Heap leach mining is an industrial hydrometallurgical process, where the metal contained in an ore body is extracted by dissolution. The target mineral is leached with an aqueous solution whereby the mineral is dissolved into what is called a “pregnant” solution which, is then captured and later recovered downstream in the processing line. Heap leach mining enables the extraction of minerals from lower grade ores that otherwise would not be economically viable to extract. However, it requires the use of many specialized techniques that aim to make the process more efficient, safe for operators, and allow minimal negative impact to the environment.
Ore initial processing
In heap leach mining the ore is prepared according to its starting particle size, and is typically crushed to achieve a particle size within the desired range of about 5mm. This particle size is desirable since it makes it easier for the leach solution to percolate through the ore, as well as achieve ideal particle size distribution for capillary effect to take place within the heap body for maximum solution saturation.
When starting off with ore that is of small particle sizes, such as sand, dust or fine powder, agglomeration must be done in order to increase solution permeability within the heap. Agglomeration is a process in which the ore is passed through a rotating drum and is mixed with cement and leaching solution that results in cementation of smaller particles into larger particle sizes to achieve the desired size of 5mm. Apart from increasing the particle size, agglomeration has the additional advantage of homogeneously distributing leach solution within the fresh ore before it is stacked into a heap, resulting in an improved rate of metal recovery.
Leaching solution composition & distribution
Leaching solutions (or lixiviants) used in heap leach mining are either highly alkaline such as sodium cyanide, or highly acidic such as sulphuric acid.
To ensure uniform distribution of the lixiviant to the ore heap and to minimize evaporation, drip irrigation systems are the preferred distribution method. As with the leach pad liner system, the drip irrigation system also has to withstand the harsh chemical stresses and environmental conditions it is subjected to. Moreover, the drip irrigation system should provide excellent anti-clogging resistance, and offer exceptional leaching solution distribution throughout every solution drop point. An example of such robust drip irrigation product is the DRTS Minoro™, a dripper designed specifically for leach mining applications that offers exceptional clogging resistance and uniform lixiviant flow.
Heap leach pad
Following the crushing and or agglomeration stage, the ore is then transferred onto a heap leach pad, where the long process of metal leaching from the ore to the lixiviant takes place. Typically leach pads are made by grading the surface with a slight down slope, or if an operation is limited in area the natural contours of the adjacent ridges will be used in what is called a “valley fill” leach pads. In both cases the base of the pads are lined with an impermeable layer so that the solution after percolating through the height of the pad is then collected at the base and is allowed to drain by means of ditches or network of drainage pipes which convey the “pregnant” solution to storage ponds for further processing.
To contain the leaching solution and prevent seepage to the environment leach pad liner systems are used. Liner systems can vary from operation to operation and are typically engineered based on surface conditions and leach pad specifications, but primarily involve the use of compacted clays, geo-textiles and geomembranes with additional use of geo-nets for lateral support. Most importantly is that the liner system must meet and fulfill strict specifications, be completely impermeable, withstand prolonged exposure to acidic and alkaline solutions, resistant to abrasions and puncture, and offer overall chemical and thermal resistance.
After the “pregnant” leach solution drains to the storage ponds, it is then conveyed to the recovery circuit where the target metal is extracted from the pregnant solution leaving behind what is called “barren” solution. The process of heap leaching is a closed-loop process, which means that no solution leaves the circuit, thus the “barren” solution is then pumped back to the top of the heaps for continual ore leaching.
Learn more with DRTS
DRTS has over 35 years of experience in supporting the production needs of drip irrigation product manufacturers for a wide range of applications, including heap leach mining. We provide leach mining project design services, uncloggable mining grade drippers and a wide range of accessories. Contact DRTS for a free consultation, schedule a call with an expert, and discover solutions tailored to your needs and budget. Or feel free to learn more about our mining products and services on our leach mining page.