Effect of FeO and CaO on the Sulfide Capacity of the Ferronickel Smelting Slag
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HE extensive global use of stainless and other specialty steels has lead to a shortage in the supply of nickel ores, increasing production costs. Nickel is conventionally produced by extraction from high quality pentlandite ore, which accounts for approximately 30 pct of the earth’s nickel resources.[1,2] But, reserves are in short supply at present, making the utilization of low-grade nickel ores such as laterite and saprolite, which account for the remainder of the nickel resources on the earth, more economically viable.[3] However, during smelting of this low-grade ores significant amounts of sulfur is present, which drastically decreases the quality of the Fe-Ni melt. Thus, the control of sulfur through slag refining is imperative to the overall process efficiency of the smelting process and production of the required quality of the Fe-Ni alloy. Ferronickel smelting slags are a low basicity slag based on MgO-SiO2, unlike the conventional carbon steel slag that is based on CaO-SiO2. The Ni content in laterite and saprolite is approximately 1.5 to 3.2 wt pct, which results in slag volumes in the Ni smelting reduction process to be 25 to 35 times that of the carbon steel process. Considering these challenges, an understanding of the thermodynamic behavior of sulfur and the mechanisms involved are essential for improving process efficiency, maximizing Ni yield, and improving product quality. Given that most carbon steel and ferro-alloy EAF processing slags KI DEOK KIM, Researcher, formerly with the Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea, is now with the Hyundai Steel Company, Seoul, Korea. WAN WOOK HUH, Research Engineer, is with the Department of Materials Science and Engineering, Yonsei University, and also with the Yonsei Institute of Green Technology, Seoul, Korea. DONG JOON MIN, Professor, is with the Department of Materials Science and Engineering, Yonsei University. Contact e-mail: [email protected] Manuscript submitted March 26, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
contain a certain amount of FeO, knowledge of the sulfide capacities of FeO-containing slags is valuable to the metallurgical industry. However, there are limited number of experimental studies carried out on these systems owing to distribution of the FeO as Fe2+ and Fe3+ dependent on the oxygen partial pressure and the experimental difficulties involved in determining the Fe2+ and Fe3+ fractions within the slag. The dissolution mechanism of sulfur into slag can be expressed by Eq. [1]. The sulfide capacity for slags is an effective concept to quantitatively determine the removal of sulfur from metal. The sulfide capacity is known to be proportional to the basicity (aO2 ) and temperature and inversely proportional to the activity coefficient of sulfide ions in slag (cS2 ), as given by Eq. [2].
CS2 ¼
½S þ ðO2 Þ ¼ ðS2 Þ þ ½O
½1
Kð3Þ aO2 ðpct S2 Þ 1 1=2 pO 2 : ¼ ½pct S fS cS2
½2
Nzotta et al.[4] measured the sulfide capacity in the FeO-SiO2, FeO-MnO, and CaO-FeO slag in equili
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