ZHANG Xun1, ZHOU Yue-fei1, XIE Qiao-qing1, QING Cheng-song1, CHEN Tian-hu1
(Laboratory of Nanominerals and Environmental Materials, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China)
Abstract: This study designed a two-staged simulating experiment to investigate the role of natural combustion in the formation of maghemite in soil. In the first stage, goethite was calcined at 300 ℃ in hydrogen atmosphere for 2 hours. In the second stage, the calcined product was oxidized at 70 ℃ in the air for 70 days. Mineralogical and magnetic properties of the samples were determined during and after the experiments. Results show that goethite was transformed into pure, porous and nano-sized magnetite under the given calcination conditions. The sizes of magnetite grains and aggregates are about 37 nm and 57 nm, respectively. The nanomagnetite quickly transformed to maghemite. After being oxidized for 70 days, the sample changed the ratio of ferrous Fe to total Fe (Fe2+/TFe) from the initial 31.4% to 5.4%. The process of magnetite-maghemite transformation was accompanied by the decrease of the mineral crystal size (ca. 17% ~ 19%) and the increase of mass volume. Both the magnetic susceptibility and frequency magnetic susceptibility kept decreasing during the second stage. The decrease of magnetic susceptibility was possibly related to mineral phase transformation, while the decrease of frequency magnetic susceptibility was related to the change of mineral particle size. This study provides experimental supports to the two-staged model of maghemite formation under natural combustion. The study further shows that controlling calcination conditions to generate nano-sized magnetite is the key step in the first stage, while that for the second stage is a suitable transformation temperature.
Keywords: Natural combustion; Magnetite; Maghemite; Calcination; Oxidation; Transformation
Email: zhangxunfighting@126.com
ACTA MINERALOGICA SINICA Vol. 35, No. 3, 2015, Page 344-350
