Conversion of crystalline oxy-hydroxides of Fe(III) to magnetite under ammoniacal conditions

M. Mohapatra1, C. Upadhyay2, S. Anand1,*, H.C.Verma2 and R.P.Das1

1 Hydro metallurgy Departments, Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India
2 Physics Departments, Indian Institute of Technology, Kanpur 208016, UP, India

Accepted 28 March 2005


In an earlier work, complete conversion of pure crystalline goethite to magnetite was reported while maintaining the reaction parameters as: NH3 25g/l, Fe(II) 45g/l, reaction temperature 130oC and time 120 minutes. In the present work some of the reaction parameters were varied to monitor the formation of magnetite. The reaction time was varied between 5 to 120 minutes. Samples were also prepared at different temperatures to observe the initiation of conversion. It was observed that the formation of magnetite starts at 90oC and increases as the temperature is increased up to 130oC. The XRD patterns suggested formation of only two crystalline phases, i.e., unreacted goethite and magnetite. No other intermediate compound formation was observed even when the Fe(II) in solids was as low as 0.55% indicating very low magnetite content in the product. The Mössbauer spectra of some of the samples confirmed these observations. Some typical microstructures obtained at different time intervals revealed that the formation of magnetite progresses through the edges of acicular type goethite. The conversion seems to follow adsorption of FeOH+ on the surface of goethite followed by release of a water molecule and a proton, though it could not be conclusively established. Behavior of other two forms of iron oxy-hydroxides namely akaganeite (b-FeOOH) and lepidocrocite (g- FeOOH) was also studied. It was observed that all the three forms of FeOOH (a-, b- and g- ) got converted to magnetite.

Keywords: Goethite, Magnetite, Ferrous sulphate, XRD, Mössbauer Spectroscopy

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