Experimental study of gold-hydrosulphide complexing in aqueous solutions at 350-500 degrees C, 500 and 1000 bars using mineral buffers

摘要

The solubility of gold was measured in KCl solutions (0.001-0.1 m) at near-neutral to weakly acidic pH in the presence of the K-feldspar-muscovite-quartz, andalusite-muscovite-quartz, and pyrite-pyrrhotite-magnetite buffers at temperatures 350 to 500 degrees C and pressures 0.5 and 1 kbar. These mineral buffers were used to simultaneously constrain pH, f(S-2), and f(H-2). The experiments were performed using a CORETEST flexible Ti-cell rocking hydrothermal reactor enabling solution sampling at experimental conditions. Measured log m(Au) (mol/kg H2O) ranges from -7.5 at weakly acid pH to -5.9 in near-neutral solutions, and increases slightly with temperature. Gold solubility in weakly basic and near-neutral solutions decreases with decreasing pH at all temperatures, which implies that Au(HS)(2) is the dominant Au species in solution. In more acidic solutions, solubility is independent of pH. Comparison of the experimentally measured solubilities with literature values for An hydrolysis constants demonstrates that at 350 degrees C AuHS(aq)0 dominates An aqueous speciation at the weakly acidic pH and f(S-2)/f(H-2) conditions imposed by the pyrite-pyrrhotite-magnetite buffer. In contrast, at temperatures > 400 degrees C AuHS0 (aq) becomes less important and AuOH(aq)0 predominates in weakly acid solutions. Solubility data collected in this study were used to calculate the following equilibrium reaction constants:Au-(cr) + H2S(aq)0 = AuHS(aq)0 + 0.5 H-2(g), K-(AuHS())0Au-(cr) + H2S(aq)0 + HS- = Au(HS)(-)(2) + 0.5 H-2(g), K-(Au(HS)(2))-The calculated log K((AuHS0)) values are -5.25 +/- 0.35 and -5.92 +/- 0.36 at 350 degrees C (500-1000 bars) and 400 degrees C/1000 bars, respectively. The log K-(Au(Hs)(2)-) values are -1.68 +/- 0.42 at 350 degrees C/500 bars, -1.40 +/- 0.41 at 350 degrees C/1000 bars, -2.14 +/- 0.42 at 400 degrees C/500 bars, -1.75 +/- 0.40 at 400 degrees C/1000 bars, -1.87 +/- 0.40 at 450 degrees C/1000 bars, band -1.80 +/- 0.40 at 500 degrees C/1000 bars. Equilibrium constants for both reactions are retrograde with respect to temperature, but this effect is more pronounced for AuHS(aq)0 than for Au(HS)(-)(2) The K values' dependence on temperature, as well as the calculated values Of K(A (Hs) 2) are consistent with the data of Benning and Seward (1996). The calculated values of K-(AuHS0) are between those determined by Benning and Seward (1996) and Gibert et al. (1998). Copyright (c) 2005 Elsevier Ltd