Heap leaching
Heap leaching is a chemical process to extract precious metals and copper compounds from ore.
Process:
The mined ore is crushed into small chunks and heaped on a leach pad. A diluted cyanide solution is then sprinkled onto the heap. The solution percolates through the heap and leaches out the precious metal. This can take several weeks. The solution containing the precious metals ("pregnant solution") is collected for further processing on the bottom of the heap in a storage pond. After stripping the precious metals from the pregnant solution, the cyanid solution (now called "barren solution") is either re-used in the heap-leach-process or sent to a water treatment facility where the residual cyanid is destroyed and all residual metals are removed. The water can then be discharged to the environment.
Although the heap leaching is a low cost process, it only has recovery rates of 60-70%. Therefor it is normally only used on low-grade ores]. (The precious metals in higher-grade ores are extracted in more complex processes, depending on the metalurgic properties of the ore.)