Solid state synthesis of Ca2Fe2O5 by reactive firing of calcite and siderite

The brownmillerite phase Ca2Fe2O5 was processed by the reactive firing of calcite and a natural siderite for prospective catalytic applications. Thermogravimetry and X-ray diffraction (XRD) was used to monitor the decomposition of precursors and its dependence on heating rate and atmosphere. Taguchi planning was used to assess the impact of firing temperature, time, and rate of heating on the conversion of precursors reflected by I121sp:I121b peak intensity ratio of main XRD reflections of secondary spinel phase CaFe2O4 and brownmillerite. The onset of additional minor phases (Ca2Al2SiO7 and Ca2SiO4) was ascribed to the reaction of a fraction of calcia with gangue components of the siderite precursor. Thermodynamic modelling was used as a guideline for synthesis in non-oxidising conditions to hinder the onset of CaFe2O4. These guidelines allowed one to obtain high conversion to Ca2Fe2O5 with enhanced crystallinity at 700 °C, in an inert Ar atmosphere. Faster reactivity at T ≥ 800 °C in this oxygen-lean atmosphere, leads to highly crystalline Ca2Fe2O5 coexisting with the onset of wustite and/or metallic Fe, which account for the oxygen balance, and calcium silicates, by reaction of calcia with silica.