High Temperature Vaporization Chemistry in the Gold-Chlorine System Including Formation of Vapor Complex Species of Gold and Silver with Copper and I ron

Abstract

The thermodynamic properties of the vaporization reactions in the gold-chlorine system have been investigated in the temperature ranges of 580 to 649 K and 882 to 1107 K. The experimental technique consisted of classical transpiration vapor pressure measurement and analysis with a newly developed Flow Reactor-Mass Spectrometer system, which connects a high temperature, ambient pressure reactor to a TOF mass spectrometer. Metallic gold reacts with chlorine gas at temperatures above 750 K, forming the vapor species Au₂Cl₂(g). The resulting reaction 2Au(c) + Cl₂(g) = Au₂Cl₂(g) was found to have a standard Gibbs free energy change of ΔG° = 89,000 (±2400) − 27.8 (±1) T (J/mole) ΔG° = 21,300 (±570) − 6.6 (±0.2) T (cal/ mole) in the temperature range 882 to 1107 K. At lower temperatures, the vaporization reaction is 2Au(c) + 3Cl₂(g) = Au₂Cl₆(g) with a standard Gibbs free energy change of ΔG° = −102,000 (±20,000) + 235 (±29) T (J/mole) ΔG° = −24,400 (±4800) +56 (±7) T (cal/mole) in the temperature range 580 to 649 K.

The above results are combined with equilibrium dissociation data obtained in the literature for condensed gold chloride phases to construct a phase stability-vapor pressure diagram for the gold-chlorine system. Consideration is given to some possible operating conditions for a gold chlorination-vaporization process to treat low grade and refractory gold ores. Transpiration vapor pressure measurements in the Au-Cu-Cl, Ag-Cu-Cl, and Ag-Fe-Cl systems showed significant enhancement of the vapor pressure of Au₂Cl₂(g) or AgCl(g) in the presence of Cu₃Cl₃(g) or FeCl₃(g). This indicates the formation of binary vapor phase complexes in these systems. The species AuCu₂Cl₃(g), AgCu₂Cl₃(g), and AgFeC₄ have been proposed.