Thermal behaviour of iron(ii) oxalate dihydrate in the atmosphere of its conversion gases

Using a device for thermogravimetric analysis, a dynamic study of thermally induced solid-state transformations of FeC2O4·2H2O in the atmosphere allowing full participation of gaseous products (CO, CO2, H2O) in the reaction system was carried out. Solid phases formed at various temperatures between 25 and 640 °C were identified and characterized using 57Fe Mössbauer spectroscopy, TG and XRD. Up to 230 °C, evolution of two molecules of the water of crystallization takes place. Superparamagnetic nanoparticles of magnetite (Fe3O4) are formed as the primary product of the decomposition of FeC2O4, together with gaseous CO and CO2. In the next stage above 380 °C, the crystallization of magnetite is accompanied by a reduction of the remaining ferrous oxalate to iron carbide (Fe3C) by carbon monoxide. Thermally induced conversion of iron carbide into α-Fe and carbon is expected between 400 and 535 °C as the major chemical process. In the last reaction step, above 535 °C, magnetite is reduced to wüstite (FeO) by carbon monoxide evolved at lower temperatures. On the grounds of quantitative Mössbauer data possible competitive reactions are discussed and a temperature dependent reaction model is suggested.