Deoxidatlon rate of copper droplet levitated in Ar-H 2 gas stream
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INTRODUCTION
THE levitation melting technique provides a unique means for examining reactions between gas and molten metal without any contamination problem from refractory materials. A number of articles dealing with this technique have been published during the 1950s and 1960s. r~-51The technique has recently been gaining greater prominence, because electromagnetic force fields can be used to control the shape of the liquid metal surface, as seen in the cold crucible technique, t6,71 or to position the samples, as in space processing experiments, is1 However, the mass transfer inherent in the levitated droplet has not been thoroughly understood in spite of the technological importance of these modem materials processing methods. Forster and Richardson t91 oxidized small levitated drops of copper in a gas mixture of carbon dioxide and carbon monoxide. They found that a small amount of silicon seriously impeded the oxidation at the temperatures up to 1773 K and that the oxidation rate of desiliconized copper droplets was controlled by surface reaction. They also concluded that a liquid film mass-transfer coefficient of less than 1 • 10 -4 m" s -1 was highly unlikely. The significance of liquid film mass-transfer resistance, on the other hand, was pointed out by Robertson e t a/. tl~ They examined the kinetics of decarburization and carburization of levitated molten iron-carbon alloy drops at 1923 K in CO-CO2 gas mixtures up to a total pressure of 4 MPa. The results showed that the decarburization rate was controlled by mass transfer in the gas phase, but the carburization was controlled by mass transfer in both the gas and liquid phases. It was presumed in their analysis that the levitated droplet behaves as if it were Y. FUKUNAKA, Research Associate, K. TAMURA, Graduate Student, and Z. ASAKI, Professor, are with the Department of Metallurgy, Kyoto University, Kyoto 606, Japan. N. TAGUCHI, formerly Student, is with Sumitomo Life Insurance Company Ltd. Manuscript submitted July 5, 1990. METALLURGICAL TRANSACTIONS B
a static liquid sphere and that the transient behavior of solute elements in the liquid phase could be described by the diffusion model with an appropriate value of the diffusivity. The significance of liquid-phase transport resistance was also supported by the mathematical analysis of the electromagnetic force field, t121 In the present article, the deoxidation kinetics of a copper droplet levitated in an Ar-H2 mixture at relatively high flow rates of gas and at high temperatures has been examined. The thermodynamics of the deoxidation reaction of molten copper in hydrogen is reasonably well understood, t~3,~4] Based on this information, the effects of initial oxygen content of the copper, partial pressure of hydrogen gas, and flow rate of the gas mixture were analyzed to understand the reaction mechanism of the deoxidation reaction and to clarify the rate-controlling step of this reaction essential to producing highly purified copper.
II.
EXPERIMENTAL
The experimental arrangement is shown in Figure
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