Direct reduction of iron ore in a rotary kiln
Brazilian hematite iron ore was reduced successfully in a rotary kiln with a gaseous reactant which was prepared by reforming butane within the kiln, using the beneficial effect of iron/iron oxide as an autocatalyst. The optimum ratio of the gaseous reactants, H2/CO2/C4H10, was used to prevent undesirable soot formation. The effects of variation in temperature, gas composition, residence time, ore particle size and particle volume were examined. The variation of each of these, or a function thereof, on the percentage reduction or function of it showed a straight line relationship, from which the percentage of reduction can be determined for any chosen value of that variable, when all the other variables were held constant. Also the effect of reducing capacity (flow rate of gas) and the oxidant in the input gas were examined. When all these operating variables were held constant, the usage of the kiln without chokes in position gave less carbon deposition on the ore, less poisoning of the autocatalytic effect and also a higher extent of reduction than when using the kiln with chokes. For the particle size range (1.7 - 4.75 mm) examined a mode of uniform internal reduction was predominant up to the temperature at which surface sintering became significant and above that temperature a limiting mixed control checked the extent of reduction. These observations were in accord with microstructural examination of the sponge iron, porosimeter measurements and the results of percentage reduction obtained at different temperatures. Sulphur and phosphorus were partially removed in gaseous form from the ore. Within the temperature range examined, sulphur removal increased with increase in temperature, whereas phosphorus removal was favoured at lower temperatures. The carbon content of the sponge iron was not in the form of pearlite or cementite. From the microprobe observations it was concluded that the carbon was in the form of soot in the pores of the sponge. A precise study of the progress of reduction from ore to a desirable sponge iron for the different stages of reduction was made including the removal of sulphur and phosphorus and carbon deposition, which was in agreement with the above observations.