Investigation of Nickel Product Structures Developed during the Gaseous Reduction of Solid Nickel Oxide

Abstract

The product microstructures developed during the reduction of dense synthetic nickel oxide in H 2 -N 2 and H 2 -H 2 O mixtures between 300°C and 1000°C are reported. A number of distinctly different types of nickel product structures have been identified, namely, coarse fibrous nickel with fissures, fine porous nickel-planar interface, large porous nickel-irregular interface, and dense nickel layer. The influences of gas composition and temperature on the conditions for formation of these microstructures, and their evolution with temperature and time, have been systematically investigated. The condition for formation of the various microstructure types was summarized on morphology maps as functions of reducing gas partial pressure, thermodynamic driving force, and reaction temperature.

On the Relationships between the Kinetics and Mechanisms of Gaseous Hydrogen Reduction of Solid Nickel Oxide

“…From the measurement of pore sizes, [10] it was found that the NiO reduction carried out in the present study takes place in the transition region between Knudsen and molecular diffusion. [13] In the transition region, both of the diffusion mechanisms are operative and the combined diffusivity of the Knudsen and molecular diffusions has been considered.…”

“…The conditions for the formation of these structures have been identified and described in a previous study. [10] In this section, an investigation of the formation mechanisms of each product is carried out by considering the formation conditions, microstructural information, and elementary processes involved.…”

“…The presence of thin, aligned Ni plates within the product has been reported in a previous study. [10] It appears that the thin platelike Ni found in the coarse fibrous nickel with fissures is the initial structure developed by the reduction process. The formation of a platelike metal structure has also been observed in the gaseous reduction of solid calciowustites.…”

“…[2] This experiment [2] shows that sintering the metal product (or physical changes) alone cannot account for the observed rate slowdown. Separate measurements [9,10] of the pore sizes of the Ni product have shown that, in the range 600°C to 1000°C, the changes taking place in the nickel product structure result in an increase in the pore size; the pore volumes, however, remained relatively constant. This indicates that the structural changes in the product took place with relatively little densification.…”

“…The conditions of the formation of these microstructures have been identified and their evolution with temperature and time has been systematically investigated. [10] Although the reduction of nickel oxide appears superficially to be a simple reaction, the factors influencing the rates and extent of reduction are still not completely characterized. In the present article, the fundamental kinetics and mechanisms of nickel oxide reduction are examined.…”