Dissolution equilibria and kinetics of fluorite and calcite mixtures

Abstract

Minerals commonly contain impurities, and their dissolution involves complicated ionic equilibria and multicomponent mass transfer. This paper describes the experiments that were carried out and proposes a mechanism for dissolution behavior of fluorspar containing calcite as a major impurity in both batchwise and continuous packed-bed systems.The mechanism is based on analysis of coupled equilibria among the soluble species. Of more than eight potentially relevant species, only three (viz., F-, HCO,-, and Ca2+) are significant. The coupled flux equations for Fand HCO,are written in terms of "main" and "cross" mass transfer coefficients, with the concentration of Ca2+ being accounted for by electroneutrality. Only one main mass transfer coefficient needs to be determined experimentally; and other coefficients can be evaluated from it by means of simple diffusion coefficient ratios, which are determined independently. The Stanton number based on the main mass transfer coefficient is correlated with Reynolds number and the Schmidt number for packed bed dissolution.A comparison of the exact solution (Eq. 9) with the approximate solution (Eq. 10) is shown in Figure 1 for a range of aqueous carbon dioxide activities. Since the discrepancies are negligible except at very low concentration of aqueous carbon dioxide, the Vol. 35, No. 8 AICbE Journal 15 and 16 then become: open system of fluorspar dissolution equilibrium (viz., = 0.0013 mM) can be treated as a dilute ternary system consisting of F-, and Ca2+. *Obtained by recirculation of effluent at 500 mL/min. Other data were measured in zero-flow rate columns (column initially filled with distilled water). All the experiments were conducted after aging process was completed, i.e., after about 80 hours dissolution in the packed column (cf. Figure Al).