Progress of Thermodynamic Modeling for Sulfide Dissolution in Molten Oxide Slags: Sulfide Capacity and Phase Diagram

Abstract

Sulfur dissolves in molten oxide slag as a sulfide (S2-) under many high-temperature metallurgical processing environments. O-2- and S-2-exchange reaction in the molten oxide has been well known. Traditionally, a sulfide capacity (C-S) was used to characterize the S-holding ability of molten slags under given sulfurizing potential (p(S2/)p(O2))-dilute dissolution behavior of sulfide in the molten slags. A number of sulfide capacity models have been reported. Those were reviewed with an emphasis on their core idea regarding the formulation of C-S models. It was pointed out that two issues should be explicitly taken into account in the modeling of sulfide dissolution in molten oxide: ''oxygen distribution'' in the slag (free, non-bridging, and bridging oxygens) and ''sulfide stability.'' These two issues are the core facts that control the sulfide dissolution in the molten oxide. The C-S models taking into account these issues resulted in improved results for the sulfide capacity calculation. In addition to this, some additional issues which were often neglected were raised: (1) sulfide dissolution in acidic slag, (2) sulfide dissolution at high S content, and (3) sulfide dissolution in multi-component slag where more than one basic oxide exist. These issues were interpreted by ''cooperative phenomena'' using the Modified Quasichemical Model (MQM) in the quadruplet approximation. This approach treats the C-S as the real S content under a given sulfurizing potential by formulating the Gibbs energy of the ''oxysulfide'' solution. The model explicitly distinguishes two distinct sublattices for cations and anions. A Second-Nearest-Neighbor (SNN) Short-Range Ordering (SRO) between two different cations over O anion was considered, which accounts for the distribution of free, bridging, and non-bridging oxygens in the slag. A First-Nearest-Neighbor (FNN) SRO between cation and anion (O and S) was taken into account, which represents the chemical stability of sulfide of different cations. By simultaneously taking into account the FNN-and SNN-SRO using the quadruplets, a configurational entropy of the mixing in the solution was described. Therefore, the approach takes into account the actual sulfide dissolution mechanism in the molten oxide more realistically, thereby resulting in superior prediction ability for the sulfide capacity calculation. In addition, the application of the MQM was extended in a highly concentrated region of sulfide, thereby describing molten ''oxysulfide.'' Recent experimental works on oxysulfide phase diagrams and the model calculations are discussed to show the wider model applicability. Sulfide dissolution in slags with more than one basic oxide component results in the sulfur association with the various cations. A quantitative description of the different cation-sulfur associations is described in terms of the FNN SRO. By considering the FNN SRO, it is possible to find which cation is more associated with sulfur than other cations. The application of the model is also useful not only in understanding the dissolution behavior of sulfide in slags but also in designing slag/flux in various metallurgical processes.