Morphological Changes and Reduction Mechanism for the Weak Reduction of the Preoxidized Panzhihua Ilmenite
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I.
INTRODUCTION
UPGRADING ilmenite to produce synthetic rutile, which is the main raw material for production of titania pigment and metallic titanium, is becoming increasingly important.[1] Hydrochloric acid leaching is one of the preponderant approaches to obtain synthetic rutile from ilmenite due to its advantages in the removal of impurities and its ability to recycle hydrochloric acid. Thermal pretreatments were broadly adopted to improve the reactivity of ilmenite toward hydrochloric acid or to prevent the pulverization of leaching products.[2–9] Strong oxidation–weak reduction, where the ferrous ions in ilmenite are oxidized to ferric ions first (the strong oxidation) and then reduced back to the ferrous state (the weak reduction), is one of the most effective pretreatment methods and has wide applicability for nearly all kinds of ilmenite concentrates.[2,5,6,9] A full understanding of the phase and microstructural changes of ilmenite during these thermal pretreatments is important and necessary for obtaining optimal process parameters. A systematic investigation on the ilmenite oxidation process has been reported in previous
JIANBO ZHANG, GENGYU ZHANG, and CHAO LEI, Ph.D. Candidates, are with the State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China, and also with the University of Chinese Academy of Sciences, Beijing 100049, P.R. China. QINGSHAN ZHU, ZHAOHUI XIE, and HONGZHONG LI, Professors, are with the State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted August 13, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
papers.[10,11] Therefore, this article addresses the weak reduction process of the preoxidized ilmenite. Actually, the weak reduction of the preoxidized ilmenite has not been fully clarified yet, although it has been adopted as a pretreatment process for several years. On the one hand, researchers concentrated mainly on its phase transitions and reduction kinetic. Many articles in the literature proposed that the ferric iron of ilmenite was reduced to a ferrous state that presents as ilmenite phase in the weak reduction process.[2,3,5,6,12] This transformation was found to have a relatively high rate above 1073 K (800 C)[13,14]; e.g., Grey et al.[14] found that the reduction completed in ~1 minute in hydrogen at 1123 K (850 C) for ilmenite that had been oxidized at 1223 K (950 C) for 45 minutes. However, few investigations gave detailed information about the morphological changes accompanying the phase transition. On the other hand, the influence of oxidation and reduction temperatures on the weak reduction process has also not been studied. Most of the previous investigations were performed (not only the reduction but also the oxidation) at relatively high temperatures[2,5,6,9]; e.g., in the study of Sinha, ilmenite was oxidized at 1173 K to 1223 K (900 C to 950 C) and then reduced at 1
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