Investigation of the effect of a nitrogen-containing atmosphere on the carbothermal reduction of titanium dioxide

In this study, the carbothermal reduction of mechanical mixtures of TiO2 and 2.7 mol C (using a furnace black or graphite) was investigated in laboratory-scale experiments. Reaction in argon, Ar-10 vol% N-2 and nitrogen atmospheres was performed in the temperature range 1300-1800 degreesC with an isothermal treatment time of 0.5 h. The kinetics of the reaction was studied in an Ar-10 vol% N-2 atmosphere at 1250 and 1350 degreesC with isothermal treatment times ranging from 0.5 to 16 h. The solid reaction products obtained in these investigations were characterized by X-ray diffraction, chemical nonmetal analysis, SEM and physical adsorption. The results confirm the validity of the reaction mechanism consisting of two interconnected solid-gas reactions (oxide with CO and carbon with CO2) and three reaction steps for the case of presence of nitrogen in the atmosphere at atmospheric pressure, leading to the formation of oxycarbonitrides and carbonitrides, with the oxide particles as the precursors. Due to its instability in nitrogen-containing atmospheres, Ti2O3 does not appear as an intermediate reaction product if nitrogen is present. Other factors leading to its appearance in noble gas atmospheres have not been determined, but a carbon deficiency in the starting mixture relative to a stoichiometric mixture for the formation of TiC can safely be excluded. Nitrogen is probably not incorporated into the oxide intermediate products; however, this is difficult to demonstrate experimentally. Nitrogen is incorporated into the solid phase as cubic TiCxOyNz is formed, its content depending on its partial pressure. At the same time, an increase in nitrogen content above 10% seems to have no effect on the oxygen content of the cubic phase. Despite their different structures, furnace black and graphite do not show clear differences with respect to their reaction kinetics. The composition and particle size of the oxycarbonitride obtained in identical reaction conditions are nearly independent of the type of carbon used. The change in the specific surface area with progressing reaction mainly results from the regeneration Of CO2 to CO, which proceeds primarily on the external surface of the carbon particles for the reaction conditions investigated in this work. (C) 2002 Elsevier Science Ltd. All rights reserved.