Adsorption of gold onto carbon

Once the gold is leached into solution, there is the problem of selectively concentrating it onto carbon. Again the metallurgists have shown how it works and the scientists are beginning to understand why. Over the last five years the AMIRA Gold Group has been looking at the practical properties of carbon for gold adsorption with postgraduate students looking at some of the fundamental characteristics of carbon.

The essential features of the carbons used today are that they are have and have a graphitic structure with a blend of macro and meso pores. Activated carbon possesses surface oxide sites which ion exchanges with aurocyanide to release OH-, and which also adsorbs cations like Ca2+ and H-.

It is known that more active sites for Au(CN)2- are formed by regenerating carbon at 750“ rather than 550°, and that the carbon potential drops after adsorption of reducible species such as CN- and I-. Carbon also catalyses the air oxidation of CN* into carbonate as discussed below.

Some of the functional groups believed to be formed during activation include carboxylic acid, phenolic, lactone, quinone, hydro-peroxide and chromenol [29] . These groups provide the ion exchange or redox properties of carbon. Aurocyanide is believed to adsorb via ion-exchange with OH” at chromenol sites, Ca2’ adsorb at phenolic sites, whilst CN- is believed to be degraded by peroxide sites.

Significantly the pH of the solution affects both the load ng or au(CN>2′ and associated cations, As shown by Tsuchida less aurocyanide but more cations are loaded at pH < 10. It is believed that adsorption of Ca2* and Mg2* provides excess positive change which allows more aurocyanide to adsorb as an ion-pair.