Thermodynamics of Gold Dissolution Behavior in CaO-SiO2-Al2O3-MgOsat Slag System

Abstract

Gold solubility in the CaO-SiO2-Al2O3-MgOsat slag system was measured at 1773 K ( 1500 degrees C) under a CO2-CO atmosphere over a wide range of compositions, i.e., 8 to 40 mass pct CaO, 26 to 50 mass pct SiO2, and 0 to 36 mass pct Al2O3, to determine the dissolution mechanism of gold in the CaO-based metallurgical slags. Gold solubility in the present slag system increased with increasing oxygen partial pressure and increasing activity of CaO. From the thermodynamic analysis, the dissolution mechanism of gold into the (alumino-)silicate melts is proposed as follows according to the activity of basic oxide, which indicates that the predominant species of gold is dependent on slag basicity. Au(s) + 1/4O(2)(g) + 1/2(O2-) = (AuO-), (a(BO) < 0.1) Au(s) + 1/4O(2)(g) + 3/2(O-2) = (AuO23-), (a(BO) > 0.1) The enthalpy change for the dissolution of gold into the CaO-SiO2-Al2O3-MgOsat slag system was measured to be about -80 kJ/mol, indicating that the gold dissolution is exothermic. From the iso-Au solubility contours, the dominant factor affecting the gold dissolution behavior is the (CaO + MgO)/SiO2 ratio, whereas the influence of Al2O3 was negligible. Consequently, less basic slags and higher processing temperatures, in conjunction with a strongly reducing atmosphere, are recommended to increase gold recovery during pyro-processing of Au-containing e-wastes. (C) The Minerals, Metals & Materials Society and ASM International 2015