Surface analyses of calcite particles reactivity in the presence of phosphoric acid

The reactivity of calcite in phosphoric acid solutions of concentrations between 0 and 0.5 M was monitored during 15 min with complementary in-situ and ex-situ analytical techniques such as Raman and infrared spectroscopies, XRD and SEM analysis. The Raman and infrared spectra show the characteristic carbonate peaks of calcite and phosphate peaks attributed to a new phosphate phase which intensity decreases and increases, respectively with increasing phosphoric acid concentration and reaction time. A similar trend was observed with increasing acid concentration in the XRD analysis of calcite which also revealed that this new phosphate phase is dicalcium phosphate dihydrate or brushite. Fine sheets of calcium phosphate particles were observed by SEM analysis covering the calcite particles at a high phosphoric acid concentration. These changes on the calcite surface were compared to the consumption of protons by the calcite particle suspension with increasing phosphoric acid concentration. The formation and stability of passivating calcium phosphate phases on the calcite surface was evidenced. The obtained results allow to define the contact time and phosphoric acid concentration intervals where the surface of calcite is not passivated by the brushite formed or adsorbed on the mineral surface.