Kinetics of removal of bismuth and lead from molten copper alloys in vacuum induction melting

An investigation was made of the rates of removal of bismuth and lead from molten copper alloys stirred differently under vacuum at 1403 K. The heating conditions, which corresponded to highest, intermediate, and lowest stirring, were referred to as HC-A, HC-B, and HC-C, respectively. The rates of removal were described by a first-order rate equation involving the final concentration of the impurity. The overall mass-transfer coefficient for bismuth or lead (i),K _i, was determined, and it was found that K_i(HC-A) > K_i(HC-B) but K_i(HC-B) < K_i(HC-C) in Cu-Bi, Cu-Pb, and Cu-Bi-Pb alloys. In general, it was found that K_i(Cu-Bi-Pb) > K_i(Cu-Bi) or K_i(Cu-Pb). Oxygen reducedK _i more than sulfur. The reducing effect of oxygen and sulfur was weakened when stirring rate increased. In vacuum melting of Cu-Bi-Pb-O and Cu-Bi-Pb-S alloys under HC-C,K _i tended to decrease with increasing oxygen and sulfur concentrations. The loss of copper indicated that the rates were determined by liquid-phase mass-transfer and evaporation. The liquid-phase mass-transfer coefficient,K _L, was calculated, and it was found thatK _L(UC- A) > K_L(HC-B) but K_L(HC-B) < K_L(HC-C). The last relation was contrary to current theoretical prediction. A new model was assumed to account for this fact. The observed relative volatility coefficients of bismuth and lead were much smaller than their respective theoretical coefficients. Oxygen and sulfur adsorbed on the surface of the melt were considered to be close to equilibrium with their respective bulk concentrations, and their effects on the evaporation mass-transfer coefficients for bismuth and lead were estimated to be fairly small. This result means that the observed reduction ofK _i is due mostly to the decrease inK _L and made possible the calculation ofK _L. Oxygen decreasedK _L more than sulfur. In vacuum melting of Cu-Bi-Pb-O and Cu-Bi-Pb-S alloys under HC-C,K _L decreased linearly with increasing oxygen or sulfur concentration. The final concentrations of bismuth and lead in these alloys under HC-C tended to increase with increasing oxygen and sulfur concentrations and tended to decrease with increasingK _i andK _L.