Ammonium thiosulfate has emerged as one of the most promising alternatives to cyanide for the leaching of gold and silver from ores. With growing environmental concerns about cyanide use in mining, ammonium thiosulfate offers a safer, more environmentally friendly approach to precious metal recovery while maintaining high extraction efficiencies.

The Growing Need for Cyanide Alternatives

Cyanide has been the dominant lixiviant for gold extraction for over a century. However, environmental concerns, regulatory pressures, and the need to process increasingly complex ores have driven the search for effective alternatives. Ammonium thiosulfate has emerged as a leading candidate due to its effectiveness, safety profile, and environmental compatibility.

The Chemistry of Thiosulfate Leaching

The core of the ammonium thiosulfate leaching process lies in the ability of the thiosulfate ion (S₂O₃²⁻) to form stable complexes with gold and silver.

Key Chemical Reactions

• Thiosulfate ions complex with gold and silver ions

• The complexes are soluble in aqueous solution

• Precious metals can be recovered from the solution through various methods

Leaching Process Requirements

pH Control

• The leaching process requires maintenance of a pH greater than 7

• For copper-ammonium thiosulfate systems, pH must be maintained at a minimum level of 9.5

Stabilization

• Sulfite ion concentration of at least 0.05% is required to stabilize the thiosulfate ion in the system

Copper Catalysis

• Copper-ammonium thiosulfate leaching requires the presence of copper ions in the lixiviating solution

Extraction Efficiencies

Research has demonstrated impressive extraction capabilities:

Gold Extraction

• Up to 90% of gold values can be solubilized in one hour

• Ammonium thiosulfate is an effective lixiviant for the hydrometallurgical treatment of sulfide gold ores

Silver Extraction

• About 70% of silver values can be solubilized in one hour

• Effective for silver recovery from difficult-to-treat ores

Ore Types

Ammonium thiosulfate leaching is particularly effective for:

Sulfide Ores

• Ammonium thiosulfate is an alternative lixiviant for the hydrometallurgical treatment of sulfide gold ores

• Studies have focused on leaching from concentrated sulfide ore

Difficult-to-Treat Ores

• Effective for ores containing manganese

• Suitable for ores that are difficult to treat with conventional cyanidation

Advantages Over Cyanide

Process Considerations

Sensitivity to Conditions

• The leaching process is quite sensitive to thiosulfate and ammonia concentrations

• Maintenance of proper Eh (oxidation-reduction potential) is critical

Secondary Leaching

• Ammonium thiosulfate can be used as a secondary leach for the recovery of silver and gold

Market Trends

The growing demand for environmentally friendly mining solutions is likely to bolster the market for ammonium thiosulfate in mining applications. As industries seek alternatives to traditional chemicals, ammonium thiosulfate presents a viable option, aligning with global trends towards sustainability and eco-friendliness.

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