| Kinetics and mechanism of direct reduction of sintered ferric oxide and of calcium ferrites containing various amount of lime placed in carbon powders at the temperature range of 800℃ to 1,200℃ were studied. The findings from the experimental work are summarized as follow: (1) The reaction of the sintered ferric oxide pellet was clearly controlled by chemical reaction at the metal-oxide interface and the reduction rates followed the [1-(1-R)^⅓] relationship. (2) The reduction rates of calcium ferrites except for dicalcium ferrite did not follow the above relationship and a general form [1-(1-R)^((1-n))] for the rate calculation of the calcium ferrites was introduced from the experimental data. (3) Dicalcium ferrite has an accelerating or inhibiting effect on the reduction of calcium ferrites. An empirical equation was shown to describe the experimental data. (4) The reaction at the iron-ferrite interface was not similar for the calcium ferrites. An adaptable reaction model was proposed to explain the calcium ferrites reduction. 1. Direct reduction of sintered ferric oxide pellets with carbon was characterized by reduction rates that were essentially independent of time during reduction over the temperature range of 900℃ to 1100℃. The rates followed the [1-(1-R)^⅓] relationship which developed by McKewan. 2. The reduction rates of sintered calcium ferrite pellets with various lime additions did not follow the [1-(1-R)^⅓] relationship with the exception of dicalcium ferrite (2CaO·Fe₂O₃). A general form adaptable for the rate calculations [1-(1-R)^((1-n))], was introduced from the experimental data. 3. At the temperature range of 900℃ to 1100℃ in carbon the reduction rate of dicalcium ferrite is nearly one-third that of ferric oxide, a half that of mono-calcium ferrite, and nearly one-fourth that of calcium diferrite. 4. Reduction rates for calcium ferrites are not a linear combination of those for each calcium ferrite, since dicalcium ferrite has an accelerating or inhibiting effect on the reduction rate of calcium ferrites. The accelerating effect increases with increasing lime content up to the lime percent identical with CaO·2Fe₂O₃ and decreases rapidly with increasing lime content up to about 30 percent. Inhibiting effect was occurred in the calcium ferrites containing lime more than 30 percent. 5. An empirical equation, K=[K₁N_(Fe2O3)/(1+K₃N_(2CaO·Fe2O3)+K₂N_(2CaO·Fe2O3), that described the experimental data was shown to be equivalent in form to a derived equation based on the theory of activated adsorption. 6. The rate-controlling reduction reaction, that is, reaction at the iron-ferrite interface, is not similar for the calcium ferrites. Two macographs and a concentric striped reaction model were proposed to verify the fact and to give interpretation for the mechanism of calcium ferrites reduction. |
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