Kinetics of Oxidation of Divalent Iron to Trivalent State in Liquid FeO-CaO-SiO 2 Slags
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TRODUCTION
IN iron- and steelmaking, nearly 12 Mt of steelmaking slag is produced every year in Europe (around 50 Mt in the world). Although less than 65 pct is used in recovering the valuable elements in the slag, the rest of the slag is used for land filling.[1,2] Potentially useful components in the latter, such as metallic iron inclusions and iron-bearing compounds, are discarded along with rest. The most widespread approach to recover the iron values existing as inclusions in the slag matrix in steelmaking slags is to crush the slag at ambient temperature, separate the metallic iron by magnetic separation, and recycle the same material by introducing it to a steelmaking process. The nonmagnetic ironbearing compounds from the slag (up to 30 pct of FeO depending on the steel grade and technology) are not recovered. Apart from the loss of Fe, the iron oxide left in the slag limits its applications in civil engineering. Wu¨stite present in these slags lowers the value for cement applications as the mechanical properties are seriously affected. Along with other problems
ANNA SEMYKINA, Ph.D. Student, is with the Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and also with the National Metallurgical Academy of Ukraine, Dnipropetrovsk, 49600 Ukraine. Contact e-mail: [email protected] VOLODYMYR SHATOKHA, Professor, is with the National Metallurgical Academy of Ukraine. MASANORI IWASE, Ph.D. Professor, is with the University of Kyoto, Sakyo-Ku, Kyoto 606-8501, Japan. SESHADRI SEETHARAMAN, Professor, is with the Royal Institute of Technology. Manuscript submitted May 13, 2010. Article published online August 26, 2010. 1230—VOLUME 41B, DECEMBER 2010
(e.g., disintegration of calcium silicates[3,4]), this is a major reason for the insufficient recycling rate. The reduction of iron-bearing oxide compounds followed by a magnetic separation of metallic iron has been investigated by a few scientists.[5,6] The reduction route using fossil fuels cannot be considered as an environmentally friendly option for the selective extraction of iron values from the slag. A more sustainable approach to use the steelmaking slag components is based on the transformation of nonmagnetic iron monooxide to magnetite by oxidation. This process allows selective recovery of iron-bearing and non iron-bearing slag constituents for specific purposes. In our previous work,[7] preliminary studies on the feasibility of oxidizing FeO in the liquid slag were carried out. The present work aims at investigating the mechanism of oxidation of Fe2+ from molten slags containing FeO. The slags investigated were synthetic ternaries in the FeO-CaO-SiO2 system.
II.
THERMODYNAMIC CONSIDERATIONS
In the present work, the oxidation of Fe2+ to Fe3+ in CaO-FeO-SiO2 slags of five different compositions was considered. The slag compositions examined are presented in Table I. The oxidation studies were carried out in the singlephase liquid region with the lowest temperature being 1623 K (1350 C). The reaction considered in these experiments can be written as follows: 1
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