Activation energy determination and kinetic modeling of thorium oxide dissolution in nitric acid/hydrofluoric acid system: Influence of fluoride ion on ThO2 dissolution

The dissolution of thorium oxide (ThO2) is investigated in nitric acid/hydrofluoric acid (HNO3/HF) system as a function of several parameters such as reaction fluoride ion content, HNO3 concentration and reaction temperature using an experimental reactor. Based on results, the dissolution of ThO2 increased with increasing of HNO3 concentration and temperature. The normalized weight losses (N-L) and normalized dissolution rate (R-L) of ThO2 were evaluated to determine the partial order related to the proton concentration (n) and apparent activation energy (E-App). The value of E-App was found to be 21.4 (kJ/mol) for pure HNO3 system and 6.9 (kJ/mol) for HNO3/HF system over the reaction temperature range of 25 degrees C-85 degrees C. The applicability of eleven different kinetic models was studied to describe ThO2 dissolution. The kinetic data was best fitted by Kabai equation and kinetic results indicated that the dissolution process is controlled by the surface chemical reaction at the liquid-solid interface. (C) 2016 Elsevier Ltd. All rights reserved.