Evaporation Mechanism of Sn and SnS from Liquid Fe: Part I: Experiment and Adsorption of S on Reaction Site
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DEPLETION of high-quality iron ore in the world is one factor which has caused the increase of price of the iron ore. This increased cost has motivated efforts to decrease hot metal ratio in conventional blast furnace—oxygen steelmaking route. On the other hand, accumulation of ferrous scrap either produced in house or collected in market encouraged an economic use of the ferrous scrap. Although it is desirable to secure high quality of ferrous scrap with less tramp elements such as Cu, Sn, Ni, and Zn in actual practice, the available scrap from the market contains higher concentration of those tramp elements. In particular, tinplate scrap is one of the most practical sources of the ferrous scrap because the tinplate can be readily collected by recycling cans. Tinplate is also a very good source of iron because it is a typical low-carbon steel coated by Sn as much as 5 9 103 kg m2.[1] SUNG-HOON JUNG, Graduate Student, and YOUN-BAE KANG, Associate Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Engineering, Pohang, 790-784, Kyungbuk, Republic of Korea. Contact e-mail: [email protected]. JEONG-DO SEO, Senior Principal Researcher, JOONG-KIL PARK and JOO CHOI, Group Leaders, are with the Steelmaking Research Group, Technical Research Laboratories, POSCO, Pohang, 790-784, Kyungbuk, Republic of Korea. Manuscript submitted February 24, 2014. Article published online August 26, 2014. 250—VOLUME 46B, FEBRUARY 2015
However, when Sn once dissolved into molten steel after the melting of the scrap, it is very hard to remove from the molten steel. Sn is not easily oxidized which prevents from being removed by a conventional oxygen refining process. Vapor pressure of pure Sn is not significantly higher than that of pure Fe (~103 bar vs ~105 bar at 1873 K (1600 C), reported in SGTE thermochemical database[2] calculated by FactSage[3,4]); therefore, vapor pressure of dissolved Sn in liquid steel would be less than that of Fe [for example, ~107 bar for 0.02 mass pct. Sn in liquid Fe-Sn alloy at 1873 K (1600 C)]. The presence of Sn as a tramp element in steel causes a loss of ductility, consequently decreases drawing and forming properties. Therefore, it is of practical importance to develop technology to remove Sn from molten steel. There have been several attempts to remove Sn from either tinplate scrap or molten steel: exfoliated detinning by oxidizing/sulfidizing, electrolytic detinning, evaporation by SnS formation, evaporation under reduced pressure, detinning by chlorination, detinning by Ca-based flux, etc. While most of these attempts are still not commercially applied, evaporation of Sn is promising technology that it does not produce any waste after the detinning except for evaporated gas from which Sn may be collected.[1] According to Kellogg,[5] most important Sn-containing vapor species are SnS(g) and its dimer Sn2S2(g), while Sn(g) exhibits a low vapor pressure compared to those of sulfide gases. In the most of previous studies METALLURGICAL AND MATERIALS TR
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