APPLICATION OF A NONISOTHERMAL THERMOGRAVIMETRIC METHOD TO THE KINETIC-STUDY OF THE REDUCTION OF METALLIC OXIDES .2. A THEORETICAL TREATMENT OF POWDER BED REDUCTION AND ITS APPLICATION TO THE REDUCTION OF TUNGSTEN-OXIDE BY HYDROGEN

Theoretical treatment of nonisothermal kinetic studies has been extended in the present work to gas-solid reactions in powder beds. An expression for the activation energies for the reaction has been derived on the basis that the reaction proceeds by the movement of the reaction front, the velocity of the movement being kept constant. The reliability of the method has been tested by applying the same to the reduction of tungsten oxide by hydrogen. The experiments were carried out using thermogravimetric technique under both isothermal and nonisothermal conditions. The reaction front is considered to consist of a thin layer of small particles. The rate of the reduction seems to be controlled by the chemical reaction between the product and the unreacted core existing in each of the small particles. Using the expression derived in the present work, the activation energy of the reaction was calculated from the results of the nonisothermal experiments to be 83.62 kJ . mol-1. This value is in good agreement with the value of 83.17 kJ . mol-1 evaluated from isothermal experiments.