3. Results and Discussions 3.1. Microstructural characterization of reduced mill scale 424 Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A. In iron-steel making industry, it is desired that sponge iron as an alternative charge material must have high degree of iron. The effect of reductant in the reduction conditions is very important in obtaining the final product. Figure 1a and b show SEM micrographs of solidified sponge iron reduced by only solid carbon (coal). The solid metal form includes coal particle in dark contrast within the matrix and it indicates that the reduction of ferrous oxides by coal is unsufficient. An EDS analysis is given in Figure 1c and it shows the degree of metallization in the selected image area. The amount of carbon is very high and the amount of iron is lower than that of traditional sponge iron (55-65 Fe%). Iron ore can also be reduced by hydrogen. Wagner et al. reported that (i) most of the reaction features are very similar to that of the reduction by carbon monoxide and many mechanisms are common to both of them, (i) the reduction with hydrogen is endothermic, whereas it is exothermic with carbon monoxide and conversely, thermodynamics are more favorable with hydrogen than with carbon monoxide above 800°C, (iii) with hydrogen, the hot gas fed has to bring enough calories to heat and maintain the solid at a temperature sufficiently high for the reaction to occur, (iv) kinetics are also faster with hydrogen [11]. Figure 2a and b show SEM micrographs of sponge iron reduced by solid + gas reductants. Hydrogen accompanies coal for the reduction mechanism and causes a high degree of metallization. EDS analysis given in Figure 2c indicates that the content of iron is higher and the amount of carbon is lower than the sponge iron reduced by coal only. 3.2. Effect of reduction time and ratio of Cfix / Fetotal Reaction kinetics in iron ore reduction deal with the rate at which iron oxides are converted to metallic iron by the removal of oxygen. The rate of a chemical reaction increase with increase in temperature. For this reason the reaction kinetics are not generally a matter of great importance in the blast furnace because of the high temperatures at which the furnace is operated. On the other hand, in DR processes where the iron is reduced in the solid state, the maximum temperature is below the melting temperature and the reaction rates are slower. For direct reduction of iron ore, the mechanisms are complex because the oxide must go through a series of step wise changes before the conversion is complete. The slowest step in the process determines the overall reaction rate and is referred to as the rate controlling step [12]. Aguilar et al. reported the reduction process for a given spherical pellet material and they illustrated a schema explaining the reduction from outside to core. The reduction of hematite to metallic iron is carried out by the following reactions (equation 9, 10 and 11) [13]. 3Fe2O3 + CO = 2Fe3O4 + CO2 (equation 9) Fe3O4 + CO = 3FeO + CO2 (equation 10) FeO + CO = Fe + CO2 (equation 11) Figure 3 shows the the effects of reduction time and ratio of Cfix / Fetotal on %-reduction. As it is seen clearly, as reduction time increases %-reduction increases. The reduction of the iron oxides takes place in a series of sequential steps. The overall rate will be determined by the slowest process in the series. Baliarsingh et al. summarized the possible consecutive steps which are (i) transport of gaseous reductant from the bulk gas phase to the particle surface through a boundary gas film, (ii) molecular diffusion of the gaseous reductant through the product layer to the reaction interface, (iii) adsorption of the gaseous reductant at the interface, (iv) reaction at the interface, (v) desorption of the gaseous products from the interface, (vi) mass transport of iron and oxygen ions and transformations in the solid phase, formation and growth of reaction products e.g magnetite, wustite, iron, (vii) molecular diffusion of gaseous products through the product layer to the particle surface, (viii) transport of the gaseous products from the particle surface through the boundary gas film to the bulk gas phase [12]. All these mechanisms require adequate reaction time and this explains why the metallization, in other words, %-reduction increases as a function of time. On the other hand, carbon and its solid/gas compounds are the driving force for all mechanisms. An increase in the ratio of Cfix / Fetotal directly affects the metallization in the positive direction and as a result a linear relationship between the ratio of Cfix / Fetotal and %-reduction is obtained. Figure 3 shows that as the ratio of Cfix / Fetotal increases %-reduction directly increases.
(a) (c) 425 Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A. Figure 1. (a) and (b) SEM micrographs of solidified sponge iron after reduction by solid reductant (coal), (c) EDS analysis of matrix illustrated in (b). (a) (c) Figure 2. (a) and (b) SEM micrographs of solidified sponge iron after reduction by solid + gas reductants (coal+H2), (c) EDS analysis of matrix illustrated in (b). (b) (b)
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