The reversibility of adsorption of gold cyanide on activated carbon
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I.
INTRODUCTION
No agreement has yet been reached regarding the mechanism by which metal cyanides are adsorbed onto activated carbon. The following factors have contributed to the disagreement among researchers on this subject: (a) the complexity of the carbon surface renders the identification of species difficult; (b) different types of carbon have been used in the various investigations; and (c) the adsorption reaction has been studied at different experimental conditions. The more important mechanisms proposed for the adsorption of gold or silver cyanide from alkaline solutions can be grouped mainly into four categories: (1) Adsorption as Mn+(Au(CN)Z ), Davidson m proposed a mechanism in which the extent of adsorption depends on the concentration and character of the "spectator" cations (M "ยง present in the solution. His theory is supported by a series of three publications by Adams and co-workers. |2-41 (2) Adsorption as M"+ (Au(CN)Z ), followed by partial reduction McDougall et al. 15Jsuggested that the initial stage of adsorption involves the adsorption of the less soluble M r t + (Au(CN)2- - )n complex (M t z + = Ca 2 + , H + , N a + , K + ). This is followed by a reduction step in which part of the Au(CN)Z is converted into an irreversibly adsorbed species such as AuCNx. (3) Adsorption of Au(CN)Z followed by partial degradation to AuCN J.S.J. VAN DEVENTER, Professor, and P.F. VAN DER MERWE, Ph.D. Graduate, are with the Department of Metallurgical Engineering, University of Stellenbosch, Stellenbosch 7600, Republic of South Africa. Manuscript submitted September 2, 1992. METALLURGICAL TRANSACTIONS B
Tsuchida and Muir 161proposed the following dual mechanism for the adsorption of Au(CN)2: The cyanide complex adsorbs onto the carbon by anion exchange with O H - , followed by the partial oxidation thereof to AuCN by chemisorbed oxygen. The AuCN does not take part in the equilibrium between Au(CN)2- in solution and on the carbon, thus allowing higher gold loading than would be the case on deoxygenated carbon. Cook et al. t71 also confirmed that both Au(CN)2- and AuCN could be present on the carbon. Yet, other workers ts'9'j~ confirmed the absence of any irreversibly adsorbed species like AuCN or Au on their carbons by eluting all of the adsorbed gold with hot sodium hydroxide solutions. This could be achieved for carbons loaded from both alkaline and acidic solutions at room temperature, t~~ These findings led them to reject a mechanism in which it is assumed that AuCN is formed on adsorption. (4) Adsorption in the graphitic structure According to Jones et al.,ts] the Au(CN)2- anion is adsorbed without change or chemical reaction in its normal linear form (N}C-Au-C}N) in a symmetrical environment parallel to the graphitic planes of the carbon. At a pH of 10 in the absence of free cyanide, the equilibrium adsorption and desorption isotherms for gold on Calgon GRC 22 were identical. As further proof, an Au:N mole ratio of 2 was measured on the loaded carbon with the use of X-ray photoelectron spectroscopy (XPS
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