Akademik Veri Yönetim Sistemi
VDI Berichte, sa.1901 I, ss.465-470, 2005 (Scopus)
Colemanite (2CaO·3B2O3·5H 2O), one of the important boron mineral, is dissolved in aqueous sulfuric acid to produce boric acid. In this reaction, gypsum (CaSO 42H2O) 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 shapes and the volume weighted mean diameters of the gypsum crystals obtained in the four continuous flow stirred slurry reactors in series were compared. During the experiments, the temperature of the reactors was kept constant at 85°C, and complete mixing was attained at the stirring rates of 400 rpm. The ratio of molar feed rates of colemanite to sulfuric acid, were kept constant either at 1.00 or 1.37. The colemanite feed rate was varied in the range of 5-15 g/min. In all the experiments, colemanite mineral, having particle size smaller than 250 μm, was used. The operating parameters were liquid residence time (17 min, 23 min and 35 min) and volume fraction of solid in the reactors (0.04 and 0.06). Solid samples were collected from each of the four continuous reactors after the reactors had reached to steady state. The reaction between colemanite and sulfuric acid was almost completed in the first reactor, however the gypsum crystal growth continued in the other reactors as well as first reactor. The maximum mean diameter of crystals was found either in the first or second reactor, whereas in the latter reactors due to breakage their mean diameters decreased. The comparison of the images showed that as the residence time increased, the crystals grew bigger. The increase of the solid hold-up from 0.04 to 0.06 at the same residence time (17 min) had also a pronounced effect on the mean diameters of the gypsum crystals.