How does the Cyanide heap leaching process work and what are the potential impacts to the environment?

Answered by Bonnie Gestring – Northwest Program Director for EARTHWORKS

Twin Creeks cyanide leach mine in Nevada

Cyanide leach mining is a type of gold mining that uses cyanide to extract gold from the surrounding rock. First, the rock that contains gold (the ore) is dug out of the ground, leaving behind a large open pit. Then, the ore is crushed into small pieces. The crushed ore is placed in large piles, called heaps, and sprayed with cyanide solution. The cyanide solution trickles down through the pile of crushed ore and dissolves the tiny gold particles away from the surrounding ore. The cyanide gold solution is collected at the bottom of the heap by a synthetic liner and sent to the mill. There, the gold is removed from the cyanide solution and the cyanide solution is sent back to the heap to be used again. As more ore is dug out the ground, it is added in layers to the top of the heap. These heaps can become very large, covering hundreds of acres.

One of the biggest risks from cyanide leach mining is the possibility of cyanide spills into rivers and streams, or cyanide solution seeping through the soil into groundwater. Although mining companies work hard to contain the cyanide solution with synthetic liners and other types of protections, cyanide spills have commonly occurred at cyanide leach mines. Cyanide can be harmful to humans if consumed, and toxic to fish at even lower levels. A cyanide spill in Europe in 2000, resulted in a fish kill and long-term health risks.

Although cyanide will break down in rivers and streams when exposed to air and sunlight, it can also form other compounds that can be harmful to fish and aquatic life. Cyanide can last for a very long time in groundwater because sunlight and oxygen aren’t present. This can be a serious problem if the groundwater is used as a source of drinking water, or if the contaminated groundwater is a source of water to nearby streams. The State of Montana prohibited open pit, cyanide leach mining in 1998 due to the water quality impacts at five major cyanide leach mines. (Zortman Landusky, Golden Sunlight, Beal Mountain, Basin Creek, and Kendall).

It is also important to understand that cyanide leach technology has made it possible to mine low-grade ores, where only very small amounts of gold are mixed in with the rock. After the gold is removed, these types of mines generate a large volume of mine waste that must be permanently stored on the land. This mine waste can be a long-term source of pollution depending on what other harmful metals, such as mercury, copper, lead, selenium, sulfides, and thallium, are in the waste and how it is managed. For example, the Zortman Landusky mine has resulted in water pollution that has impaired drinking water sources, fish and wildlife habitat, recreation, and agricultural uses. The Kendall Mine resulted in water quality impacts to neighboring ranchers, and the Beal Mountain Mine resulted in impacts to native trout streams.

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