The influence of fluoride ions on the equilibrium between titanium ions and titanium metal in fused alkali chloride melts

Abstract

KF is employed as a source of fluoride ions added to the melt to disclose the influence of fluoride on the disproportionation reactions of titanium ions, 3Ti²⁺ = 2Ti³⁺ + Ti, and 4Ti³⁺ = 3Ti⁴⁺ + Ti. The results reveal that the equilibrium transferred to the right direction for the first reaction and the apparent equilibrium constant increased sharply, mainly because of the formation of coordination compounds: TiF_i³⁻ⁱ. The accurate values of the equilibrium constants referring to the formation reactions of Ti³⁺ + iF⁻ = TiF_i³⁻ⁱ (i = 1–6) in NaCl–KCl melt at 1023 K were evaluated with a best fit least squares method. It is also revealed that the stable states of the coordination compounds are TiF²⁺, TiF₂⁺, TiF₄⁻ and TiF₆³⁻. Moreover, the Gibbs free energies for complex formation were estimated. Ti²⁺ was undetectable when the concentration of fluoride ion was high enough. The equilibrium constant for the formation reaction, Ti⁴⁻ + 6F⁻ = TiF₆²⁻, was evaluated. The equilibrium constant, K_c2, for the disproportionation reaction 4Ti³⁺ = 3Ti⁴⁺ + Ti, in chloride melt was determined as 0.015.