Stability of chloridogold(I) complexes in aqueous solutions from 300 to 600°C and from 500 to 1800 bar

Abstract

The solubility of gold has been measured in the system H 2O+H 2+HCl+NaCl+NaOH at temperatures from 300 to 600°C and pressures from 500 to 1800 bar in order to determine the stability and stoichiometry of chloride complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave system. This approach permitted the independent determination of the concentrations of all critical aqueous components in solution for the determination of the stability and stoichiometry of gold(I) complexes. The solubilities (i.e. total dissolved gold) were in the range 9.9 × 10 −9 to 3.26 × 10 −5 mol kg −1 (0.002–6.42 mg kg −1) in solutions of total dissolved chloride between 0.150 and 1.720 mol kg −1, total dissolved sodium between 0.000 and 0.975 mol kg −1 and total dissolved hydrogen between 4.34 × 10 −6 and 7.87 × 10 −4 mol kg −1. A nonlinear least squares treatment of the data demonstrates that the solubility of gold in chloride solutions is accurately described by the reactions, Au(s) + 2Cl − + H + = AuCl 2 − + 0.5 H 2( g) K s,020 Au(s) + H 2 O = AuOH(aq) + 0.5 H 2( g) K s,001 where AuCl 2 − predominates in acidic chloride solutions and AuOH(aq) in neutral to alkaline chloride and chloride-free solutions. The solubility constant, log K s,020, increases with increasing temperature and decreases with increasing pressure from a minimum of −5.43 (±0.29) at 300°C and 500 bar to a maximum of −0.15 (±0.16) at 600°C and 1000 bar, with the pressure effects becoming more important with increasing temperature. The equilibrium solubility constant for AuOH(aq) has been previously determined by Stefánsson and Seward (2003). The solubility of gold at pH >5 was found to be independent of chloride concentration up to 1 mol kg −1 and identical to the solubility of gold with respect to AuOH(aq). The stability of AuClOH − was estimated to be 3 to 6 orders of magnitude less stable than AuOH(aq) and AuCl 2 − in hydrothermal solutions. Hence, gold(I) chloride complexes play an important role in transporting gold in aqueous acidic chloride solutions above 400°C.