A kinetic study on the leaching of Fe₂O₃ in HNO₃ solutions has been made by using agitated vessel. Three types of powder specimens, (A) reagent grade Fe₂O₃, (B) calcinated FeC₂O₄ and (C) sintered Fe₂O₃, were sized by a wet classification. The specific areas of these specimens were measured before and after the leaching experiments. Leaching rates of the powder specimens were measured at various conditions of rotation speed of the stirrer (n), concentration of HNO₃(C_HNO3), and liquid temperature (T). A microscopic observation of a polished surface of a sintered Fe₂O₃ disk was also made after exposing the surface in HNO₃ solution.
The initial leaching rate was found to be independent of n and proportional to C_HNO3 and initial specific area. The apparent activation energy was 82 kJ/mol. The leaching rate beyond initial stage increased with increasing time, in spite of very small change in the specific area with leaching. From the micrograph of a partly dissolved surface of a sintered Fe₂O₃ disk, only the (0001) oriented crystal planes were dissolved and its surface showed coarse irregularities. From these results, the acceleration of the leaching rate was thought to be caused by the anisotropic dissolution and the increase in the surface area of the (0001) plane at the surface of each Fe₂O₃ particle.