Limitation of Sulfide Capacity Concept for Molten Slags
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is one of the most critical impurities in steel, its refining technology has been well developed in steelmaking. In hot metal pre-treatment, sulfur is controlled by the addition of different types of fluxes such as CaO, Mg, CaC2, and Na2O which eventually form top slags. In the secondary steelmaking, the CaO-Al2O3-based slag is often used for the further reduction of sulfur in the liquid steel. Therefore, the sulfide capacity of a molten slag, which represents the slag potential to remove sulfur from steel, is a very important chemical property of slag, and it has been intensively investigated. Under ironmaking and steelmaking conditions, sulfur is mostly dissolved in molten slags in the form of sulfide (S2) rather than sulfate (SO42),[1–4] thus sulfate has not been concerned too much in steel industry. The sulfide capacity concept was first introduced by Fincham and Richardson[1]. They rearranged the equilibrium constant of the following ionic chemical Reaction [1] between the slag and gas, and singled out the quantities which are measurable or controllable to define the sulfide capacity, CS:
IN-HO JUNG, Associate Professor, and ELMIRA MOOSAVIKHOONSARI, Post Doctoral Fellow, are with the Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada. Contact e-mail: [email protected] Manuscript submitted September 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
O2 þ 1=2 S2 ðgÞ ¼ S2 þ 1=2 O2 ðgÞ
CS ¼ ðwt pctSÞ
1=2 a 2 K1 pO 2 ¼ O p S2 fS2
½1
½2
where K1 is the equilibrium constant of the chemical Reaction [1], pO2 and pS2 are the partial pressures of O2 and S2 in the gas phase in equilibrium with the oxide melt, aO2 is the activity of free oxygen in the melt, and fS2 is the activity coefficient of S2 with respect to the weight percent standard state. Of course, it is difficult to measure aO2 and fS2 . When the slag/ liquid Fe equilibrium is taken into account rather than the slag/gas equilibrium, pO2 and pS2 of the system can be calculated from oxygen and sulfur contents of the liquid Fe. The sulfide capacity concept was developed to objectively define the sulfur dissolution capacity in a slag regardless of pO2 and pS2 (that is, oxygen and sulfur contents of the liquid Fe). As seen in Eq. [2], however, the slag with a given composition has a unique sulfide capacity value at a given temperature only if aO2 and fS2 are constant. When the sulfide content of slag is reasonably small, aO2 and fS2 of slag would be almost constant. The reaction constant depends only on temperature. From Reaction [1], it is easily understood that the sulfide amount in molten slags can increase with increasing pS2 and decreasing pO2 . Then, aO2 and fS2 of slag can change with increasing the sulfide amount by (i) diluting oxide components of slag and (ii) the interaction between oxides and sulfides in the slag. That is, the sulfide capacity cannot be a unique property of the molten slag anymore if the sulfide solubility is relatively large. Therefore, the sulfide capac
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