Kinetics of gypsum formation and growth during the dissolution of colemanite in sulfuric acid

Abstract

One of the most important boron minerals, colemanite, is dissolved in aqueous sulfuric acid to produce boric acid whereby gypsum is formed as byproduct. Filtration of gypsum has an important role in boric acid production because gypsum affects the efficiency, purity and crystallization of boric acid. The formation and growth kinetics of gypsum during the dissolution of colemanite in aqueous sulfuric acid were studied in a batch reactor by varying the temperature (60–90°C), stirring rate (150–400 rpm), and initial concentrations of the reactants. The initial CaO/H 2SO 4 molar ratio was varied between 0.21–0.85 by keeping the initial concentration of sulfate ion at [SO 4 2−] o=0.623 mol/l, and 0.85–3.41 by keeping the initial concentration of colemanite at [B 2O 3] o=0.777 mol/l. The crystallization of gypsum from the solution was followed by monitoring the calcium ion concentration in the solution as it is decreased by the formation of calcium sulfate precipitate. The calcium ion concentration in the liquid phase first undergoes a rapid exponential decay and then slowly approaches an asymptotic value of the saturation concentration at the respective temperature. The saturation concentration decreases with the increasing temperature from 5.2 mmol/l at 60°C to 3.1 mmol/l at 80°C, however, further increase in the temperature up to 90°C causes an increase in the saturation concentration to 5.1 mmol/l. The stirring rate was found to have no significant effect on dissolution in the range of 150–400 rpm. The minimum saturation concentration of the calcium ion was obtained at 80°C when the initial CaO/H 2SO 4 molar ratio is 0.85. The boric acid concentration in the solution decreases with the decreasing initial concentration of sulfuric acid. After the fast dissolution reaction of colemanite in aqueous sulfuric acid, the nucleation of the gypsum crystals first occurs from the supersaturated solution and then the crystals grow on these nuclei. The needle like crystals become wider and taller on prolong crystallization. The rate of gypsum crystallization reaction was second order with respect to saturation level. The evaluation of the kinetic data in an Arhenius plot gives an activation energy of 34±2 kJ/mol for the crystal growth of gypsum from the supersaturated solution obtained by dissolution of colemanite in aqueous sulfuric acid.