THE MECHANISM OF THE OXIDATIVE DISSOLUTION OF COLLOIDAL GOLD IN CYANIDE MEDIA

The kinetics of the dissolution of colloidal gold in aerated cyanide s olutions has been investigated with a view to developing a fundamental understanding of the mechanism involved. Experimental results obtaine d are consistent with an electrochemical mixed potential model, where the mixed potential is set up in the passive or passive interpeak regi on of the anodic polarization curve, giving rise to a rate of dissolut ion which is controlled by the rate of a surface reaction. Electron mi croscope studies suggest that the colloidal particles dissolve in a un iform shrinking particle manner. Mass-transfer coefficients for the tr ansfer of dissolved oxygen to the colloidal surface appear to be very high, with estimates for gelatin stabilized colloids being of the orde r of 10(-4) m/s, and for unstabilized colloids 0.1 m/s. These latter e stimates are in agreement with values predicted from a hydrodynamic tr eatment of the diffusion layer.