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