An unusually thermally stable magnetite from a niobium mine in Brazil

Magnetite-rich waste from a niobium mine near Araxa, State of Minas Gerais, Brazil, was heated to 500 degrees C and 1000 degrees C under an O-2 atmosphere. The original waste and its oxidized products were characterized by energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), room-temperature Fe-57 Mossbauer spectroscopy, thermogravimetric analysis (TO) and temperature-programmed reduction (TPR). Semiquantitative analyses by EDS and quantitative chemical analyses showed the waste to be constituted primarily of Fe with minor amounts of Ti, Ba, Al, Si, Nb, Mn, S and P. Mossbauer and XRD showed the waste to consist predominantly of magnetite and hematite. The magnetite content decreases when the temperature increases due to its direct conversion to hematite. However, at 500 degrees C only 10 wt.% of original magnetite was converted to hematite, confirming the high stability of this magnetite, which could still be detected at 1000 degrees C. The TO profile shows no significant weight gain on heating, indicating a high stability of the magnetite. The TP12 profiles show that the hematite in the waste is sintered after treatment at 1000 degrees C and the reduction peaks are consequently shifted to higher temperatures. This high thermal stability is attributed to a moderate isomorphous replacement of Fe by other cations present in the Nb mining waste.