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INTRODUCTION

TITANIUM-BEARING iron ores are distributed worldwide and represent an important mineral resource for the extraction of both iron and titanium.[1–3] The ilmenite (FeTiO3)-hematite (Fe2O3) solid solutions in these ores increase the difficulty of pyrometallurgical extraction, as the process becomes time-consuming and requires high temperatures and large reducing potentials. While the hematite-ilmenite solid solution is one of the minor phases in ilmenite, it is a major phase in ironsand, which is a titanomagnetite species. Although the hematite-ilmenite (titanohematite) solid solution is a minor phase, it becomes predominant upon long-term weathering of titanomagnetite (ironsand) and hence is an important topic of research. Most studies regarding the reduction process have used titanium-bearing iron ores, rather than ilmenite-hematite solid solutions. The studies using ilmenite-hematite solid solutions largely focused on mineralogy and magnetism. Accordingly, elucidation of the factors that control the phase stability and transformation in the carbothermic reduction of xFeTiO3Æ(1  x)Fe2O3 solid solution is critical to the efficient utilization of titanium-containing iron ores.

YIRAN LIU is with the School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. JIANLIANG ZHANG, ZHENGJIAN LIU, XINGLE LIU, and NAIYAO LI are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China. Contact email: [email protected] XIANGDONG XING is with the School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an, China. YANSONG SHEN is with the School of Chemical Engineering, The University of New South Wales. Manuscript submitted October 18, 2016.

METALLURGICAL AND MATERIALS TRANSACTIONS B

Mineralogical studies on hematite-ilmenite solid solutions[4] revealed that the xFeTiO3Æ(1  x)Fe2O3 solid solution could be present over the complete range from hematite to ilmenite at temperatures higher than 1323 K (1050 C). A systematic investigation of the solid solution of hematite and ilmenite, xFeTiO3Æ(1  x) Fe2O3, was first carried out by Pouillard.[5] It was found that hematite and ilmenite form solid solutions only over a limited composition range (0 < x < 0.33 and 0.66 < x < 1), when the specimens are prepared by sintering at 1223 K (950 C) for 2 h. Nicholls[6] proposed a phase diagram of hematite and ilmenite. Akimoto et al. found that when the specimens were quenched from a sintering temperature above 1373 K (1100 C), it is possible to prepare solid solutions covering the entire range of composition (0 £ x £ 1).[7,8] Carbothermic reduction based on natural hematite and ilmenite,[9–13] the two end members of the solid solutions, has been investigated extensively. However, xFeTiO3Æ(1  x)Fe2O3 solid solutions have not been studied comprehensively in the past. Hence, this paper summarizes the general carbothermic reduction behavior using synthesized samples of xFeTiO3Æ(1  x)Fe