Chlorination kinetics of a niobium pyrochlore in the Gas-Solid phase
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IN a previous paper, the kinetics of the chlorination of a niobium pyrochlore in the liquid phase has been described2 The present paper deals with the chlorination of a pyrochlore mineral of the same origin (St-Honorr, P.Q., Canada), in the solid phase. The mineral is of the approximate formula (Na0.73, Ca0.95 , Ao.32) (Nb~ ~2, Ti0.u, B0.03) (06.25 , F0.75) with A representing rare earth metals and Sr, while B is written for Fe and Zr. The composition of the mineral used for the experiments is shown in Table I. The kinetics of the reaction has been studied between 1373 and 1573 K, temperatures well below the fusion point of the starting material. At these temperatures, the chlorination of NaNbO3, which is a component of the mineral, occurs with evolution of NaC1, NbO2F and NbOC13, this reaction proceeding at a much faster rate than does the chlorination of the main component CaNb2062. Since the Nb:O 5 rate of departure resulting from the CaNb206 chlorination is measured by means of X-fluorescence before and after chlorination, the possible chlorination of the minor metal oxide components shown in Table I does not interfere in the Nb205 determination. Also, C12 is available in excess. As a result, the study reduces to the chlorination of CaNb~O 6 in the solid state, after a short time. With the exception of what is available on Nb205 chlorination, information on the subject of CaNb206 chlorination kinetics is scarce. The paucity may be traced, like in many other similar cases, to the difficulty in identifying the true rate control mechanism in such reactions. Many attempts have been made to study the kinetics of gas-solid reactions 3,4,5in which pellets are PAUL MEUBUS is Professor, Applied Sciences Department, and Centre de Recherche du Moyen Nord, Universit6 du Qurbec ~t Chicoutimi, Chicoutimi, Canada. Manuscript submitted May 7, 1980. METALLURGICAL TRANSACTIONS B
exposed to a gas stream. However, the interplay of physical and chemical factors makes it difficult to identify the true chemical reaction rate and, in addition, the extension to the case of single small particles is dubious. In this respect, two interesting papers 6,7 describe a model for gas-solid reactions and an application case. The model and derived experimental data provide information on the intrinsic kinetic parameters, the experiments being conducted with pellets of initial variable size. However, a number of restrictive assumptions decrease the application range of the model: first order reaction is assumed, no structural changes should occur in the course of the reaction, and isothermal conditions are required. In the present study, a method is used which is also based on the principle of decreasing amounts of solid reactant (niobium pyrochlore) subjected to chlorine reaction. However, experimental curves are obtained as a preliminary step, showing the function (weight of sample residue after reaction) v s (initial sample weight). Extrapolation of this function to nil sample value yields the reaction rate for the given operating conditions.
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