Influence of Basicity on Anodic Reaction in CaO-SiO₂-Al₂O₃Melts

Abstract

A cyclic voltammetry study on the anodic reaction between carbon electrodes and CaO-SiO 2 -Al 2 O 3 melts at 1773 K was carried out to understand the electrochemical behavior of free oxygen ions in silicate melts. The potential of free oxygen ions corresponding to CO evolution in the totally diffusion controlled region was obtained from the voltammetric data. An empirical relationship between the current density on the logarithm scale and the basicity (B) of each silicate melt was established through Tafel analysis in the non-diffusion-controlled region. The obtained electrochemical data showed good correlation with thermodynamic properties such optical basicity and sulfide capacity of the silicate melts as a relative index of oxygen ion activity in molten oxides. The use of this electrochemical method to evaluate the relative potential of oxygen ions in molten oxides is discussed in detail. Bockris et al.1 carried out pioneering research on the anion species in silicate melts and postulated that discharge of free oxygen ions is related to the concentration of free oxygen ions in silicate melts and the structural equilibrium of the silicate units. Moreover, basicity, which is most appropriate to be defined by the activity of a free oxygen ion, could be used to monitor free oxygen ions and understand the equilibrium state of silicate melts. Electrochemical research on the anodic reaction of free oxygen ions has been continuously pursued to determine the potential of molten oxides as electrolytes in various electrochemical applications, namely, molten oxide electrolysis (MOE), 2-12 direct current electro-slag remelting (DC-ESR), 13 and electrorefining. [14][15][16][17] Since basicity as a measure of oxygen ions in molten oxides is a fundamental index for defining the equilibrium state of chemical reactions in molten oxides, extensive research has been carried out using basicity measurements to define the chemical potential of oxygen ions in molten oxides qualitatively. However, it is difficult to directly evaluate the activity of free oxygen ions because it is impossible to experimentally measure the chemical potential of ionic species in solution due to the constrain of electro-neutrality.18 Therefore, many studies have used indirect methods to quantify free oxygen ions in molten oxides. Wagner 19 proposed an indirect basicity concept using sulfide capacity as a quantitative index. A linear correlation between sulfide capacity and basicity is observed in a certain composition range. However, it is assumed that the activity coefficient of the reaction product is independent from the composition, which is the standard state defined by Henry's law. Therefore, for various oxide systems with wide ranges of basicity, the capacity only allows a qualitative comparison owing to change in the activity coefficient of the product. Therefore, Nakamura and Sano 20 measured the solubility of platinum in CaO-SiO 2 and BaO-SiO 2 systems and in CaO-Al 2 O 3 and BaO-Al 2 O 3 systems, while Park and Min 21 measured the solubility of...

The Influence of Iron Concentration on the Anodic Charge Transfer in Molten Oxide Electrolysis

“…The current density in Fe#5, Fe#7.5 and Fe#10 acquired above 1 V is characterized by a slope that flattens out toward higher cell voltages, Figure 3. Despite not reaching a distinct plateau, this trend can be interpreted as a mass transfer limitation of the free oxide anions, which is in coherence with results published elsewhere 7,9 for lime based molten oxides. In contrast, charge transfer in Fe#15 increases almost linearly and starts significantly below 1 V. Either Reaction 6 or Reaction 7 are possible to explain the charge transfer in this cell voltage range.…”

“…Evidence has shown that the anode half reaction in MOE is controlled by the diffusion of free oxide anions (O II− ) in the melt toward the electrode, 7 which infers a strong dependence on the electrolyte's optical basicity. 8,9 However, the oxygen production itself was not monitored during these experiments and either the conditions (i.e. PO 2 , reactive cell constituents) did not allow multivalence of any of the present components or their concentration was kept too low for them to interfere with the anode half reaction.…”

Electrochemical Behaviour of Iron in Molten Oxides

“…In calciumaluminates, the anodic reaction is not likely to be rate limiting owing to the higher concentration of free O 2-. Limiting anodic current densities for oxygen evolution in molten oxide electrolytes have been observed to vary with the basicity, as suggested here (15,16).…”

Study of CMAS infiltration and evaporation behaviour under water vapour/sulphur oxide conditions in EB-PVD 7YSZ

“…The acidity of the CMAS is vital for the solubility mechanism but also for the physical properties of the melt. The basicity index is often related to the viscosity of the melts [36].…”