Investigation of the structural and mineralogical changes of Tunisian phosphorite during calcinations S. Elgharbi • K. Horchani-Naifer • M. Fe ´rid Received: 2 April 2014 / Accepted: 25 August 2014 Ó Akade ´miai Kiado ´, Budapest, Hungary 2014 Abstract The thermal behavior of Tunisian phosphorite was investigated with X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), and Raman spectros- copies 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 disso- ciation 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. Keywords Tunisian phosphorite  Calcinations  Fluorapatite  Calcite Introduction Most of the commercial phosphate rocks are of sedimen- tary origin and are mainly used in the manufacture of phosphate fertilizers. The composition of these rocks varies from one deposit to another. These rocks are composed essentially of the apatite group in association with a wide assortment of accessory minerals mainly fluorides, car- bonates (calcite and/or dolomite), clays, quartz, silicates, and metal oxides as well as organic matter. Most of Tunisian phosphorites are considered to be carbonate-apatite or francolite with association of wide impurities such as calcite, dolomite, and quartz [1, 2]. It is more beneficial to remove the carbonate minerals because it consumes an additional acidulant (sulfuric acid) during the production of phosphoric acid by wet process. Also, if considerable amount of organic matter is present in the gangue material, it is necessary to eliminate this matter because it causes the reduction of sulfates in sulfide thus involving an intense corrosion of reactor during the man- ufacture of phosphoric acid [3]. Calcination was found to be one of the suitable methods to eliminate water and organic matter, to decompose all or part of carbonates [4]. By heating the ore to a high tem- perature ranging from 800 to 1000 °C, CaCO 3 and MgCO 3 are decomposed to CaO, MgO, and CO 2 . The upgrading by heat treatment has been considered by many authors [5, 6]. Calcination of phosphate ores is necessary for obtaining a phosphate product suitable for the production of light green phosphoric acid, which is in turn suitable for the production of pure, edible, super phosphoric acid. This last product is used in the preparation of pure chemical reagents, food, livestock, soft drinks, and other pharma- ceutical preparations. In a previous study, thermal treatment of mixtures of Tunisian phosphorite and additives of aluminum silicates was investigated [7]. The results show that the P 2 O 5 sol- ubility in various extracting agents in the case of the mixture of calcined and mechanically activated Tunisian phosphorite with Medet’s steryl (Aluminum silicate waste) S. Elgharbi (&)  K. Horchani-Naifer  M. Fe ´rid Laboratory of Physical Chemistry of Mineral Materials and their Applications, National Research Center in Materials Sciences, Technopole Borj Cedria, B.P. 73, 8027 Soliman, Tunisia e-mail: elgharbisarra@gmail.com 123 J Therm Anal Calorim DOI 10.1007/s10973-014-4132-5