Change in the Surface Area and Dissolution Rate during Acid Leaching of Phosphate Particles at 25 °C

This paper is part of a series of investigations into the reaction rate of acid dissolution of solids and particularly the influence of particle size distribution. In the case of acid dissolution of raw phosphate particles, preliminary results show that the reaction rate is significantly affected by the particle size distribution. In this paper, the dissolution into a 0.3 M H3PO4 solution of one size fraction 125−200 μm (d p = 162.5 μm) prepared by sieving has been studied at 25 °C using a batch method. Experimental data were fitted to an empirical equation of the form m/m ∞ = 1 − e- kt , where m is the mass of phosphate dissolved at time t and m ∞ is the final mass of phosphate dissolved. The dissolution rate constant k was found to be equal to 0.2118 min-1. Variation of the physical properties of the remaining solid with reaction time was studied. Experimental methods such as scanning electron microscopy (SEM), mercury porosimetry, argon adsorption/desorption isotherms, and volume displacement using helium for the determination of the true density were used. As a result, a significant change in the Brunauer−Emmett−Teller surface area (S BET) occurred during the reaction. S BET was 16 m2·g-1 at t = 0 min, it reached a minimum value of 10 m2·g-1 at 7 min, and it increased again to 24 m2·g-1 at t = 50 min. Moreover, SEM observations revealed that phosphate particles dissolve beginning from their surface without any disintegration.