Reduction of mixed spinel oxides. Final report, April 1, 1976-August 31, 1978

Abstract

Dense ferrites of nickel and cobalt, pure and alloyed with various amounts of aluminum in solid solution were reduced in hydrogen between 500 and 1000/sup 0/C. The reaction is topochemical and a product layer proceeds from the surface inward. The rate of the interface advance is determined by a variety of factors including the gas pressure, the temperature, and ferrite composition. In cobalt ferrites, a reaction rate anomaly was observed between 600 and 800/sup 0/C, depending on the cobalt ferrite composition. In this temperature range, the reaction rate was found to decrease with increasing temperature. This is common in the reduction of iron-bearing oxides. The phenomena is explained in terms of the rate of the scale/oxide interface advance in comparison with the rate at which the wustite subscale forms. The study of the reaction kinetics at various temperatures and pressures permitted the evaluation of a number of important kinetic constants. The relative importance of the gas oxide interface reaction and the solid state diffusion processes is discussed. Much of the reduction rates can be understood in terms of a relatively simple model containing only 2 explicit materials parameters.