KINETICS OF REDUCTION OF DENSE SINTERED SPHERICAL PELLETS OF NICKEL OXIDE IN FLOWING HYDROGEN.

The kinetics of reduction of dense sintered spherical pellets of nickel oxide in flowing hydrogen has been investigated by simultaneous measurement of the changes in the sample weight and temperature. The maximum flow rate sensitivity of the reaction rate was estimated theoretically and the experiments were carried out at a flow rate above the critical value. The reduction experiments were performed in the temperature range 400-800 degree C and at partial pressures of hydrogen ranging from 0. 2 to 1 atm. The reaction was found to be topochemical and the results were analysed on the basis of heat- and mass-transfer concepts. The reduction was found to be controlled by gaseous diffusion through the porous nickel metal formed. The pore surface area was measured by the BET technique and the measurements showed Knudsen diffusion to be important. A mixed transport mechanism involving Knudsen and molecular diffusion is proposed. A detailed analysis of the results, incorporating bulk flow and viscous flow terms in the flux equations, showed the possibility of a pressure build-up inside the pellet. The proposed transport-control mechanism was confirmed by evaluation of the transport parameters from the experimental data and comparison of them with those obtained by independent experiments. The measured values were found to be in agreement with those estimated from single pellet reduction experiments on the assumption of transport-controlled reduction.