thiosulfaatleaching

Thiosulfate leaching is a hydrometallurgical process that dissolves gold and some other metals from ores and concentrates using thiosulfate ions as complexing agents. The most common leachants are sodium thiosulfate or ammonium thiosulfate, typically used under mildly alkaline conditions (pH around 9–10). The method is pursued as an alternative to cyanide in settings where cyanide use is restricted or where ores are difficult to dissolve with cyanide alone.

Chemistry and mechanism: Gold dissolves in thiosulfate solutions to form soluble gold–thiosulfate complexes, principally [Au(S2O3)2]3−. Silver can form similar thiosulfate complexes such as [Ag(S2O3)2]3−. Thiosulfate itself is oxidized during leaching, generating sulfur- and sulfide-containing species such as tetrathionate, which can consume thiosulfate and reduce efficiency. To sustain leaching, an oxidant is maintained to regenerate thiosulfate and to oxidize sulfide phases that may impede dissolution. Common oxidants include dissolved oxygen, hydrogen peroxide, or copper(II) ions used as catalysts.

Process forms and applications: Thiosulfate leaching can be carried out in tank leaching or heap leaching configurations and is often pursued for deposits where cyanidation is undesirable due to toxicity concerns or environmental regulations. It is particularly explored for refractory or carbonaceous ores and for recovering gold from materials with high preg-robbing tendencies where cyanide consumption is problematic.

Advantages and limitations: The approach offers a less toxic alternative to cyanide and can be more selective for certain ores. However, thiosulfate solutions are chemically unstable, and high reagent consumption, complex oxidation chemistry, and the need for reliable oxidants pose economic and operational challenges. As a result, thiosulfate leaching remains mainly in pilot testing or niche industrial applications rather than as a universal replacement for cyanide.