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Studies on the mechanism of gold adsorption and elution in the carbon-in-pulp process

Tsuchida, N. (1984) Studies on the mechanism of gold adsorption and elution in the carbon-in-pulp process. PhD thesis, Murdoch University.

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This thesis is a fundamental study of the adsorption and desorption of gold cyanide from activated carbon. Emphasis was placed on defining the adsorption mechanism of Au(CN)2 on activated carbon and in understanding the chemistry of the elution procedure which uses organic solvents to promote the desorption of Au(CN)2 • The results improve our understanding of the Carbon-in-Pulp process, and have practical applications for potential improvements to process efficiency.

A comparative study has been conducted on the adsorption of Au(CN)2-, Ag(CN)2-, Cu(CN)32- and Hg(CN)2 and cations like K+ , Ca2+, Mg2+ both on normal activated carbon and carbon which had been deoxygenated at high temperature and vacuum. It has been found that the adsorption of Au(CN)2 is dependent on the equilibrium pH and is enhanced by adsorption of Ca2+ and Mg2+’ at pH > 9. It is also dependent on the oxygen content in carbon. In general the molar ratio of Au(CN)2 :cations is >> 1 on normal carbon at pH 10, but this ratio 1 on deoxygenated carbon. These results suggest that the adsorption of Au(CN)2 involves separate anion and cation adsorption at different carbon sites and that some of the adsorbed Au(CN)2 on normal activated carbon is oxidatively decomposed to AuCN by chemisorbed oxygen and quinone-type surface oxide groups.

Potential measurements of the activated carbon show that carbon potentials also lie between 0.2 ~ 0.6V and decrease according to the adsorption of Au(CN)2-. Comparative studies of activated carbon electrodes with platinum and glassy carbon electrodes have shown that the activated carbon electrode catalyses the oxidation of cyanide. This appears to be attributed to strong adsorption of cyanide onto the oxidised carbon surface and oxidation via a free radical mechanism. A carbon catalysed mechanism is likewise proposed for the oxidative decomposition of Au(CN)2- and Ag(CN)2- to account for the decrease of the carbon potential upon adsorption of these ions at pH 7-10. However, the oxidative decomposition of Au(CN)2- is also dependent on the pH of the carbon surface and on carbon loading. In practical solutions, at pH 10 it is believed that much of the gold is adsorbed via anion exchange or Au(CN)2- with OH- at chromenol sites on the carbon surface; whilst cations adsorb via cation exchange with H+ at carboxylic acid and phenol sites. A model of the activated sites for the adsorption of Au(CN)2- is proposed.

Several classes of aqueous organic solvents have been found to effectively strip gold from carbon at ambient temperature in the presence of different anions. The cyanide anion proved to be the best anion for the desorption of gold, but lauryl sulphate and sulphide ions were also effective. Cyanide is an excellent carbon neucleophile and is effective both for anion exchange with Au(CN)2 and for the dissolution of any AuCN arising from decomposition of Au(CN)2 on carbon. Of the various organic solvent mixtures tested, aqueous acetonitrile was found to be the most efficient in desorbing gold from carbon. The kinetics of gold desorption from carbon in the aqueous acetonitrile is believed to be controlled by diffusion of CN and Au(CN)2 in the pore structure of carbon.

Measurements of the free energy of transfer of CN- from water to aqueous acetonitrile indicate that the activity of CN- increases according to the acetonitrile content in the aqueous organic mixtures much more than the activity of Au(CN)2-. This provides an effective driving force for the desorption of Au(CN)2- from carbon.

This knowledge is used to investigate and develop a flow sheet and method for the effective elution of gold cyanide from carbon using aqueous acetonitrile solutions. Column elution tests were carried out on samples of plant loaded carbon and the gold was recovered from the eluate both by electrowinning and zinc cementation. However, due to the strong adsorption of organics onto carbon, and consequent loss in carbon activity, the solvent must be recovered from the carbon. Consequently, a solvent distillation elution procedure was tested and developed which recovers the solvent both from the eluate and the carbon. This procedure has potential commercial benefits.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Parker, Jim and Muir, David
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