Thiocyanate hydrometallurgy for the recovery of goldPart III: Thiocyanate stability

Abstract The effects of metal ions, minerals, temperature, thiocyanate concentration, activated carbon, and pH on the rate of thiocyanate oxidation were determined. The rate of ferrous ion generation from the redox reaction between thiocyanate and ferric ion was found to be significant at 50 °C. The reaction constant (k) at 25 °C was found to be 1.43 × 10 − 5  L 0.4  mol − 0.4  min − 1 . Ferric oxidation of thiocyanate was sensitive to temperature with an activation energy of 76.4 kJ/mol, typical of homogenous chemical reactions. Based on the kinetic data, the empirical rate equation for thiocyanate consumption and/or ferrous ion generation was found to have the following form: d [ F e 2 + ] d t = − 8 d [ S C N − ] d t = k [ S C N − ] 1.36 [ F e 3 + ] 0 [ H + ] 0 = k [ S C N − ] 1.36 Oxide minerals did not have a profound effect on the oxidation of thiocyanate by ferric ion. Sulfide minerals, especially pyrite and galena catalyzed the redox reaction. The addition of cupric ion resulted in the oxidation of thiocyanate and formation of an insoluble cuprous thiocyanate compound.