Experimental evaluation of a kinetic method for the study of non-catalysed heterogeneous reactions in solid–liquid systems: Leaching of apatite by dilute nitric acid

A method for evaluating the formal reaction order n of a chemical reaction is proposed, based on a kinetic equation describing chemical dissolution of solid particles in a batch reactor. The evaluation is based on the fact that the function expressing the dependence of the conversion, X_B, on the term c_A0ⁿt has the same course for different initial concentrations of the liquid reactant, c_A0 (t denotes time), at constant temperature and ratio of reactants. The measured dependences of the conversion on the time give a function, f_s(X_B), characterizing changes of the solid surface area during the reaction. Minimization of deviations of this function permits the influence of mass transfer between the bulk of the liquid and the solid surface on the measured formal reaction order to be corrected. Knowledge of the value of n and the function f_s(X_B) enables the derived kinetic equation to be integrated numerically and the dependence of the conversion on time to be calculated. A set of experimental data measured during dissolution of natural apatite in dilute nitric acid (0.1–1.5 mol dm⁻³) at 20°C was used for demonstration of the proposed method. © 1998 SCI