Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction2017-04-04T06:57:36-04:00
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Thiocyanate Leaching of Gold (3 replies and 2 comments)

4 years ago
emanus 4 years ago

hello metallurgist;

please, what is the function of iron (Fe) in gold leaching process with thiocyanate as a Gold Dressing Agent composition?


4 years ago
David 4 years ago

The leaching of gold ores by acid solutions of thiocyanate produces two complexes, Au(SCN)2- and Au(SCN)4-.

The reaction involves oxidation. It is believed that the addition of Iron III is a good oxidant.

Trying to add oxygen is not a good option because the gold dissolution will be very slow.

Also the addition of iron tends to make more stable the thiocyanate.

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4 years ago

That's interesting, David. Is this process actually used industrially? If so, can the Fe II be recovered and re-oxidised, giving iron the role of catalyst rather than reactant? Otherwise, it seems to me that using up multiple Fe atoms per Au atom could add significant cost and waste stream to the process..

4 years ago
David 4 years ago

The possible use of thiocyanate was studied the first years of the twenty century. The reagent was only evaluated in laboratory tests, but not in an industrial gold operation. During the dissolution of gold by iron, it was established that Fe III is reduced to Fe II, and the oxidizing of thiocyanate is promoted, producing (SCN)3- and (SCN)2, the reaction is fast and iron is not recovered, these SCN compounds are not very stable, this is one of the main disadvantages. Other point to comment is the effect of temperature, gold dissolution is better at high temperatures. It is easy to realize that to build a plant could involve a higher Capex and Opex than a regular cyanidation plant.

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Marc LeVier
4 years ago

Newmont was granted a patent on use of SCN around 2010/11. Patent description is available but application is a narrow window.

2 years ago
Unterstarm 2 years ago

Thiocyanate as a leaching agent for gold has been studied by many. The overall reaction for gold dissolution by thiocyanate can be written as:

Au + 2SCN- + Fe+3 ⇔ Au(SCN)2- + Fe+2.........................................[2]

The mechanism of dissolution of gold by thiocyanate in ferric sulfate solutions was found to be directly related to the oxidation of thiocyanate by the reduction of Fe+3 to Fe+2. The oxidation of SCN- proceeds through the formation of several intermediate species, in particular trithiocyanate (SCN)3- and thiocyanogen (SCN)2, which act both as oxidants and complexants for gold.

noncyanide leaching eh-ph diagram

Figure 2 presents an Eh-pH diagram for the Au-SCN-H2O system at 25°C, assuming an activity of 10 -5 for all dissolved gold species and a thiocyanate activity of 10 -1. Thermodynamic analysis suggests that gold can be leached by thiocyanate in ferric sulfate solutions at potentials of 600-700 mV (SHE) at pH 1-3.

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