetics experiments, the overall reduction process was found to be controlled up to 0.5 to 0.8 fractional reduction by the Boudouard reaction, depending on the particle size and temperature; from then on, the kinetics were controlled essentially by the reduction reaction. The activation energies obtained were 224 kJ/mole for the Boudouard reaction, using sub-bituminous coal char, and 264 kJ/mole using lignite coal char; these values correspond to that of a catalyzed reaction. The catalytic effect of the coal ash on the Boudouard reaction was found to be much larger than the respective effect of metallic iron. The presence of a diluent gas extended the fractional reduction over which Boudouard reaction control is exerted. The activation energy obtained for the reduction of wustite by CO is 116.4 kJ/mole. The analysis of the Pco/Pco₂ ratio produced by the reaction proved to be a powerful tool in elucidating the rate controlling step. Smaller ore particles were found to agglomerate considerably more, in the non-catalyzed experiments; the addition of a catalyst for the Boudouard reaction also produced larger agglomerates. In neither case did agglomeration retard the reduction rate to a considerable extent. No accretion growth was observed on the reactor wall. Estimative calculations showed that similar throughputs can be obtained by processing the unagglomerated concentrate, as compared to operations which utilize indurated pellets under the same conditions. An advantage of a process using concentrates is the lower temperature at which it can be operated.

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