Employing low-quality raw materials to lower steel production costs is an effective way for many iron and steel enterprises to achieve the sustainable development in China. Meanwhile, this also raises higher requirement for the desulfurization ability of blast furnace slag with diverse components. In this work, the effect of Na2O on the sulfide capacity of CaO-SiO2-10wt% MgO-12wt% Al2O3-2wt% BaO slag was studied at 1773 K by employing the gas-slag equilibrium technique. The distribution of bridged oxygen (O-0), non-bridged oxygen (O-), and free oxygen (O2-) in slag was analyzed by XPS. The results indicated that the sulfide capacity increased constantly with Na2O content rising from 0 to 5wt% due largely to an increase in the activity of O2-. Na+ would be consumed to compensate preferentially the [AlO4]-tetrahedron structural units to maintain electrical neutrality of molten slag, releasing some Ca2+ ions for sulfur stabilization. Assuming that the activity of O2- and the stability of S2- in molten slag were proportional to the activity of CaO and the activity coefficient of CaS, respectively, the increased range of the activity of CaO was larger than that of the activity coefficient of CaS on the whole, which improved the sulfide capacity of slag. XPS analysis indicated that the relative fraction of free oxygen and non-bridged oxygen increased gradually, and bridged oxygen showed a decreasing trend, which supported the changes of sulfide capacity based on the dominant effect for free oxygen on sulfide capacity.
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