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How does heap leaching extract gold from low-grade ore?

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A diagram showing the periodic table entry for gold, with the symbol Au, atomic number 79, atomic weight 196.96657, electron configuration, and labels for its properties as a solid transition metal with a face-centered cubic crystal structure. — gold The precious metal gold is a chemical element with the symbol Au. Its square on the periodic table details some of its properties, including its atomic weight, atomic number, and crystal structure.

Heap leaching is a cost-effective mining process used to extract gold from low-grade ores, avoiding the high capital expenses associated with traditional vat leaching equipment.1

In this process, crushed ore is piled into large heaps. These heaps are sprayed with a dilute, alkaline solution of sodium cyanide. As the solution percolates downward through the piled ore, it oxidizes and dissolves the metallic gold, forming a gold-bearing solution.1

This method is specifically utilized for low-grade ores because it circumvents the need for complex solids-liquids separation equipment required in higher-grade processing. Once the gold is dissolved, it is typically recovered from the solution using techniques such as adsorption onto activated carbon or the Merrill-Crowe process, where the gold is displaced by zinc metal powder.1