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Cyanidation has been the industry standard process for leaching of gold since the beginning of the 20th century. Cyanide is a powerful lixiviant that forms extremely stable complexes with gold. The cyanide processing takes advantage of the ability of dilute cyanide solutions to selectively dissolve gold from the gangue material and for simple recovery of the gold by carbon adsorption on precipitation on zinc. The disadvantages of cyanidation of the toxicity associated with cyanide and its ineffectiveness in treating refractory ores and concentrates.
Other alternative lixiviants have been study to overcome these weaknesses of cyanidation. Of these lixiviants, the most commonly studied are the halides (Cl, Br, and I), thiourea, and thiosulfate. Other lixiviants which have received less attention include malononitrile, 3-bromo-1-chloro-5, 5 dimethylhydantoin, a-hydroxynitriles, N, N'-ethylenethiourea, and polysulfides. All of these lixiviants have shown promise, but the most promising appear to the halides and thiosulfate based on the number of technical articles that have been generated about these lixiviants.
The purpose of this investigation was to collect and review a through technical bibliography concerning alternative lixiviants for gold with particular interest give to the halides. This report will include the result of the literature search, a discussion about the pertinent process chemistry, the development potential of the lixiviants and recommendations for future work.
Pertinent Process Chemistry
The dissolution of gold in aqueous solution involves oxidation of gold into an ionic specie coupled with complexation to stabilize the gold ion in solution. In the dissolution of gold using cyanide as the lixiviant this can be written as the following chemical reactions:
(Oxidation of Gold) Au = Au+ + e-
(Complexation of Gold) Au+ + 2 CN- = Au(CN)2-
(Net Reaction) Au + 2 CN- = Au(CN)2- + e-
The most common oxidizing agent in alkaline cyanide solution is dissolved oxygen. The overall leaching of gold depends on the reaction
4 Au + 8 CN- + 2 H2O = 4 Au(CN)2- + 4 OH-
Therefore, it is imperative that an oxidizer and complexing agent are present in solution to extract gold. Because of the need for a complexing agent, pH is also crucial in maximizing the ability of lixiviant to extract gold. The solution's pH will dictate what specie the complexing agent forms in solution and thus the form of the gold complex. In extreme case, as with cyanide, pH not only controls the leaching ability of the solution, but also the toxicity.
With these three factors in mind, the process chemistry involving the halogen complexers will not discussed in the order of Cl, Br, and I. Dissolution of gold using chlorine or chloride ion has been known from several centuries. Gold will dissolve in the presence of a strong oxidant and chlorine ion only in highly acidic solutions. Examination of the Eh - pH diagram for the Au-Cl-H2O system reveals that pH < 3 and E > 1.1 V is needed to stabilize AuCl4- in solutions containing [Au] = 10-5 M and [Cl] = 10-2 M. In dilute chloride solutions, AuCl2- will not be stable due to the following disproportionation reaction
3 AuCl2- = AuCl4- + 2 Auo + 2 Cl-
At higher concentrations of chloride, AuCl4- becomes stable at lower potentials and AuCl2- can be stabilized. Examples of adequate oxidants for gold in chloride media are nitric acid (aqua regia), chlorine, hypochlorous acid, and hypochlorite acid.
Complexation of gold with bromide ions can generate stable Au(I) and Au(III) complexes. AuBr2- is stable between 0.90 V and 0.95 at pH < 9 with [Au] = 10-5 M and [Br] = 10-2 M. AuBr4- is stable above 0.95 V and is stable over a larger set of potential and pH than AuCl4-. In acidic solutions, bromine can oxidize gold in the presence of bromide ions.
Of the halides, iodine exhibits the widest range oxidation and pH conditions for stabilization of gold complexes. At [Au] = 10-5 M and [I] = 10-2 M, AuI2- is stable between 0.51 V and 0.69 V and at pH < 12. AuI4- is stable above 0.69 V and is stable over a larger set of potential and pH than AuBr4- or AuCl4-. Gold iodide complexes are the most stable relative to the halide complexes in aqueous solutions.
Iodine and Bromine also exhibits the ability to exist as a polyhalide. The formation of the triodide ion is shown in the following chemical reaction
I2(aq) + I- = I3-
The formation of the I3- allows for high solubility of iodine in solutions containing iodide ion. Thus, I3- can serve as an oxidant for gold according to these half-cell reactions
(Anodic) Au + 2 I- = AuI2- + e
(Cathodic) I3- + 2e = 3 I- _
(Net Reaction) 2Au + I3- + I- = 2AuI2-
Thiosulfate ion, S2O32-, has a structure similar to sulfate with one oxygen replaced by a sulfur atom. It is this second sulfur atom that dominates the complexing tendencies and reducing properties of the thiosulfate ion. Thiosulfate form complexes with a variety of metals including gold. The chemistry associated with the leaching of gold with thiosulfate can be rather involved. Gold will dissolve in the presence of ammonium thiosulfate as follows
4 Au + 8 S2O32- + 2 H2O + O2 = 4 Au(S2O3)23- + 4 OH-
Examination the Eh-pH diagram of the gold-ammonia-thiosulfate-water system at 25 oC with [S2O32-] = 0.1 M, [NH3] = 0.1 M, and [Au] = 10-5 M indicates that the stability of Au(S2O3)23- at pH less than approximately 9. The lower limit of oxidation potential varies with pH but is a minimum of about 0.1 V occurs between pH 4 and 9.
Leaching gold with thiosulfate, however, was found to be sensitive to the concentration of copper, thiourea, and temperature of the solution. The relationship between gold dissolution and copper concentration is complex and involves the complexation of copper by thiosulfate. At low concentrations, copper can have a catalytic influence, but at elevated levels it can be detrimental to the leaching process. To assure the catalytic effect of copper the ratio of ammonia to thiosulfate must be maintained. Degradation of thiosulfate to tetrathionate by air can also occur which then results in various side reactions with can further complicate the leaching reactions.
Here is a more scientific explanation of thiosulfate leaching
A considerable amount of study has occurred involving alternative lixiviants to extract gold. The halogens have received the most attention with numerous articles and patents regarding leaching of gold ores. Several systems using chloride, hypochlorite, and/or ferric chloride have been devised for heap and in situ leaching of gold. These processes claim to generate less cost due to lower reagent consumption particularly in ores with cyanide-soluble copper, less environmental impact, and faster dissolution. Processes also exist for the use of bromine, bromide, perbromate, iodine, and iodide to extract gold. These processes again appear to be beneficial in processing refractory or preg-robbing ores. With the growing use of bacteria assisted leaching of refractory ores, the ability of these halogen based leachants to extract gold under acidic conditions provides a valuable benefit. From the literature, the most serious problem associated with the use of halides is the recovery of gold from solution.