Colemanite is one of the most important boron minerals used for production of boric acid. Boric acid (H3BO3) is produced by the reaction of colemanite (2CaO (.) 3B(2)O(3) (.) 5H(2)O) with sulfuric acid in a heterogeneous solid-liquid reaction leading to crystallization of gypsum as a byproduct. The influence of ultrasound on the dissolution of colernanite in H2SO4 solution and on the precipitation of the gypsum during the reaction was investigated. Experiments have been carried out in a batch stirred vessel at 85 degrees C in the absence and presence of ultrasound. The stirring rates were chosen as 600 rpm (for 500-600 mu m) and 800 rpm (for 1000-1180 mu m) to provide a homogeneous suspension during the reaction. The boric acid and calcium ion concentrations in the solution were determined as a function of time. The results showed that ultrasound enhances the dissolution of colemanite and precipitation rate of the gypsum in the solution after 1 h. It has been shown that the use of ultrasound decreases the size of gypsum crystals. (c) 2006 Elsevier B.V. All rights reserved.