SOLUTION CHEMISTRY OF IODIDE LEACHING OF GOLD

The thermodynamic equilibria and kinetic aspects of gold dissolution in iodide electrolytes have been studied with emphasis on the effect of different oxidants on the system. In conjunction with kinetic measurements, the CHEMIX computer program was used to predict the concentration profiles of the predominant species at equilibrium in different solution conditions for the systems Au-I--I2-H2O and Au-I--OCl--H2O. The thermodynamic study showed that I3- is the predominant oxidant species in both systems. However, if the concentrations of OCl- and I- are equal, solid iodine is formed. In these systems iodide (I-) is used to form 13- (responsible for the gold oxidation) and more free iodide needed for the gold complexation is destroyed in the I--OCl- system than the I--I2 System. The formation of solid AuI also explains the lower rate of gold dissolution determined for certain conditions in the kinetic study. The thermodynamic modelling supports the kinetic measurements which show that, although the I--OCl- system has a higher oxidation capacity, it does not extract gold as well as the I--I2 system. In all cases there exist optimum oxidant/iodide ratios for achieving maximum gold extraction rates. A mixture which has the highest I3- and free I- concentration will attain the best gold extraction rate.