Thermodynamics of the Miscibility Gap in the Ag-Se System

The miscibility gap in the Ag-Cu-Se system plays a central role in the smelting of copper anode slimes for the recovery of silver; however, its position in the Ag-Se and Cu-Se component binary systems is poorly defined. In this work, the miscibility gap in the Ag-Se system has been accurately defined by holding samples at selected temperatures, then quenching them and analyzing the two phases. The miscibility gap has been shown to extend from 6.7 to 25.0 pet at the monotectic temperature of 890 °C. As the temperature increases to 1150 °C, the boundaries do not converge very rapidly. The limiting activity coefficient of Ag2Se in molten silver was calculated to be 7.8 at 1100 °C, confirming earlier work. The limiting activity coefficient of hypothetical liquid selenium at the same temperature was found to be approximately 0.008. Finally, the activity of hypothetical liquid selenium in the liquids on either side of the miscibility gap, from the monotectic temperature to 1150 °C, was mapped.