Effect of different carbon sources on the conversion of ilmenite into synthetic rutile via ball milling induced carbothermic reduction

Abstract

Carbothermic reduction reduces ilmenite into TiO2 or acid-soluble intermediates. This method gained more attention due to several industrial applications of TiO2 and metal Ti. However, the efficiency of carbothermic reduction of ilmenite into synthetic rutile is poorly investigated for different carbon sources. In this study, several advanced characterization techniques were used to examine the effect of carbon on the carbothermic reduction of ilmenite via ball milling and isothermal annealing. Fourier transform infrared spectroscopic (FTIR) analysis showed new broad bands appearing in ball milled mixtures (ranging from ∼650 cm−1 to 1200 cm−1), due to temporary bonding of Ti–O–C overlapped with Ti–O–Ti bonding. Scanning electron microscopic (SEM) analysis also suggested that carbon was incorporated in the crystal lattice of ilmenite during ball milling. However, no new phases were observed in the X-ray diffraction (XRD) patterns of ball milled ilmenite-carbon mixtures. Therefore, the energy transferred during ball milling is not sufficient to overcome the activation energy of carbothermic reduction of ilmenite. Phase transformation of ilmenite occurred after annealing at 1000 °C by forming new phases such as rutile, pseudorutile, and pseudobrookite. Intermediate phases such as pseudorutile and pseudobrookite decreased with the use of carbon. Consequently, the efficiency of carbothermic reduction decreased in the order of coconut shells (amorphous)> activated carbon> vein graphite (crystalline). The absence of temporary bonding of Ti–O–C in annealed samples further confirmed the phase transformation of ilmenite. This study also showed that ball milled, and annealed samples resulted in highly crystalline rutile. Thermodynamic behavior of carbothermic reduction was finally simulated using the calculated enthalpy, entropy, and the Gibb’s free energy for each reaction.