Cyanide-free Biocatalyzed Leaching of
Gold and Silver Ore



Process which uses microorganisms for the recovery of precious metals from metal-containing processes and streams. This technology is targeted at using naturally-occurring, sulfate-reducing bacteria for the recovery of gold and silver from ores, instead of the more traditional -- and more toxic -- cyanidation process.

How it works:

Step 1: Aerobic bacteria catalyze bio-oxidation of low-grade, hard to treat (refractory), sulfidic gold ore. This step is currently being adopted by gold producers for freeing gold from base-metal sulfides (e.g., pyrite). A waste stream from the bio-oxidation step is used in a second step.

Step 2: Naturally-occurring, sulfate-reducing bacteria are used to convert the dissolved sulfate in acid mine drainage into a dissolved bisulfide leaching agent and to neutralize the acid mine drainage. These bacteria can use wood alcohol, grain alcohol or vinegar as food, and they are also capable of consuming hydrogen produced by the gold dissolution process. Gold dissolves in the bisulfide solution and is recovered with activated carbon or zinc dust. If needed, excess sulfur can be recovered as a byproduct. By using the natural sulfur cycle, the process provides a complete solution to the gold recovery problem.

Potential Competitive Advantages:

  1. More environmentally friendly -- the bisulfide leaching agent used is about 200 times less toxic than cyanide. Since its invention in 1899, cyanidation and its variants have been the processes of choice for extraction of gold and silver from oxidized ores. Cyanide, however, is legendary in its toxic potency.
  2. Improved economics -- with increasing awareness for the environment worldwide, containment, treatment costs and time spent on environment impact studies associated with cyanidation plants have skyrocketed. These factors have raised the economic hurdle necessary to justify a working mine. Environmentally acceptable alternatives could broaden the definition of an attractive mine, through reduction of the economic and environmental risks. In addition, preliminary results indicate chemical reagent costs could be 80 percent lower than cyanide.

Examples of Applications and Markets:

Applicable to both new and existing mines in the $10 billion/year international gold and silver mining industry. A second application of the process is recovery of gold and silver from scrap.


Our studies in Phase I revealed no theoretical or practical reasons why an optimized YES process cannot achieve the efficiency and efficacy on par with cyanidation, with a concurrent reduction in chemical reagent costs of about 80 percent. The National Science Foundation awarded a $300,000 Phase II grant to advance the research into this process. YES has achieved 75 percent gold extraction during two-stage leaching experiments. Additional R&D funding is being sought to expand research on specific steps critical to the process.

Commercial Interest:

YES can recommend labs who have permission to use this novel bisulfide leaching process to perform amenability testing on bio-oxidized ore samples. We are open to negotiating an R&D evaluation license with either a single company or with a consortium of companies, allowing a credit against future royalties for any investment made in process development. We will also grant a nonexclusive, royalty-free license to practice our patents to the first company to build and successfully operate a full-scale plant based on our technology.


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