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Figure 1.3 Limit of equilibrium potential‐pO2– graphs in molten alkali carbonates and sulfates, at 600°C
(modified from Trémillon, 1974).
It is then possible to define pO2– = ‐logaO2– and introduce E‐pO2– diagrams, in which acid species will be located at high pO2– values. These diagrams were first introduced by Littlewood (1962) to present the electrochemical behaviour of molten salt systems and provide an understanding of the stability fields of the different forms taken by metals in these systems. Reference potential for molten salt is chosen either from anion or from cation, but anion, making up the ligand, is normally selected because there may be several different cations in the system.
For molten solvent diagrams, such as carbonate and sulfate melts, the stability area of the bath depends on the salt itself and can be seen by using as examples oxyanion solvents (Figure 1.3). Limitations on the pO2– scale of oxoacidity (Reaction 1.27) are given by the values of the Gibbs free energy of the formation reactions of alkali carbonates or sulfates at the liquid state, which depends on temperature as well as on pressure. On the basic side (low pO2– side) the limit is imposed by the solubility threshold of the generic Mν+Oν/2 oxide in the electrolyte medium, i.e., pO2–min ≈ Mν+Oν/2 solubility, whereas on the acidic side the limit is imposed by PCO2 or PSO3 = 1 bar. For example, it is 11 units in the case of the ternary eutectic Li2CO3+Na2CO3+K2CO3 at 600°C and 19.7 units in the case of the ternary eutectic Li2SO4 + Na2SO4 + K2SO4 at the same temperature (Trémillon 1974; Figure 1.3).
The upper stability limit is related to the O–II/O2(g) redox system (Reaction 1.6), i.e., to the oxidation of CO32– and SO42– anions:
which results from acid–base exchanges of the type:
(1.31)
(1.32)
coupled to half‐reaction 1.6.
Both Reactions 1.29 and 1.30 yield the E‐pO2– relationship:
(1.33)
The lower stability limit of the solvent can be given by either the reduction of alkaline cation in the corresponding metal:
whose potential is independent of pO2–, or the reduction of CO32– and SO42– anions given by:
(1.35)
Figure 1.4 Log fO2 ‐ Ph diagrams for 290°C (left panel) and 145°C (right panel) at saturated vapor pressure, showing predominance fields for aqueous sulfur species (dashed lines), stability fields for Fe–O–S minerals and bornite–chalcopyrite (solid grey lines). The solubility contours (left panel: 1, 10, 100 ppm; right panel: 0.1, 1 ppm) are for gold in the form Au(HS)2–.
Modified from Raymond et al. (2005).
(1.36)
and to which the following E‐pO2– relationships correspond (CO32– and SO42– anions having unitary activity):
(1.37)
(1.38)
Figure 1.3 shows the results on carbonate and sulfate melts (modified from Trémillon, 1974, and references therein). The utilizable regions appear as quadrilaterals on the E‐pO2 graph. If in sulfates, the region is a parallelogram similar to the E‐pH region in aqueous solution, the theoretical range of potential in molten carbonates appears more restricted in an oxoacidic medium than in an oxobasic medium, because the lower limit varies versus pO2‐ with a slope greater than that of the upper limit (Trémillon, 1974).
Silicate melts have been so far an underestimated electrolytic medium acting as a solvent for oxides. This is mainly because E‐pO2– diagrams cannot be based on predictive thermodynamics and physical chemistry assessments such as the dilute electrolyte concept and its developments in the case of previous solvents, aqueous solutions particularly (Allanore, 2015). In silicate melts, and more generally molten oxides, oxygen tout‐court cannot be identified as the solvent, despite its abundance. Silicate melts are in fact a high‐temperature highly interconnected (polymerized) matrix in which solvation units cannot be easily defined and both ionic and covalent bonds rule the reactive entities that make up the melt network. Because of this, some approaches have been formalized in terms of the Lewis acid–base definition (network formers and their oxides, such as SiO2 and Al2O3 are acids; network modifiers and their oxides such as MgO, CaO, Na2O are bases) by using electronegativity
