Thermodynamic calculation for cyanide speciation at CN/Cu mole ratio of 2/1 is presented in Fig. 2. The initial cyanide concentration was fixed at 10 ppm and Eh at 300 mV (SHE). The only dominant species dissolved in the solution from pH 7 to pH 9 is cuprous dicyanide Cu(CN)2−. When pH is increased to 10, a small amount of Cu(CN)32− species appears, while Cu(CN)2− is still dominant. There is no free cyanide species either HCN or CN− over the whole range of pH calculated. However, it has been suggested that Cu(CN)2− is not stable in contact with air, oxidising to cupric species according to Eq. (7) (Vukcevic, 1996).