DIRECT REDUCTION OF FERROUS OXIDES TO FORM AN IRON ...

3. Results and Discussions
3.1. Microstructural characterization of reduced mill scale
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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.