High temperature chlorination kinetics of a niobium pyrochlore

Abstract

The chlorination kinetics of a niobium (Cb) pyrochlore has been studied between 1873 and 2223 K, the chlorine concentration in helium varying between 0 and 20 pct. The pyrochlore was subjected to a preliminary thermal treatment at 1473 K in order to remove fluorine which escaped under the form of niobium oxyfluorides. This left NaNbO3, CaNb2O6 and residual refractory oxides. The large chlorination reaction rate difference between NaNbO3 and CaNb2O6 made possible the definition of distinct chlorination reaction rates for these constituents. It was found that the decomposition of CaNb2O6 is the controlling step in the chlorination of this constituent, while Nb2O5 (NbO2 + NbO2 at the prevailing temperatures) chlorination is very fast. The reaction is second order with respect to CaNb2O6 concentration and first order with respect to chlorine partial pressure between 1873 and 2023 K, a distinct reaction rate equation being obtained at 2223 K. Reaction rate constants have been calculated and vary between 3 and 10 moleJ.kg ·min for the temperature range considered. The NaNbO3 reaction rate is first order with respect to total Nb2O5 concentration and 2.5 order with respect to chlorine partial pressure for the temperature range covered (1973 to 2223 K). Reaction rate constants are much higher than in the former case, being respectively 148 (1873 K), 214 (2023 K) and 518 (2223 K) mole/kg-min. Reaction orders may be affected by an error varying between 16 and 40 pct. The reaction rate constants are found accurate within 40 pct for CaNb2O6 and 25 pct for NaNbO3.