Investigation of the structural and mineralogical changes of Tunisian phosphorite during calcinations

The thermal behavior of Tunisian phosphorite was investigated with X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), and Raman spectroscopies and DTA-TG measurements. The XRD patterns showed that the chief mineral constituents of calcined samples are calcium oxide and fluorapatite, while those in the raw phosphorite are calcite, dolomite, fluorapatite, and carbonate-fluorapatite. Physicochemical transformations result in the oxidation of organic matter, disappearance of calcite and dolomite crystalline phases, and partial dissociation of structural carbonates. The FT-IR and Raman spectra showed modifications of some bands; a decrease in the intensity of the v 2 and v 3 vibrations of carbonate groups and the appearance of new bands at 520 and 926 cm −1 after calcination of phosphorites at 800 °C. These bands were assigned to isomorphous substitutions of PO 4 3− in apatite by SiO 4 4− . Heat treatment alters the qualitative composition of the mineral as a result; the solubility of apatite in dilute citric acid was decreased.