Kinetically enhanced metallothermic redox of TiO2 by Mg in molten salt

Molten salt is used as a reaction medium in a number of thermochemical and electrochemical processes including a new method named “hydrogen assisted magnesiothermic reduction (HAMR)” for the production of Ti metal powder from TiO2. A key enabling element of the HAMR process is based on the thermodynamic effect of hydrogen on the relative stability of MgO versus that of Ti-O solid solutions. Another key of the process is the catalytic effect of molten salt, which is used as the reaction medium. Although the general positive effect of the molten salt in the HAMR has been recognized, the role and mechanisms of molten salt during the reaction are still not well understood. The focus of this work is thus placed on the effects of the molten salt on the kinetics of the redox of TiO2 by Mg, and the possible mechanisms by which the molten salt facilitates the reaction. Experiments were specifically designed to elucidate the role of molten salt when TiO2 is separated from the reducing agent Mg when Mg is in vapor, liquid, or solid states. The results show that the kinetic rate of the reduction can be significantly enhanced by increasing the amount of salt. Although the reduction can take place directly between vapor, liquid, or even solid Mg and TiO2, the ionization of Mg metal and the transport of Mg22+ through the molten salt play a major role in the reduction of TiO2 by Mg. The molten salt must be a Mg2+-bearing salt. A molten salt depleted of Mg2+ is not effective. The possible pathways by which the reaction species form and react with TiO2 are also discussed.