Aqueous reduction of crystalline goethite under ammoniacal conditions

Pure crystalline goethite was laboratory-prepared and its reduction under various experimental conditions was studied. Both goethite and the reduced products obtained by the action of ammonium sulphite and/or ferrous sulphate were characterized using several techniques such as XRD, TG-DTA, TEM and Mössbauer spectroscopy and these compounds were also chemically analyzed. During reduction of goethite with ammonium sulphite, parameters such as temperature, ammonium sulphate concentration and addition of Cu(II), Ni(II) and Co(II) ions were studied. No reduction of goethite was observed during this study up to a temperature of 403 K. At this temperature, addition of ferrous sulphate promotes reduction of goethite and formation of magnetite was observed. The presence of ammonium sulphate and/or ammonium sulphite hindered the reduction reaction. Best results for quantitative reduction of goethite to magnetite were obtained with the following conditions: NH3 25 g L−1, Fe(II) 45 g L−1, reduction temperature 403 K and reduction time 2 h.