- Cyanide

Cyanide is used in gold mining to extract the gold from the ore. This is done by leaching the rock on engineered, lined pads or by crushing the rock to the consistency of sand (milling), then adding water to form a slurry that is mixed with a cyanide solution. Gold particles bond with the cyanide in solution and are extracted from the slurry.

The slurry of crushed rock left after processing is called tailings (see link to tailings section). Low-grade ore is treated differently because of the cost of milling. It is broken up into smaller pieces and placed in layers on a leach pad made of an engineered, compacted clay lined with high density polyethylene liners placed on top of the clay.

A weak cyanide solution is dripped onto the leach pad to extract the gold and the gold-laden solution is collected on the lining at the bottom of the pad and flows to double – or triple – lined ponds for processing. Leach pads are specially designed and constructed to ensure the solution is contained to prevent any leakage into ground and surface water.

Risks of cyanide use:

The use and disposal of cyanide solutions used to dissolve and extract gold from ore is another environmental concern. Cyanide is a well known poison. Hydrogen cyanide is acutely toxic to humans and, in its gaseous state, can be fatal at exposure levels of 100 to 300 parts per million (ppm).

Cyanide is likewise harmful to wildlife. Mammals, birds, and fish all have acute toxicity reactions to very low cyanide exposures. Cyanide does not, however, accumulate or biomagnify, so prolonged exposure to sub-toxic levels does not, in most cases, appear to pose health risks.

The most significant environmental risk from the cyanide solutions used in gold mining is the possible leaching into soil and groundwater of cyanide at toxic concentrations, or catastrophic cyanide spills that might inundate an ecosystem with toxic levels of cyanide.

However, mining operations in the U.S. take precautions to prevent the cyanide solution from escaping into the environment; the layers of ore and ground up rock are contained in special leach pads lined with a plastic membrane to prevent the cyanide from leaching into the soil. The cyanide is captured and recycled.

  In order to minimize the environmental impact of cyanide that is not recycled, mining facilities treat the cyanide waste through several processes. They will allow it to degrade naturally, which will occur in sunlight (photodegredation), and through hydrolysis and oxidation, among other natural processes. Mine managers can also seek to enhance this natural degradation, as well as using non-natural methods to accelerate the cyanide breakdown.

In addition, researchers are investigating alternate methods of separating metals from ore, including using microorganisms. While these methods are promising, microrganisms work very slowly and may be impractical.

The generally accepted reference for the use of cyanide is the International Cyanide Management Code