The electro-deoxidation of porous titanium dioxide precursors in molten calcium chloride under cathodic potential control

Abstract

A study on the electro-deoxidation of porous titanium dioxide precursors in molten calcium chloride is reported. Experiments were performed with a three-terminal electrochemical cell, comprising a molten salt electrolyte of calcium chloride with additions of calcium oxide, a cathode of compact titanium dioxide with a significant degree of open porosity, as well as a graphite anode and a graphite pseudo-reference electrode. Reductions were carried out under cathodic potential control and at different applied potentials. The results reveal that the formation of titanium metal occurs at electrode potentials significantly more positive than that of calcium deposition, whilst the realisation of very low residual oxygen contents requires potentials around that of calcium deposition. It is demonstrated that oxygen contents in the titanium metal prepared of below 5000 ppm by mass may be achieved reproducibly within processing times of 16 h and at current efficiencies between 10 and 40%. The kinetic pathway is investigated, by analysing the compositions of samples prepared at different cathode potentials, and compared against the results from foregoing studies. It is found that the presence of calcium oxide in the calcium chloride accelerates the overall rate of electro-deoxidation, and that the temporary occurrence in the cathode of the calcium-containing compounds calcium titanate, CaTiO 3, and calcium titanite, CaTi 2O 4, are inherent features of the reaction path. The overpotential at the anode is shown to be of significant magnitude.