A study of gaseous reduction of calcium ferrites

Abstract

The reduction behaviour of pure, dense binary calcium ferrites, and calcium ferrites containing selective Si4+ and Al3+ additions, has been examined. Reduction has been carried out in hydrogen, carbon monoxide and CO/CO2 gas mixtures at 500, 800 and 1100°C. Phase equilibria in the Ca-Fe-O ternary system have been determined for Fe/Ca ratios of 1, 2 and 4 using the results of reduction equilibria of binary calcium ferrites. Ternary compound C7.2W0.8F15 is a stable phase in the system. Other phases are similar to those previously reported by other workers. Dicalcium ferrite is reduced to calcium oxide and metallic iron in single reaction step below 1070°C whereas mono calcium ferrite and calcium diferrite are reduced in a number of reaction steps. The reduction reaction sequences of mono calcium ferrite and calcium diferrite determined in the present investigation, however, differ from those previously reported. A number of previously unreported product morphologies have been observed. Present investigations have revealed that the product morphologies and reduction reaction kinetics are dependent on the rates of chemical reactions, rates of calcium and iron ion diffusion (surface and bulk), oxygen potential of the reducing gas and inherent structural and surface defects of the samples. Reducibilities of binary calcium ferrites have been determined. A general theory of reduction mechanism of binary calcium ferrites, consistent with the present observations, is proposed.