Potentiometric studies on the adsorption of Au(CN) 2 and Ag(CN) 2 onto activated carbon
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I.
INTRODUCTION
THE carbon-in-pulp
(CIP) process is a recent hydrometallurgical development for the extraction of gold and silver. However, according to McDougall and Hancock, ~ there is no generally acceptable mechanism for the adsorption of Au(CN)~ onto activated carbon. Surface functional groups on activated carbon undoubtedly play an important role in the adsorption of ions on carbon. Although the exact nature of the functional groups on carbon is unknown, phenolic, carboxylic, benzpyran, and quinone groups are believed to act as sites for the adsorption of ions .2 Accordingly, for many years carbons have been characterized by acid-base titrimetric and potentiometric measurements. The electrochemical potential of activated carbon clearly indicates the presence of some redox couple such as the oxygen/hydroxide or quinone/hydroquinone couple. Jankowska et al. 3 recently carried out potential measurements of activated carbon in solutions containing F-, CI-, Br-, and I-. It was found that the adsorption of these anions onto carbon is accompanied by the release of OHion from the carbon surface and a characteristic change in carbon potential. Hence they concluded that the adsorption of anions onto carbon involves both an ion exchange and an oxidative reaction coupled to reduction of chemisorbed oxygen on the carbon surface. More recent work by Fleming and Nicol4 supports the view that activated carbon behaves like an ion-exchange resin in the adsorption of Au(CN)~ anion. On the other hand, the observed carbon catalyzed oxidation of CN- to CNO- by air 5 supports the redox character of carbon. A study of the adsorption of various anions onto carbon, particularly CN-, Ag(CN) 2, and Au(CN) ~, and their effect on the carbon potential is therefore of interest in characterizing the active sites on carbon and in developing an understanding of the mechanism of adsorption of silver and gold cyanide. In this work the adsorption of ions was measured according to pH, and the change in pH observed upon adsorption was monitored. The carbon potential was then measured in a flow-through cell as various halide and cyanide N. TSUCHIDA, formerly Postgraduate Student, Murdoch University, is Metallurgist with Beshi Nickel Refinery (Sumitomo Metal and Mining Co.), Niihama, Japan. D.M. MUIR is Associate Professor in Mineral Science, Murdoch University, Murdoch, Western Australia. Manuscript submitted June 24, 1985. METALLURGICALTRANSACTIONSB
complexes were injected into the solution flowing through the carbon. Changes in potential are discussed in relation to the adsorption equilibrium of each ion. II.
EXPERIMENTAL
A. Chemicals and Activated Carbon Potassium dicyanoaurate and dicyanoargentate were used as supplied by Johnson-Matthey, while all other chemicals were of AR grade, dissolved in double distilled deionized water. Deaerated solutions were prepared by sparging with N2 gas for 1 to 2 hours. All experimental work used demineralized Norit R2020 carbon which had been manufactured from peat by a steam activation process. It was cho
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