Gold Leaching with acidic thiocyanate
The oxidation of gold in acidic thiocyanate solutions can be expected to produce either the gold (I) complex Au(SCN)2- or the gold (III) complex Au(SCN)4-.
Au(SCN)2- + e = Au + 2SCN-, ORP = 662 mV
Au(SCN)4- + 3e = Au + 4SCN-, ORP = 636 mV
The potentials required for these reactions are below those for the reduction of oxygen and ferric ions, and these reagents can be expected to be effective oxidants for gold in thiocyanate solutions; however, ferric iron is the most suitable oxidant for the reaction since the kinetics of dissolution are slow if oxygen is used and thiocyanate is oxidized rapidly by hydrogen peroxide. The stability of the thiocyanate ion is increased in the presence of ferric ions due to the many complexes it forms with these ions. The standard potential for the reduction of oxygen is 700 mV and this value is higher than the required for the oxidation of gold in thiocyanate solutions, the irreversible nature of the oxygen reduction reaction results in insignificantly slow leaching under normal conditions. The use of elevated temperatures and pressures could be expected to produce acceptable leaching rates, but the decomposition of thiocyanate by oxidation to thiocyanogen is a possible complication.
Employing ferric ion as oxidant, thiocyanate forms relatively strong complexes with ferric ions, which reduces both the oxidizing potential of ferric ions and the concentration of free thiocyanate needed for the formation of complexes with gold.
Fe(SCN)4 + Au = Fe2+ + Au(SCN)2- + 2SCN-
The optimum pH range is 2.0-3.0. The stability of thiocyanate is potential dependent, with stability achieved below approximately 640 mV. Also, higher potentials are required to achieve satisfactory gold leaching rates at practical thiocyanate concentrations. There is a real relationship between these two requirements. The rate of gold dissolution increases with increasing thiocyanate and to a lesser extent ferric ion concentration. Concentrations of 10-15 g/l and 2-10 g/l for the two species have been used. Increasing temperature increases the rate of thiocyanate consumptions and, in view of the high reagent consumption even at ambient temperatures, elevating pulp temperatures is not a good option. A significant drop in gold extraction has been obtained at temperatures above 40 oC when using thiocyanate. Silver trends to form an almost insoluble product, silver thiocyanate and is unrecoverable by this process.