Effect of temperatures and residence time of calcinations on substitution process during enrichment of Tunisian phosphorites

The present study deals with the effects of temperature and residence time of calcinations on substitution process. The upgrading of sedimentary phosphorites coming from the Ras-Dhraa deposit which belongs to the Gafsa-Metlaoui basin (Tunisia) was conducted. The characterization of raw and calcined phosphorites and the evolution of rare earth elements and yttrium (REEs+Y) content and associated gangue were evaluated with different techniques (X-ray powder diffraction (XRD), scanning electron microscopy (SEM/EDX), Fourier transform infrared (FT-IR) and inductively coupled plasma mass spectrometry (ICP-MS)). The effect of increasing residence time and temperature of calcinations on phosphate structure were determined. FT-IR spectroscopy indicates that PO 4 3- was substituted by SiO 4 4- and SO 4 2- and these substitutions cause a worsening of phosphorites properties for sulfuric acid production. A temperature of calcination approximately of 900°C and a period of 30 minutes is sufficient, because higher temperature and longer residence times accentuate these substitutions. Indeed, in these conditions we obtain a phosphate with a better grade of P 2 O 5 . It permits to increase the phosphate content from 29.58% to 33.07%. Moreover, REEs+Y contents increased by 5%.