How to Leach Gold From Low-Grade Ore Using Cyanide Alternatives

Gold extraction from low-grade ore is an essential part of the mining industry, particularly as higher-grade deposits become depleted. Traditionally, cyanide has been the go-to chemical for gold leaching due to its effectiveness in dissolving gold from ore. But, due to environmental concerns and regulatory pressures, alternative leaching methods have gained traction. This article explores various cyanide alternatives and their implementation in gold extraction.

The Problem with Cyanide

Cyanide, while effective, poses significant environmental risks, including the toxic effects on wildlife and potential contamination of water sources. The International Cyanide Management Code regulates the use of cyanide in gold mining, pressing the industry to explore safer alternatives.

Alternatives to Cyanide

Several alternatives to cyanide researched and developed for gold leaching. The most prominent among these are:

Thiosulfate: A more environmentally friendly option, thiosulfate can dissolve gold in aqueous solutions without the harmful effects associated with cyanide.
Chlorine Leaching: This method uses chlorine gas to leach gold and is considered a less toxic alternative to cyanide.
Bioleaching: Utilizing bacteria to extract gold, bioleaching presents a method that is sustainable and employable in low-grade deposits.
Acid Leaching: Sulfuric acid and nitric acid can be used in conjunction to enhance gold recovery from low-grade ores.

Thiosulfate Leaching

Thiosulfate leaching has gained attention for its lower environmental impact compared to cyanide. In laboratory experiments, thiosulfate can recover up to 93% gold from certain ores. For example, the Betze-Post and Goldstrike mines in Nevada have shown successful implementation of this method. The overall process involves:

Preparing a thiosulfate solution.
Adding the solution to crushed ore.
Separating the leachate containing dissolved gold.

Chlorine Leaching

Chlorine leaching is another viable method, especially in the context of oxidized ores. This method has shown to yield results comparable to cyanide leaching. For example, the use of chlorine in the Kalgoolie Gold Mine in Australia has illustrated the feasibility of this method. The process generally includes:

Crushing and grinding the ore.
Injecting chlorine gas into a slurry of the ore.
Recovering gold through precipitation or electrolysis.

Bioleaching

Bioleaching employs microorganisms to extract metal from ores, making it a promising alternative for low-grade deposits. For example, the use of Acidithiobacillus ferrooxidans bacteria can significantly improve gold recovery by breaking down sulfide minerals. This method is beneficial as it requires milder conditions and can be scaled for large operations.

Real-World Applications

Real-world applications demonstrate the practicality of these cyanide alternatives. The use of thiosulfate at Barrick Gold Corporations Goldstrike operations, and bioleaching in some South American copper-gold projects have showcased not only improved gold recovery rates but also minimized environmental impacts.

Potential Challenges

Despite the promise of these alternatives, some challenges persist. Thiosulfate is less stable than cyanide, requiring careful handling and conditions to maintain efficacy. Also, chlorine technology demands significant investment in terms of infrastructure and operational changes. The biological leaching process can also be slow and requires specific environmental conditions to be efficient.

Conclusion

The shift from cyanide to alternative leaching methods is not just a trend but a necessity for sustainable gold mining. As the industry faces increased scrutiny over environmental practices, embracing alternatives such as thiosulfate, chlorine, and bioleaching presents viable and responsible approaches to gold recovery from low-grade ores.

Actionable Takeaways

For mining companies considering these alternatives, the following steps are recommended:

Conduct pilot studies to assess the effectiveness of thiosulfate or bioleaching on specific ore types.
Invest in training for employees on new technologies and safety practices for handling chemicals like chlorine.
Collaborate with environmental experts to ensure that all methods comply with international regulations and environmental standards.

By adopting these strategies, mining operations can improve gold recovery while committing to environmentally sustainable practices.