Hironori Mine1, Masanori Tokuda2 and Masayasu Ohtani2
1Graduate School, Tohoku University, Sendai. Present address: Nippon Light Metal Corp. Ltd., Tomakomai
The reduction of plate specimens of Cu2O and NiO with H2 and CO has been studied over the temperature range of 200° ∼ 1000°C. The reduction process was observed by a high temperature microscope, and the pore structure of the reduced metal was also observed by a scanning electron microscope. The results obtained are summarized as follows:
(1) At the initial stage of the reduction of Cu2O, the incubation period was observed below 300°C. In the low temperature range, the chemical reaction is the rate determining step, and the activation energy of the reaction being 14 ± 2, and 29 ± 2 kcal/mol in the reduction with H2 and CO respectively. With rising temperature, the effect of pore diffusion appears gradually with its increasing contribution to the rate determining step.
(2) The shape and size of pores in the reduced metals are strongly influenced by the mechanism of the formation of the metallic phase which depends on temperature. Sintering may not be considered as an effective factor for the pore distribution except in a high temperature range near the melting point.
(3) In the reduction with H2, many nuclei of metallic Cu appear on the surface, then grow almost uniformly and the reduction proceeds topochemically. On the contrary, in the reduction with CO, a few nuclei appear locally and grow over the surface and the reduction was not observed topochemically in macroscopic structure.
(4) The reduction processes of NiO with H2 and CO are quite different each other. In the reduction with H2, the incubation period was not observed. Below 400°C, the chemical reaction is considered as the rate determining step and 12 ± 1 kcal/mol is obtained as the activation energy.
In a high temperature range, pore diffusion is an important factor in the determination of the rate determining step. The plate specimen of NiO is hardly reduced by CO below 600°C because of the formation of the protective Ni3C layer. At a higher temperature, the protecting effect of Ni3C becomes weak and reduction is able to proceed steadily, but the rate of the reduction with CO is much smaller than that of H2 reducion.
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