Studies on the kinetics of apatite dissolution in sulfuric acid have shown that the formation of insoluble compound films on the surface of the solid phase hinders solvent access to it and leads to a slowdown and even complete cessation of the dissolution process. In the concentration range of 16–25 N H₂SO₄, the dissolution reaction is sharply retarded. At concentrations above 25 N, apatite dissolves rapidly. This is attributed to the different physical structure of the calcium sulfate film, which depends on the concentration of the solvent. ...