Comparison of the Oxidation Behaviors of High FeO Chromite and Magnetite Concentrates Relevant to the Induration of Ferr
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WITH the increasing depletion of high grade chromite ores, especially for ‘‘lumpy ores,’’ pioneering endeavor that aims at reducing the production cost and promoting the competiveness of stainless steel products in the market now is carried out by utilizing cost-effective chromite fines or concentrates with low Cr2O3/FeO ratio.[1,2] In an earlier study, pelletization of high FeO South African chromite concentrates with coexistence of domestic magnetite concentrates for a novel blast furnace (BF) smelting process was investigated and the consolidation mechanism of oxidized pellets was revealed.[3,4] It was found that the non-fluxed chromite–magnetite composite pellets were predominantly solid-state bonded relying on the recrystallization of hematite and formation of solid solutions in adjacent areas of both chromite–magnetite and chromite–chromite particles, which are largely affected by the oxidizability of chromite and magnetite spinels. However, the oxidation behaviors of high FeO chromite concentrates
DEQING ZHU, Professor, CONGCONG YANG, Ph.D. Student, JIAN PAN, Associate Professor, and XIAOBO LI, Graduate Student, are with the School of Minerals Processing and Bioengineering, Central South University, Changsha, 410083, P.R. China. Contact e-mails: [email protected], [email protected] Manuscript submitted December 20, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
in relation to the hardening of pellets have not been well investigated. It has been stressed that oxidation of magnetite is extremely important to the induration of pellets only when a defective structure occurs and then undergoes phase transformation at higher temperatures, resulting in more efficient consolidation.[5–7] Generally, oxidation of both magnetite and chromite spinels to sesquioxides in air follows a two-stage process.[8–11] It begins with the oxidation of ferrous spinels to an intermediate defect phase (known as the gamma phase with a spinel structure, in which a certain amount of vacancies locate on the octahedral and tetrahedral sites) at low temperature and then transforms to stable alpha phase with a corundum structure when temperature increases to a higher level. However, with increasing substitution of cations like Cr3+, Mg2+, Al3+, and Ti4+ for Fe2+, Fe3+ ions in magnetite crystal structure, ideally as pure ferrous spinels or natural complex chromite spinels (Fe2+, Mg2+)[Cr3+, Fe3+, Al3+, Ti4+]2O4, the spinel structure tends to be stabilized and moreover, both oxidation of spinel phase to metastable c-phase and its transition to stable a-phase are found to shift to higher temperatures.[4,12–18] The discrepancy of oxidation behaviors will inevitably affect the hardening of oxidized pellets. In this work, particular attention was fixed on the comparison of oxidation behaviors of high FeO chromite and magnetite concentrates in relation with the hardening of their pellets. A set of methods, for example differential scanning calorimetry (DSC),
thermogravimetric analysis (TG), X-ray diffraction (XRD), thermodynamic calcu
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