Abstract
Apatite (Ap) dissolution in diluted acids is well described in the literature, but in technological processes which use more concentrated acids, the reaction is fast, and it is complicated to follow the process kinetics. The relationship between pH change and the apatite dissolution rate depending on HCl concentration was studied by thermodynamic calculations and experiments with synthetic fluorapatite (FAp). On the basis of experimental pH measurements, the kinetics of dissolution was analyzed. The solution composition (P, Ca, F) was determined by wet chemical methods and the solid part was characterized by XRD and FTIR. It was shown that the amount of HCl needed for FAp dissolution depends on acid concentration. FAp dissolution rate cannot be deduced from solubility data of P, Ca or F as the secondary reactions of CaF2 and CaHPO4 formation take place simultaneously. It was found that the Ap dissolution rate can be followed by pH change.
Highlights
The main source of phosphorus for mankind is natural apatites
The thermodynamic calculations (HsC) were performed for HCl solutions with concentrations from 0.05 to 2 M adding FAp by the step of 0.02 mol and with different compositions of the solid phase formed in the reaction: HsC1—with the formation of CaF2, and HsC2—
The correlation between the pH change and the apatite dissolution rate depending on HCl concentration was studied by thermodynamic calculations and experiments with synthetic fluorapatite
Summary
The main source of phosphorus for mankind is natural apatites. The biggest part of natural calciumfluorapatite (CaFAp) is used for the production of phosphorus fertilizers and phosphoric acid via acidic digestion processes [1].The studies of CaAp dissolution processes in acids have been comprehensively reviewed by Dorozkin up to the year 2010, and a general description of the dissolution stages has been given [2]. The main source of phosphorus for mankind is natural apatites. The biggest part of natural calciumfluorapatite (CaFAp) is used for the production of phosphorus fertilizers and phosphoric acid via acidic digestion processes [1]. The dissolution of apatite is described by different models:. Most of the models have been elaborated for apatite dissolution in either slightly acidic or nearly neutral (4 < pH < 8) aqueous solutions and at relatively low concentrations. The validity of these models for apatite dissolution in strong acids (HCl, HNO3 , H2 SO4 ) at pH < 2 used in the production of fertilizers and phosphorus acid as well as in the recovery technologies of rare earth elements needs specification
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