Kinetic and Passivation Limitations in Titanium Reduction via Molten Oxide-Fluoride Electrolysis

Molten oxide-fluoride electrolysis enables direct metal extraction from ores while producing only oxygen. Titanium and its alloys are highly valued for their exceptional high- and low-temperature properties, enabling broad applications in metallurgy and other fields. However, the direct electrolytic deposition of titanium from its oxides via molten oxide electrolysis is a significant challenge. This article systematically investigates the limitations of this process in a CaF₂-CaO-TiO₂ melt. Electrochemical experiments were conducted using tungsten and iron electrodes with Fe₂O₃ deposition; in all cases, no significant metallic titanium deposition was observed. Cyclic and square-wave voltammetry analyses revealed a multistep reduction mechanism. Results show negligible Ti deposition on the W electrode and only micrometer-thick layers on the Fe electrode or with Fe₂O₃. The primary limiting factors are the following: (1) The reduction rate of titanium ions is close to the dissolution rate of metallic titanium (both ~10^-4cm·s^-1) and (2) rapid formation of CaTiO₃ passivates the cathode, impeding continuous Ti deposition, with CaTiO₃ reduction as the rate-controlling step. This article clarifies the fundamental barriers to efficient Ti extraction via molten oxide-fluoride electrolysis and offers strategic insights for optimizing titanium metallurgy from mineral sources.