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

One of the most important boron minerals, colemanite, is dissolved in aqueo us sulfuric acid to produce boric acid whereby gypsum is formed as byproduc t. Filtration of gypsum has an important role in boric acid production beca use gypsum affects the efficiency, purity and crystallization of boric acid . The formation and growth kinetics of gypsum during the dissolution of col emanite in aqueous sulfuric acid were studied in a batch reactor by varying the temperature (60-90 degreesC), stirring rate (150-400 rpm), and initial concentrations of the reactants. The initial CaO/H2SO4 molar ratio was var ied between 0.21-0.85 by keeping the initial concentration of sulfate ion a t [SO42-](o) = 0.623 mol/l, and 0.85-3.41 by keeping the initial concentrat ion of colemanite at [B2O3](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 prec ipitate. 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 degreesC to 3.1 mmol/l at 80 degreesC, however, further increase in the temperature up to 90 degreesC causes an increase in the saturation concent ration to 5.1 mmol/l. The stirring rate was found to have no significant ef fect on dissolution in the range of 150-400 rpm. The minimum saturation con centration of the calcium ion was obtained at 80 degreesC when the initial CaO/H2SO4 molar ratio is 0.85. The boric acid concentration in the solution decreases with the decreasing initial concentration of sulfuric acid. Afte r the fast dissolution reaction of colemanite in aqueous sulfuric acid, the nucleation of the gypsum crystals first occurs from the supersaturated sol ution and then the crystals grow on these nuclei. The needle like crystals become wider and taller on prolong crystallization. The rate of gypsum crys tallization reaction was second order with respect to saturation level. The evaluation of the kinetic data in an Arhenius plot gives an activation ene rgy of 34 +/-2 kJ/mol for the crystal growth of gypsum from the supersatura ted solution obtained by dissolution of colemanite in aqueous sulfuric acid . (C) 2001 Elsevier Science B.V. All rights reserved.