Selective oxidation of copper from liquid copper-silver alloys

Abstract

In this work, the oxygen refining of liquid copper-silver alloys with a borosilicate slag was studied. First, a comprehensive thermodynamic analysis was performed using the data available in the literature. The results indicate that since silver oxide is relatively unstable in silicate-based slags, then it should be thermodynamically feasible to oxidize copper from copper-silver alloys with a very low silver loss to the silicate slag. In actual practice, although relatively low copper levels can be achieved in the metal phase, the silver losses to the slag are excessive. Therefore, in the present work, both kinetic and equilibrium experiments were performed on a molten copper-silver alloy containing 12.68 mass pct silver in order to elucidate the mechanism of silver loss to the slag. The kinetic experimental results indicated that copper levels of less than 2 mass pct could be achieved with silver recoveries of about 95 pct after relatively short refining times of 15 minutes. In the equilibrium experiments, the copper contents of the metal were less than 1 mass pct, and these values were in good agreement with those which were calculated from the data of previous researchers. In order to explain the relatively high silver losses to the slag, a model was developed which is based on the transport of silver from the metal phase to the slag phase both in metallic form and as silver oxide in the copper oxide oxidation product. The copper and silver oxides and the metallic copper-silver alloy are all transported into the slag by the oxidizing gas bubbles. It is proposed that once in the slag, the silver oxide is unstable and decomposes into metallic silver which is not easily recovered in the metal phase. Also, the transfer of the copper-silver alloy into the slag, by the gas bubbles, promotes the slag-metal exchange reaction, which again results in the generation of silver particles in the slag.