Decomposition, Reduction and Reoxidation of Iron Oxide (II)

According to theoretical calculations, in the equilibrium gas phase in which Fe 2 O 3 is reduced to Fe 3 O 4 , the CO% content requirement is very low, that is, the ratio of CO 2 /CO is large. Of CO in the mixed gas is sufficient to completely reduce hematite become magnetite. The reduction reaction may be carried out in a sintered pre-tropical zone. Especially in the combustion zone of solid fuels. Laboratory studies have shown that the presence of CO 2 (in a CO-CO 2 mixed gas) in the gas phase does not slow the reduction of hematite. For the reduction of FeO 4 to FeO, the equilibrium gas phase has a CO 2 /CO ratio of 1.84 at 700 ° C and 10.76 at 1300 ° C. For the reduction of FeO to Fe, the equilibrium gas phase at 700 ° C, CO 2 /CO The ratio is 0.67, and the temperature rises to 0.297 at 1300 °C, and the ratio is continuously decreasing. In the actual sintering process, when sintered with an inert material such as quartz sand, the fuel combustion product has a CO 2 /CO ratio of between 0.76 and 1.00. Therefore, from the viewpoint of thermodynamics, Fe 3 O 4 may be reduced to FeO, and FeO may not be reduced to Fe. However, it must be noted that the distribution of the gas composition in the sinter layer is extremely uneven. The CO 2 /CO may be small around the coke breeze, while the CO 2 /CO may be large in the region far from the coke powder particles, and the oxygen content may be large. In the former case, even iron oxides to metallic iron reduction wave. In the latter case, Fe 3 O 4 and FeO may be oxidized. It is therefore impossible to reduce all Fe 3 O 4 or even all Fe 2 O 3 under sintering conditions. In addition, the actual reduction process depends on the kinetic conditions of the process, such as the reducibility of the ore itself, the contact time and surface area of ​​the ore and reducing agent. Although the iron ore in the sintered material has a small particle size and a large specific surface area, since the high temperature duration is short (1 to 1.5 minutes), (the diffusion condition of CO to the ore particles is poor, and the reduction property of Fe 3 O 4 itself is not good, so Fe The reduction of 3 O 4 will be limited. Therefore, it is possible to analyze Fe 3 O 4 from thermodynamics to reduce to FeO. In fact, the amount of reduction depends on the average gas phase composition and kinetic conditions in the high temperature region.

The final effect of the reduction is also determined by the temperature of the sintering process and the fuel consumption. When limestone is added to the sinter, it is advantageous to form a fusible compound to lower the combustion temperature, and the reduction reaction process is limited, and as a result, FeO decreases in the sinter. On the contrary, when MgO is added to the material to form a refractory compound, the temperature of the combustion zone rises, so FeO in the sinter also rises, as shown in Fig. 4. [next]
(III) Reoxidation of iron oxide during sintering The sintering test using high-grade hematite powder shows that under normal carbon conditions (4.75% C), hematite is almost completely reduced to magnetite, but burning The upper part of the belt is oxidized, and the sinter Fe ++ is gradually reduced. As the fixed carbon is reduced, the oxidation is more intense and can be returned to the level of hematite. Figure 5 shows the distribution of Fe ++ in the layer of hematite powder when sintered with different carbons.

It can be seen from the figure that when the coke powder is 7%, the Fe ++ of the combustion zone can reach more than 30%, which indicates that Fe x O appears. At 5.7% of the coke powder, Fe ++ is up to 23%, indicating that all of the hematite is reduced to magnetite. It is reoxidized to hematite in an area about 60% from the combustion zone. For high fuel ratios (7% coke breeze), the weaker oxidation only reoxidizes the reduced oxide to the level of magnetite.
When the magnetite is sintered, the oxidation reaction is also considerably developed. Especially in the case of low fuel, the combustion zone temperature is below 1350 ° C, and the oxidation is very intense. The oxidation zone of the magnetite begins in the pre-tropical zone and then in the non-fuel-containing sinter of the combustion zone and finally in the sinter cooling zone.
When the fuel consumption is slightly higher than normal, this reoxidation process has no effect on the final structure of the sinter. The sinter structure obtained at lower fuel depletion usually contains the first magnetite particles oxidized along the cleavage plane because the heat and reducing atmosphere are weak under such strips, making them insufficient reduction. This type of structure is often characterized by natural oxidized magnetite and imaginary hematite. It is worth noting that magnetite precipitated in the liquid phase also tends to have oxidation along the cleavage plane, which may be related to the law of crystallization heterogeneity. The oxidation of the float body is often carried out along the edge of the particle.
When the final structure of the sinter is formed, the sinter is subjected to a very weak second oxidation. In general, magnetite crystals distributed between the silicate liquid phases are not as oxidized. The partial oxidation of magnetite can only occur on the pore surface of the sintered ore, cracks and various particles in the vicinity of defects.

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