Kinetics of Estonian Phosphate Rock Dissolution in Hydrochloric Acid

The kinetics of the dissolution of Estonian phosphate rock and the governing reaction mechanisms in hydrochloric acid in technological processes were investigated. The influences of particle size and acid concentrations of 0.5–1.5 M on the reaction rate and the pH variation during the process were studied at a dosage of 2.1 moles of HCl per mole of calcium for 60 min. The results indicated that the solubility of phosphorus reached 94%–100% for the fine samples and 82%–99% for the coarse samples. The time required to achieve an apparent steady-state pH reduced with the increasing acid concentrations and decreasing particle sizes. It was determined that the CaF2 precipitation in solutions starting at 1 M was faster at higher concentrations. The SEM surface analysis of the insoluble particles proved the existence of etch pit formation. The XPS and EDX analyses affirmed that the dissolution was incongruent. The surface composition of the unreacted particles gave a stoichiometry of CaF1.8, showing the formation of CaF2 on the surface. The dissolution kinetics were analyzed using the shrinking core model and showed a combination of chemical reaction, diffusion or interfacial transfer, and diffusion, sequentially for coarse particles or simultaneously for fine fractions.