Thermal Decomposition of Coprecipitated Fe-Al Hydroxides

Abstract

Thermal decomposition of coprecipitated Fe-Al hydroxides has been studied with reference to precipitating conditions by means of X-ray diffraction, magnetic analyses, DTA and TGA. Is is shown that an increase in Al content restrains the crystallization of the amorphous phase to hematite or corundum. The decomposition behavior depends on the degree of dispersion of Fe and Al ions. Thus, well-dispersed coprecipitates yielded an intermediate phase X on heating, whose structure has not been known so far. Poor dispersion of Fe and Al ions, on the other hand, resulted in the occurrence of r-Al₂O₃, as an intermediate phase after crystallization, finally leading to the corundum phase. Although a variety of precipitants were found to exert little influence on the dispersion of Fe and Al ions, both oxidation of (Fe³⁺-Al³⁺) precipitates and ageing of (Fe²⁺-Al³⁺) precipitates were found to promote the segregation of Fe and Al ions. Discussion on the formation of the unknown phase X has been made.