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Reaction Mechanism and Kinetics of Evaporation of Cu and Sn from Liquid Steel

용강 내 구리와 주석의 기화 반응 기구 및 반응 속도에 관한 연구

정성훈 (Sung-Hoon Jung, 포항공과대학교)

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As ferrous scrap consumption is increasing in steel industry due to depletion of high quality iron ore and increase of price of iron ore, tramp elements accumulation in ferrous scrap become more important. Among tramp elements, Cu and Sn cannot be removed by conventional oxidation refining process ...
As ferrous scrap consumption is increasing in steel industry due to depletion of high quality iron ore and increase of price of iron ore, tramp elements accumulation in ferrous scrap become more important. Among tramp elements, Cu and Sn cannot be removed by conventional oxidation refining process and these elements cause deterioration of mechanical property of steel. In order to solve problems with Cu and Sn accumulation in ferrous scrap, Cu and Sn evaporation removal in liquid steel were kinetically investigated. Fundamental studies for Cu and Sn evaporation removal give information to evaluate the feasibility of Cu and Sn evaporation removal process. Previous investigation for various Cu removal (sulfide flux, evaporation, low melting point bath, chlorination removal) and Sn removal (evaporation, stripping, Ag liquid bath, unidirectional solidification removal) methods have been discussed and advantages and disadvantages of previous suggestions were critically evaluated. Cu evaporation reaction mechanism has been investigated and rate determining step for Cu evaporation reaction was carefully examined. Based on the assumption that chemical reaction controls Cu evaporation reaction, Cu evaporation reaction rate equation was developed. Furthermore, the effect of alloying elements, gas atmosphere, vacuum degree on Cu evaporation rate were thermodynamically and kinetically investigated. Sn evaporation reaction mechanism in the presence of S has been investigated. In literature, Sn evaporation reaction mechanism was not clearly investigated and proper Sn evaporation reaction rate equation was not established as the result. Based on the assumption that chemical reaction on the surface is controlling the evaporation reaction, Sn evaporation rate equation was successfully formulated by applying surface adsorption by S with residual sites. Therefore, it was first investigation that can successfully explain Sn evaporation reaction mechanism in the presence of S. However, it has been revealed that Sn evaporates as Sn(g) gas rather than SnS(g) at low S content region by experiments and it causes suggested Sn evaporation rate equation considering only SnS(g) evaporation is not valid for wide range of S contents. Therefore, unified Sn evaporation model which is considering the evaporation reaction of Sn(g) and SnS(g) simultaneously has been formulated. It was the first time to develop Sn evaporation rate equation which is valid for 0 to high S contents. By calculating Sn evaporation model, the evaporation of Sn and S can be quantitatively evaluated. Also, the effect of C on Sn evaporation rate has been studied. It has been revealed that S in C-saturated liquid Fe is evaporated no only as SnS(g) but also as CS2(g). Furthermore, the effects of C on Sn evaporation rate were explained by increasing activity of Sn and S and intensification of surface adsorption of S. Based on the experimental result, evaporation model for Fe-C-Sn-S alloy has been developed by considering Sn(g), SnS(g) and CS2(g). Calculation result of evaporation model for Fe-C-Sn-S alloy showed that Sn evaporation rate can be strongly accelerated by the addition of C and 50% detinning time can be improved 15 times compared to Fe-Sn alloy and 7 times compared to Fe-Sn-S alloy. Furthermore, simultaneous evaporation of Cu and Sn has been investigated to extend evaporation model to Fe-C-Cu-Sn-S liquid alloy. It has been experimentally observed that Cu can be evaporated as Cu(g) and CuS(g) in the presence of S. Cu evaporation rate equations were formulated by considering surface adsorption of S with residual sites for Cu(g) and CuS(g) which were in consistence with Fe-C-Sn-S evaporation model. As the result, Fe-C-Cu-Sn-S evaporation model has been developed considering Cu(g), CuS(g), Sn(g), SnS(g) and CS2(g) and the model can be applicable for simultaneous evaporation of Cu and Sn in the presence of C and S.