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

Abstract

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.