Reduction of Iron Oxides by Carbon and Water Vapor

Abstract

The complete reduction of iron oxide on heating the initial Fe₃O₄–H₂O–C system with isothermal holding is subjected to thermodynamic analysis. (Note that the initial system contains e_o moles of Fe₃O₄ and b_o moles of H₂O, with excess carbon.) The processes in the system may be divided into four stages. First, the gasification of carbon by water vapor at temperatures below 880 K activates the reaction of the water gas and the dissociation of CO with the formation of lamp black (solid carbon). The composition of the H₂–H₂O–CO–CO₂ gas mixture obtained depends only on the temperature. The carbon consumption at 880 K is ~0.445 mole per mole of water. The second stage, in the range 880–917 K, is the reduction of Fe₃O₄ to wustite FeO_1+x with different degrees of oxidation. Hydrogen reduces the oxide at temperatures above 888 K. The proportion of the oxide reduced by hydrogen in this temperature range increases from zero to ~63%. The total quantity of Fe₃O₄ reduced to wustite at 917 K is ~123 moles per mole of water. This is only possible with repeated regeneration of the reducing agents CO and H₂ as a result of the gasification of carbon by water vapor and carbon dioxide CO₂. The carbon consumption is approximately 78 moles. In the third stage, the wustite FeO_1.092 is reduced solely by carbon monoxide CO in the range 917–955°C to wustite FeO_1.054, with a lower degree of oxidation. The carbon is only gasified by CO₂; the carbon consumption is around 18 moles. In the fourth stage, with isothermal holding at ~955 K, the wustite is reduced to iron by carbon monoxide alone. The carbon consumption is around 257 moles. In a closed system at 1 atm, 1 mole of water is sufficient for the complete reduction of around 123 moles of Fe₃O₄ in a mixture with excess carbon. The total carbon consumption is ~353 moles, with the production of 368 moles of Fe; that is, the carbon consumption is ~0.21 kg/kg of iron.