Insight into the Consolidation Mechanism of Oxidized Pellets Made from the Mixture of Magnetite and Chromite Concentrate
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TO produce more competitive stainless steel products, the blast furnace smelting process is considered to be a promising way to produce cost-effective stainless steel master alloys containing 5 to 21.3 pct chromium content, which helps lay a foundation for using the low-cost chromite concentrates.[1] High-quality fired pellets made from mixtures of chromite and magnetite concentrates can be manufactured by using high-pressure grinding rollers (HPGR) to pretreat pellet feed before balling.[2] However, the consolidation mechanism of this type of pellets has not been well understood. Investigating the consolidation mechanism of fired pellets may help gain a better understanding of interparticle reactions and has a potential to improve the efficiency of the pelletization processes. As with the fired
DEQING ZHU, Professor, CONGCONG YANG, BENJING SHI, and FENG ZHANG, Ph.D Candidates, JIAN PAN, Associate Professor, and QIANG ZHANG, Graduate Student, are with the School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 10, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
pellet made from the magnetite or hematite concentrates, its hardening is mostly attributed to the recrystallization and the growth of the hematite crystals, while some slag phases are of minor importance to the hardening.[3–6] This kind of hardening is generally identified as a solid-state consolidation process in which iron-bearing mineral particles are with each other and consolidated into a solid body below their melting temperature. By looking into the microstructures of the fired pellets made from magnetite–hematite mixtures, it was found that the activity of newly formed hematite crystals (oxidized from the magnetite) was much higher than the original particles and the recrystallization process was heavily dependent on the conditions of preheating and roasting, e.g., temperature, time, and oxygen partial pressure.[7,8] It was found that the sintering of magnetite was important only when the defective structure occurred under the induration temperatures.[9] The recrystallization process accompanied with the phase and structural transformation of the defective spinel c-Fe2O3 to the stable rhombohedral a-Fe2O3 was reported to begin at about 1173 K (900 C), but evident crystallization of hematite occurs only when the temperature is above 1473 K (1200 C).[5] This process might be hindered if the magnetitie spinel
lattice was substituted by cations such as Ti4+, Cr3+, Al3+, and Mg2+ because they tended to stabilize the spinel structure and made it more difficult for the oxidation to happen.[10–14] In contrast, the investigations of pure ferrous spinel oxidation[15–17] and natural chromite spinel oxidation[18,19] showed that the oxidizability of chromite spinel was much poorer than that of magnetite spinel on account of their different crystallographic properties, cation distributions, diffusion mechanisms, etc.[20–22] Because of the industrial consider
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