Reduction kinetics of porous zinc oxide pellet with CO–N2 gas mixture

Abstract

Effect of purity on reduction kinetics of porous zinc oxide bearing pellets in a 2 : 1 nitrogen–carbon monoxide gas mixture has been studied at 1273 K, and the experimental results have been compared with the data available in literature. Two types of materials have been selected for experimentation: laboratory grade 99% pure zinc oxide and industrial grade zinc calcine containing 78% zinc oxide. In each experiment, a single pellet with an average porosity of 60% has been subjected to reduction inside a vertical reactor under a constant flow rate of N2–CO gas mixture. Subsequently, the reduced pellets have been studied under a scanning electron microscope and an image analyser for determination of the extent of reaction at different depths of the pellet. The topmost surface layer of the reacted pellet has been found to be friable indicating the presence of loosely bound particles. Image analysis study has established that the individual zinc oxide particle of any single pellet at different depths from top surface of the pellet has not undergone any appreciable size change during the course of reaction. Such evidence suggests that no appreciable reduction has taken place below the top surface of the pellets. Assuming the process to be topochemical and irreversible with reaction rate as the rate controlling step, the rate constants have been estimated and found to be the same for both the materials, agreeing well with those reported in literature for a non-porous pure zinc oxide pellet.