Changes in Effective Thermal Conductivity During the Carbothermic Reduction of Magnetite Using Graphite
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RECENTLY, carbothermic reduction processes where carbonaceous material mixed with metallic oxide powder are heated to achieve reduction is being explored to supplement gas-based reduction processes.[1] These reduction reactions are highly endothermic and hence, the overall rate of these reactions is greatly influenced by the heat transfer through powder mixture. Therefore, in such processes, the knowledge of the effective thermal conductivity of the composite pellet made of powder mixture is essential for process understanding and control. In addition, in such processes, the reaction is quite often accompanied by sample shrinkage, change of porosity, partial melting, and other physical changes apart from the chemical reaction. Traditional method to study gas-solid or solid-solid reactions accompanied by mass changes is thermogravimetry. While this method SAEED KIAMEHR is with the Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden. HESHAM AHMED is with the Lulea˚ Technical University, 971 87 Lulea˚, Sweden and Central Metallurgical Research and Development Institute, P.O. Box 87Helwan, Cairo, Egypt. Contact e-mail: [email protected] NURNI VISWANATHAN is with the Lulea˚ Technical University, and also with Indian Institute of Technology Bombay, 400 076 Mumbai, India. SESHADRI SEETHARAMAN is with the Royal Institute of Technology (KTH), and also with Indian Institute of Technology Bombay. Manuscript submitted April 27, 2016. Article published online February 27, 2017. 1502—VOLUME 48B, JUNE 2017
provides knowledge of the chemistry involved in the reaction by monitoring the mass change (due to loss of the gaseous product), it does not provide information regarding the physical changes and their impact on the overall reaction. For example, decrease of porosity will result in the decrease of the permeability of the product gases in the carbothermic reduction and consequently on the reaction rate. In situ measurement of thermal properties of the composite pellet also gives insights into the physicochemical changes occurring in the pellet during carbothermic reduction. Attempts were made earlier to understand the physicochemical changes based on corresponding changes of thermal properties. Akiyama et al.[2] studied temperature dependency of thermal conductivity of iron ore agglomerates. Thermal conductivities of dense pure hematite, magnetite, wu¨stite, and metallic iron were measured as a function of temperature, and a linear relationship between thermal resistivity (reciprocal of thermal conductivity) and temperature was proposed. Further, the thermal conductivity of iron ore agglomerates from different origins has been measured by the same authors while being reduced to magnetite, wu¨stite and finally to metallic iron by CO/CO2 or H2. It was found that the obtained thermal conductivity values for the processed iron ore agglomerates were remarkably lower than those of pure dense ones which of course could be attributed mainly to the existence of pores originating from oxygen removal. Expressions in literature[2–6
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