Stability and calorimetric studies of silico‐ferrites of calcium aluminum and magnesium

In a systematic study on silico-ferrites of calcium aluminum and magnesium (SFCA phases) this investigation focuses on synthesis of silicon-free SFCA-type compounds with low-MgO contents (similar to 1.00 apfuatoms per formula unit). Previous studies revealed the existence of iron-rich SFCA phases similar to the SFCA-I structure with the chemical composition Ca3MgAl6Fe10O28 (Metall Mater Trans B. 2017;48:2207). The experimental results in the quaternary Fe2O3-CaO-Al2O3-MgO system confirm large stability fields of 2 silicon-free ferrites FCAM-I and FCAM-III, which are members of the homologous series M14+6nO20+8n (n = 1, 2). Starting with compositions corresponding to Ca3MgAlxFe16-xO28 (with increasing aluminum content from x = 0-12 apfu), it was possible to synthesize these phases with an x-value 2 apfu, which corresponds to Al2O3 contents 7.14 wt%. Synthesis of pure silicon-free ferrites with n = 1 (FCAM-I) and 2 (FCAM-III) and silicon-bearing ferrites with n = 0 (SFCA) was possible. Samples were characterized by electron probe microanalysis, powder diffraction, and subsequently studied using relaxation calorimetry measurements in combination with differential scanning calorimetry for determination of the heat capacities and standard entropies S degrees(298). The corresponding values are S degrees(298) = 650.3 +/- 4.6 J/molK for SFCA, S degrees(298) = 864.5 +/- 6.1 J/molK for FCAM-I, and S degrees(298) = 1206.2 +/- 8.4 J/molK for FCAM-III. These thermodynamic data are a step toward a rigorous quantitative thermodynamic modeling of the iron ore sintering process.