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Solubility Control, Solubility Limitation, the Coexistence of Multiple Solid Phases, and Multiple-Solid Predominance Diagrams
Published in James F. Pankow, Aquatic Chemistry Concepts, 2018
The type of diagram that we are drawing is called a “predominance diagram.” In any solution phase region, the species used to label the region is the predominant Fe species. However, inside a region labelled with a solid, all we know from the diagram is that the solid is present and is controlling the concentrations of all of the Fe(II) species. Thus, the diagram that we are drawing might be more correctly described as a “predominance and/or solubility controlling diagram.” To illustrate this matter further, consider the case of the Fe(OH)2(s) domain. Labelling that domain with “Fe(OH)2(s)” does not mean that, everywhere inside that domain, there are more mols of Fe(OH)2(s)2 per L of water than there are of total dissolved Fe(II). In fact, inside the domain and very near the solution/solid boundaryline, we know that the concentration of some solution phase Fe species will always exceed the total FW of Fe(OH)2(s) present per liter of aqueous phase. When the value of FeT for the diagram is sufficiently large, then well inside the Fe(OH)2(s) domain, the FW of the solid per liter of aqueous phase can however exceed the concentration of any solution phase species. Therefore, inside a region labeled with a solid, that solid will be solubility controlling, but may not predominate over other forms of iron anywhere in that region.
Thermodynamic analysis of hot water leaching of sulphur from desulphurisation slag by Eh–pH diagrams of the Ca-S-H2O system
Published in Mineral Processing and Extractive Metallurgy, 2019
Huanyu He, Jieqi Wang, Yang Li, Zeyu Song, Mooni Sivaprasada
Per previous literature, sulphur exists as CaS and 11CaO·7Al2O3·CaS crystals in the slag. It is difficult to remove sulphur from residues under normal conditions because of the stability of the sulphur-containing species (Posch et al. 2003; He et al. 2009; Lv et al. 2013). Removal of sulphur from the exhausted slag by oxidation has been investigated by many researchers. However, oxidation not only resulted in an unsatisfactory removal efficiency of sulphur but also resulted in environmental pollution (Hiraki et al. 2012; Kobayashi, Hiraki, and Nagasaka 2012). Hydrothermal leaching has successfully removed sulphur based on the hydrolysis properties of sulphur-containing slag. This approach utilises the hydrolysis character of Ca-S compounds in saturated steam to selectively remove sulphur with water as the solvent (He et al. 2010; Uehara et al. 2014). An understanding of the thermodynamic behaviours of the sulphur-containing species in hydrothermal leaching systems is essential to promote the application of hydrothermal leaching. Eh–pH diagrams are a well-known predominance diagram used to describe the relationship between potential and pH under specific thermodynamic conditions, and directly depict regions of stable species existing in a given system. Recently, Eh–pH diagrams such as sulfide-H2O, (non-metal) metal-H2O and metal-complex-H2O systems have been used to guide production and inform metallurgical research (Jiang 1984; Zhong and Mei 1986).