Reduction kinetics of iron Ore pellets and the effect of binders

Abstract

Reduction of Iron Ore Pellets was carried out for the temperature range 900 to 1100°C. In reduction kinetic study the most satisfactory model was to take the slope of the initial linear region of fractional reduction vs. time curve as a measure of rate constant (k). In k vs. 1/T plots were straight line from which Activation Energy was calculated. Pellets having different percentages of binders were reduced and compared to find the effect of binders.
Introduction
Iron ore in a finely ground state is not easily transported or readily processed. Thus it is necessary to agglomerate the fine ground ore into pellet using binders. Inorganic binders introduce silica which decreases the final ore content of the pellet. Hence organic binders were developed. Use of pellets increases the productivity in blast furnace and reduces coke consumption. Iron ore from M.G.Mohanty mines and charcoal were used in the experiment. Analysis of the iron ore showed that it contained 63.7%Fe, 1.6%SiO2, and 1.4% Al2O3. The reaction carried out in the experiment is mainly of direct reduction because here we have used charcoal. The iron ore was crushed , ground and screened to 100# size and charcoal to 72# size. Pellets were made by hand rolling method by using water and different quantities of binder. The pellets were fired at different firing temperature. Each crucible containing iron ore pellets were placed in a furnace and reduction was carried out at different temperature from 900-1100°C with time intervals of 5,10, 15………. minutes. Six crucibles containing pellets of different % binder(0.5%,1%,2%)were taken & reduction was carried out at a constant temperature of 950°C and pellets were taken out at a time interval of 10,20,30,40,50,and 60 minutes. The product obtained after reduction was then taken for the study and analysis of reduction behaviour.
Analysis
Percentage reduction is found as R = Initial oxygen content – Final oxygen content × 100% Total oxygen content initially
With the help of the Arrhenius equation K = A e -E/RT, we can calculate the activation energy. Where, K = Rate Constant, A= Arrehenius Constant, E= Activation Energy, R= Gas Constant, T= Temperature. We plotted the graph of ln (K) vs. (1/Tx10 for pellets with binder, with bentonite and with dextrin. Slope of this graph x (universal gas constant) = activation energy (E)
Results
With increase in temperature, the percentage reduction increases with increase in time.
The percentage reduction in case of pellet with Dextrin was found more. This may be because of the lower activation energy of the pellets with dextrin as a binder.
A number of Models were considered out of which the model 1-(1-R) 1/3 =kT exactly fits to our experimental values.
Lower the binder percentage, greater the reducibility (.5% >1%>2%). This is due to a
decrease in the porosity with increasing binder percentage.
The activation energy of a pellet with 0.5% bentonite binder is Ea= 20.701 Kcal/mole and the activation energy for pellet with dextrin binder is Ea=17.423 Kcal/mole. So the rate of reaction in case of pellets with Dextrin is faster than the rate of reaction of pellets with Bentonite.
Conclusions
1. Topochemical reaction phenomenon was observed.
2. With the increase of time, percentage reduction (O2 removal) increases in all the pellet samples.
3. With the increase of temperature, percentage reduction increases up to a certain extent.
4. Almost a mixed controlled reaction was obtained as the activation energy is in between 10 kcals/mole and 30 kcals/mole.
5. Reduction reaction is temperature dependant.
6. Lower the binder percentage, greater the reducibility (.5% >1%>2%).
References
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3. Ripke S.J., Kawatra S. K. “Can fly-ash extend Bentonite binder for iron ore agglomeration” Int. J. of Mineral Process. 60 (2000) 181-198
4. Dutta S.K., Ghosh A., “Kinetics of gaseous reduction of Iron ore fines” ISIJ Intenational, Vol.33 (1993), No-11, 1168-1173