The successful application of heap leaching to the extraction of gold from low-grade deposits has been one of the main factors in higher output since the 1970s, especially in the United States. It is a low cost process that extracts a soluble precious metal or copper compound by dissolving the metal content from the crushed ore.
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
Cyanide has a natural affinity for gold
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.