Abstract
The present study was undertaken to investigate the effect of the stirring of molten copper alloys by a rotating molybdenum disk on the rates of removal of silver, lead, bismuth, and sulfur, which are impurities of crude copper, under reduced pressure. Liquid Cu–Ag, Cu–Pb, Cu–Bi, Cu–Ag–O, Cu–Bi–O and Cu–Si–S alloys were maintained at 1468 and 1573 K or 1573 K under reduced pressures of the order of 10−1 to 10−2 mmHg while being stirred at two different speeds in a molybdenum-wire-wound resistance furnace. The evaporation of silver from Cu–Ag alloys, bismuth from Cu–Bi and Cu–Bi–O alloys, and sulfur from Cu–Si–S alloys follow first-order kinetics. The evaporation of lead from Cu–Pb alloys follows the same kinetics at least initially. Any of the evaporation-rate constants hardly depend on the stirring rate, and those of silver, lead, and bismuth are smaller than their respective rate constants of vaporization at the interface. These facts suggest that their evaporation rates are determined by gas-phase mass transfer and vaporization at the interface or the former under the present experimental conditions. At 1573 K, the evaporation-rate constants of lead, bismuth, and sulfur are much greater than those of silver. Oxygen in the melts increases the rate of evaporation of silver at about 0.09% O at 1573 K and slightly decreases the evaporation-rate constants of bismuth at 0.03 to 0.07% O at 1468 K.
