Precipitation of α-Fe2O3 from dense β-FeOOH suspensions with added ammonium amidosulfonate

Concentrated FeCl3 solutions containing starting concentrations 0.1, 0.01 or 0.001 M of ammonium amidosulfonate (AAS) were partially neutralized with concentrated NaOH solution and the dense suspensions so obtained were autoclaved at 160 °C for different times. Fine β-FeOOH particles were formed at the beginning of the precipitation process and they transformed to α-Fe2O3 the through dissolution/recrystallization process. The transformation was monitored by XRD and 57Fe Mössbauer spectroscopy. FT-IR spectra showed the presence of sulfate groups in the precipitates, which was interpreted as a consequence of the decomposition of amidosulfonate groups. For specific experimental conditions in the precipitates small amounts of α-FeOOH were found, as evidenced by FT-IR spectroscopy. The corresponding FE-SEM images confirmed the presence of α-FeOOH whiskers. FE-SEM also demonstrated the importance of the aggregation mechanism in the formation of peanut-type α-Fe2O3 particles, as well as in the formation of interconnected (with the neck) double cupolas. The last α-Fe2O3 particles were also porous and they consisted of linear chains of small α-Fe2O3 particles (also interconnected) which were directed from the center towards the surface of cupola. EDS spectra showed a gradual decrease of sulfur content in the precipitates with a prolonged time of autoclaving. It was suggested that the preferential adsorption of sulfate groups on selected planes was responsible for the elongation of the particles and formation of linear chains of small α-Fe2O3 particles (interconnected).