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Experimental studies of the kinetics and mechanism of the process, decomposition of apatite by phosphoric acid, in the ApatiteH3PO4-H2O system without the addition of sulfuric acid have been performed. The study of the decomposition process of Kovdorsky apatite with certain particle sizes was carried out in a batch reactor with a volume of 1 dm with stirring of the reaction mixture, and an initial concentration of phosphoric acid of 17% by weight, at a temperature of 78–82 °C. Observation of the process was carried out by determining the concentration of phosphoric acid and the concentration of monocalcium phosphate. The acidity of the reaction mixture was determined by the pH meter readings (pH-105 MA with a glass combined-ESC-10603 electrode). It was shown that during the whole process a constant smooth increase in the pH value of the reaction mixture to pH 6 occurs. Comparison of the pH values of the reaction mixture during the actual at the time of determining the concentration of phosphoric acid and pH of phosphoric acid of the corresponding concentration in the aqueous solution shows that the pH value of the reaction mixture is significantly affected by the presence of monocalcium phosphate gel. During the process, during the first thirty minutes, the concentration of phosphoric acid decreases from 17 to 10% by weight, the corresponding quantitative formation of monocalcium phosphate gel and a proportional increase in the pH of the reaction mixture. Then, as the concentration of phosphoric acid decreases, the process slows down and does not proceed to the end under the experimental conditions. The dependence of the concentration of hydrogen ions in the reaction mixture on the time of the process of decomposition of apatite in phosphoric acid, which is presented in logarithmic coordinates, shows that the mechanism of formation of hydrogen ions during the whole process does not change. Thus, it is shown that the process of decomposition of apatite by phosphoric acid in the Apatite-H3PO4-H2O system proceeds with the formation of an intermediate product monocalcium phosphate gel. When this occurs, a corresponding significant change in the pH values of the reaction mixture occurs. During the whole process there is a constant decrease in the concentration of phosphoric acid.

Mechanism of the process of decomposition of apatitis with phosphoric acid