Kinetics of gold cementation on copper in ammoniacal thiosulfate solutions

The rate of cementation of gold on rotating copper discs was investigated in ammoniacal thiosulfate solutions as a function of rotational speed of the copper disc, initial gold concentration, initial copper concentration, and temperature. The cementation reaction exhibited two distinct kinetic regions, an initially slow rate followed by an enhanced rate. With a disc rotating at 1500 rpm in a solution containing 0.25 M [S₂O₃^2-], 30 ppm [Au]₀, and 30 ppm [Cu]₀, the apparent rate constant was 4.7 × 10^-3 and 1.1 × 10^-2 cm s^-1 for the initial and enhanced regions, respectively. At low initial copper concentrations, the temperature dependence was well behaved, yielding an activation energy of 5.9 kJ mol^-1. In a solution containing 30 ppm [Cu]₀, there was a noticeable decrease in the reaction rate between 25 and 50 °C. This may be attributed to the decomposition of Cu(S₂O₃)₃^5- and the formation of Cu₂S, which creates a passivating film. The deposit morphology is relatively nodular under the conditions of this investigation. Importantly, EDS and XRD analysis confirmed that the deposits are Au-Cu alloys instead of pure gold. The composition ranged from Au₃Cu to AuCu₃ intermetallic alloys depending on the initial Cu/Au ratio in solution.