Niobium and titanium electrowinning in fused salts

Abstract

In the frame of the preparation of refractory metals, thermodynamics and kinetics properties of salts of refractory metals (mainly titanium, and niobium) dissolved in alkali chloride melts were determined by transient electrochemical techniques. Two kinds of reactions occur in the melts: homogeneous reactions such as: Me^(n+z)+ + n^e− ⇌ Me⁺ where Me^(n+z)+ and Me^z+ are two oxidation states of metal ions; heterogeneous reactions corresponding to the metal deposition: Meⁿ⁺ + n^e− ⇌ Me. The potential corresponding to the reactions are linked to the Gibbs free energy of formation of salt solutions. The formal standard potentials of the various redox reactions were measured by transient electrochemical techniques. These techniques were also used to determine the reaction kinetics and to measure the diffusion coefficients of the active ions which control the deposition process. In pure chloride melts, the metal is obtained from the reduction of lower oxidation states, Ti²⁺ and Nb³⁺. However, disproportionation reactions and the formation of insoluble subhalides perturb the reduction reaction. When a small amount of NaF was added to the bath, the direct reduction of Ti³⁺ or Nb⁴⁺ to metal was obtained, which greatly improved the quality of the deposit.