Cyanide Leaching

Gold particles in crushed rock can be brought into solution as Au(CN)2 ion by leaching the rock with aerated sodium cyanide solution (see Exercise 15.2) the aluminosilicates remain undissolved  

The reaction is rather slow, requiring several days, after which the gold may be recovered by electrolysis (Section 15.7) or by reaction with zinc ( cementation )  

When zinc cementation is to be used, the gold-bearing cyanide solution must first be deoxygenated to prevent oxidation of Zn(s) to Zn(CN)4 by O2 rather than Au(CN)2.  

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Sulfur dioxide is usefiil as a solvent for sulfur trioxide in sulfonation reactions for example, in the large-scale production of alkylbenzenesulfonate surfactant (329). A newer use for sulfur dioxide is in cyanide detoxification in connection with cyanide leaching of precious metals from mine dumps. 

The capability of zinc to reduce the ions of many metals to theh metallic state is the basis of important appHcations. However, metals are removed from zinc solutions by displacement with finely divided zinc before winning by electrolysis. Gold and silver are displaced from cyanide leach solutions with zinc and the following metals are similarly recovered from various solutions platinum group, cadmium, indium, thallium, and sometimes copper. 

Gold telluride Thermal treatment AuTe2 —> Au + 2 Te Cyanide leaching... 

Schmitz, P. A. Duyvesteyn, S. Johnson, W. P. Enloe, L. McMullen, J. Ammoniacal thiosulfate and sodium cyanide leaching of preg-robbing Goldstrike ore carbonaceous matter. Hydrometallurgy 2001, 60, 25 40. 

Kordosky, G. A. Sierakoski, J. M. Virnig, M. J. Mattison, P. L. Gold solvent extraction from typical cyanide leach... 

AuPLUS A hydrometallurgical process for extracting gold. Addition of calcium peroxide to the cyanide leaching liquor increases the rate and amount of gold extracted. 

CIL [Carbon In Leach] A process for extracting gold from cyanide leach liquors using activated carbon. 

CIP (1) [Carbon In Pulp] A general name for hydrometallurgical extraction processes that use activated carbon in slurries of ground ores. One such process is used for extracting gold from cyanide leach liquors. First operated on a large scale at the Homestake lead mine in South Dakota in 1974. See also RIP. 

The National Science Foundation is funding research at the South Dakota School of Mines, Rapid City, South Dakota, to develop an alternative approach to the extraction of precious metals from refractory ores which will pose a much lesser threat to environmental quality than the presently predominant cyanide leaching process. 

Brankovic et al. [510] have described the surfactant-mediated electrochemical deposition of Ag on Au(lll). Also, Reyes-Cruz etal. [511] have studied electrochemical deposition of Ag and Au from cyanide leaching solutions. 

In the extraction process, the aurocyanide complex plus other metal cyanide complexes that are common in cyanide leach liquors (e.g. Fe(CN) 4-, Fe(CN)63-, Zn(CN)42-, Ni(CN)42-, Cu(CN)43-and Co(CN)63-) load onto the resin by simple ion exchange ... 

The elution of aurocyanide and other anions from a weak-base resin is achieved by neutralization of the amine functional groups (equation 96). The method is cheap and very effective 366 this is a major factor favouring the use of weak-base resins, particularly since recent research has shown that modem weak-base resins extract "aurocyanide efficiently from cyanide leach liquors that have undergone minimal adjustment of pH.358 368... 

The coordination chemistry of both metal ions involved plays a vital role in governing the thermodynamics and the kinetics of such reactions. This will be illustrated by reference to the chemistry involved in the precipitation of gold from cyanide leach liquors by metallic zinc powder — the most widely applied method for the recovery of gold from solutions.415 The overall reaction is... 

A cyanide leaching pool for extracting gold at the Picacho Gold Mine in the Mohave Desert. 

Other general applications have included catalyst carriers and decolorizing for gold recovery from cyanide leach solutions. Of particular interest is its effective use for military and industrial clothing where protection from noxious vapors is necessary. Recent studies have shown its ability to separate chemical isomers, such as the separation of dioxin isomers. It has been used worldwide for analytical methods for determining trace (PPB) toxic substances in water,... 

It is well known that gold is extracted and also assayed by the familiar cyanide leaching. 

After flotation or other concentration processes, the crucial chemical steps are cyanide leaching and zinc precipitation, for example,... 

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