Role of Calcium Content in Dissolution Kinetics of Iranian Borate Minerals in Sulfuric Acid

The dissolution kinetics of pandermite and hydroboracite, present in an Iranian borate ore, in sulfuric acid has been comparatively studied. The effect of particle size, temperature, and acid concentration on their dissolution rate was investigated. Dissolution rates of both minerals increased by reducing the particle size and by raising the temperature. Although, an increase in the acid concentration from 0.5 to 1 N accompanied an increase in the dissolution rate, the concentration beyond 1N did not produce any significant change. The dissolution of hydroboracite occurred appreciably faster than pandermite, which was attributed to its lower calcium content. This could be due to the formation of a product layer namely, CaSO4.2H2O, acting as a diffusion barrier, which in the case of hydroboracite was less effective. The rate data obtained was in agreement with a kinetic model proposed for diffusion- controlled heterogeneous reactions.