Used in the past for gold dissolution, iodine trends to forms the most stable gold complexes of all halogens. As a result, the redox potential at which gold dissolves in an iodine solution is about half that at which it dissolves in acidified chlorine. The redox potential at which gold dissolves in iodine is, however, considerably higher than in cyanide. Iodine leaches gold over a wide pH range. See the next table.
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Leaching Agent
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Redox Potential, Volts
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Cl-
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+1.15
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Br-
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+0.96
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I-
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+0.56
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Thiourea
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+0.36
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CN-
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-0.61
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Iodine is an element very expensive, it cost about $7 to10/lb, but it can be regenerated from the electrolyte. The high cost demands the efficiency recovery and recycling of iodine. Less pure grades of iodine are said to be available at around $1 to 3/lb. Iodine does not absorbs to any great extent on gangue particles, so good recoveries of reagent is possible.
Iodine at very low concentrations leaches gold. If the ore or pulp has to be washed with a lot of water to recover the lixiviant, iodine can be recovered from the resulting dilute solution using a simple ion exchange technique. When recovered in that manner, iodine is readily eluted by hot water, with leaves it in a recyclable form.
When iodine attacks a gold ore where there is pyrite or other reducing agents, gold is solubilized as iodize complex AuI2- and AuI4-, and pyrite and other minerals are oxidized by iodine forming iodide. For an economical process iodine must be recycled, thus iodine must return to its oxidizing state and the gold have to be removed. These requirements are difficult and expensive. The difficulty is on the fact that iodine and gold-complex behave very similar in the presence of reducing materials such as resins or activated carbon which are employed to recover gold from liquors. Ergo, gold cementation with iron or zinc dust results in solubilization of big amounts of these elements because iodine and gold will be oxidized. Recovery by resins or activated carbon has a problem, gold and iodine competes for the same places at the same time.
In this process gold is leaches by iodine which is complexed by iodide. The oxidation of sulphide minerals produces iodide as a reaction product. The latter is combined with iodine favoring its dissolution because iodine has a low solubility (0.3 g/l). Oxidants are no necessary to be added. Iodine concentration must be in the range 2 to 10 gr/l.
The reactions are:
FeS2 +I2 = Fe2+ + 2S + 2I-
Au + I- + I2 = AuI2-
Iodine penetrates gold particles particularly well and, above pH 8, it does no complex iron and attacks sulphides weakly. Below pH 7, iodine does attack sulphides, at which time is converted in iodine ions rather than combining with sulphur. Iodine can be regenerated to I2 electrolytically in a diaphragm cell. During this electrolytical process iodide is reduce to iodine in the anode and gold is deposited in the cathode. Thus, iodine can be recycled and gold can be recovered.
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