Mechanism of Gold Recovery by Activated Carbon Adsorption

1 Mechanism of Gold Recovery by Activated Carbon Adsorption 

This could not be explained on the basis of the Garten and Weiss mechanism, because both Au(CN)2 and Ag(CN)2 are univalent negative ions. Similarly, the adsorption of silver cyanide Ag(CN)2 at pH 11 to 11.5, where the zeta potential of the carbon is -50 to -60 mv, and where the carbon surface will have more negative than positive sites, could not be explained by the electrostatic forces. 

FIGURE 5.3 Schematic representation of the adsorption model for silver cyanide on activated carbons. (Adapted from Radovic, L.R., in Chemistry and Physics of Carbon, Vol. 27 L.R. Radovic, Ed., Marcel Dekker, New York, 2001. With permission.) 

FIGURE 5.4 Adsorption isotherms of gold cyanide in the presence of equal strengths of different ions. (Adapted from McDougaU, G.J., Hancock, R.D., Nicol, M.J., Wellington, O.L., and Copperthwaite, R.G., J. South African Inst. Min MetalL, 80, 344, 1980. With permission.) 

McDougall et al. also found that the neutral Hg(CN)2 and the anion Au(CN)2 competed for the adsorption sites on the carbon surface when present together in the solution, and Hg(CN)2 could even displace some of the adsorbed Au(CN)2 anion. This indicated that the negative charge of the gold anion was not essential for the adsorption of gold. The authors, therefore, concluded that the initial stage of adsorption of aurocyanide on carbon involves adsorption of aurocyanide as the less soluble M + [Au(CN)2 ] complex, where M + could be an H+ ion or a metal ion (Na+, K+, Ca +), which is followed by a reduction of the cyanide complex either into a substoichiometiic 

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