Kinetic investigation of reaction between ulexite ore and phosphoric acid

Abstract

One of the most important boron minerals, ulexite, is dissolved in aqueous phosphoric acid to produce boric acid whereby sodium and calcium phosphates are formed as byproduct. The dissolution of ulexite in aqueous phosphoric acid solutions was investigated in a batch reactor employing the parameters of stirring speed, particle size, temperature, solid/liquid ratio and acid concentration. It was found that the dissolution rate increased with stirring speed and reaction temperature. However, increasing the particle size and solid/liquid ratio decreased the dissolution rate. The conversion rate increased until 0.7 mol L − 1 with increasing acid concentration and then decreased with increasing acid concentration. The dissolution kinetics of ulexite was examined according to heterogeneous and pseudo-homogeneous reaction models, and it was found that the dissolution rate was controlled by product layer (or ash layer) diffusion process. The experimental data were tested by graphical and statistical methods and it was found that the following models for two different concentration levels were best fitted to the experimental data. 1 − 3 ( 1 − x ) 2 / 3 + 2 ( 1 − x ) = [ 197.72 D − 1.74 C 0.63 ( S / L ) − 1.1 ( W ) 2.01 e − 26.17 / R T ] . t ( C ≤ 0.7 M ) 1 − 3 ( 1 − x ) 2 / 3 + 2 ( 1 − x ) = [ 46.97 D − 1.51 C − 0.37 ( S / L ) − 1 ( W ) 1.87 e − 26.17 / R T ] . t ( 0.7 M ≤ C ) where X is the conversion fraction, D is the particle size, C is the acid concentration, S/ L is the solid-to-liquid ratio, W is the stirring speed, t is the reaction time, and T is the reaction temperature. The activation energy for the process was found to be approximately 26.17 kJ mol − 1.