Vacuum refining copper melts to remove bismuth, arsenic, and antimony

Abstract

Experiments were carried out on 35 kg melts of doped cathode copper and anode copper in a 3 m3, 150 kW vacuum induction furnace. Rates of removal of bismuth, arsenic, and antimony were measured over temperature and pressure ranges of 1450 to 1610 K and 3 to 30 pascals, respectively. Bismuth removal was found to be rapid: 1 to 18 x 10-5 m/s. Arsenic and antimony removal were quite slow: 0.2 to 3 x 10-5 and 0.1 x 10-5 m/s, respectively, and evaporation controlled rates of refining. It is shown that, at typical concentrations of these elements in copper, monatomic evaporation is the predominant evaporation mechanism. An expression for the melt phase mass transport rate coefficient is developed from Machlin’s model. In this expression, melt diffusion is a function of melt temperature, and melt surface velocity is a function of the square root of melt surface area to volume ratio and the square of melt temperature, i.e.: it = 1.11 x 10-7[(A/V)/]1/4Tr1/2 exp(-2515/T). This coefficient is used to examine rate control in previous small scale studies and in the present and previous pilot scale studies. The gas phase mass transport coefficient is found to be proportional to the overpressure ratio defined as: total initial melt vapor pressure/chamber pressure, and is also found to be dependent on the geometry of the gas space immediately above the melt.