Chlorination kinetics of a niobium pyrochlore in the Gas-Solid phase

Abstract

The chlorination kinetics of a niobium (columbium) pyrochlore has been studied in the gas-solid phase, for temperatures between 1373 and 1573 K, using a high temperature differential tungsten reactor. Chlorine-helium mixtures were used which contained between 0 and 60 pet helium. It is shown that the kinetic study reduces to one of CaNb₂O₆ chlorination. In order to obtain information on the true reaction mechanisms involved and avoid side effects like difficulty of access of the reactant gas throughout the sample mass subjected to reaction, the reaction rate has been determined from decreasing amounts of initial solid sample. The reaction rate obtained by extrapolation to nil sample values was considered to be the true reaction rate that would be observed if a single particle were subjected to chlorination with the prevailing conditions. Using a reactant gas flow rate which provided a purely chemical reaction process (no film diffusion effects), it has been found that the reaction is of the continuous-reaction type model, while the reaction rate is nearly first order with respect to the chlorine concentration at the solid-gas interface. The rate constants are 0.21 (at 1373 K), 0.46 (at 1473 K) and 0.92 (at 1573 K) min⁻¹.atm⁻¹. The energy of activation was found equal to 129 KJ/mol. The theoretical maximum error, calculated from a knowledge of the error made on temperature, time, sample weight and Nb₂O₅ analysis, does affect the reaction order by ± 20 pct, the reaction rate by ± 20 pct and the energy of activation by ± 25 pct.