Heap leaching is an industrial mining process of separating precious metals, copper, and other minerals, from ores. It involves a series of chemical processes, through which the mineral ores are piled into the form of a heap, and a leach solution is spread over the ore surface to leach metal from the heap.
Heap leaching may take several weeks, or months, for metal recovery. The leaching time may increase if the heap material is compact, such that the leach solution percolates very slowly via the heap. Low permeability of the heap results in the formation of channels, where fine particles often tend to agglomerate.
Heap Leaching Process
The mined ore is crushed and stacked over a leach pad lined with an impermeable plastic and/or clay. The leach pad is then wetted with the leach solution, under atmospheric conditions, to dissolve the precious metals. The drip irrigation method is commonly used to wet the pad, which reduces the chance of evaporation, thereby providing uniform distribution of the leach solution. The leach solution then penetrates the heap and leaches the precious metals. Simple oxide ores, such as gold ores, can be leached in one or two months, while it takes nearly two years to leach complex ores, such as nickel laterite.
Finally, the percolated leach solution, having dissolved the minerals, is then collected and treated to recover the desired mineral. In some cases, the solution is again recycled to recover minerals precipitated in the solution. The percentage of metal recovery of the target mineral using heap leaching can range from 30 to 90%.
Cyanide Method of Heap Leaching
Cyanide technique is another heap leaching method of extracting precious metals. The crushed ore is wetted with a dilute alkaline cyanide solution, which is then allowed to percolate through the heap. The solution, containing precious metals, continues to percolate via the crushed ore until it reaches the liner beneath the heap, where it drains into a storage pond. After extracting the precious metals, the dilute cyanide solution is usually reused in the heap-leach-process, or passed to an industrial water treatment facility - in some cases to remove residual metals.