Reaction Kinetics of Desulfurization of Molten Pig Iron Using CaO-SiO2-Al2O3-Na2O Slag Systems.

Abstract

Desulfurization kinetics of molten pig iron were studied using CaO-SiO 2 -Al 2 O 3 -Na 2 O quaternary slag systems at 1 350°C. The concentration of Na 2 O in slag decreases with time due to evaporation. The rate of desulfurization increases by increasing the Na 2 O content in slag, decreasing Al 2 O 3 content, increasing the slag basicity, i.e., the ratio of CaO to SiO 2 , and increasing the temperature. A mathematical model has been developed which enables the effect of decrease in Na 2 O content over time to be taken into consideration. The model provides a rate equation that can represent all conceivable rate controlling steps, such as interfacial chemical reaction, mass transport in the metal phase, and in the slag phase. The apparent rate coefficient-a parameter employed in the model-was dependent on temperature and slag composition, especially Na 2 O content. An equation is suggested which relates the coefficient to the sulfide capacity of slag. The activation energy of the apparent rate coefficient was found to be 127 kJ mol Ϫ1 . It was concluded that the desulfurization of molten pig iron using the present slag system is controlled either by the interfacial chemical reaction or slag phase mass transfer.KEY WORDS: desulfurization; pig iron; kinetics; rate; Na 2 O; slag. cible, along with the reaction crucibles filled with the samples was preheated by holding it above the hot zone using a molybdenum wire. It was then lowered to the zone for reaction, and placed on the support. As soon as the crucible assembly (holding crucibleϩreaction crucibles) was placed on the support, one of the reaction crucibles was taken out from the furnace by using a specially designed tong, which was taken as the starting point for kinetic study. The reaction crucibles were taken out of the furnace one at a time according to a predetermined time schedule. These samples were then subjected to chemical analysis. ResultsEffects of basicity (CaO/SiO 2 ), Na 2 O, Al 2 O 3 and temperature were examined, and experimental results are summarized in Table 2. Figure 2 shows the effect of Na 2 O in slag on the rate of sulfur transfer from metal to slag at 1 350°C. In the experiments the basicity of CaO/SiO 2 in mass% was fixed at unity, and the Al 2 O 3 content was adjusted so as to maintain the ratio of Al 2 O 3 to CaOϩSiO 2 ϩAl 2 O 3 in mass% at 0.18. It is clear that Na 2 O greatly enhances the rate of desulfurization. It is also seen that the desulfurization reaction is ISIJ International, Vol. 41 (2001) completed in about 20 min and thereafter a decrease in sulfur is hardly seen. The efficiency of desulfurization was calculated by taking the ratio of the amount of sulfur removed from the metal in 60 min to the initial amount of sulfur, and the results are given in Fig. 3. The efficiency increases along with increasing the Na 2 O content in the slag, and reaches 90 % with 20 mass% Na 2 O. Figure 4 shows the effect of Al 2 O 3 in the slag on the rate of sulfur transfer from metal to slag at 1 350°C. In the experiments the basicity of C...

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