Spraying cyanide on leach pads
at the Zortman-Landusky mine
in Montana (Credit: Timothy
Green)

Ore is heaped onto open-air leach pads with a base of asphalt or impervious plastic sheeting. A sprinkler system is then laid along the top of the ore pile through which a solution of dilute cyanide is sprayed. The cyanide percolates down through the heap for several weeks, leaching out the gold. This solution, now enriched with gold, drains off the bottom of the pad into what is known as the ‘pregnant pond’, from which it is pumped to the recovery plant.

Heap Leaching: extraction of gold using heap leaching and carbon recovery

Heap leaching of gold was pioneered in the United States in 1973 at Placer Development’s Cortez open pit in Nevada and proved on a larger scale at Pegasus Gold’s Zortman Landusky mine in Montana. Although it is low cost, recovery rates average only sixty to seventy per cent, significantly less than with conventional milling. But it has enabled low-grade ores, which otherwise might not be economically viable, to be processed. In the United States, where heap leaching is used most extensively, half of all production is won by this method. See also Biological Leaching.