Reduction of ZnO Using Hydrogen: A Kinetic Study and Microstructure Evolution Analysis

Abstract

This study investigates the kinetics of ZnO reduction using hydrogen gas mixtures of 15% H2-Ar, 50% H2-Ar, and 100% H2 as the reducing agent. The reaction kinetics were analyzed at temperatures ranging from 600 to 1000 °C to determine the reaction rate and possible rate-controlling mechanism. Weight-loss data of pellets obtained from reduction experiments conducted in a resistance heating furnace were used as input for the kinetic analysis. Kinetic model-fitting analysis indicated that the ZnO reduction reaction appeared to be controlled by chemical reaction at the gas–solid interface. Higher temperatures and hydrogen concentrations significantly enhance the reaction rate, leading to an increased overall reduction degree. The activation energies for experiments carried out at 15% H2-Ar, 50% H2-Ar, and 100% H2 were calculated to be 118 kJ/mol, 110 kJ/mol, and 104 kJ/mol, respectively. This small variation in the activation energy indicates that the rate-limiting mechanism remains largely unchanged within the investigated hydrogen concentration range. Microstructure evolution analysis of the reduced ZnO pellet shows that the reduction reaction occurred simultaneously at both the center and surface of the pellet. The reduced pellet showed a porous morphology. Initially, the pellet develops a fine, uniform porous microstructure. However, over time, larger pores form, accompanied by sintering of the unreduced ZnO pellet. Meanwhile, the sublimated Zn gas was successfully captured and condensed as metallic zinc in the condenser.