Nitridation of titanium minerals

Abstract

Reactions between powders of carbon and the titanium dioxide minerals rutile and anatase have been studied attemperatures up to 1500°C under flowing nitrogen. Nine carbon samples and nine titanium dioxide samples from a variety of sources were included in the study. The reactionsof the two titania minerals are similar and show that TiO2 is reduced by carbon to lower oxides in the series TinO2n-1 (10≥ n ≥ 4) according to the reaction: nTiO2 + nC → nCO + TinO2n-1 Successively lower oxides (i.e. with smaller n) occur as the reaction proceeds. Ti4O7, the most reduced oxide in the series, is then further reduced to Ti3O5 which is converted directly to TiN rather than Ti2O3. This result can be understood in terms of the crystal structures of the oxides and TiN. Evidence is presented to suggest that the transformationfrom Ti4O7 to Ti3O5 is the rate limiting step in the reaction sequence. The overall reaction may be represented as: TiO2 + 2C + 1/2 → TiN + 2CO Thermodynamic predictions for a closed system at equilibrium were found to agree surprisingly well with the results of the dynamic experiment in flowing nitrogen. The reactivity of a range of carbon samples from coal to activated carbon varied widely. The reactivity of the TiO2 samples also varied but not to the same extent. The kinetics and mechanism of the nitridation step at temperatures in the range 1090-1230°C have been studied by X-ray diffraction for a number of pigment grade samples and one reagent grade TiO2 sample. The activation energies varied with the TiO2 samples in the range 260-390 kJ/mole at temperatures greater than 1100°C. Below this temperature they are approximately 1000 kJ/mole. The mechanism is shown to involve both contact reduction by carbon and indirect reductioninvolving the gases CO and CO2. The reaction sequence is so complex that the kinetic parameters of the final nitridation step are virtually independent of the physical or chemical properties of the titanium dioxide.