Colemanite, one of the important boron minerals, is dissolved in aqueous sulfuric acid to produce boric acid. In this reaction, gypsum is obtained as a by-product. Gypsum crystals are in the shape of thin needles. These crystals should be grown to an easily filterable size in order to increase the production yield and purity of boric acid. In this paper, the particle size distributions and the volume-weighted mean diameters of the gypsum crystals obtained in batch and continuous flow systems were compared. Experiments in both batch and continuous reactors were performed at a temperature of 85 degrees C, a stirring rate of 400 rpm, and the inlet CaO to SO4-2 molar ratio of 1.0 using colemanite mineral in particle size smaller than 150 mu m. The average diameter of the gypsum crystals obtained at 3.5 h from the batch reactor was found to be 37-41 mu m. This value for the continuous system at steady state was observed to change between 44-163 mu m. The particle size of the gypsum crystals was found to increase with the residence time of the solid in the continuous system. (c) 2006 Elsevier B.V. All rights reserved.