Reduction equilibria of ternary calcium ferrite with CO-CO₂ gas mixture

The reduction behaviors of CaO-Fe₂O₃ binary calcium ferrite and CaO-Fe₂O₃-SiO₂-Al₂O₃ quaternary calcium ferrite are known to be quite different from each other. To clarify the difference, the reduction sequence and reduction equilibrium with CO-CO₂ gas mixture were investigated by using two kinds of synthesized CaO-Fe₂O₃-Al₂O₃ ternary calcium ferrite. One of the samples had an Al₂O₃ content as high as 16 mass% and a composition of CaO·3(Fe_0.74Al_0.26)₂O₃. The other had a low Al₂O₃ content of 2 mass% and was essentially CaO·Fe₂O₃ containing Al₂O₃ as solid solution. The results are summarized as follows: 1) The reduction behavior of the ternary calcium ferrite with higher Al₂O₃ content was essentially the same as that of the quaternary calcium ferrite. It was reduced to iron stepwise via magnetite and wustite, both containing Al₂O₃ and CaO as solid solution. In the course of reduction, a calcium aluminate, CaO·Al₂O₃, was precipitated. The equilibrium gas composition vs. temperature curve, of each reduction step was almost the same as the corresponding curve for the quaternary calcium ferrite. 2) The reduction behavior of the ternary calcium ferrite with lower Al₂O₃ content was essentially the same as that of the binary calcium ferrite. But the equilibrium CO contents were considerably higher than those of the binary calcium ferrite. 3) From the results, the difference in reduction behavior between the binary and the quaternary calcium ferrites was attributed to the difference in their crystal structures.