Displacement and Migration Behavior of Al³⁺ in Ca₂Fe_2−xAl_xO₅ Solid Solution During Reduction Process

Ca₂Fe_2−xAl_xO₅ solid solution is an intermediate product for reduction of complex calcium ferrites, which is a restrictive step for efficient metallurgy of high-aluminum iron ore for ironmaking. In this work, three Ca₂Fe_2−xAl_xO₅ (x = 0.00, 0.21, 0.41) solid solutions were synthesized to investigate the reduction mechanism of Ca₂Fe_2−xAl_xO₅ solid solution. Combined with the Rietveld refinement method, it was confirmed Ca₂Fe_2−xAl_xO₅ is a substitutional solid solution, where the Al³⁺ could replace the position of Fe³⁺ to lower the cell size with orthorhombic crystal. The reduction result of Ca₂Fe_2−xAl_xO₅ shows that the solid solution of Al³⁺ in Ca₂Fe_2−xAl_xO₅ increased the structure stability, in which the reduction-beginning temperature increased from 820 °C to 1020 °C when x increased to 0.41, which was also verified by first principle calculation. It was found the reduction products of Ca₂Fe_2−xAl_xO₅ were iron, CaO, and Ca₃Al₂O₆, where the CaO belongs to a solid solution with the lower melting-temperature, resulting in decrease of the reduction rate at the higher temperature. Furthermore, it was revealed that the Al³⁺ has been gradually dissolved into Ca₂Fe_2−xAl_xO₅ remained with the reduction in progress at the earlier stage, but the Al³⁺ separates from Ca₂Fe_2−xAl_xO₅ and reacts with CaO to generate Ca₃Al₂O₆ at the later stage of reduction due to the Al³⁺ amount exceeding the solid solution limit (x = 0.63 at 1050 °C).