The solution containing the precious metals ("pregnant solution") continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage (pregnant solution) pond. After separating the precious metals from the pregnant solution, the dilute cyanide solution (now called "barren solution") is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. The water is then discharged to the environment, posing possible water pollution.
During the extraction phase, the gold ions form complex ions with the cyanide:
Recuperation of the gold is readily achieved with a redox-reaction:
Although the heap leaching is a low cost-process, it normally has recovery rates of 60-70%, although there are exceptions. It is normally most profitable with low-grade ores. Higher-grade ores are usually put through more complex milling processes where higher recoveries justify the extra cost. The process chosen depends on the properties of the ore.
Exeter Resource Corporation: Caspiche Gold Property Metallurgical Testwork Indicates Potential for Heap Leaching of Oxide Mineralization
Nov 07, 2007; VANCOUVER, BRITISH COLUMBIAMarketwire - Nov. 7, 2007) - Exeter Resource Corporation (TSX VENTURE:XRC)(AMEX:XRA)(FRANKFURT:EXB)...