Leaching gold and silver from ore heap leaching

Heap leach mining is an industrial hydrometallurgical process that extracts metals contained in ore bodies through dissolution. The target minerals are leached with an aqueous solution, thereby dissolving the minerals into a so-called "enriched" solution, which is then captured and subsequently recovered downstream of the processing line. Heap leach mining enables the extraction of minerals from low-grade ore.

Leaching process

Ore primary processing

In heap leach mining, the ore is prepared according to its starting particle size and is usually crushed to achieve a particle size in the desired range of about 5 mm. This particle size is ideal as it allows the leach solution to penetrate the ore more easily and achieves an ideal particle size distribution for maximum solution saturation.

In addition to increasing particle size, agglomeration has the added advantage of evenly distributing the leachate throughout the new ore before it is piled into piles, thereby increasing metal recovery.

Leachate composition and distribution

The leach solutions used in heap leach mining are either highly alkaline, such as sodium cyanide, or highly acidic, such as sulfuric acid.

To ensure even distribution of the leachant into the heap and to minimize evaporation, a drip irrigation system is the preferred distribution method. Like leach pad lining systems, drip irrigation systems must withstand harsh chemical stresses and environmental conditions. In addition, the drip irrigation system should provide excellent resistance to clogging and provide excellent distribution of leach solution at each solution drop point.

Heap leach pad

After the crushing stage, the ore is transferred to a heap leach pad, where the lengthy process of leaching metal from the ore to the leachant occurs. Typically, leach pads are made by grading the surface and sloping it slightly downwards, or if the operating area is limited, the natural contours of adjacent ridges are used for what is called a "valley fill" leach pad. In both cases, the bottom of the mat is lined with an impermeable layer so that the solution, after penetrating through the height of the mat, is then collected at the bottom and allowed to drain through a network of ditches or drains that carry the "pregnant" The solution is stored in a tank for further processing.

To contain the leach solution and prevent leaching into the environment, a leach pad lining system is used. 

Metal extraction 

After the "enriched" leachate is discharged into the storage tank, it is sent to a recovery loop to extract the target metals from the enriched solution, leaving behind a so-called "lean" solution. The heap leach process is a closed loop process, meaning no solution leaves the loop, so the "lean" solution is then pumped back to the top of the heap for continuous ore leaching.