Reaction kinetics of gold dissolution in acid thiourea solution using ferric sulfate as oxidant

Reaction kinetics of gold dissolution in acid (pH 1–2) thiourea solutions using ferric sulfate as the oxidant were investigated with the rotating disk technique. Experiments for the kinetic study on the intrinsic reaction of gold dissolution were designed to measure the influence of system variables on the rate of gold dissolution and to determine the important parameters which govern the kinetic response of the gold leaching system. The variables tested included gold disk rotational speed, ferric sulfate concentration, thiourea concentration, pH, and temperature.Reaction control regions for the kinetics of gold dissolution were clarified under certain conditions. In the region of mass transfer control by ferric ion, the diffusion coefficient of ferric ion in acid thiourea solution was determined from the Levich theory and found to be 4.67 × 10– 6 cm2/s. When the dissolution reaction was in the region controlled by a surface reaction, the dissolution rate was found to be approximately one-fourth order with respect to thiourea concentration, zero order with respect to ferric concentration, and minus one-fifth order with respect to hydrogen ion concentration within the pH range from pH 1 to pH 2. The empirical kinetic expression under the surface reaction control has the following form:dCAu(I)dt=k[Tu]0.25[H+]−0.2The surface reaction had a reaction constant, k, at 25 °C of 5.93 × 10− 7 mol0.95/l0.95 min (or 3.58 × 10− 7 mol0.95 l0.05/cm2 min) and an apparent activation energy of 13.9 kcal/mol or 58.1 kJ/mol.