THE MECHANISM OF ELUTION OF GOLD CYANIDE FROM ACTIVATED CARBON

Numerous articles have appeared on the mechanism of the adsorption of gold cyanide onto activated carbon. In contrast, little information is available on the mechanism of elution of the adsorbed gold. It is the objective of this article to formulate such a mechanism on the basis of batch and column elution tests without analyzing adsorbed species o n the carbon directly. The presence of spectator cations (M(n+)) enhan ces the formation of M(n+){Au(CN)(2)(-)}(n) ion pairs on the carbon, w hich in turn suppress the elution of gold cyanide. The dynamics of rem oval of these cations determine the horizontal position of the gold pe ak in an elution profile. When the concentration of cations in the elu ant is high and no cyanide-is present in the solution or on the carbon , very little desorption of gold is observed. The quantitative effect of the concentration of spectator cations on the equilibrium for desor ption of aurocyanide can be estimated from the elution profiles for go ld and cations. Free cyanide in the eluant, which causes some competit ive adsorption of cyanide with aurocyanide, therefore plays a minor ro le at the elevated temperatures used in industry. A more important eff ect of cyanide is its reaction with functional groups on the carbon, t he products of which passivate the surface for adsorption of aurocyani de, and thereby cyanide promotes the elution of aurocyanide. The degre e of passivation, which is determined to a large extent by the tempera ture of pretreatment, also affects the elution of cations and the degr adation/adsorption of cyanide itself. Reactivation of the carbon surfa ce occurs;when the adsorbed/decomposed cyanide is removed by the eluan t. At high temperatures of pretreatment, such as used in practice, it is not necessary to include a reactivation term in the mathematical mo del for elution.