Kinetics of removal of bismuth and lead from molten copper alloys in vacuum induction melting
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I.
INTRODUCTION
THE rates of removal of bismuth and lead from molten copper alloys in vacuum melting and the effects of oxygen and sulfur on the rates have already been investigated in small and large scales as fundamental studies of refining of crude copper, fl-4J It was found that bismuth and lead were rapidly eliminated. In a previous study by the author, t21 the following were observed: (1) A decrease in stirring rate of the melt does not necessarily decrease the rates of removal of bismuth and lead, and (2) oxygen and sulfur decrease the rates of removal. The former observation was contrary to current theoretical prediction. The present study was undertaken in order to investigate the mechanisms by which bismuth and lead are removed and by which oxygen and sulfur decrease the rates of removal of bismuth and lead. The vacuum induction melting was carried out at 1403 K on copper alloys containing bismuth and/or lead and either oxygen or sulfur under three different heating conditions considered to differ from one another in the level of stirring. The observed rate of removal of bismuth or lead was expressed by a first-order rate equation involving the final concentration of the impurity. The overall mass-transfer coefficients defined by this equation were then determined. The rate-determining step was REIICHI OHNO, Associate Professor, is with the Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan. Manuscript submitted December 5, 1989. METALLURGICAL TRANSACTIONS B
determined from the loss of c o p p e r f r o m the melt, and the liquid-phase mass-transfer coefficients of the impurities were calculated. For bismuth and lead in copper alloys, except those containing oxygen, the liquid-phase mass-transfer coefficient at the lowest stirring was greater than that at intermediate stirring. A new model for liquidphase mass transfer was developed to explain this fact. Consideration was given to the effects of stirring on the adsorption of oxygen and sulfur on the surface of the melt and of adsorbed oxygen and sulfur on the evaporation mass-transfer coefficients for bismuth and lead. An investigation was made of the effects of oxygen and sulfur on the overall mass-transfer coefficients, liquidphase mass-transfer coefficients, and final concentrations of bismuth and lead. II.
THEORY
A. Kinetics of Solute Removal The removal of solute atoms from a liquid metal under reduced pressure involves the following three processes: (a) transfer of the solute atoms through the metal to the metal-gas interface; (b) evaporation at the interface; and (c) transfer through the gas to a condensing surface.
1. Transfer from bulk liquid to interface [process (a)] The molar flux of i in process (a) is expressed in the form[S] J~ = KL(C,- C*) [1] VOLUME 22B, AUGUST 1991--447
The mass-transfer coefficient within the liquid under forced convection can be expressed by a correlation of the form Sh = f ( R e , Sc), where Sh represents the Sherwood number (KtZ/D), Re the Reynolds number (UZ/v), and Sc the Schmi
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