The adsorption of the aurocyanide ion onto highly orientated pyrolytic graphite: A scanning tunnelling microscopy investigation

G.E. Poinen

Gold is an important mineral for its value, its properties and its natural beauty. In Australia, gold is a significant resource mineral. It is expected that within a few years Australia will be the second largest producer of gold in the world. The present gold mining practices though efficient at recovering minute amounts of gold, can still be improved. The present study is concerned with two major aspects of gold mining processing: gold dissolution and gold recovery. The thesis is divided into two parts. The manufacture of single crystals of gold using the Clavilier method is the focus of the first part of the study. The Clavilier gold single crystals were used for testing and optimising the newly acquired scanning probe microscopy instruments. In addition, it was expected that the single crystals would be used as a substrate for gold dissolution experiments in the multi component Ultra-High-Vacuum/Scanning-Tunneling-Microscope/Low- Energy-Electron-Diffraction/AUGER/Electrochemical system. The second part of the thesis is devoted to the adsorption of dissolved gold onto a carbon substrate. The current process makes use of activated carbon as the adsorbent for the gold cyanide complex (Au(CN)-2) produced by the dissolution of the gold ore in an alkaline cyanide solution. Activated carbon has been shown to have a special affinity for the gold complex even in the presence of other metal-cyanide complexes at much higher concentration in the pulp formed. This process is efficient and is able to recover gold at a concentration of 1 part per million in the ore. It is mainly the adoption of the carbon technology which has increased the output of Australian gold mines to the extent that within 15 years the local production has increased more than ten-fold. The special affinity of the gold cyanide complex for activated carbon has spurred several investigations into the gold cyanide-activated carbon system over several years. In recent years, largely due to the advent of new analytical systems some significant pieces of information about the gold-carbon system have been exposed. Different analytical tools such as Mossbauer spectroscopy, X-ray photoelectron spectroscopy and fast-Fourier transform infrared have been applied to the gold-carbon system. The results have shown mostly that a linear gold cyanide complex (aurocyanide ion) is attached to a small plate of graphitic region in the activated carbon. The attachment to the carbon substrate is only through the central gold atom with the cyanide ligands remaining linear. Several investigations have revealed the strong link between the increased loading capacity of carbons with increased graphitic content of these carbons. Other studies have suggested an ion pair of the form Mn+[AufCN)-2]n to be the manner in which the aurocyanide ion is adsorbed onto activated carbon under plant conditions. Our investigation of this system has been directed towards the study of the aurocyanide complex ion onto pure graphite. The adsorption process was studied in situ with a Scanning Tunnelling Microscope. Several investigations have revealed that the adsorption of the gold complex on Highly Orientated Pyrolytic Graphite (HOPG) is slow, the surface remaining bare for several hours in solution. When Ca-+ ions were added to the aurocyanide-HOPG system, adsorption although low, was seen to be enhanced as in the case of gold plant practices using activated carbon. Images of the gold adsorbate have been obtained with atomic/molecular resolution on the carbon substrate. The gold adsorbate was found in or near defect regions on the HOPG surface after several hours in contact with solution. It is apparent that the gold species is adsorbed on top as well as at the edge of a graphite sheet. It is clear from the results obtained so far, that calcium ions have a significant role to play in the attachment of the aurocyanide ion onto the graphite substrate. In addition, X-ray Photoelectron Spectroscopy measurements have been made on the HOPG samples. These samples were treated in the same way as for the STM studies. These analyses has confirmed that gold adsorption is extremely low and indicated that H-,0 is intercalated between the graphite sheets. Although no K or Ca was detected in the XPS analysis, the gold adsorbate was found to be associated with calcium when analysed by an energy dispersive X-ray analyser. The present study has shown that the manner in which the aurocyanide is bonded to pure graphite is different to the way in which the gold complex bonds to activated carbon. From the results obtained it is proposed that partially solvated calcium ions are intercalated in the expanded graphitic sheets and that the top graphite sheet electronic structure is modified in such a way that the latter is favourable for the attachment of the aurocyanide ion onto pure graphite.

The adsorption of the aurocyanide ion onto highly orientated pyrolytic graphite: A scanning tunnelling microscopy investigation

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