Mechanism and Influencing Factors of Iron Nuggets Forming in Rotary Hearth Furnace Process at Lower Temperature
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THE ironmaking process of direct reduction and smelting reduction using non-coking coal as a fuel and reductant is widespread around the world. A variety of methods are emerging.[1–3] Rotary hearth furnaces (RHF), direct reduction processes using carbon-containing pellet feeds, with its improved environmental footprint, extensive raw material and, flexible operating performance, are developing rapidly around the world.[4–6] In addition, the rotary hearth furnace iron nuggets process (ITmk3) is preferred by the domestic and international metallurgical workers due to the advantages of the final product.[7,8] In the ITmk3 process, iron ore powder, pulverized coal, and binders are mixed in the required proportions HONGLIANG HAN and SIMING CHEN, Assistant Researchers, are with the Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190 P.R. China. Contact e-mail: [email protected] DONGPING DUAN, Researcher, is with the Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences and also with the Qinghai Institute of Salt Lake, Chinese Academy of Sciences, Xining, 810008, Qinghai, P.R. China. PENG YUAN, Student, is with the College of Metallurgy and Energy Resources, Hebei United University, Tangshan, 063009, Hebei, P.R. China. Manuscript submitted September 5, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
and subsequently briquetted, and the briquettes are reduced at 1623 K and 1723 K (1350 C and 1450 C).[9–12] Under these conditions, both the iron and slag phases are liquid, and separation occurs with sufficient holding time at temperature. Finally, iron nuggets with a similar composition to pig iron, fewer impurities (such as S), and without gangue would be produced.[13–15] However, separation of metallic iron was conducted by melting both the metal and slag phases, and the high separation temperature is a disadvantage. In addition, the surface tension of melting slag is low, and it would cover the iron nuggets, hindering from coalescing and growing into much larger particles. Therefore, there are difficulties in the separation of small size iron nuggets and slag.[16–20] In recent years, in order to reduce the separation temperature, reducing carbon ratio to generate low melting point compound from the reaction of FeO and slag can achieve the separation of melting iron and slag, but people encountered the same difficulties in the separation of iron nuggets and slag.[21,22] Separation of iron and slag is closely related to iron phase structures (reduction level, degree of carburization) and slag phase structure. In theory, whether the slag is in a solid state or molten state, the separation of iron and slag can be realized when the metallic iron is in a molten state. When the temperature is above 1424 K (1151 C), metallic iron can transform from solid state
to molten state by carburization. As a consequence, a new idea of ‘‘separation of slag (solid state) and iron (molten state) at low temperature du
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