Processing of ilmenite into synthetic rutile using ball milling induced sulphurisation and carbothermic reduction

Abstract

Sri Lanka is a major global producer of raw material-heavy minerals from beach placer deposits. This study focusses on upgrading Sri Lankan ilmenite sands to synthetic rutile via a mechanochemical method. Ball milling-induced sulphurisation and carbothermic reductions were examined using several geochemical techniques. Raw ilmenite concentrate contained over 95 wt% FeTiO3, ∼2 wt% SiO2, and ∼ 1 wt% Al2O3. Ilmenite was mixed with additives (i.e. sulphur, vein graphite, and a 1:1 mix of sulphur and vein graphite), separately in three ratios by weight, namely 1:1, 2:1 and 4:1, respectively. Each mixture was milled for 4 and 6 h. A high percentage of ilmenite/sulphur + graphite (ISG) was converted to nanoparticles (average particle size = 124 nm) in samples milled for 6 h. X-ray diffraction (XRD) spectra indicate that the intensities of sulphur and graphite peaks decrease after ball milling. This suggests possible dissolution of sulphur and graphite into the ilmenite structure, with the formation of mixed composite structures. The optimum ilmenite to additives ratio for ilmenite to rutile conversion was found to be 4:1. The occurrence of pyrite and pseudorutile peaks in XRD spectra of milled samples indicates that ilmenite reduction was initiated during ball milling. Fourier-transform infrared (FTIR) spectra of milled samples confirm the octahedral metal-ion stretching vibrations of carbon-incorporated ilmenite structures. The ilmenite-additive mixtures containing both sulphur and graphite (ISG) showed the best results. Consequently, ISG samples (mixing ratio of 4:0.5:0.5, and milled for 6 h) were isothermally annealed at 800 °C, 1000 °C and 1200 °C, respectively. Sharpening of rutile peaks and disappearance of pyrrhotite peaks in the annealed samples indicated the completion of the mechanochemical reactions that were initiated during ball milling. The optimum temperature for ilmenite reduction was determined to be 1000 °C. At that temperature pseudorutile peaks had disappeared from the XRD spectra of the annealed samples, whereas brookite and rutile peaks appeared. The method introduced in this study thus can be utilised to convert ilmenite to synthetic rutile.