Effect of Addition of Mill Scale on Sintering of Iron Ores
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INTRODUCTION
Mill scale is a waste product containing wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3), which is formed on the surface of steel as a result of oxidation of the metal that occurs during continuous casting, reheating, and hot-rolling operations.[1] Mill scale is a valuable ferrous raw material, containing 65 to 70 pct iron[2] and is often recycled through the sintering process.[3] During iron ore sintering, iron ore fines are mixed with limestone and coke breeze. The combustion of coke breeze supplies the necessary heat to achieve partial melting and reaction of the raw materials. It also produces a locally, relatively reducing atmosphere
ZHE WANG, formerly Ph.D. Student with the School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia, is now Lecturer with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. DAVID PINSON, Senior Research Engineer, and SHENG CHEW, Principal Research Engineer, are with Steelmaking Technology and Planning, BlueScope, P. O. Box 202, Port Kembla, NSW 2505, Australia. BRIAN J. MONAGHAN, Professor, HAROLD ROGERS, Senior Research Fellow, and GUANGQING ZHANG, Senior Lecturer, are with the School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong. Contact e-mail: [email protected] MARK I. POWNCEBY, Principal Research Scientist, and NATHAN A.S. WEBSTER, Research Scientist, are with CSIRO Mineral Resources, Private Bag 10, Clayton South, VIC 3169, Australia. Manuscript submitted March 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
within the sinter bed during the heating stage. Once peak temperatures are reached, the agglomerated, semimolten material slowly cools under a relatively oxidizing atmosphere.[4,5] The final sinter predominantly consists of primary and secondary hematite, magnetite, silicoferrites of calcium and aluminum (SFCA), and glassy silicate phases. Their relative proportions depend on the sintering conditions, such as temperature, raw sinter mix composition, oxygen partial pressure, basicity (CaO/SiO2 mass ratio), and sintering time, which are controlled by operating parameters such as sinter blend composition, coke breeze rate, humidity, windbox pressure, and bed height. As mill scale contains a high amount of Fe and low amounts of silica and alumina, recycling it through the sintering process reduces the consumption of raw materials like iron ore and limestone. A further advantage with the use of mill scale is that assimilation during sintering can provide a reduction in the rate of coke breeze addition due to the exothermic oxidation of wustite and magnetite.[3] The oxidation of magnetite ore has been demonstrated to reduce coke consumption as observed by several investigators. Button and Lundh,[6] using an ore blend containing 30 wt pct magnetite in pot tests, found that compared to a 100 wt pct hematite ore blend, the maximum sintering temperature at a given coke rate was about 60 K (60 C)
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