Fundamental studies on the mechanisms and kinetics of reduction of dense synthetic nickel oxide have been carried out in H2-N2 and H2-H2O mixtures. The influences of temperature, hydrogen partial pressure, and hydrogen-steam ratio on the reduction process were systematically investigated. The kinetics of the reduction process were followed metallographically by measuring the advance of the nickel product layer. The microstructures of the reduction products and their changes during heating were characterized using a high resolution scanning electron microscopy. In H2-N2 mixtures and H2-H2O mixtures with low steam content, it was found that the initial reduction rates were first order with respect to hydrogen partial pressure. In both sets of mixtures, it was found that the progress of NiO reduction was not a monotonic function of temperature. A minimum rate of advancement of NiO reduction was observed in the temperature range 700oC – 800oC depending on the hydrogen partial pressures and reduction time. A number of distinctly different nickel product microstructures, originating at the Ni-NiO interface were observed under various reduction conditions, namely coarse fibrous nickel with fissures, fine porous nickel-planar interface, large porous nickel-irregular interface and dense nickel product layer. The microstructures of reduction product were found to change with temperature and time. It was found that heating the coarse fibrous nickel structure resulted in a re-crystallization, grain growth and densification of nickel product. When the heat treatments were carried out on the porous nickel structures, the number of pores decreases with increasing temperature and time, which was accompanied by the increase in the pore sizes. The microstructures and kinetics of the reduction of nickel oxide were found to be a function of temperature, gas composition and reaction time. The study provides strong evidence for a link between the reduction kinetics and the changes in the reduction product microstructures. Mechanisms and kinetics of the reduction of nickel oxide have been discussed by considering reduction conditions, information on the structures and elementary processes involving in the reduction process.
| Identifer | oai:union.ndltd.org:ADTP/253978 |
| Creators | Taufiq Hidayat |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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