Investigation on fluoroacidity of molten fluorides solutions in relation with mass transport

Abstract

Equilibria between species in solution and its corresponding gaseous species were investigated in var ious molten fluorides mixtures. From the measurements of baron and silicon kinetic constant of gas release, a relative fluoroacidity scale of solvents was proposed. Due ta very high kinetic rates of SiF 4 c 8 i and BF 3 (gJ release for the most acidic melts, another approach ta evaluate fluoroacidity based on mass transport was considered. Relationships between bridging fluorine atoms, viscosity and fluoroacidity was demonstrated. Diffusion coefficients of Si(IV) and B(111) ions were measured and correlated ta the viscosity through the Schmidt number (Sc) defined as the ratio between solvent viscosity and solute diffusivity. These results confirmed the fluoroacidity scale previously obtained, validating the original approach developed on mass transport.

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF–LaF3–Li2O systems

“…57 have also been plotted for comparison. 72,73 The authors have shown that the CaF 2 -LiF is more acidic than LiF, i.e. 4a).…”

Generalized method for determining fluoroacidity by electrochemical diffusion coefficient measurement (application to HfF4)

“…The effect of fluoroacidity on Si(IV) and B(III) ions mass transports was previously demonstrated [5]. Results showed that an increase of the fluoroacidity leads to a decrease of diffusion coefficients due to coordinancy and bridging solvent phenomena.…”

“…One electrochemical method to sort molten fluorides mixtures in regard of their fluoroacidity was proposed in our laboratory by Bieber et al [4] and Kergoat et al [5]. Based on silicon [4] and completed with boron [5], it consists in studying the equilibrium between a dissolved species and a gaseous compounds given by Eq.…”

“…To avoid these drastic conditions, a universal method for fluoroacidity evaluation was developed and based on a mass transport approach: it simply consists in measuring the diffusion coefficient of a species in various molten media [5]. However, small differences were observed between the diffusion coefficients and the discrimination was complicated by a lack of selectivity.…”

“…Based on silicon [4] and completed with boron [5], it consists in studying the equilibrium between a dissolved species and a gaseous compounds given by Eq. (3) for Si and Eq.…”