Mechanisms and Kinetics of Reduction of Solid NiO in CO/CO 2 and CO/Ar Gas Mixtures
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INTRODUCTION
THERE have been many studies examining the gaseous reduction of nickel oxide since the behavior of this system is of fundamental scientific interest as well of relevance to the industrial production of nickel metal. Most of the experimental studies of NiO reduction have been undertaken using hydrogen as the reductant; there have been relatively few studies of the reduction of solid NiO in CO/CO2 gas mixtures and, as a result, the reaction mechanisms and kinetics of reduction in this system are poorly understood. The formation and growth of Ni metal on the surface of dense NiO plates during reduction in flowing CO gas was observed directly using an optical microscope in conjunction with a hot stage apparatus.[1] The reduction was undertaken at temperatures from 873 K to 1273 K (600 °C to 1000 °C) in pure CO (1 atm.). At 973 K (700 °C) and below, following initial Ni nuclei formation and growth of a dense nickel metal layer on the NiO surface the rates of reduction were found to be drastically reduced. In contrast, in the same study, the complete reduction of NiO to Ni in H2 was observed in the temperature range from 473 K to 1273 K (200 °C to 1000 °C). An experiment was undertaken at 673 K (400 °C) that involved switching the reactant gas from
JIANG CHEN and PETER C. HAYES are with the PYROSEARCH, Pyrometallurgy Innovation Centre, School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted February 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
initially hydrogen to CO, then back to hydrogen. The sample was observed to reduce under the initial hydrogen H2 atmosphere but no reduction was observed with CO; the reduction resumed when the H2 was reintroduced into the system. The authors claimed that the reduction in CO was halted through the formation of Ni3C, but this phase was not explicitly identified. The reduction of pure NiO pellets, with open porosities ranging from approximately 3 to 37 vol pct, was carried out in CO/CO2 gas mixtures at temperatures between 839 K and 1069 K (566 °C and 796 °C).[2] The temperatures and gas compositions were selected to ensure that no deposition of carbon or Ni3C formation took place. This was confirmed by X-ray powder diffraction analysis of some of the pellets after reduction; these measurements verified the absence of carbon and of Ni3C in the products. The rates and extents of reduction were measured by weight changes of the samples. The apparent reaction rates were estimated from the initial rates of reduction and were found to be first order with respect to CO partial pressure. The apparent activation energy between 839 K and 955 K (566 °C and 682 °C) was reported to be 196.5 kJ mol1; the reaction rate constants were found to be independent of temperature between approximately 953 K and 1069 K (680 °C and 796 °C). The rates of reduction of NiO pellets in pure CO (1 atm.) were measured for temperatures ranging from 1120 K to 1272 K (847 °C to 1099 °C).[3] The porosity of
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