ROLE OF FREE F- ANIONS IN THE ELECTROREFINING OF TITANIUM IN MOLTEN ALKALI-HALIDE MIXTURES

The influence of the F/Cl atomic ratio on the morphology of titanium m etal, deposited by electrorefining in various alkali halide solvent mi xtures and K2TiF6 solute, was investigated. A continuous transition fr om powdery deposits in chloride mixtures (F/Cl = 0), through dendritic deposits in chloride-fluoride mixtures (0 < C/Cl < infinity), to cohe rent and adherent deposits in fluoride mixtures (F/Cl = infinity), was evidenced. This dependence was explained in terms of the increase of the thermodynamic stability of TiF63- anion (and the consequent decrea se of the activity of Ti3+, i.e. electrocrystallization centres availa ble in the melt), with increase of the parameter F/Cl. This is support ed by the observed increase in the decomposition potential, E(d), and the shift of the Ti3+ discharge potential, E(p), toward more negative values, in the same sequence. The influence of the F/Cl atomic ratio o n current efficiency, eta, on the grain size distribution and on the c hemical purity of electrodeposited titanium metallic powder, was also discussed.