The mechanisms and kinetics of reduction of zinc oxide solid solutions

Abstract

The reduction of dense zinc oxide (ZnO) samples with and without impurity oxide additions in CO/CO 2 and CO/N 2 gas mixtures has been investigated at temperatures between 1173 and 1373 K. The microstructural characterization of the reacted and partially reduced samples was complemented with measurements of the rates of the reactions. The kinetics of reduction of ZnO were found to be dependent upon the CO partial pressure, temperature, degree of reduction, and the type of impurity oxides (i.e., FeO, MgO, CaO, SiO 2 , MnO, Al 2 O 3 ) in the samples. The reduction of ZnO solid solutions resulted in the formation of a planar ZnO reaction interface and a porous product layer composed of impurity oxides, whereas the reduction of pure ZnO resulted in selective attack of particular crystal planes and an irregular growth interface. An interface reaction model has been proposed for the observed reaction kinetics and product microstructures for the reduction of ZnO solid solutions. From the analysis of the relative rates of the various chemical reactions and mass transport processes, the rates of reduction of ZnO with and without impurity oxide addition were found to be controlled jointly by chemical reaction at the ZnO reaction interface and pore diffusion of reducing gas through the porous ZnO layer or reaction product layer.